The Sun is a G-type main-sequence star (G2V) based on its spectral class. As such, it is informally referred to as a yellow dwarf. It formed approximately 4.6 billion[a] years ago from the gravitational collapse of matter within a region of a large molecular cloud. Most of this matter gathered in the center, whereas the rest flattened into an orbiting disk that became the Solar System. The central mass became so hot and dense that it eventually initiated nuclear fusion in its core. It is thought that almost all stars form by this process.
The Sun is roughly middle-aged; it has not changed dramatically for more than four billion[a] years, and will remain fairly stable for more than another five billion years. After hydrogen fusion in its core has diminished to the point at which it is no longer in hydrostatic equilibrium, the core of the Sun will experience a marked increase in density and temperature while its outer layers expand to eventually become a red giant. It is calculated that the Sun will become sufficiently large to engulf the current orbits of Mercury and Venus, and render Earth uninhabitable.
The enormous effect of the Sun on Earth has been recognized since prehistoric times, and the Sun has been regarded by some cultures as a deity. The synodic rotation of Earth and its orbit around the Sun are the basis of solar calendars, one of which is the predominant calendar in use today.The English proper name Sun developed from Old English sunne and may be related to south. Cognates to English sun appear in other Germanic languages, including Old Frisian sunne, sonne, Old Saxon sunna, Middle Dutch sonne, modern Dutch zon, Old High German sunna, modern German Sonne, Old Norse sunna, and Gothic sunnō. All Germanic terms for the Sun stem from Proto-Germanic *sunnōn.
The English weekday name Sunday stems from Old English (Sunnandæg; "Sun's day", from before 700) and is ultimately a result of a Germanic interpretation of Latin dies solis, itself a translation of the Greek ἡμέρα ἡλίου (hēméra hēlíou). The Latin name for the Sun, Sol, is not common in general English language use; the adjectival form is the related word solar. The term sol is also used by planetary astronomers to refer to the duration of a solar day on another planet, such as Mars. A mean Earth solar day is approximately 24 hours, whereas a mean Martian 'sol' is 24 hours, 39 minutes, and 35.244 seconds.Solar deities play a major role in many world religions and mythologies. The ancient Sumerians believed that the sun was Utu, the god of justice and twin brother of Inanna, the Queen of Heaven, who was identified as the planet Venus. Later, Utu was identified with the East Semitic god Shamash. Utu was regarded as a helper-deity, who aided those in distress, and, in iconography, he is usually portrayed with a long beard and clutching a saw, which represented his role as the dispenser of justice.
From at least the 4th Dynasty of Ancient Egypt, the Sun was worshipped as the god Ra, portrayed as a falcon-headed divinity surmounted by the solar disk, and surrounded by a serpent. In the New Empire period, the Sun became identified with the dung beetle, whose spherical ball of dung was identified with the Sun. In the form of the Sun disc Aten, the Sun had a brief resurgence during the Amarna Period when it again became the preeminent, if not only, divinity for the Pharaoh Akhenaton.
In Proto-Indo-European religion, the sun was personified as the goddess *Seh2ul. Derivatives of this goddess in Indo-European languages include the Old Norse Sól, Sanskrit Surya, Gaulish Sulis, Lithuanian Saulė, and Slavic Solntse. In ancient Greek religion, the sun deity was the male god Helios, but traces of an earlier female solar deity are preserved in Helen of Troy. In later times, Helios was syncretized with Apollo.
In the Bible, Malachi 4:2 mentions the "Sun of Righteousness" (sometimes translated as the "Sun of Justice"), which some Christians have interpreted as a reference to the Messiah (Christ). In ancient Roman culture, Sunday was the day of the Sun god. It was adopted as the Sabbath day by Christians who did not have a Jewish background. The symbol of light was a pagan device adopted by Christians, and perhaps the most important one that did not come from Jewish traditions. In paganism, the Sun was a source of life, giving warmth and illumination to mankind. It was the center of a popular cult among Romans, who would stand at dawn to catch the first rays of sunshine as they prayed. The celebration of the winter solstice (which influenced Christmas) was part of the Roman cult of the unconquered Sun (Sol Invictus). Christian churches were built with an orientation so that the congregation faced toward the sunrise in the East.
Tonatiuh, the Aztec god of the sun, was usually depicted holding arrows and a shield and was closely associated with the practice of human sacrifice. The sun goddess Amaterasu is the most important deity in the Shinto religion, and she is believed to be the direct ancestor of all Japanese emperors. The Sun is a G-type main-sequence star that comprises about 99.86% of the mass of the Solar System. The Sun has an absolute magnitude of +4.83, estimated to be brighter than about 85% of the stars in the Milky Way, most of which are red dwarfs. The Sun is a Population I, or heavy-element-rich,[b] star. The formation of the Sun may have been triggered by shockwaves from one or more nearby supernovae. This is suggested by a high abundance of heavy elements in the Solar System, such as gold and uranium, relative to the abundances of these elements in so-called Population II, heavy-element-poor, stars. The heavy elements could most plausibly have been produced by endothermic nuclear reactions during a supernova, or by transmutation through neutron absorption within a massive second-generation star.
The Sun is by far the brightest object in the Earth's sky, with an apparent magnitude of −26.74. This is about 13 billion times brighter than the next brightest star, Sirius, which has an apparent magnitude of −1.46. The mean distance of the Sun's center to Earth's center is approximately 1 astronomical unit (about 150,000,000 km; 93,000,000 mi), though the distance varies as Earth moves from perihelion in January to aphelion in July. At this average distance, light travels from the Sun's horizon to Earth's horizon in about 8 minutes and 19 seconds, while light from the closest points of the Sun and Earth takes about two seconds less. The energy of this sunlight supports almost all life[c] on Earth by photosynthesis, and drives Earth's climate and weather.
The Sun does not have a definite boundary, but its density decreases exponentially with increasing height above the photosphere. For the purpose of measurement, however, the Sun's radius is considered to be the distance from its center to the edge of the photosphere, the apparent visible surface of the Sun. By this measure, the Sun is a near-perfect sphere with an oblateness estimated at about 9 millionths, which means that its polar diameter differs from its equatorial diameter by only 10 kilometres (6.2 mi). The tidal effect of the planets is weak and does not significantly affect the shape of the Sun. The Sun rotates faster at its equator than at its poles. This differential rotation is caused by convective motion due to heat transport and the Coriolis force due to the Sun's rotation. In a frame of reference defined by the stars, the rotational period is approximately 25.6 days at the equator and 33.5 days at the poles. Viewed from Earth as it orbits the Sun, the apparent rotational period of the Sun at its equator is about 28 days.[The solar constant is the amount of power that the Sun deposits per unit area that is directly exposed to sunlight. The solar constant is equal to approximately 1,368 W/m2 (watts per square meter) at a distance of one astronomical unit (AU) from the Sun (that is, on or near Earth). Sunlight on the surface of Earth is attenuated by Earth's atmosphere, so that less power arrives at the surface (closer to 1,000 W/m2) in clear conditions when the Sun is near the zenith. Sunlight at the top of Earth's atmosphere is composed (by total energy) of about 50% infrared light, 40% visible light, and 10% ultraviolet light. The atmosphere in particular filters out over 70% of solar ultraviolet, especially at the shorter wavelengths. Solar ultraviolet radiation ionizes Earth's dayside upper atmosphere, creating the electrically conducting ionosphere.
The Sun's color is white, with a CIE color-space index near (0.3, 0.3), when viewed from space or when the Sun is high in the sky. When measuring all the photons emitted, the Sun is actually emitting more photons in the green portion of the spectrum than any other. When the Sun is low in the sky, atmospheric scattering renders the Sun yellow, red, orange, or magenta. Despite its typical whiteness, most people mentally picture the Sun as yellow; the reasons for this are the subject of debate. The Sun is a G2V star, with G2 indicating its surface temperature of approximately 5,778 K (5,505 °C, 9,941 °F), and V that it, like most stars, is a main-sequence star. The average luminance of the Sun is about 1.88 giga candela per square metre, but as viewed through Earth's atmosphere, this is lowered to about 1.44 Gcd/m2.[d] However, the luminance is not constant across the disk of the Sun (limb darkening). The Sun is composed primarily of the chemical elements hydrogen and helium; they account for 74.9% and 23.8% of the mass of the Sun in the photosphere, respectively. All heavier elements, called metals in astronomy, account for less than 2% of the mass, with oxygen (roughly 1% of the Sun's mass), carbon (0.3%), neon (0.2%), and iron (0.2%) being the most abundant.
The Sun inherited its chemical composition from the interstellar medium out of which it formed. The hydrogen and helium in the Sun were produced by Big Bang nucleosynthesis, and the heavier elements were produced by stellar nucleosynthesis in generations of stars that completed their stellar evolution and returned their material to the interstellar medium before the formation of the Sun. The chemical composition of the photosphere is normally considered representative of the composition of the primordial Solar System. However, since the Sun formed, some of the helium and heavy elements have gravitationally settled from the photosphere. Therefore, in today's photosphere the helium fraction is reduced, and the metallicity is only 84% of what it was in the protostellar phase (before nuclear fusion in the core started). The protostellar Sun's composition is believed to have been 71.1% hydrogen, 27.4% helium, and 1.5% heavier elements.
Today, nuclear fusion in the Sun's core has modified the composition by converting hydrogen into helium, so the innermost portion of the Sun is now roughly 60% helium, with the abundance of heavier elements unchanged. Because heat is transferred from the Sun's core by radiation rather than by convection (see Radiative zone below), none of the fusion products from the core have risen to the photosphere.
The reactive core zone of "hydrogen burning", where hydrogen is converted into helium, is starting to surround an inner core of "helium ash". This development will continue and will eventually cause the Sun to leave the main sequence, to become a red giant.
The solar heavy-element abundances described above are typically measured both using spectroscopy of the Sun's photosphere and by measuring abundances in meteorites that have never been heated to melting temperatures. These meteorites are thought to retain the composition of the protostellar Sun and are thus not affected by settling of heavy elements. The two methods generally agree well. n the 1970s, much research focused on the abundances of iron-group elements in the Sun. Although significant research was done, until 1978 it was difficult to determine the abundances of some iron-group elements (e.g. cobalt and manganese) via spectrography because of their hyperfine structures.
The first largely complete set of oscillator strengths of singly ionized iron-group elements were made available in the 1960s, and these were subsequently improved. In 1978, the abundances of singly ionized elements of the iron group were derived. Various authors have considered the existence of a gradient in the isotopic compositions of solar and planetary noble gases, e.g. correlations between isotopic compositions of neon and xenon in the Sun and on the planets.
Prior to 1983, it was thought that the whole Sun has the same composition as the solar atmosphere. In 1983, it was claimed that it was fractionation in the Sun itself that caused the isotopic-composition relationship between the planetary and solar-wind-implanted noble gases.The core of the Sun extends from the center to about 20–25% of the solar radius. It has a density of up to 150 g/cm3 (about 150 times the density of water) and a temperature of close to 15.7 million kelvins (K). By contrast, the Sun's surface temperature is approximately 5,800 K. Recent analysis of SOHO mission data favors a faster rotation rate in the core than in the radiative zone above. Through most of the Sun's life, energy has been produced by nuclear fusion in the core region through a series of steps called the p–p (proton–proton) chain; this process converts hydrogen into helium. Only 0.8% of the energy generated in the Sun comes from the CNO cycle, though this proportion is expected to increase as the Sun becomes older.
The core is the only region in the Sun that produces an appreciable amount of thermal energy through fusion; 99% of the power is generated within 24% of the Sun's radius, and by 30% of the radius, fusion has stopped nearly entirely. The remainder of the Sun is heated by this energy as it is transferred outwards through many successive layers, finally to the solar photosphere where it escapes into space as sunlight or the kinetic energy of particles.
The proton–proton chain occurs around 9.2×1037 times each second in the core, converting about 3.7×1038 protons into alpha particles (helium nuclei) every second (out of a total of ~8.9×1056 free protons in the Sun), or about 6.2×1011 kg/s. Fusing four free protons (hydrogen nuclei) into a single alpha particle (helium nucleus) releases around 0.7% of the fused mass as energy, so the Sun releases energy at the mass–energy conversion rate of 4.26 million metric tons per second (which requires 600 metric megatons of hydrogen ), for 384.6 yottawatts (3.846×1026 W), or 9.192×1010 megatons of TNT per second. Theoretical models of the Sun's interior indicate a power density of approximately 276.5 W/m3, a value that more nearly approximates that of reptile metabolism or a compost pile than of a thermonuclear bomb.[e]
The fusion rate in the core is in a self-correcting equilibrium: a slightly higher rate of fusion would cause the core to heat up more and expand slightly against the weight of the outer layers, reducing the density and hence the fusion rate and correcting the perturbation; and a slightly lower rate would cause the core to cool and shrink slightly, increasing the density and increasing the fusion rate and again reverting it to its present rate. From the core out to about 0.7 solar radii, thermal radiation is the primary means of energy transfer. The temperature drops from approximately 7 million to 2 million kelvins with increasing distance from the core. This temperature gradient is less than the value of the adiabatic lapse rate and hence cannot drive convection, which explains why the transfer of energy through this zone is by radiation instead of thermal convection. Ions of hydrogen and helium emit photons, which travel only a brief distance before being reabsorbed by other ions. The density drops a hundredfold (from 20 g/cm3 to 0.2 g/cm3) from 0.25 solar radii to the 0.7 radii, the top of the radiative zone.The radiative zone and the convective zone are separated by a transition layer, the tachocline. This is a region where the sharp regime change between the uniform rotation of the radiative zone and the differential rotation of the convection zone results in a large shear between the two—a condition where successive horizontal layers slide past one another. Presently, it is hypothesized (see Solar dynamo) that a magnetic dynamo within this layer generates the Sun's magnetic field. The Sun's convection zone extends from 0.7 solar radii (200,000 km) to near the surface. In this layer, the solar plasma is not dense enough or hot enough to transfer the heat energy of the interior outward via radiation. Instead, the density of the plasma is low enough to allow convective currents to develop and move the Sun's energy outward towards its surface. Material heated at the tachocline picks up heat and expands, thereby reducing its density and allowing it to rise. As a result, an orderly motion of the mass develops into thermal cells that carry the majority of the heat outward to the Sun's photosphere above. Once the material diffusively and radiatively cools just beneath the photospheric surface, its density increases, and it sinks to the base of the convection zone, where it again picks up heat from the top of the radiative zone and the convective cycle continues. At the photosphere, the temperature has dropped to 5,700 K and the density to only 0.2 g/m3 (about 1/6,000 the density of air at sea level).
The thermal columns of the convection zone form an imprint on the surface of the Sun giving it a granular appearance called the solar granulation at the smallest scale and supergranulation at larger scales. Turbulent convection in this outer part of the solar interior sustains "small-scale" dynamo action over the near-surface volume of the Sun. The Sun's thermal columns are Bénard cells and take the shape of hexagonal prisms. The visible surface of the Sun, the photosphere, is the layer below which the Sun becomes opaque to visible light. Above the photosphere visible sunlight is free to propagate into space, and almost all of its energy escapes the Sun entirely. The change in opacity is due to the decreasing amount of H− ions, which absorb visible light easily. Conversely, the visible light we see is produced as electrons react with hydrogen atoms to produce H− ions. The photosphere is tens to hundreds of kilometers thick, and is slightly less opaque than air on Earth. Because the upper part of the photosphere is cooler than the lower part, an image of the Sun appears brighter in the center than on the edge or limb of the solar disk, in a phenomenon known as limb darkening. The spectrum of sunlight has approximately the spectrum of a black-body radiating at about 6,000 K, interspersed with atomic absorption lines from the tenuous layers above the photosphere. The photosphere has a particle density of ~1023 m−3 (about 0.37% of the particle number per volume of Earth's atmosphere at sea level). The photosphere is not fully ionized—the extent of ionization is about 3%, leaving almost all of the hydrogen in atomic form.
During early studies of the optical spectrum of the photosphere, some absorption lines were found that did not correspond to any chemical elements then known on Earth. In 1868, Norman Lockyer hypothesized that these absorption lines were caused by a new element that he dubbed helium, after the Greek Sun god Helios. Twenty-five years later, helium was isolated on Earth. During a total solar eclipse, when the disk of the Sun is covered by that of the Moon, parts of the Sun's surrounding atmosphere can be seen. It is composed of four distinct parts: the chromosphere, the transition region, the corona and the heliosphere.
The coolest layer of the Sun is a temperature minimum region extending to about 500 km above the photosphere, and has a temperature of about 4,100 K. This part of the Sun is cool enough to allow the existence of simple molecules such as carbon monoxide and water, which can be detected via their absorption spectra.
The chromosphere, transition region, and corona are much hotter than the surface of the Sun. The reason is not well understood, but evidence suggests that Alfvén waves may have enough energy to heat the corona.
Above the temperature minimum layer is a layer about 2,000 km thick, dominated by a spectrum of emission and absorption lines. It is called the chromosphere from the Greek root chroma, meaning color, because the chromosphere is visible as a colored flash at the beginning and end of total solar eclipses. The temperature of the chromosphere increases gradually with altitude, ranging up to around 20,000 K near the top. In the upper part of the chromosphere helium becomes partially ionized. Above the chromosphere, in a thin (about 200 km) transition region, the temperature rises rapidly from around 20,000 K in the upper chromosphere to coronal temperatures closer to 1,000,000 K. The temperature increase is facilitated by the full ionization of helium in the transition region, which significantly reduces radiative cooling of the plasma. The transition region does not occur at a well-defined altitude. Rather, it forms a kind of nimbus around chromospheric features such as spicules and filaments, and is in constant, chaotic motion. The transition region is not easily visible from Earth's surface, but is readily observable from space by instruments sensitive to the extreme ultraviolet portion of the spectrum.
The corona is the next layer of the Sun. The low corona, near the surface of the Sun, has a particle density around 1015 m−3 to 1016 m−3.[f] The average temperature of the corona and solar wind is about 1,000,000–2,000,000 K; however, in the hottest regions it is 8,000,000–20,000,000 K. Although no complete theory yet exists to account for the temperature of the corona, at least some of its heat is known to be from magnetic reconnection. The corona is the extended atmosphere of the Sun, which has a volume much larger than the volume enclosed by the Sun's photosphere. A flow of plasma outward from the Sun into interplanetary space is the solar wind.
The heliosphere, the tenuous outermost atmosphere of the Sun, is filled with the solar wind plasma. This outermost layer of the Sun is defined to begin at the distance where the flow of the solar wind becomes superalfvénic—that is, where the flow becomes faster than the speed of Alfvén waves, at approximately 20 solar radii (0.1 AU). Turbulence and dynamic forces in the heliosphere cannot affect the shape of the solar corona within, because the information can only travel at the speed of Alfvén waves. The solar wind travels outward continuously through the heliosphere, forming the solar magnetic field into a spiral shape, until it impacts the heliopause more than 50 AU from the Sun. In December 2004, the Voyager 1 probe passed through a shock front that is thought to be part of the heliopause. In late 2012 Voyager 1 recorded a marked increase in cosmic ray collisions and a sharp drop in lower energy particles from the solar wind, which suggested that the probe had passed through the heliopause and entered the interstellar medium. High-energy gamma-ray photons initially released with fusion reactions in the core are almost immediately absorbed by the solar plasma of the radiative zone, usually after traveling only a few millimeters. Re-emission happens in a random direction and usually at a slightly lower energy. With this sequence of emissions and absorptions, it takes a long time for radiation to reach the Sun's surface. Estimates of the photon travel time range between 10,000 and 170,000 years. In contrast, it takes only 2.3 seconds for the neutrinos, which account for about 2% of the total energy production of the Sun, to reach the surface. Because energy transport in the Sun is a process that involves photons in thermodynamic equilibrium with matter, the time scale of energy transport in the Sun is longer, on the order of 30,000,000 years. This is the time it would take the Sun to return to a stable state, if the rate of energy generation in its core were suddenly changed.
Neutrinos are also released by the fusion reactions in the core, but, unlike photons, they rarely interact with matter, so almost all are able to escape the Sun immediately. For many years measurements of the number of neutrinos produced in the Sun were lower than theories predicted by a factor of 3. This discrepancy was resolved in 2001 through the discovery of the effects of neutrino oscillation: the Sun emits the number of neutrinos predicted by the theory, but neutrino detectors were missing 2⁄3 of them because the neutrinos had changed flavor by the time they were detected. The Sun has a magnetic field that varies across the surface of the Sun. Its polar field is 1–2 gauss (0.0001–0.0002 T), whereas the field is typically 3,000 gauss (0.3 T) in features on the Sun called sunspots and 10–100 gauss (0.001–0.01 T) in solar prominences.
The magnetic field also varies in time and location. The quasi-periodic 11-year solar cycle is the most prominent variation in which the number and size of sunspots waxes and wanes.
Sunspots are visible as dark patches on the Sun's photosphere, and correspond to concentrations of magnetic field where the convective transport of heat is inhibited from the solar interior to the surface. As a result, sunspots are slightly cooler than the surrounding photosphere, and, so, they appear dark. At a typical solar minimum, few sunspots are visible, and occasionally none can be seen at all. Those that do appear are at high solar latitudes. As the solar cycle progresses towards its maximum, sunspots tend form closer to the solar equator, a phenomenon known as Spörer's law. The largest sunspots can be tens of thousands of kilometers across.
An 11-year sunspot cycle is half of a 22-year Babcock–Leighton dynamo cycle, which corresponds to an oscillatory exchange of energy between toroidal and poloidal solar magnetic fields. At solar-cycle maximum, the external poloidal dipolar magnetic field is near its dynamo-cycle minimum strength, but an internal toroidal quadrupolar field, generated through differential rotation within the tachocline, is near its maximum strength. At this point in the dynamo cycle, buoyant upwelling within the convective zone forces emergence of toroidal magnetic field through the photosphere, giving rise to pairs of sunspots, roughly aligned east–west and having footprints with opposite magnetic polarities. The magnetic polarity of sunspot pairs alternates every solar cycle, a phenomenon known as the Hale cycle.
During the solar cycle's declining phase, energy shifts from the internal toroidal magnetic field to the external poloidal field, and sunspots diminish in number and size. At solar-cycle minimum, the toroidal field is, correspondingly, at minimum strength, sunspots are relatively rare, and the poloidal field is at its maximum strength. With the rise of the next 11-year sunspot cycle, differential rotation shifts magnetic energy back from the poloidal to the toroidal field, but with a polarity that is opposite to the previous cycle. The process carries on continuously, and in an idealized, simplified scenario, each 11-year sunspot cycle corresponds to a change, then, in the overall polarity of the Sun's large-scale magnetic field.
The solar magnetic field extends well beyond the Sun itself. The electrically conducting solar wind plasma carries the Sun's magnetic field into space, forming what is called the interplanetary magnetic field. In an approximation known as ideal magnetohydrodynamics, plasma particles only move along the magnetic field lines. As a result, the outward-flowing solar wind stretches the interplanetary magnetic field outward, forcing it into a roughly radial structure. For a simple dipolar solar magnetic field, with opposite hemispherical polarities on either side of the solar magnetic equator, a thin current sheet is formed in the solar wind. At great distances, the rotation of the Sun twists the dipolar magnetic field and corresponding current sheet into an Archimedean spiral structure called the Parker spiral. The interplanetary magnetic field is much stronger than the dipole component of the solar magnetic field. The Sun's dipole magnetic field of 50–400 μT (at the photosphere) reduces with the inverse-cube of the distance to about 0.1 nT at the distance of Earth. However, according to spacecraft observations the interplanetary field at Earth's location is around 5 nT, about a hundred times greater. The Sun's magnetic field leads to many effects that are collectively called solar activity. Solar flares and coronal-mass ejections tend to occur at sunspot groups. Slowly changing high-speed streams of solar wind are emitted from coronal holes at the photospheric surface. Both coronal-mass ejections and high-speed streams of solar wind carry plasma and interplanetary magnetic field outward into the Solar System. The effects of solar activity on Earth include auroras at moderate to high latitudes and the disruption of radio communications and electric power. Solar activity is thought to have played a large role in the formation and evolution of the Solar System.
With solar-cycle modulation of sunspot number comes a corresponding modulation of space weather conditions, including those surrounding Earth where technological systems can be affected. Long-term secular change in sunspot number is thought, by some scientists, to be correlated with long-term change in solar irradiance, which, in turn, might influence Earth's long-term climate. For example, in the 17th century, the solar cycle appeared to have stopped entirely for several decades; few sunspots were observed during a period known as the Maunder minimum. This coincided in time with the era of the Little Ice Age, when Europe experienced unusually cold temperatures. Earlier extended minima have been discovered through analysis of tree rings and appear to have coincided with lower-than-average global temperatures.
A recent theory claims that there are magnetic instabilities in the core of the Sun that cause fluctuations with periods of either 41,000 or 100,000 years. These could provide a better explanation of the ice ages than the Milankovitch cycles. The Sun today is roughly halfway through the most stable part of its life. It has not changed dramatically for over four billion[a] years, and will remain fairly stable for more than five billion more. However, after hydrogen fusion in its core has stopped, the Sun will undergo severe changes, both internally and externally. The Sun formed about 4.6 billion years ago from the collapse of part of a giant molecular cloud that consisted mostly of hydrogen and helium and that probably gave birth to many other stars. This age is estimated using computer models of stellar evolution and through nucleocosmochronology. The result is consistent with the radiometric date of the oldest Solar System material, at 4.567 billion years ago. Studies of ancient meteorites reveal traces of stable daughter nuclei of short-lived isotopes, such as iron-60, that form only in exploding, short-lived stars. This indicates that one or more supernovae must have occurred near the location where the Sun formed. A shock wave from a nearby supernova would have triggered the formation of the Sun by compressing the matter within the molecular cloud and causing certain regions to collapse under their own gravity. As one fragment of the cloud collapsed it also began to rotate because of conservation of angular momentum and heat up with the increasing pressure. Much of the mass became concentrated in the center, whereas the rest flattened out into a disk that would become the planets and other Solar System bodies. Gravity and pressure within the core of the cloud generated a lot of heat as it accreted more matter from the surrounding disk, eventually triggering nuclear fusion. Thus, the Sun was born. The Sun is about halfway through its main-sequence stage, during which nuclear fusion reactions in its core fuse hydrogen into helium. Each second, more than four million tonnes of matter are converted into energy within the Sun's core, producing neutrinos and solar radiation. At this rate, the Sun has so far converted around 100 times the mass of Earth into energy, about 0.03% of the total mass of the Sun. The Sun will spend a total of approximately 10 billion years as a main-sequence star. The Sun is gradually becoming hotter during its time on the main sequence, because the helium atoms in the core occupy less volume than the hydrogen atoms that were fused. The core is therefore shrinking, allowing the outer layers of the Sun to move closer to the centre and experience a stronger gravitational force, according to the inverse-square law. This stronger force increases the pressure on the core, which is resisted by a gradual increase in the rate at which fusion occurs. This process speeds up as the core gradually becomes denser. It is estimated that the Sun has become 30% brighter in the last 4.5 billion years. At present, it is increasing in brightness by about 1% every 100 million years. The Sun does not have enough mass to explode as a supernova. Instead it will exit the main sequence in approximately 5 billion years and start to turn into a red giant. As a red giant, the Sun will grow so large that it will engulf Mercury, Venus, and probably Earth.
Even before it becomes a red giant, the luminosity of the Sun will have nearly doubled, and Earth will receive as much sunlight as Venus receives today. Once the core hydrogen is exhausted in 5.4 billion years, the Sun will expand into a subgiant phase and slowly double in size over about half a billion years. It will then expand more rapidly over about half a billion years until it is over two hundred times larger than today and a couple of thousand times more luminous. This then starts the red-giant-branch phase where the Sun will spend around a billion years and lose around a third of its mass.
Evolution of a Sun-like star. The track of a one solar mass star on the Hertzsprung–Russell diagram is shown from the main sequence to the post-asymptotic-giant-branch stage. After the red-giant branch the Sun has approximately 120 million years of active life left, but much happens. First, the core, full of degenerate helium ignites violently in the helium flash, where it is estimated that 6% of the core, itself 40% of the Sun's mass, will be converted into carbon within a matter of minutes through the triple-alpha process. The Sun then shrinks to around 10 times its current size and 50 times the luminosity, with a temperature a little lower than today. It will then have reached the red clump or horizontal branch, but a star of the Sun's mass does not evolve blueward along the horizontal branch. Instead, it just becomes moderately larger and more luminous over about 100 million years as it continues to burn helium in the core.
When the helium is exhausted, the Sun will repeat the expansion it followed when the hydrogen in the core was exhausted, except that this time it all happens faster, and the Sun becomes larger and more luminous. This is the asymptotic-giant-branch phase, and the Sun is alternately burning hydrogen in a shell or helium in a deeper shell. After about 20 million years on the early asymptotic giant branch, the Sun becomes increasingly unstable, with rapid mass loss and thermal pulses that increase the size and luminosity for a few hundred years every 100,000 years or so. The thermal pulses become larger each time, with the later pulses pushing the luminosity to as much as 5,000 times the current level and the radius to over 1 AU. According to a 2008 model, Earth's orbit is shrinking due to tidal forces (and, eventually, drag from the lower chromosphere), so that it will be engulfed by the Sun near the tip of the red giant branch phase, 1 and 3.8 million years after Mercury and Venus have respectively suffered the same fate. Models vary depending on the rate and timing of mass loss. Models that have higher mass loss on the red-giant branch produce smaller, less luminous stars at the tip of the asymptotic giant branch, perhaps only 2,000 times the luminosity and less than 200 times the radius. For the Sun, four thermal pulses are predicted before it completely loses its outer envelope and starts to make a planetary nebula. By the end of that phase – lasting approximately 500,000 years – the Sun will only have about half of its current mass.
The post-asymptotic-giant-branch evolution is even faster. The luminosity stays approximately constant as the temperature increases, with the ejected half of the Sun's mass becoming ionised into a planetary nebula as the exposed core reaches 30,000 K. The final naked core, a white dwarf, will have a temperature of over 100,000 K, and contain an estimated 54.05% of the Sun's present day mass. The planetary nebula will disperse in about 10,000 years, but the white dwarf will survive for trillions of years before fading to a hypothetical black dwarf. The Sun lies close to the inner rim of the Milky Way's Orion Arm, in the Local Interstellar Cloud or the Gould Belt, at a distance of 7.5–8.5 kpc (25,000–28,000 light-years) from the Galactic Center.  The Sun is contained within the Local Bubble, a space of rarefied hot gas, possibly produced by the supernova remnant Geminga. The distance between the local arm and the next arm out, the Perseus Arm, is about 6,500 light-years. The Sun, and thus the Solar System, is found in what scientists call the galactic habitable zone. The Apex of the Sun's Way, or the solar apex, is the direction that the Sun travels relative to other nearby stars. This motion is towards a point in the constellation Hercules, near the star Vega. Of the 50 nearest stellar systems within 17 light-years from Earth (the closest being the red dwarf Proxima Centauri at approximately 4.2 light-years), the Sun ranks fourth in mass.
The Sun orbits the center of the Milky Way, and it is presently moving in the direction of the constellation of Cygnus. The Sun's orbit around the Milky Way is roughly elliptical with orbital perturbations due to the non-uniform mass distribution in Milky Way, such as that in and between the galactic spiral arms. In addition, the Sun oscillates up and down relative to the galactic plane approximately 2.7 times per orbit. It has been argued that the Sun's passage through the higher density spiral arms often coincides with mass extinctions on Earth, perhaps due to increased impact events. It takes the Solar System about 225–250 million years to complete one orbit through the Milky Way (a galactic year), so it is thought to have completed 20–25 orbits during the lifetime of the Sun. The orbital speed of the Solar System about the center of the Milky Way is approximately 251 km/s (156 mi/s). At this speed, it takes around 1,190 years for the Solar System to travel a distance of 1 light-year, or 7 days to travel 1 AU.
The Milky Way is moving with respect to the cosmic microwave background radiation (CMB) in the direction of the constellation Hydra with a speed of 550 km/s, and the Sun's resultant velocity with respect to the CMB is about 370 km/s in the direction of Crater or Leo. The temperature of the photosphere is approximately 6,000 K, whereas the temperature of the corona reaches 1,000,000–2,000,000 K. The high temperature of the corona shows that it is heated by something other than direct heat conduction from the photosphere.
It is thought that the energy necessary to heat the corona is provided by turbulent motion in the convection zone below the photosphere, and two main mechanisms have been proposed to explain coronal heating. The first is wave heating, in which sound, gravitational or magnetohydrodynamic waves are produced by turbulence in the convection zone. These waves travel upward and dissipate in the corona, depositing their energy in the ambient matter in the form of heat. The other is magnetic heating, in which magnetic energy is continuously built up by photospheric motion and released through magnetic reconnection in the form of large solar flares and myriad similar but smaller events—nanoflares.
Currently, it is unclear whether waves are an efficient heating mechanism. All waves except Alfvén waves have been found to dissipate or refract before reaching the corona. In addition, Alfvén waves do not easily dissipate in the corona. Current research focus has therefore shifted towards flare heating mechanisms. Theoretical models of the Sun's development suggest that 3.8 to 2.5 billion years ago, during the Archean eon, the Sun was only about 75% as bright as it is today. Such a weak star would not have been able to sustain liquid water on Earth's surface, and thus life should not have been able to develop. However, the geological record demonstrates that Earth has remained at a fairly constant temperature throughout its history, and that the young Earth was somewhat warmer than it is today. One theory among scientists is that the atmosphere of the young Earth contained much larger quantities of greenhouse gases (such as carbon dioxide, methane) than are present today, which trapped enough heat to compensate for the smaller amount of solar energy reaching it.
However, examination of Archaean sediments appears inconsistent with the hypothesis of high greenhouse concentrations. Instead, the moderate temperature range may be explained by a lower surface albedo brought about by less continental area and the "lack of biologically induced cloud condensation nuclei". This would have led to increased absorption of solar energy, thereby compensating for the lower solar output. The Sun has been an object of veneration in many cultures throughout human history. Humanity's most fundamental understanding of the Sun is as the luminous disk in the sky, whose presence above the horizon creates day and whose absence causes night. In many prehistoric and ancient cultures, the Sun was thought to be a solar deity or other supernatural entity. Worship of the Sun was central to civilizations such as the ancient Egyptians, the Inca of South America and the Aztecs of what is now Mexico. In religions such as Hinduism, the Sun is still considered a god. Many ancient monuments were constructed with solar phenomena in mind; for example, stone megaliths accurately mark the summer or winter solstice (some of the most prominent megaliths are located in Nabta Playa, Egypt; Mnajdra, Malta and at Stonehenge, England); Newgrange, a prehistoric human-built mount in Ireland, was designed to detect the winter solstice; the pyramid of El Castillo at Chichén Itzá in Mexico is designed to cast shadows in the shape of serpents climbing the pyramid at the vernal and autumnal equinoxes.
The Egyptians portrayed the god Ra as being carried across the sky in a solar barque, accompanied by lesser gods, and to the Greeks, he was Helios, carried by a chariot drawn by fiery horses. From the reign of Elagabalus in the late Roman Empire the Sun's birthday was a holiday celebrated as Sol Invictus (literally "Unconquered Sun") soon after the winter solstice, which may have been an antecedent to Christmas. Regarding the fixed stars, the Sun appears from Earth to revolve once a year along the ecliptic through the zodiac, and so Greek astronomers categorized it as one of the seven planets (Greek planetes, "wanderer"); the naming of the days of the weeks after the seven planets dates to the Roman era. In the early first millennium BC, Babylonian astronomers observed that the Sun's motion along the ecliptic is not uniform, though they did not know why; it is today known that this is due to the movement of Earth in an elliptic orbit around the Sun, with Earth moving faster when it is nearer to the Sun at perihelion and moving slower when it is farther away at aphelion.
One of the first people to offer a scientific or philosophical explanation for the Sun was the Greek philosopher Anaxagoras. He reasoned that it was not the chariot of Helios, but instead a giant flaming ball of metal even larger than the land of the Peloponnesus and that the Moon reflected the light of the Sun. For teaching this heresy, he was imprisoned by the authorities and sentenced to death, though he was later released through the intervention of Pericles. Eratosthenes estimated the distance between Earth and the Sun in the 3rd century BC as "of stadia myriads 400 and 80000", the translation of which is ambiguous, implying either 4,080,000 stadia (755,000 km) or 804,000,000 stadia (148 to 153 million kilometers or 0.99 to 1.02 AU); the latter value is correct to within a few percent. In the 1st century AD, Ptolemy estimated the distance as 1,210 times the radius of Earth, approximately 7.71 million kilometers (0.0515 AU).
The theory that the Sun is the center around which the planets orbit was first proposed by the ancient Greek Aristarchus of Samos in the 3rd century BC, and later adopted by Seleucus of Seleucia (see Heliocentrism). This view was developed in a more detailed mathematical model of a heliocentric system in the 16th century by Nicolaus Copernicus.
Observations of sunspots were recorded during the Han Dynasty (206 BC–AD 220) by Chinese astronomers, who maintained records of these observations for centuries. Averroes also provided a description of sunspots in the 12th century. The invention of the telescope in the early 17th century permitted detailed observations of sunspots by Thomas Harriot, Galileo Galilei and other astronomers. Galileo posited that sunspots were on the surface of the Sun rather than small objects passing between Earth and the Sun.
Arabic astronomical contributions include Albatenius' discovery that the direction of the Sun's apogee (the place in the Sun's orbit against the fixed stars where it seems to be moving slowest) is changing. (In modern heliocentric terms, this is caused by a gradual motion of the aphelion of the Earth's orbit). Ibn Yunus observed more than 10,000 entries for the Sun's position for many years using a large astrolabe. From an observation of a transit of Venus in 1032, the Persian astronomer and polymath Avicenna concluded that Venus is closer to Earth than the Sun. In 1672 Giovanni Cassini and Jean Richer determined the distance to Mars and were thereby able to calculate the distance to the Sun.
In 1666, Isaac Newton observed the Sun's light using a prism, and showed that it is made up of light of many colors. In 1800, William Herschel discovered infrared radiation beyond the red part of the solar spectrum. The 19th century saw advancement in spectroscopic studies of the Sun; Joseph von Fraunhofer recorded more than 600 absorption lines in the spectrum, the strongest of which are still often referred to as Fraunhofer lines. In the early years of the modern scientific era, the source of the Sun's energy was a significant puzzle. Lord Kelvin suggested that the Sun is a gradually cooling liquid body that is radiating an internal store of heat. Kelvin and Hermann von Helmholtz then proposed a gravitational contraction mechanism to explain the energy output, but the resulting age estimate was only 20 million years, well short of the time span of at least 300 million years suggested by some geological discoveries of that time. In 1890 Joseph Lockyer, who discovered helium in the solar spectrum, proposed a meteoritic hypothesis for the formation and evolution of the Sun.
Not until 1904 was a documented solution offered. Ernest Rutherford suggested that the Sun's output could be maintained by an internal source of heat, and suggested radioactive decay as the source. However, it would be Albert Einstein who would provide the essential clue to the source of the Sun's energy output with his mass-energy equivalence relation E = mc2. In 1920, Sir Arthur Eddington proposed that the pressures and temperatures at the core of the Sun could produce a nuclear fusion reaction that merged hydrogen (protons) into helium nuclei, resulting in a production of energy from the net change in mass. The preponderance of hydrogen in the Sun was confirmed in 1925 by Cecilia Payne using the ionization theory developed by Meghnad Saha, an Indian physicist. The theoretical concept of fusion was developed in the 1930s by the astrophysicists Subrahmanyan Chandrasekhar and Hans Bethe. Hans Bethe calculated the details of the two main energy-producing nuclear reactions that power the Sun. In 1957, Margaret Burbidge, Geoffrey Burbidge, William Fowler and Fred Hoyle showed that most of the elements in the universe have been synthesized by nuclear reactions inside stars, some like the Sun. The first satellites designed to observe the Sun were NASA's Pioneers 5, 6, 7, 8 and 9, which were launched between 1959 and 1968. These probes orbited the Sun at a distance similar to that of Earth, and made the first detailed measurements of the solar wind and the solar magnetic field. Pioneer 9 operated for a particularly long time, transmitting data until May 1983.
In the 1970s, two Helios spacecraft and the Skylab Apollo Telescope Mount provided scientists with significant new data on solar wind and the solar corona. The Helios 1 and 2 probes were U.S.–German collaborations that studied the solar wind from an orbit carrying the spacecraft inside Mercury's orbit at perihelion. The Skylab space station, launched by NASA in 1973, included a solar observatory module called the Apollo Telescope Mount that was operated by astronauts resident on the station. Skylab made the first time-resolved observations of the solar transition region and of ultraviolet emissions from the solar corona. Discoveries included the first observations of coronal mass ejections, then called "coronal transients", and of coronal holes, now known to be intimately associated with the solar wind.
In 1980, the Solar Maximum Mission was launched by NASA. This spacecraft was designed to observe gamma rays, X-rays and UV radiation from solar flares during a time of high solar activity and solar luminosity. Just a few months after launch, however, an electronics failure caused the probe to go into standby mode, and it spent the next three years in this inactive state. In 1984 Space Shuttle Challenger mission STS-41C retrieved the satellite and repaired its electronics before re-releasing it into orbit. The Solar Maximum Mission subsequently acquired thousands of images of the solar corona before re-entering Earth's atmosphere in June 1989.
Launched in 1991, Japan's Yohkoh (Sunbeam) satellite observed solar flares at X-ray wavelengths. Mission data allowed scientists to identify several different types of flares, and demonstrated that the corona away from regions of peak activity was much more dynamic and active than had previously been supposed. Yohkoh observed an entire solar cycle but went into standby mode when an annular eclipse in 2001 caused it to lose its lock on the Sun. It was destroyed by atmospheric re-entry in 2005.
One of the most important solar missions to date has been the Solar and Heliospheric Observatory, jointly built by the European Space Agency and NASA and launched on 2 December 1995. Originally intended to serve a two-year mission, a mission extension through 2012 was approved in October 2009. It has proven so useful that a follow-on mission, the Solar Dynamics Observatory (SDO), was launched in February 2010. Situated at the Lagrangian point between Earth and the Sun (at which the gravitational pull from both is equal), SOHO has provided a constant view of the Sun at many wavelengths since its launch. Besides its direct solar observation, SOHO has enabled the discovery of a large number of comets, mostly tiny sungrazing comets that incinerate as they pass the Sun. All these satellites have observed the Sun from the plane of the ecliptic, and so have only observed its equatorial regions in detail. The Ulysses probe was launched in 1990 to study the Sun's polar regions. It first travelled to Jupiter, to "slingshot" into an orbit that would take it far above the plane of the ecliptic. Once Ulysses was in its scheduled orbit, it began observing the solar wind and magnetic field strength at high solar latitudes, finding that the solar wind from high latitudes was moving at about 750 km/s, which was slower than expected, and that there were large magnetic waves emerging from high latitudes that scattered galactic cosmic rays.
Elemental abundances in the photosphere are well known from spectroscopic studies, but the composition of the interior of the Sun is more poorly understood. A solar wind sample return mission, Genesis, was designed to allow astronomers to directly measure the composition of solar material.
The Solar Terrestrial Relations Observatory (STEREO) mission was launched in October 2006. Two identical spacecraft were launched into orbits that cause them to (respectively) pull further ahead of and fall gradually behind Earth. This enables stereoscopic imaging of the Sun and solar phenomena, such as coronal mass ejections.
The Indian Space Research Organisation has scheduled the launch of a 100 kg satellite named Aditya for 2017–18. Its main instrument will be a coronagraph for studying the dynamics of the Solar corona. The brightness of the Sun can cause pain from looking at it with the naked eye; however, doing so for brief periods is not hazardous for normal non-dilated eyes. Looking directly at the Sun causes phosphene visual artifacts and temporary partial blindness. It also delivers about 4 milliwatts of sunlight to the retina, slightly heating it and potentially causing damage in eyes that cannot respond properly to the brightness. UV exposure gradually yellows the lens of the eye over a period of years, and is thought to contribute to the formation of cataracts, but this depends on general exposure to solar UV, and not whether one looks directly at the Sun. Long-duration viewing of the direct Sun with the naked eye can begin to cause UV-induced, sunburn-like lesions on the retina after about 100 seconds, particularly under conditions where the UV light from the Sun is intense and well focused; conditions are worsened by young eyes or new lens implants (which admit more UV than aging natural eyes), Sun angles near the zenith, and observing locations at high altitude.
Viewing the Sun through light-concentrating optics such as binoculars may result in permanent damage to the retina without an appropriate filter that blocks UV and substantially dims the sunlight. When using an attenuating filter to view the Sun, the viewer is cautioned to use a filter specifically designed for that use. Some improvised filters that pass UV or IR rays, can actually harm the eye at high brightness levels. Herschel wedges, also called Solar Diagonals, are effective and inexpensive for small telescopes. The sunlight that is destined for the eyepiece is reflected from an unsilvered surface of a piece of glass. Only a very small fraction of the incident light is reflected. The rest passes through the glass and leaves the instrument. If the glass breaks because of the heat, no light at all is reflected, making the device fail-safe. Simple filters made of darkened glass allow the full intensity of sunlight to pass through if they break, endangering the observer's eyesight. Unfiltered binoculars can deliver hundreds of times as much energy as using the naked eye, possibly causing immediate damage. It is claimed that even brief glances at the midday Sun through an unfiltered telescope can cause permanent damage. Partial solar eclipses are hazardous to view because the eye's pupil is not adapted to the unusually high visual contrast: the pupil dilates according to the total amount of light in the field of view, not by the brightest object in the field. During partial eclipses most sunlight is blocked by the Moon passing in front of the Sun, but the uncovered parts of the photosphere have the same surface brightness as during a normal day. In the overall gloom, the pupil expands from ~2 mm to ~6 mm, and each retinal cell exposed to the solar image receives up to ten times more light than it would looking at the non-eclipsed Sun. This can damage or kill those cells, resulting in small permanent blind spots for the viewer. The hazard is insidious for inexperienced observers and for children, because there is no perception of pain: it is not immediately obvious that one's vision is being destroyed.
Partial solar eclipses are hazardous to view because the eye's pupil is not adapted to the unusually high visual contrast: the pupil dilates according to the total amount of light in the field of view, not by the brightest object in the field. During partial eclipses most sunlight is blocked by the Moon passing in front of the Sun, but the uncovered parts of the photosphere have the same surface brightness as during a normal day. In the overall gloom, the pupil expands from ~2 mm to ~6 mm, and each retinal cell exposed to the solar image receives up to ten times more light than it would looking at the non-eclipsed Sun. This can damage or kill those cells, resulting in small permanent blind spots for the viewer. The hazard is insidious for inexperienced observers and for children, because there is no perception of pain: it is not immediately obvious that one's vision is being destroyed. During sunrise and sunset, sunlight is attenuated because of Rayleigh scattering and Mie scattering from a particularly long passage through Earth's atmosphere, and the Sun is sometimes faint enough to be viewed comfortably with the naked eye or safely with optics (provided there is no risk of bright sunlight suddenly appearing through a break between clouds). Hazy conditions, atmospheric dust, and high humidity contribute to this atmospheric attenuation.
An optical phenomenon, known as a green flash, can sometimes be seen shortly after sunset or before sunrise. The flash is caused by light from the Sun just below the horizon being bent (usually through a temperature inversion) towards the observer. Light of shorter wavelengths (violet, blue, green) is bent more than that of longer wavelengths (yellow, orange, red) but the violet and blue light is scattered more, leaving light that is perceived as green. Ultraviolet light from the Sun has antiseptic properties and can be used to sanitize tools and water. It also causes sunburn, and has other biological effects such as the production of vitamin D and sun tanning. Ultraviolet light is strongly attenuated by Earth's ozone layer, so that the amount of UV varies greatly with latitude and has been partially responsible for many biological adaptations, including variations in human skin color in different regions of the globe. The Sun has eight known planets. This includes four terrestrial planets (Mercury, Venus, Earth, and Mars), two gas giants (Jupiter and Saturn), and two ice giants (Uranus and Neptune). The Solar System also has at least five dwarf planets, an asteroid belt, numerous comets, and a large number of icy bodies which lie beyond the orbit of Neptune. A star is a luminous sphere of plasma held together by its own gravity. The nearest star to Earth is the Sun. Many other stars are visible to the naked eye from Earth during the night, appearing as a multitude of fixed luminous points in the sky due to their immense distance from Earth. Historically, the most prominent stars were grouped into constellations and asterisms, the brightest of which gained proper names. Astronomers have assembled star catalogues that identify the known stars and provide standardized stellar designations. However, most of the stars in the Universe, including all stars outside our galaxy, the Milky Way, are invisible to the naked eye from Earth. Indeed, most are invisible from Earth even through the most powerful telescopes.
For at least a portion of its life, a star shines due to thermonuclear fusion of hydrogen into helium in its core, releasing energy that traverses the star's interior and then radiates into outer space. Almost all naturally occurring elements heavier than helium are created by stellar nucleosynthesis during the star's lifetime, and for some stars by supernova nucleosynthesis when it explodes. Near the end of its life, a star can also contain degenerate matter. Astronomers can determine the mass, age, metallicity (chemical composition), and many other properties of a star by observing its motion through space, its luminosity, and spectrum respectively. The total mass of a star is the main factor that determines its evolution and eventual fate. Other characteristics of a star, including diameter and temperature, change over its life, while the star's environment affects its rotation and movement. A plot of the temperature of many stars against their luminosities produces a plot known as a Hertzsprung–Russell diagram (H–R diagram). Plotting a particular star on that diagram allows the age and evolutionary state of that star to be determined.
A star's life begins with the gravitational collapse of a gaseous nebula of material composed primarily of hydrogen, along with helium and trace amounts of heavier elements. When the stellar core is sufficiently dense, hydrogen becomes steadily converted into helium through nuclear fusion, releasing energy in the process. The remainder of the star's interior carries energy away from the core through a combination of radiative and convective heat transfer processes. The star's internal pressure prevents it from collapsing further under its own gravity. A star with mass greater than 0.4 times the Sun's will expand to become a red giant when the hydrogen fuel in its core is exhausted. In some cases, it will fuse heavier elements at the core or in shells around the core. As the star expands it throws a part of its mass, enriched with those heavier elements, into the interstellar environment, to be recycled later as new stars. Meanwhile, the core becomes a stellar remnant: a white dwarf, a neutron star, or if it is sufficiently massive a black hole.
Binary and multi-star systems consist of two or more stars that are gravitationally bound and generally move around each other in stable orbits. When two such stars have a relatively close orbit, their gravitational interaction can have a significant impact on their evolution. Stars can form part of a much larger gravitationally bound structure, such as a star cluster or a galaxy. Historically, stars have been important to civilizations throughout the world. They have been part of religious practices and used for celestial navigation and orientation. Many ancient astronomers believed that stars were permanently affixed to a heavenly sphere and that they were immutable. By convention, astronomers grouped stars into constellations and used them to track the motions of the planets and the inferred position of the Sun. The motion of the Sun against the background stars (and the horizon) was used to create calendars, which could be used to regulate agricultural practices. The Gregorian calendar, currently used nearly everywhere in the world, is a solar calendar based on the angle of the Earth's rotational axis relative to its local star, the Sun.
The oldest accurately dated star chart was the result of ancient Egyptian astronomy in 1534 BC. The earliest known star catalogues were compiled by the ancient Babylonian astronomers of Mesopotamia in the late 2nd millennium BC, during the Kassite Period (ca. 1531–1155 BC).
The first star catalogue in Greek astronomy was created by Aristillus in approximately 300 BC, with the help of Timocharis. The star catalog of Hipparchus (2nd century BC) included 1020 stars, and was used to assemble Ptolemy's star catalogue. Hipparchus is known for the discovery of the first recorded nova (new star). Many of the constellations and star names in use today derive from Greek astronomy.
In spite of the apparent immutability of the heavens, Chinese astronomers were aware that new stars could appear. In 185 AD, they were the first to observe and write about a supernova, now known as the SN 185. The brightest stellar event in recorded history was the SN 1006 supernova, which was observed in 1006 and written about by the Egyptian astronomer Ali ibn Ridwan and several Chinese astronomers. The SN 1054 supernova, which gave birth to the Crab Nebula, was also observed by Chinese and Islamic astronomers.
Medieval Islamic astronomers gave Arabic names to many stars that are still used today and they invented numerous astronomical instruments that could compute the positions of the stars. They built the first large observatory research institutes, mainly for the purpose of producing Zij star catalogues. Among these, the Book of Fixed Stars (964) was written by the Persian astronomer Abd al-Rahman al-Sufi, who observed a number of stars, star clusters (including the Omicron Velorum and Brocchi's Clusters) and galaxies (including the Andromeda Galaxy). According to A. Zahoor, in the 11th century, the Persian polymath scholar Abu Rayhan Biruni described the Milky Way galaxy as a multitude of fragments having the properties of nebulous stars, and also gave the latitudes of various stars during a lunar eclipse in 1019.
According to Josep Puig, the Andalusian astronomer Ibn Bajjah proposed that the Milky Way was made up of many stars that almost touched one another and appeared to be a continuous image due to the effect of refraction from sublunary material, citing his observation of the conjunction of Jupiter and Mars on 500 AH (1106/1107 AD) as evidence. Early European astronomers such as Tycho Brahe identified new stars in the night sky (later termed novae), suggesting that the heavens were not immutable. In 1584 Giordano Bruno suggested that the stars were like the Sun, and may have other planets, possibly even Earth-like, in orbit around them, an idea that had been suggested earlier by the ancient Greek philosophers, Democritus and Epicurus, and by medieval Islamic cosmologists such as Fakhr al-Din al-Razi. By the following century, the idea of the stars being the same as the Sun was reaching a consensus among astronomers. To explain why these stars exerted no net gravitational pull on the Solar System, Isaac Newton suggested that the stars were equally distributed in every direction, an idea prompted by the theologian Richard Bentley. The Italian astronomer Geminiano Montanari recorded observing variations in luminosity of the star Algol in 1667. Edmond Halley published the first measurements of the proper motion of a pair of nearby "fixed" stars, demonstrating that they had changed positions since the time of the ancient Greek astronomers Ptolemy and Hipparchus.
William Herschel was the first astronomer to attempt to determine the distribution of stars in the sky. During the 1780s he established a series of gauges in 600 directions and counted the stars observed along each line of sight. From this he deduced that the number of stars steadily increased toward one side of the sky, in the direction of the Milky Way core. His son John Herschel repeated this study in the southern hemisphere and found a corresponding increase in the same direction. In addition to his other accomplishments, William Herschel is also noted for his discovery that some stars do not merely lie along the same line of sight, but are also physical companions that form binary star systems.
The science of stellar spectroscopy was pioneered by Joseph von Fraunhofer and Angelo Secchi. By comparing the spectra of stars such as Sirius to the Sun, they found differences in the strength and number of their absorption lines—the dark lines in a stellar spectra caused by the atmosphere's absorption of specific frequencies. In 1865 Secchi began classifying stars into spectral types. However, the modern version of the stellar classification scheme was developed by Annie J. Cannon during the 1900s. The first direct measurement of the distance to a star (61 Cygni at 11.4 light-years) was made in 1838 by Friedrich Bessel using the parallax technique. Parallax measurements demonstrated the vast separation of the stars in the heavens. Observation of double stars gained increasing importance during the 19th century. In 1834, Friedrich Bessel observed changes in the proper motion of the star Sirius and inferred a hidden companion. Edward Pickering discovered the first spectroscopic binary in 1899 when he observed the periodic splitting of the spectral lines of the star Mizar in a 104-day period. Detailed observations of many binary star systems were collected by astronomers such as Friedrich Georg Wilhelm von Struve and S. W. Burnham, allowing the masses of stars to be determined from computation of orbital elements. The first solution to the problem of deriving an orbit of binary stars from telescope observations was made by Felix Savary in 1827. The twentieth century saw increasingly rapid advances in the scientific study of stars. The photograph became a valuable astronomical tool. Karl Schwarzschild discovered that the color of a star and, hence, its temperature, could be determined by comparing the visual magnitude against the photographic magnitude. The development of the photoelectric photometer allowed precise measurements of magnitude at multiple wavelength intervals. In 1921 Albert A. Michelson made the first measurements of a stellar diameter using an interferometer on the Hooker telescope at Mount Wilson Observatory.
Important theoretical work on the physical structure of stars occurred during the first decades of the twentieth century. In 1913, the Hertzsprung-Russell diagram was developed, propelling the astrophysical study of stars. Successful models were developed to explain the interiors of stars and stellar evolution. Cecilia Payne-Gaposchkin first proposed that stars were made primarily of hydrogen and helium in her 1925 PhD thesis. The spectra of stars were further understood through advances in quantum physics. This allowed the chemical composition of the stellar atmosphere to be determined.
With the exception of supernovae, individual stars have primarily been observed in the Local Group, and especially in the visible part of the Milky Way (as demonstrated by the detailed star catalogues available for our galaxy). But some stars have been observed in the M100 galaxy of the Virgo Cluster, about 100 million light years from the Earth. In the Local Supercluster it is possible to see star clusters, and current telescopes could in principle observe faint individual stars in the Local Group (see Cepheids). However, outside the Local Supercluster of galaxies, neither individual stars nor clusters of stars have been observed. The only exception is a faint image of a large star cluster containing hundreds of thousands of stars located at a distance of one billion light years—ten times further than the most distant star cluster previously observed. The concept of a constellation was known to exist during the Babylonian period. Ancient sky watchers imagined that prominent arrangements of stars formed patterns, and they associated these with particular aspects of nature or their myths. Twelve of these formations lay along the band of the ecliptic and these became the basis of astrology. Many of the more prominent individual stars were also given names, particularly with Arabic or Latin designations.
As well as certain constellations and the Sun itself, individual stars have their own myths. To the Ancient Greeks, some "stars", known as planets (Greek πλανήτης (planētēs), meaning "wanderer"), represented various important deities, from which the names of the planets Mercury, Venus, Mars, Jupiter and Saturn were taken. (Uranus and Neptune were also Greek and Roman gods, but neither planet was known in Antiquity because of their low brightness. Their names were assigned by later astronomers.)
Circa 1600, the names of the constellations were used to name the stars in the corresponding regions of the sky. The German astronomer Johann Bayer created a series of star maps and applied Greek letters as designations to the stars in each constellation. Later a numbering system based on the star's right ascension was invented and added to John Flamsteed's star catalogue in his book "Historia coelestis Britannica" (the 1712 edition), whereby this numbering system came to be called Flamsteed designation or Flamsteed numbering.
The only internationally recognized authority for naming celestial bodies is the International Astronomical Union (IAU). The International Astronomical Union maintains the Working Group on Star Names (WGSN) which catalogs and standardizes proper names for stars. A number of private companies sell names of stars, which the British Library calls an unregulated commercial enterprise. The IAU has disassociated itself from this commercial practice, and these names are neither recognized by the IAU, professional astronomers, nor the amateur astronomy community. One such star-naming company is the International Star Registry, which, during the 1980s, was accused of deceptive practice for making it appear that the assigned name was official. This now-discontinued ISR practice was informally labeled a scam and a fraud, and the New York City Department of Consumer Affairs issued a violation against ISR for engaging in a deceptive trade practice. Although stellar parameters can be expressed in SI units or CGS units, it is often most convenient to express mass, luminosity, and radii in solar units, based on the characteristics of the Sun. In 2015, the IAU defined a set of nominal solar values (defined as SI constants, without uncertainties) which can be used for quoting stellar parameters:
nominal solar luminosity: L⊙ = 3.828 × 1026 W  nominal solar radius R⊙ = 6.957 × 108 m  The solar mass M⊙ was not explicitly defined by the IAU due to the large relative uncertainty (10−4) of the Newtonian gravitational constant G. However, since the product of the Newtonian gravitational constant and solar mass together (GM⊙) has been determined to much greater precision, the IAU defined the nominal solar mass parameter to be:
nominal solar mass parameter: GM⊙ = 1.3271244 × 1020 m3 s−2  However, one can combine the nominal solar mass parameter with the most recent (2014) CODATA estimate of the Newtonian gravitational constant G to derive the solar mass to be approximately 1.9885 × 1030 kg. Although the exact values for the luminosity, radius, mass parameter, and mass may vary slightly in the future due to observational uncertainties, the 2015 IAU nominal constants will remain the same SI values as they remain useful measures for quoting stellar parameters.
Large lengths, such as the radius of a giant star or the semi-major axis of a binary star system, are often expressed in terms of the astronomical unit — approximately equal to the mean distance between the Earth and the Sun (150 million km or approximately 93 million miles). In 2012, the IAU defined the astronomical constant to be an exact length in meters: 149,597,870,700 m. Stars condense from regions of space of higher matter density, yet those regions are less dense than within a vacuum chamber. These regions – known as molecular clouds – consist mostly of hydrogen, with about 23 to 28 percent helium and a few percent heavier elements. One example of such a star-forming region is the Orion Nebula. Most stars form in groups of dozens to hundreds of thousands of stars. Massive stars in these groups may powerfully illuminate those clouds, ionizing the hydrogen, and creating H II regions. Such feedback effects, from star formation, may ultimately disrupt the cloud and prevent further star formation.
All stars spend the majority of their existence as main sequence stars, fueled primarily by the nuclear fusion of hydrogen into helium within their cores. However, stars of different masses have markedly different properties at various stages of their development. The ultimate fate of more massive stars differs from that of less massive stars, as do their luminosities and the impact they have on their environment. Accordingly, astronomers often group stars by their mass:
Very low mass stars, with masses below 0.5 M☉, are fully convective and distribute helium evenly throughout the whole star while on the main sequence. Therefore, they never undergo shell burning, never become red giants, which cease fusing and become helium white dwarfs and slowly cool after exhausting their hydrogen. However, as the lifetime of 0.5 M☉ stars is longer than the age of the universe, no such star has yet reached the white dwarf stage. Low mass stars (including the Sun), with a mass between 0.5 M☉ and 1.8–2.5 M☉ depending on composition, do become red giants as their core hydrogen is depleted and they begin to burn helium in core in a helium flash; they develop a degenerate carbon-oxygen core later on the asymptotic giant branch; they finally blow off their outer shell as a planetary nebula and leave behind their core in the form of a white dwarf. Intermediate-mass stars, between 1.8–2.5 M☉ and 5–10 M☉, pass through evolutionary stages similar to low mass stars, but after a relatively short period on the red giant branch they ignite helium without a flash and spend an extended period in the red clump before forming a degenerate carbon-oxygen core. Massive stars generally have a minimum mass of 7–10 M☉ (possibly as low as 5–6 M☉). After exhausting the hydrogen at the core these stars become supergiants and go on to fuse elements heavier than helium. They end their lives when their cores collapse and they explode as supernovae. The formation of a star begins with gravitational instability within a molecular cloud, caused by regions of higher density – often triggered by compression of clouds by radiation from massive stars, expanding bubbles in the interstellar medium, the collision of different molecular clouds, or the collision of galaxies (as in a starburst galaxy). When a region reaches a sufficient density of matter to satisfy the criteria for Jeans instability, it begins to collapse under its own gravitational force. As the cloud collapses, individual conglomerations of dense dust and gas form "Bok globules". As a globule collapses and the density increases, the gravitational energy converts into heat and the temperature rises. When the protostellar cloud has approximately reached the stable condition of hydrostatic equilibrium, a protostar forms at the core. These pre-main-sequence stars are often surrounded by a protoplanetary disk and powered mainly by the conversion of gravitational energy. The period of gravitational contraction lasts about 10 to 15 million years.
A cluster of approximately 500 young stars lies within the nearby W40 stellar nursery. Early stars of less than 2 M☉ are called T Tauri stars, while those with greater mass are Herbig Ae/Be stars. These newly formed stars emit jets of gas along their axis of rotation, which may reduce the angular momentum of the collapsing star and result in small patches of nebulosity known as Herbig–Haro objects. These jets, in combination with radiation from nearby massive stars, may help to drive away the surrounding cloud from which the star was formed. Early in their development, T Tauri stars follow the Hayashi track—they contract and decrease in luminosity while remaining at roughly the same temperature. Less massive T Tauri stars follow this track to the main sequence, while more massive stars turn onto the Henyey track.
Most stars are observed to be members of binary star systems, and the properties of those binaries are the result of the conditions in which they formed. A gas cloud must lose its angular momentum in order to collapse and form a star. The fragmentation of the cloud into multiple stars distributes some of that angular momentum. The primordial binaries transfer some angular momentum by gravitational interactions during close encounters with other stars in young stellar clusters. These interactions tend to split apart more widely separated (soft) binaries while causing hard binaries to become more tightly bound. This produces the separation of binaries into their two observed populations distributions. Stars spend about 90% of their existence fusing hydrogen into helium in high-temperature and high-pressure reactions near the core. Such stars are said to be on the main sequence, and are called dwarf stars. Starting at zero-age main sequence, the proportion of helium in a star's core will steadily increase, the rate of nuclear fusion at the core will slowly increase, as will the star's temperature and luminosity. The Sun, for example, is estimated to have increased in luminosity by about 40% since it reached the main sequence 4.6 billion (4.6 × 109) years ago.
Every star generates a stellar wind of particles that causes a continual outflow of gas into space. For most stars, the mass lost is negligible. The Sun loses 10−14 M☉ every year, or about 0.01% of its total mass over its entire lifespan. However, very massive stars can lose 10−7 to 10−5 M☉ each year, significantly affecting their evolution. Stars that begin with more than 50 M☉ can lose over half their total mass while on the main sequence.
An example of a Hertzsprung–Russell diagram for a set of stars that includes the Sun (center). (See "Classification" below.) The time a star spends on the main sequence depends primarily on the amount of fuel it has and the rate at which it fuses it. The Sun is expected to live 10 billion (1010) years. Massive stars consume their fuel very rapidly and are short-lived. Low mass stars consume their fuel very slowly. Stars less massive than 0.25 M☉, called red dwarfs, are able to fuse nearly all of their mass while stars of about 1 M☉ can only fuse about 10% of their mass. The combination of their slow fuel-consumption and relatively large usable fuel supply allows low mass stars to last about one trillion (1012) years; the most extreme of 0.08 M☉) will last for about 12 trillion years. Red dwarfs become hotter and more luminous as they accumulate helium. When they eventually run out of hydrogen, they contract into a white dwarf and decline in temperature. However, since the lifespan of such stars is greater than the current age of the universe (13.8 billion years), no stars under about 0.85 M☉ are expected to have moved off the main sequence.
Besides mass, the elements heavier than helium can play a significant role in the evolution of stars. Astronomers label all elements heavier than helium "metals", and call the chemical concentration of these elements in a star, its metallicity. A star's metallicity can influence the time the star takes to burn its fuel, and controls the formation of its magnetic fields, which affects the strength of its stellar wind. Older, population II stars have substantially less metallicity than the younger, population I stars due to the composition of the molecular clouds from which they formed. Over time, such clouds become increasingly enriched in heavier elements as older stars die and shed portions of their atmospheres. As stars of at least 0.4 M☉ exhaust their supply of hydrogen at their core, they start to fuse hydrogen in a shell outside the helium core. Their outer layers expand and cool greatly as they form a red giant. In about 5 billion years, when the Sun enters the helium burning phase, it will expand to a maximum radius of roughly 1 astronomical unit (150 million kilometres), 250 times its present size, and lose 30% of its current mass.
As the hydrogen shell burning produces more helium, the core increases in mass and temperature. In a red giant of up to 2.25 M☉, the mass of the helium core becomes degenerate prior to helium fusion. Finally, when the temperature increases sufficiently, helium fusion begins explosively in what is called a helium flash, and the star rapidly shrinks in radius, increases its surface temperature, and moves to the horizontal branch of the HR diagram. For more massive stars, helium core fusion starts before the core becomes degenerate, and the star spends some time in the red clump, slowly burning helium, before the outer convective envelope collapses and the star then moves to the horizontal branch.
After the star has fused the helium of its core, the carbon product fuses producing a hot core with an outer shell of fusing helium. The star then follows an evolutionary path called the asymptotic giant branch (AGB) that parallels the other described red giant phase, but with a higher luminosity. The more massive AGB stars may undergo a brief period of carbon fusion before the core becomes degenerate. During their helium-burning phase, a star of more than 9 solar masses expands to form first a blue and then a red supergiant. Particularly massive stars may evolve to a Wolf-Rayet star, characterised by spectra dominated by emission lines of elements heavier than hydrogen, which have reached the surface due to strong convection and intense mass loss.
When helium is exhausted at the core of a massive star, the core contracts and the temperature and pressure rises enough to fuse carbon (see Carbon-burning process). This process continues, with the successive stages being fueled by neon (see neon-burning process), oxygen (see oxygen-burning process), and silicon (see silicon-burning process). Near the end of the star's life, fusion continues along a series of onion-layer shells within a massive star. Each shell fuses a different element, with the outermost shell fusing hydrogen; the next shell fusing helium, and so forth.
The final stage occurs when a massive star begins producing iron. Since iron nuclei are more tightly bound than any heavier nuclei, any fusion beyond iron does not produce a net release of energy. To a very limited degree such a process proceeds, but it consumes energy. Likewise, since they are more tightly bound than all lighter nuclei, such energy cannot be released by fission. As a star's core shrinks, the intensity of radiation from that surface increases, creating such radiation pressure on the outer shell of gas that it will push those layers away, forming a planetary nebula. If what remains after the outer atmosphere has been shed is less than 1.4 M☉, it shrinks to a relatively tiny object about the size of Earth, known as a white dwarf. White dwarfs lack the mass for further gravitational compression to take place. The electron-degenerate matter inside a white dwarf is no longer a plasma, even though stars are generally referred to as being spheres of plasma. Eventually, white dwarfs fade into black dwarfs over a very long period of time. In massive stars, fusion continues until the iron core has grown so large (more than 1.4 M☉) that it can no longer support its own mass. This core will suddenly collapse as its electrons are driven into its protons, forming neutrons, neutrinos, and gamma rays in a burst of electron capture and inverse beta decay. The shockwave formed by this sudden collapse causes the rest of the star to explode in a supernova. Supernovae become so bright that they may briefly outshine the star's entire home galaxy. When they occur within the Milky Way, supernovae have historically been observed by naked-eye observers as "new stars" where none seemingly existed before.
A supernova explosion blows away the star's outer layers, leaving a remnant such as the Crab Nebula. The core is compressed into a neutron star, which sometimes manifests itself as a pulsar or X-ray burster. In the case of the largest stars, the remnant is a black hole greater than 4 M☉. In a neutron star the matter is in a state known as neutron-degenerate matter, with a more exotic form of degenerate matter, QCD matter, possibly present in the core. Within a black hole, the matter is in a state that is not currently understood. The blown-off outer layers of dying stars include heavy elements, which may be recycled during the formation of new stars. These heavy elements allow the formation of rocky planets. The outflow from supernovae and the stellar wind of large stars play an important part in shaping the interstellar medium. The post–main-sequence evolution of binary stars may be significantly different from the evolution of single stars of the same mass. If stars in a binary system are sufficiently close, when one of the stars expands to become a red giant it may overflow its Roche lobe, the region around a star where material is gravitationally bound to that star, leading to transfer of material to the other. When the Roche lobe is violated, a variety of phenomena can result, including contact binaries, common-envelope binaries, cataclysmic variables, and type Ia supernovae. Stars are not spread uniformly across the universe, but are normally grouped into galaxies along with interstellar gas and dust. A typical galaxy contains hundreds of billions of stars, and there are more than 100 billion (1011) galaxies in the observable universe. In 2010, one estimate of the number of stars in the observable universe was 300 sextillion (3 × 1023). While it is often believed that stars only exist within galaxies, intergalactic stars have been discovered.
A multi-star system consists of two or more gravitationally bound stars that orbit each other. The simplest and most common multi-star system is a binary star, but systems of three or more stars are also found. For reasons of orbital stability, such multi-star systems are often organized into hierarchical sets of binary stars. Larger groups called star clusters also exist. These range from loose stellar associations with only a few stars, up to enormous globular clusters with hundreds of thousands of stars. Such systems orbit their host galaxy. It has been a long-held assumption that the majority of stars occur in gravitationally bound, multiple-star systems. This is particularly true for very massive O and B class stars, where 80% of the stars are believed to be part of multiple-star systems. The proportion of single star systems increases with decreasing star mass, so that only 25% of red dwarfs are known to have stellar companions. As 85% of all stars are red dwarfs, most stars in the Milky Way are likely single from birth.
The nearest star to the Earth, apart from the Sun, is Proxima Centauri, which is 39.9 trillion kilometres, or 4.2 light-years. Travelling at the orbital speed of the Space Shuttle (8 kilometres per second—almost 30,000 kilometres per hour), it would take about 150,000 years to arrive. This is typical of stellar separations in galactic discs. Stars can be much closer to each other in the centres of galaxies and in globular clusters, or much farther apart in galactic halos. Due to the relatively vast distances between stars outside the galactic nucleus, collisions between stars are thought to be rare. In denser regions such as the core of globular clusters or the galactic center, collisions can be more common. Such collisions can produce what are known as blue stragglers. These abnormal stars have a higher surface temperature than the other main sequence stars with the same luminosity of the cluster to which it belongs. Most stars are between 1 billion and 10 billion years old. Some stars may even be close to 13.8 billion years old—the observed age of the universe. The oldest star yet discovered, HD 140283, nicknamed Methuselah star, is an estimated 14.46 ± 0.8 billion years old. (Due to the uncertainty in the value, this age for the star does not conflict with the age of the Universe, determined by the Planck satellite as 13.799 ± 0.021).
The more massive the star, the shorter its lifespan, primarily because massive stars have greater pressure on their cores, causing them to burn hydrogen more rapidly. The most massive stars last an average of a few million years, while stars of minimum mass (red dwarfs) burn their fuel very slowly and can last tens to hundreds of billions of years. When stars form in the present Milky Way galaxy they are composed of about 71% hydrogen and 27% helium, as measured by mass, with a small fraction of heavier elements. Typically the portion of heavy elements is measured in terms of the iron content of the stellar atmosphere, as iron is a common element and its absorption lines are relatively easy to measure. The portion of heavier elements may be an indicator of the likelihood that the star has a planetary system.
The star with the lowest iron content ever measured is the dwarf HE1327-2326, with only 1/200,000th the iron content of the Sun. By contrast, the super-metal-rich star μ Leonis has nearly double the abundance of iron as the Sun, while the planet-bearing star 14 Herculis has nearly triple the iron. There also exist chemically peculiar stars that show unusual abundances of certain elements in their spectrum; especially chromium and rare earth elements. Stars with cooler outer atmospheres, including the Sun, can form various diatomic and polyatomic molecules. Due to their great distance from the Earth, all stars except the Sun appear to the unaided eye as shining points in the night sky that twinkle because of the effect of the Earth's atmosphere. The Sun is also a star, but it is close enough to the Earth to appear as a disk instead, and to provide daylight. Other than the Sun, the star with the largest apparent size is R Doradus, with an angular diameter of only 0.057 arcseconds.
The disks of most stars are much too small in angular size to be observed with current ground-based optical telescopes, and so interferometer telescopes are required to produce images of these objects. Another technique for measuring the angular size of stars is through occultation. By precisely measuring the drop in brightness of a star as it is occulted by the Moon (or the rise in brightness when it reappears), the star's angular diameter can be computed.
Stars range in size from neutron stars, which vary anywhere from 20 to 40 km (25 mi) in diameter, to supergiants like Betelgeuse in the Orion constellation, which has a diameter 887±203 to 950 times that of our sun. Betelgeuse, however, has a much lower density than the Sun. The motion of a star relative to the Sun can provide useful information about the origin and age of a star, as well as the structure and evolution of the surrounding galaxy. The components of motion of a star consist of the radial velocity toward or away from the Sun, and the traverse angular movement, which is called its proper motion.
Radial velocity is measured by the doppler shift of the star's spectral lines, and is given in units of km/s. The proper motion of a star, its parallax, is determined by precise astrometric measurements in units of milli-arc seconds (mas) per year. With knowledge of the star's parallax and its distance, the proper motion velocity can be calculated. Together with the radial velocity, the total velocity can be calculated. Stars with high rates of proper motion are likely to be relatively close to the Sun, making them good candidates for parallax measurements.
When both rates of movement are known, the space velocity of the star relative to the Sun or the galaxy can be computed. Among nearby stars, it has been found that younger population I stars have generally lower velocities than older, population II stars. The latter have elliptical orbits that are inclined to the plane of the galaxy. A comparison of the kinematics of nearby stars has allowed astronomers to trace their origin to common points in giant molecular clouds, and are referred to as stellar associations. The magnetic field of a star is generated within regions of the interior where convective circulation occurs. This movement of conductive plasma functions like a dynamo, wherein the movement of electrical charges induce magnetic fields, as does a mechanical dynamo. Those magnetic fields have a great range that extend throughout and beyond the star. The strength of the magnetic field varies with the mass and composition of the star, and the amount of magnetic surface activity depends upon the star's rate of rotation. This surface activity produces starspots, which are regions of strong magnetic fields and lower than normal surface temperatures. Coronal loops are arching magnetic field flux lines that rise from a star's surface into the star's outer atmosphere, its corona. The coronal loops can be seen due to the plasma they conduct along their length. Stellar flares are bursts of high-energy particles that are emitted due to the same magnetic activity.
Young, rapidly rotating stars tend to have high levels of surface activity because of their magnetic field. The magnetic field can act upon a star's stellar wind, functioning as a brake to gradually slow the rate of rotation with time. Thus, older stars such as the Sun have a much slower rate of rotation and a lower level of surface activity. The activity levels of slowly rotating stars tend to vary in a cyclical manner and can shut down altogether for periods of time. During the Maunder Minimum, for example, the Sun underwent a 70-year period with almost no sunspot activity. One of the most massive stars known is Eta Carinae, which, with 100–150 times as much mass as the Sun, will have a lifespan of only several million years. Studies of the most massive open clusters suggests 150 M☉ as an upper limit for stars in the current era of the universe. This represents an empirical value for the theoretical limit on the mass of forming stars due to increasing radiation pressure on the accreting gas cloud. Several stars in the R136 cluster in the Large Magellanic Cloud have been measured with larger masses, but it has been determined that they could have been created through the collision and merger of massive stars in close binary systems, sidestepping the 150 M☉ limit on massive star formation. The first stars to form after the Big Bang may have been larger, up to 300 M☉, due to the complete absence of elements heavier than lithium in their composition. This generation of supermassive population III stars is likely to have existed in the very early universe (i.e., they are observed to have a high redshift), and may have started the production of chemical elements heavier than hydrogen that are needed for the later formation of planets and life. In June 2015, astronomers reported evidence for Population III stars in the Cosmos Redshift 7 galaxy at z = 6.60.
With a mass only 80 times that of Jupiter (MJ), 2MASS J0523-1403 is the smallest known star undergoing nuclear fusion in its core. For stars with metallicity similar to the Sun, the theoretical minimum mass the star can have and still undergo fusion at the core, is estimated to be about 75 MJ. When the metallicity is very low, however, the minimum star size seems to be about 8.3% of the solar mass, or about 87 MJ. Smaller bodies called brown dwarfs, occupy a poorly defined grey area between stars and gas giants. The combination of the radius and the mass of a star determines its surface gravity. Giant stars have a much lower surface gravity than do main sequence stars, while the opposite is the case for degenerate, compact stars such as white dwarfs. The surface gravity can influence the appearance of a star's spectrum, with higher gravity causing a broadening of the absorption lines. The rotation rate of stars can be determined through spectroscopic measurement, or more exactly determined by tracking their starspots. Young stars can have a rotation greater than 100 km/s at the equator. The B-class star Achernar, for example, has an equatorial velocity of about 225 km/s or greater, causing its equator to be slung outward and giving it an equatorial diameter that is more than 50% greater than between the poles. This rate of rotation is just below the critical velocity of 300 km/s at which speed the star would break apart. By contrast, the Sun rotates once every 25 – 35 days, with an equatorial velocity of 1.994 km/s. A main sequence star's magnetic field and the stellar wind serve to slow its rotation by a significant amount as it evolves on the main sequence.
Degenerate stars have contracted into a compact mass, resulting in a rapid rate of rotation. However they have relatively low rates of rotation compared to what would be expected by conservation of angular momentum—the tendency of a rotating body to compensate for a contraction in size by increasing its rate of spin. A large portion of the star's angular momentum is dissipated as a result of mass loss through the stellar wind. In spite of this, the rate of rotation for a pulsar can be very rapid. The pulsar at the heart of the Crab nebula, for example, rotates 30 times per second. The rotation rate of the pulsar will gradually slow due to the emission of radiation.
The surface temperature of a main sequence star is determined by the rate of energy production of its core and by its radius, and is often estimated from the star's color index. The temperature is normally given in terms of an effective temperature, which is the temperature of an idealized black body that radiates its energy at the same luminosity per surface area as the star. Note that the effective temperature is only a representative of the surface, as the temperature increases toward the core. The temperature in the core region of a star is several million kelvins.
The stellar temperature will determine the rate of ionization of various elements, resulting in characteristic absorption lines in the spectrum. The surface temperature of a star, along with its visual absolute magnitude and absorption features, is used to classify a star (see classification below).
Massive main sequence stars can have surface temperatures of 50,000 K. Smaller stars such as the Sun have surface temperatures of a few thousand K. Red giants have relatively low surface temperatures of about 3,600 K; but they also have a high luminosity due to their large exterior surface area. The energy produced by stars, a product of nuclear fusion, radiates to space as both electromagnetic radiation and particle radiation. The particle radiation emitted by a star is manifested as the stellar wind, which streams from the outer layers as electrically charged protons and alpha and beta particles. Although almost massless, there also exists a steady stream of neutrinos emanating from the star's core.
The production of energy at the core is the reason stars shine so brightly: every time two or more atomic nuclei fuse together to form a single atomic nucleus of a new heavier element, gamma ray photons are released from the nuclear fusion product. This energy is converted to other forms of electromagnetic energy of lower frequency, such as visible light, by the time it reaches the star's outer layers.
The color of a star, as determined by the most intense frequency of the visible light, depends on the temperature of the star's outer layers, including its photosphere. Besides visible light, stars also emit forms of electromagnetic radiation that are invisible to the human eye. In fact, stellar electromagnetic radiation spans the entire electromagnetic spectrum, from the longest wavelengths of radio waves through infrared, visible light, ultraviolet, to the shortest of X-rays, and gamma rays. From the standpoint of total energy emitted by a star, not all components of stellar electromagnetic radiation are significant, but all frequencies provide insight into the star's physics.
Using the stellar spectrum, astronomers can also determine the surface temperature, surface gravity, metallicity and rotational velocity of a star. If the distance of the star is found, such as by measuring the parallax, then the luminosity of the star can be derived. The mass, radius, surface gravity, and rotation period can then be estimated based on stellar models. (Mass can be calculated for stars in binary systems by measuring their orbital velocities and distances. Gravitational microlensing has been used to measure the mass of a single star.) With these parameters, astronomers can also estimate the age of the star. The luminosity of a star is the amount of light and other forms of radiant energy it radiates per unit of time. It has units of power. The luminosity of a star is determined by its radius and surface temperature. Many stars do not radiate uniformly across their entire surface. The rapidly rotating star Vega, for example, has a higher energy flux (power per unit area) at its poles than along its equator.
Patches of the star's surface with a lower temperature and luminosity than average are known as starspots. Small, dwarf stars such as our Sun generally have essentially featureless disks with only small starspots. Giant stars have much larger, more obvious starspots, and they also exhibit strong stellar limb darkening. That is, the brightness decreases towards the edge of the stellar disk. Red dwarf flare stars such as UV Ceti may also possess prominent starspot features. The apparent brightness of a star is expressed in terms of its apparent magnitude. It is a function of the star's luminosity, its distance from Earth, and the altering of the star's light as it passes through Earth's atmosphere. Intrinsic or absolute magnitude is directly related to a star's luminosity, and is what the apparent magnitude a star would be if the distance between the Earth and the star were 10 parsecs (32.6 light-years). Both the apparent and absolute magnitude scales are logarithmic units: one whole number difference in magnitude is equal to a brightness variation of about 2.5 times (the 5th root of 100 or approximately 2.512). This means that a first magnitude star (+1.00) is about 2.5 times brighter than a second magnitude (+2.00) star, and about 100 times brighter than a sixth magnitude star (+6.00). The faintest stars visible to the naked eye under good seeing conditions are about magnitude +6.
On both apparent and absolute magnitude scales, the smaller the magnitude number, the brighter the star; the larger the magnitude number, the fainter the star. The brightest stars, on either scale, have negative magnitude numbers. The variation in brightness (ΔL) between two stars is calculated by subtracting the magnitude number of the brighter star (mb) from the magnitude number of the fainter star (mf), then using the difference as an exponent for the base number 2.512; that is to say: Relative to both luminosity and distance from Earth, a star's absolute magnitude (M) and apparent magnitude (m) are not equivalent; for example, the bright star Sirius has an apparent magnitude of −1.44, but it has an absolute magnitude of +1.41.
The Sun has an apparent magnitude of −26.7, but its absolute magnitude is only +4.83. Sirius, the brightest star in the night sky as seen from Earth, is approximately 23 times more luminous than the Sun, while Canopus, the second brightest star in the night sky with an absolute magnitude of −5.53, is approximately 14,000 times more luminous than the Sun. Despite Canopus being vastly more luminous than Sirius, however, Sirius appears brighter than Canopus. This is because Sirius is merely 8.6 light-years from the Earth, while Canopus is much farther away at a distance of 310 light-years.
As of 2006, the star with the highest known absolute magnitude is LBV 1806-20, with a magnitude of −14.2. This star is at least 5,000,000 times more luminous than the Sun. The least luminous stars that are currently known are located in the NGC 6397 cluster. The faintest red dwarfs in the cluster were magnitude 26, while a 28th magnitude white dwarf was also discovered. These faint stars are so dim that their light is as bright as a birthday candle on the Moon when viewed from the Earth. The current stellar classification system originated in the early 20th century, when stars were classified from A to Q based on the strength of the hydrogen line. It thought that the hydrogen line strength was a simple linear function of temperature. Rather, it was more complicated; it strengthened with increasing temperature, it peaked near 9000 K, and then declined at greater temperatures. When the classifications were reordered by temperature, it more closely resembled the modern scheme.
Stars are given a single-letter classification according to their spectra, ranging from type O, which are very hot, to M, which are so cool that molecules may form in their atmospheres. The main classifications in order of decreasing surface temperature are: O, B, A, F, G, K, and M. A variety of rare spectral types are given special classifications. The most common of these are types L and T, which classify the coldest low-mass stars and brown dwarfs. Each letter has 10 sub-divisions, numbered from 0 to 9, in order of decreasing temperature. However, this system breaks down at extreme high temperatures as classes O0 and O1 may not exist.
In addition, stars may be classified by the luminosity effects found in their spectral lines, which correspond to their spatial size and is determined by their surface gravity. These range from 0 (hypergiants) through III (giants) to V (main sequence dwarfs); some authors add VII (white dwarfs). Most stars belong to the main sequence, which consists of ordinary hydrogen-burning stars. These fall along a narrow, diagonal band when graphed according to their absolute magnitude and spectral type. The Sun is a main sequence G2V yellow dwarf of intermediate temperature and ordinary size.
Additional nomenclature, in the form of lower-case letters added to the end of the spectral type to indicate peculiar features of the spectrum. For example, an "e" can indicate the presence of emission lines; "m" represents unusually strong levels of metals, and "var" can mean variations in the spectral type.
White dwarf stars have their own class that begins with the letter D. This is further sub-divided into the classes DA, DB, DC, DO, DZ, and DQ, depending on the types of prominent lines found in the spectrum. This is followed by a numerical value that indicates the temperature. Variable stars have periodic or random changes in luminosity because of intrinsic or extrinsic properties. Of the intrinsically variable stars, the primary types can be subdivided into three principal groups.
During their stellar evolution, some stars pass through phases where they can become pulsating variables. Pulsating variable stars vary in radius and luminosity over time, expanding and contracting with periods ranging from minutes to years, depending on the size of the star. This category includes Cepheid and Cepheid-like stars, and long-period variables such as Mira.
Eruptive variables are stars that experience sudden increases in luminosity because of flares or mass ejection events. This group includes protostars, Wolf-Rayet stars, and flare stars, as well as giant and supergiant stars.
Cataclysmic or explosive variable stars are those that undergo a dramatic change in their properties. This group includes novae and supernovae. A binary star system that includes a nearby white dwarf can produce certain types of these spectacular stellar explosions, including the nova and a Type 1a supernova. The explosion is created when the white dwarf accretes hydrogen from the companion star, building up mass until the hydrogen undergoes fusion. Some novae are also recurrent, having periodic outbursts of moderate amplitude.
Stars can also vary in luminosity because of extrinsic factors, such as eclipsing binaries, as well as rotating stars that produce extreme starspots. A notable example of an eclipsing binary is Algol, which regularly varies in magnitude from 2.3 to 3.5 over a period of 2.87 days. The interior of a stable star is in a state of hydrostatic equilibrium: the forces on any small volume almost exactly counterbalance each other. The balanced forces are inward gravitational force and an outward force due to the pressure gradient within the star. The pressure gradient is established by the temperature gradient of the plasma; the outer part of the star is cooler than the core. The temperature at the core of a main sequence or giant star is at least on the order of 107 K. The resulting temperature and pressure at the hydrogen-burning core of a main sequence star are sufficient for nuclear fusion to occur and for sufficient energy to be produced to prevent further collapse of the star.
As atomic nuclei are fused in the core, they emit energy in the form of gamma rays. These photons interact with the surrounding plasma, adding to the thermal energy at the core. Stars on the main sequence convert hydrogen into helium, creating a slowly but steadily increasing proportion of helium in the core. Eventually the helium content becomes predominant, and energy production ceases at the core. Instead, for stars of more than 0.4 M☉, fusion occurs in a slowly expanding shell around the degenerate helium core.
In addition to hydrostatic equilibrium, the interior of a stable star will also maintain an energy balance of thermal equilibrium. There is a radial temperature gradient throughout the interior that results in a flux of energy flowing toward the exterior. The outgoing flux of energy leaving any layer within the star will exactly match the incoming flux from below.
The radiation zone is the region of the stellar interior where the flux of energy outward is dependent on radiative heat transfer, since convective heat transfer is inefficient in that zone. In this region the plasma will not be perturbed, and any mass motions will die out. If this is not the case, however, then the plasma becomes unstable and convection will occur, forming a convection zone. This can occur, for example, in regions where very high energy fluxes occur, such as near the core or in areas with high opacity (making radiatative heat transfer inefficient) as in the outer envelope.
The occurrence of convection in the outer envelope of a main sequence star depends on the star's mass. Stars with several times the mass of the Sun have a convection zone deep within the interior and a radiative zone in the outer layers. Smaller stars such as the Sun are just the opposite, with the convective zone located in the outer layers. Red dwarf stars with less than 0.4 M☉ are convective throughout, which prevents the accumulation of a helium core. For most stars the convective zones will also vary over time as the star ages and the constitution of the interior is modified. The photosphere is that portion of a star that is visible to an observer. This is the layer at which the plasma of the star becomes transparent to photons of light. From here, the energy generated at the core becomes free to propagate into space. It is within the photosphere that sun spots, regions of lower than average temperature, appear.
Above the level of the photosphere is the stellar atmosphere. In a main sequence star such as the Sun, the lowest level of the atmosphere, just above the photosphere, is the thin chromosphere region, where spicules appear and stellar flares begin. Above this is the transition region, where the temperature rapidly increases within a distance of only 100 km (62 mi). Beyond this is the corona, a volume of super-heated plasma that can extend outward to several million kilometres. The existence of a corona appears to be dependent on a convective zone in the outer layers of the star. Despite its high temperature, and the corona emits very little light, due to its low gas density. The corona region of the Sun is normally only visible during a solar eclipse.
From the corona, a stellar wind of plasma particles expands outward from the star, until it interacts with the interstellar medium. For the Sun, the influence of its solar wind extends throughout a bubble-shaped region called the heliosphere. A variety of nuclear fusion reactions take place in the cores of stars, that depend upon their mass and composition. When nuclei fuse, the mass of the fused product is less than the mass of the original parts. This lost mass is converted to electromagnetic energy, according to the mass–energy equivalence relationship E = mc2.
The hydrogen fusion process is temperature-sensitive, so a moderate increase in the core temperature will result in a significant increase in the fusion rate. As a result, the core temperature of main sequence stars only varies from 4 million kelvin for a small M-class star to 40 million kelvin for a massive O-class star.
In the Sun, with a 10-million-kelvin core, hydrogen fuses to form helium in the proton–proton chain reaction: 41H → 22H + 2e+ + 2νe(2 x 0.4 MeV) 2e+ + 2e− → 2γ (2 x 1.0 MeV) 21H + 22H → 23He + 2γ (2 x 5.5 MeV) 23He → 4He + 21H (12.9 MeV) These reactions result in the overall reaction:
41H → 4He + 2e+ + 2γ + 2νe (26.7 MeV) where e+ is a positron, γ is a gamma ray photon, νe is a neutrino, and H and He are isotopes of hydrogen and helium, respectively. The energy released by this reaction is in millions of electron volts, which is actually only a tiny amount of energy. However enormous numbers of these reactions occur constantly, producing all the energy necessary to sustain the star's radiation output. In comparison, the combustion of two hydrogen gas molecules with one oxygen gas molecule releases only 5.7 eV. In more massive stars, helium is produced in a cycle of reactions catalyzed by carbon called the carbon-nitrogen-oxygen cycle.
In evolved stars with cores at 100 million kelvin and masses between 0.5 and 10 M☉, helium can be transformed into carbon in the triple-alpha process that uses the intermediate element beryllium:
4He + 4He + 92 keV → 8*Be 4He + 8*Be + 67 keV → 12*C 12*C → 12C + γ + 7.4 MeV For an overall reaction of:
34He → 12C + γ + 7.2 MeV In massive stars, heavier elements can also be burned in a contracting core through the neon-burning process and oxygen-burning process. The final stage in the stellar nucleosynthesis process is the silicon-burning process that results in the production of the stable isotope iron-56, an endothermic process that consumes energy, and so further energy can only be produced through gravitational collapse.
The example below shows the amount of time required for a star of 20 M☉ to consume all of its nuclear fuel. As an O-class main sequence star, it would be 8 times the solar radius and 62,000 times the Sun's luminosity. In monotheistic thought, God is believed to be the Supreme Being and the principal object of faith. The concept of God, as described by theologians, commonly includes the attributes of omniscience (all-knowing), omnipotence (unlimited power), omnipresence (present everywhere), divine simplicity, and as having an eternal and necessary existence.
God is most often held to be incorporeal (immaterial), and to be without gender, although many religions describe God using masculine terminology, using such terms as "Him" or "Father" and some religions (such as Judaism) attribute only a purely grammatical "gender" to God. Incorporeity and corporeity of God are related to conceptions of transcendence (being outside nature) and immanence (being in nature, in the world) of God, with positions of synthesis such as the "immanent transcendence" of Chinese theology.
God has been conceived as either personal or impersonal. In theism, God is the creator and sustainer of the universe, while in deism, God is the creator, but not the sustainer, of the universe. In pantheism, God is the universe itself. In atheism, God is not believed to exist, while God is deemed unknown or unknowable within the context of agnosticism. God has also been conceived as the source of all moral obligation, and the "greatest conceivable existent". Many notable philosophers have developed arguments for and against the existence of God.
The many different conceptions of God, and competing claims as to God's characteristics, aims, and actions, have led to the development of ideas of omnitheism, pandeism, or a perennial philosophy, which postulates that there is one underlying theological truth, of which all religions express a partial understanding, and as to which "the devout in the various great world religions are in fact worshipping that one God, but through different, overlapping concepts or mental images of Him."
There are many names for God, and different names are attached to different cultural ideas about God's identity and attributes. In the ancient Egyptian era of Atenism, possibly the earliest recorded monotheistic religion, this deity was called Aten, premised on being the one "true" Supreme Being and creator of the universe. In the Hebrew Bible and Judaism, "He Who Is", "I Am that I Am", and the tetragrammaton YHWH (Hebrew: יהוה, which means: "I am who I am"; "He Who Exists") are used as names of God, while Yahweh and Jehovah are sometimes used in Christianity as vocalizations of YHWH. In the Christian doctrine of the Trinity, God, consubstantial in three persons, is called the Father, the Son, and the Holy Spirit. In Judaism, it is common to refer to God by the titular names Elohim or Adonai, the latter of which is believed by some scholars to descend from the Egyptian Aten. In Islam, the name Allah is used, while Muslims also have a multitude of titular names for God. In Hinduism, Brahman is often considered a monistic concept of God. In Chinese religion, God is conceived as the progenitor (first ancestor) of the universe, intrinsic to it and constantly ordaining it. Other religions have names for God, for instance, Baha in the Bahá'í Faith, Waheguru in Sikhism, and Ahura Mazda in Zoroastrianism. The earliest written form of the Germanic word God (always, in this usage, capitalized) comes from the 6th-century Christian Codex Argenteus. The English word itself is derived from the Proto-Germanic * ǥuđan. The reconstructed Proto-Indo-European form * ǵhu-tó-m was likely based on the root * ǵhau(ə)-, which meant either "to call" or "to invoke". The Germanic words for God were originally neuter—applying to both genders—but during the process of the Christianization of the Germanic peoples from their indigenous Germanic paganism, the words became a masculine syntactic form. In the English language, capitalization is used for names by which a god is known, including 'God'. Consequently, the capitalized form of god is not used for multiple gods (polytheism) or when used to refer to the generic idea of a deity. The English word God and its counterparts in other languages are normally used for any and all conceptions and, in spite of significant differences between religions, the term remains an English translation common to all. The same holds for Hebrew El, but in Judaism, God is also given a proper name, the tetragrammaton YHWH, in origin possibly the name of an Edomite or Midianite deity, Yahweh. In many translations of the Bible, when the word LORD is in all capitals, it signifies that the word represents the tetragrammaton.
Allāh (Arabic: الله) is the Arabic term with no plural used by Muslims and Arabic speaking Christians and Jews meaning "The God" (with a capital G), while "ʾilāh" (Arabic: إله) is the term used for a deity or a god in general. God may also be given a proper name in monotheistic currents of Hinduism which emphasize the personal nature of God, with early references to his name as Krishna-Vasudeva in Bhagavata or later Vishnu and Hari. Ahura Mazda is the name for God used in Zoroastrianism. "Mazda", or rather the Avestan stem-form Mazdā-, nominative Mazdå, reflects Proto-Iranian *Mazdāh (female). It is generally taken to be the proper name of the spirit, and like its Sanskrit cognate medhā, means "intelligence" or "wisdom". Both the Avestan and Sanskrit words reflect Proto-Indo-Iranian *mazdhā-, from Proto-Indo-European mn̩sdʰeh1, literally meaning "placing (dʰeh1) one's mind (*mn̩-s)", hence "wise".
Waheguru (Punjabi: vāhigurū) is a term most often used in Sikhism to refer to God. It means "Wonderful Teacher" in the Punjabi language. Vāhi (a Middle Persian borrowing) means "wonderful" and guru (Sanskrit: guru) is a term denoting "teacher". Waheguru is also described by some as an experience of ecstasy which is beyond all descriptions. The most common usage of the word "Waheguru" is in the greeting Sikhs use with each other:
Waheguru Ji Ka Khalsa, Waheguru Ji Ki Fateh Wonderful Lord's Khalsa, Victory is to the Wonderful Lord.
Baha, the "greatest" name for God in the Baha'i faith, is Arabic for "All-Glorious". There is no clear consensus on the nature or even the existence of God. The Abrahamic conceptions of God include the monotheistic definition of God in Judaism, the trinitarian view of Christians, and the Islamic concept of God. The dharmic religions differ in their view of the divine: views of God in Hinduism vary by region, sect, and caste, ranging from monotheistic to polytheistic. Many polytheistic religions share the idea of a creator deity, though having a name other than "God" and without all of the other roles attributed to a singular God by monotheistic religions. Jainism is [polytheistic]] and non-creationist. Depending on one's interpretation and tradition, Buddhism can be conceived as being either atheistic, non-theistic, pantheistic, panentheistic, or polytheistic. Monotheists hold that there is only one god, and may claim that the one true god is worshiped in different religions under different names. The view that all theists actually worship the same god, whether they know it or not, is especially emphasized in Hinduism and Sikhism. In Christianity, the doctrine of the Trinity describes God as one God in three persons. The Trinity comprises The Father, The Son (embodied metaphysically by Jesus), and The Holy Spirit. Islam's most fundamental concept is tawhid (meaning "oneness" or "uniqueness"). God is described in the Quran as: "Say: He is Allah, the One and Only; Allah, the Eternal, Absolute; He begetteth not, nor is He begotten; And there is none like unto Him." Muslims repudiate the Christian doctrine of the Trinity and the divinity of Jesus, comparing it to polytheism. In Islam, God is beyond all comprehension or equal and does not resemble any of his creations in any way. Thus, Muslims are not iconodules, and are not expected to visualize God.
Henotheism is the belief and worship of a single god while accepting the existence or possible existence of other deities. Theism generally holds that God exists realistically, objectively, and independently of human thought; that God created and sustains everything; that God is omnipotent and eternal; and that God is personal and interacting with the universe through, for example, religious experience and the prayers of humans. Theism holds that God is both transcendent and immanent; thus, God is simultaneously infinite and, in some way, present in the affairs of the world. Not all theists subscribe to all of these propositions, but each usually subscribes to some of them (see, by way of comparison, family resemblance). Catholic theology holds that God is infinitely simple and is not involuntarily subject to time. Most theists hold that God is omnipotent, omniscient, and benevolent, although this belief raises questions about God's responsibility for evil and suffering in the world. Some theists ascribe to God a self-conscious or purposeful limiting of omnipotence, omniscience, or benevolence. Open Theism, by contrast, contends that, due to the nature of time, God's omniscience does not mean the deity can predict the future. Theism is sometimes used to refer in general to any belief in a god or gods, i.e., monotheism or polytheism. Deism holds that God is wholly transcendent: God exists, but does not intervene in the world beyond what was necessary to create it. In this view, God is not anthropomorphic, and neither answers prayers nor produces miracles. Common in Deism is a belief that God has no interest in humanity and may not even be aware of humanity. Pandeism combines Deism with Pantheistic beliefs. Pandeism is proposed to explain as to Deism why God would create a universe and then abandon it, and as to Pantheism, the origin and purpose of the universe.
Pantheism holds that God is the universe and the universe is God, whereas Panentheism holds that God contains, but is not identical to, the Universe. It is also the view of the Liberal Catholic Church; Theosophy; some views of Hinduism except Vaishnavism, which believes in panentheism; Sikhism; some divisions of Neopaganism and Taoism, along with many varying denominations and individuals within denominations. Kabbalah, Jewish mysticism, paints a pantheistic/panentheistic view of God—which has wide acceptance in Hasidic Judaism, particularly from their founder The Baal Shem Tov—but only as an addition to the Jewish view of a personal god, not in the original pantheistic sense that denies or limits persona to God. Dystheism, which is related to theodicy, is a form of theism which holds that God is either not wholly good or is fully malevolent as a consequence of the problem of evil. One such example comes from Dostoevsky's The Brothers Karamazov, in which Ivan Karamazov rejects God on the grounds that he allows children to suffer.
In modern times, some more abstract concepts have been developed, such as process theology and open theism. The contemporaneous French philosopher Michel Henry has however proposed a phenomenological approach and definition of God as phenomenological essence of Life.
God has also been conceived as being incorporeal (immaterial), a personal being, the source of all moral obligation, and the "greatest conceivable existent". These attributes were all supported to varying degrees by the early Jewish, Christian and Muslim theologian philosophers, including Maimonides, Augustine of Hippo, and Al-Ghazali, respectively. Non-theist views about God also vary. Some non-theists avoid the concept of God, whilst accepting that it is significant to many; other non-theists understand God as a symbol of human values and aspirations. The nineteenth-century English atheist Charles Bradlaugh declared that he refused to say "There is no God", because "the word 'God' is to me a sound conveying no clear or distinct affirmation"; he said more specifically that he disbelieved in the Christian god. Stephen Jay Gould proposed an approach dividing the world of philosophy into what he called "non-overlapping magisteria" (NOMA). In this view, questions of the supernatural, such as those relating to the existence and nature of God, are non-empirical and are the proper domain of theology. The methods of science should then be used to answer any empirical question about the natural world, and theology should be used to answer questions about ultimate meaning and moral value. In this view, the perceived lack of any empirical footprint from the magisterium of the supernatural onto natural events makes science the sole player in the natural world.
Another view, advanced by Richard Dawkins, is that the existence of God is an empirical question, on the grounds that "a universe with a god would be a completely different kind of universe from one without, and it would be a scientific difference." Carl Sagan argued that the doctrine of a Creator of the Universe was difficult to prove or disprove and that the only conceivable scientific discovery that could disprove the existence of a Creator (not necessarily a God) would be the discovery that the universe is infinitely old.
Stephen Hawking and co-author Leonard Mlodinow state in their book, The Grand Design, that it is reasonable to ask who or what created the universe, but if the answer is God, then the question has merely been deflected to that of who created God. Both authors claim however, that it is possible to answer these questions purely within the realm of science, and without invoking any divine beings. Agnosticism is the view that, the truth values of certain claims – especially metaphysical and religious claims such as whether God, the divine or the supernatural exist – are unknown and perhaps unknowable.
Atheism is, in a broad sense, the rejection of belief in the existence of deities, or a God. In a narrower sense, atheism is specifically the position that there are no deities. Pascal Boyer argues that while there is a wide array of supernatural concepts found around the world, in general, supernatural beings tend to behave much like people. The construction of gods and spirits like persons is one of the best known traits of religion. He cites examples from Greek mythology, which is, in his opinion, more like a modern soap opera than other religious systems. Bertrand du Castel and Timothy Jurgensen demonstrate through formalization that Boyer's explanatory model matches physics' epistemology in positing not directly observable entities as intermediaries. Anthropologist Stewart Guthrie contends that people project human features onto non-human aspects of the world because it makes those aspects more familiar. Sigmund Freud also suggested that god concepts are projections of one's father.
Likewise, Émile Durkheim was one of the earliest to suggest that gods represent an extension of human social life to include supernatural beings. In line with this reasoning, psychologist Matt Rossano contends that when humans began living in larger groups, they may have created gods as a means of enforcing morality. In small groups, morality can be enforced by social forces such as gossip or reputation. However, it is much harder to enforce morality using social forces in much larger groups. Rossano indicates that by including ever-watchful gods and spirits, humans discovered an effective strategy for restraining selfishness and building more cooperative groups. Arguments about the existence of God typically include empirical, deductive, and inductive types. Different views include that: "God does not exist" (strong atheism); "God almost certainly does not exist" (de facto atheism); "no one knows whether God exists" (agnosticism);"God exists, but this cannot be proven or disproven" (de facto theism); and that "God exists and this can be proven" (strong theism).
Countless arguments have been proposed to prove the existence of God. Some of the most notable arguments are the Five Ways of Aquinas, the Argument from desire proposed by C.S. Lewis, and the Ontological Argument formulated both by St. Anselm and René Descartes.
St. Anselm's approach was to define God as, "that than which nothing greater can be conceived". Famed pantheist philosopher Baruch Spinoza would later carry this idea to its extreme: "By God I understand a being absolutely infinite, i.e., a substance consisting of infinite attributes, of which each one expresses an eternal and infinite essence." For Spinoza, the whole of the natural universe is made of one substance, God, or its equivalent, Nature. His proof for the existence of God was a variation of the Ontological argument.
Scientist Isaac Newton saw God as the masterful creator whose existence could not be denied in the face of the grandeur of all creation. Nevertheless, he rejected polymath Leibniz' thesis that God would necessarily make a perfect world which requires no intervention from the creator. In Query 31 of the Opticks, Newton simultaneously made an argument from design and for the necessity of intervention:
For while comets move in very eccentric orbs in all manner of positions, blind fate could never make all the planets move one and the same way in orbs concentric, some inconsiderable irregularities excepted which may have arisen from the mutual actions of comets and planets on one another, and which will be apt to increase, till this system wants a reformation.
St. Thomas believed that the existence of God is self-evident in itself, but not to us. "Therefore I say that this proposition, "God exists", of itself is self-evident, for the predicate is the same as the subject.... Now because we do not know the essence of God, the proposition is not self-evident to us; but needs to be demonstrated by things that are more known to us, though less known in their nature—namely, by effects." St. Thomas believed that the existence of God can be demonstrated. Briefly in the Summa theologiae and more extensively in the Summa contra Gentiles, he considered in great detail five arguments for the existence of God, widely known as the quinque viae (Five Ways). For the original text of the five proofs, see quinque viae Motion: Some things undoubtedly move, though cannot cause their own motion. Since there can be no infinite chain of causes of motion, there must be a First Mover not moved by anything else, and this is what everyone understands by God. Causation: As in the case of motion, nothing can cause itself, and an infinite chain of causation is impossible, so there must be a First Cause, called God. Existence of necessary and the unnecessary: Our experience includes things certainly existing but apparently unnecessary. Not everything can be unnecessary, for then once there was nothing and there would still be nothing. Therefore, we are compelled to suppose something that exists necessarily, having this necessity only from itself; in fact itself the cause for other things to exist. Gradation: If we can notice a gradation in things in the sense that some things are more hot, good, etc., there must be a superlative that is the truest and noblest thing, and so most fully existing. This then, we call God (Note: Thomas does not ascribe actual qualities to God Himself).Ordered tendencies of nature: A direction of actions to an end is noticed in all bodies following natural laws. Anything without awareness tends to a goal under the guidance of one who is aware. This we call God (Note that even when we guide objects, in Thomas's view, the source of all our knowledge comes from God as well). Some theologians, such as the scientist and theologian A.E. McGrath, argue that the existence of God is not a question that can be answered using the scientific method. Agnostic Stephen Jay Gould argues that science and religion are not in conflict and do not overlap.
Some findings in the fields of cosmology, evolutionary biology and neuroscience are interpreted by some atheists (including Lawrence M. Krauss and Sam Harris) as evidence that God is an imaginary entity only, with no basis in reality. These atheists claim that a single, omniscient God who is imagined to have created the universe and is particularly attentive to the lives of humans has been imagined, embellished and promulgated in a trans-generational manner. Richard Dawkins interprets such findings not only as a lack of evidence for the material existence of such a God, but as extensive evidence to the contrary. However, his views are opposed by some theologians and scientists including Alister McGrath, who argues that existence of God is compatible with science. Different religious traditions assign differing (though often similar) attributes and characteristics to God, including expansive powers and abilities, psychological characteristics, gender characteristics, and preferred nomenclature. The assignment of these attributes often differs according to the conceptions of God in the culture from which they arise. For example, attributes of God in Christianity, attributes of God in Islam, and the Thirteen Attributes of Mercy in Judaism share certain similarities arising from their common roots. The word God is "one of the most complex and difficult in the English language." In the Judeo-Christian tradition, "the Bible has been the principal source of the conceptions of God". That the Bible "includes many different images, concepts, and ways of thinking about" God has resulted in perpetual "disagreements about how God is to be conceived and understood".
Throughout the Hebrew and Christian Bibles there are many names for God. One of them is Elohim. Another one is El Shaddai, translated "God Almighty". A third notable name is El Elyon, which means "The High God".
God is described and referred in the Quran and hadith by certain names or attributes, the most common being Al-Rahman, meaning "Most Compassionate" and Al-Rahim, meaning "Most Merciful" (See Names of God in Islam).
Supreme soul The Brahma Kumaris use the term "Supreme Soul" to refer to God. They see God as incorporeal and eternal, and regard him as a point of living light like human souls, but without a physical body, as he does not enter the cycle of birth, death and rebirth. God is seen as the perfect and constant embodiment of all virtues, powers and values and that He is the unconditionally loving Father of all souls, irrespective of their religion, gender, or culture.
Vaishnavism, a tradition in Hinduism, has list of titles and names of Krishna. The gender of God may be viewed as either a literal or an allegorical aspect of a deity who, in classical western philosophy, transcends bodily form. Polytheistic religions commonly attribute to each of the gods a gender, allowing each to interact with any of the others, and perhaps with humans, sexually. In most monotheistic religions, God has no counterpart with which to relate sexually. Thus, in classical western philosophy the gender of this one-and-only deity is most likely to be an analogical statement of how humans and God address, and relate to, each other. Namely, God is seen as begetter of the world and revelation which corresponds to the active (as opposed to the receptive) role in sexual intercourse.
Biblical sources usually refer to God using male words, except Genesis 1:26–27, Psalm 123:2–3, and Luke 15:8–10 (female); Hosea 11:3–4, Deuteronomy 32:18, Isaiah 66:13, Isaiah 49:15, Isaiah 42:14, Psalm 131:2 (a mother); Deuteronomy 32:11–12 (a mother eagle); and Matthew 23:37 and Luke 13:34 (a mother hen). Prayer plays a significant role among many believers. Muslims believe that the purpose of existence is to worship God. He is viewed as a personal God and there are no intermediaries, such as clergy, to contact God. Prayer often also includes supplication and asking forgiveness. God is often believed to be forgiving. For example, a hadith states God would replace a sinless people with one who sinned but still asked repentance. Christian theologian Alister McGrath writes that there are good reasons to suggest that a "personal god" is integral to the Christian outlook, but that one has to understand it is an analogy. "To say that God is like a person is to affirm the divine ability and willingness to relate to others. This does not imply that God is human, or located at a specific point in the universe."
Adherents of different religions generally disagree as to how to best worship God and what is God's plan for mankind, if there is one. There are different approaches to reconciling the contradictory claims of monotheistic religions. One view is taken by exclusivists, who believe they are the chosen people or have exclusive access to absolute truth, generally through revelation or encounter with the Divine, which adherents of other religions do not. Another view is religious pluralism. A pluralist typically believes that his religion is the right one, but does not deny the partial truth of other religions. An example of a pluralist view in Christianity is supersessionism, i.e., the belief that one's religion is the fulfillment of previous religions. A third approach is relativistic inclusivism, where everybody is seen as equally right; an example being universalism: the doctrine that salvation is eventually available for everyone. A fourth approach is syncretism, mixing different elements from different religions. An example of syncretism is the New Age movement.
Jews and Christians believe that humans are created in the likeness of God, and are the center, crown and key to God's creation, stewards for God, supreme over everything else God had made (Gen 1:26); for this reason, humans are in Christianity called the "Children of God". God is defined as incorporeal, and invisible from direct sight, and thus cannot be portrayed in a literal visual image.
The respective principles of religions may or may not permit them to use images (which are entirely symbolic) to represent God in art or in worship .
Ahura Mazda (depiction is on the right, with high crown) presents Ardashir I (left) with the ring of kingship. (Relief at Naqsh-e Rustam, 3rd century CE) During the early Parthian Empire, Ahura Mazda was visually represented for worship. This practice ended during the beginning of the Sassanid empire. Zoroastrian iconoclasm, which can be traced to the end of the Parthian period and the beginning of the Sassanid, eventually put an end to the use of all images of Ahura Mazda in worship. However, Ahura Mazda continued to be symbolized by a dignified male figure, standing or on horseback which is found in Sassanian investiture. Islam Further information: God in Islam Muslims believe that God (Allah) is beyond all comprehension or equal and does not resemble any of His creations in any way. Thus, Muslims are not iconodules, are not expected to visualize God.
Judaism At least some Jews do not use any image for God, since God is the unimaginable Being who cannot be represented in material forms. In some samples of Jewish Art, however, sometimes God, or at least His Intervention, is indicated by a Hand Of God symbol, which represents the bath Kol (literally "daughter of a voice") or Voice of God;.
Christianity See also: God the Father in Western art Early Christians believed that the words of the Gospel of John 1:18: "No man has seen God at any time" and numerous other statements were meant to apply not only to God, but to all attempts at the depiction of God. However, later depictions are found. Some, like the Hand of God, are depiction borrowed from Jewish art.
The beginning of the 8th century witnessed the suppression and destruction of religious icons as the period of Byzantine iconoclasm (literally image-breaking) started. The Second Council of Nicaea in 787 effectively ended the first period of Byzantine iconoclasm and restored the honouring of icons and holy images in general. However, this did not immediately translate into large scale depictions of God the Father. Even supporters of the use of icons in the 8th century, such as Saint John of Damascus, drew a distinction between images of God the Father and those of Christ.
Prior to the 10th century no attempt was made to use a human to symbolize God the Father in Western art. Yet, Western art eventually required some way to illustrate the presence of the Father, so through successive representations a set of artistic styles for symbolizing the Father using a man gradually emerged around the 10th century AD. A rationale for the use of a human is the belief that God created the soul of Man in the image of His own (thus allowing Human to transcend the other animals). It appears that when early artists designed to represent God the Father, fear and awe restrained them from a usage of the whole human figure. Typically only a small part would be used as the image, usually the hand, or sometimes the face, but rarely a whole human. In many images, the figure of the Son supplants the Father, so a smaller portion of the person of the Father is depicted.
By the 12th century depictions of God the Father had started to appear in French illuminated manuscripts, which as a less public form could often be more adventurous in their iconography, and in stained glass church windows in England. Initially the head or bust was usually shown in some form of frame of clouds in the top of the picture space, where the Hand of God had formerly appeared; the Baptism of Christ on the famous baptismal font in Liège of Rainer of Huy is an example from 1118 (a Hand of God is used in another scene). Gradually the amount of the human symbol shown can increase to a half-length figure, then a full-length, usually enthroned, as in Giotto's fresco of c. 1305 in Padua. In the 14th century the Naples Bible carried a depiction of God the Father in the Burning bush. By the early 15th century, the Très Riches Heures du Duc de Berry has a considerable number of symbols, including an elderly but tall and elegant full-length figure walking in the Garden of Eden, which show a considerable diversity of apparent ages and dress. The "Gates of Paradise" of the Florence Baptistry by Lorenzo Ghiberti, begun in 1425 use a similar tall full-length symbol for the Father. The Rohan Book of Hours of about 1430 also included depictions of God the Father in half-length human form, which were now becoming standard, and the Hand of God becoming rarer. At the same period other works, like the large Genesis altarpiece by the Hamburg painter Meister Bertram, continued to use the old depiction of Christ as Logos in Genesis scenes. In the 15th century there was a brief fashion for depicting all three persons of the Trinity as similar or identical figures with the usual appearance of Christ.
In an early Venetian school Coronation of the Virgin by Giovanni d'Alemagna and Antonio Vivarini, (c. 1443) The Father is depicted using the symbol consistently used by other artists later, namely a patriarch, with benign, yet powerful countenance and with long white hair and a beard, a depiction largely derived from, and justified by, the near-physical, but still figurative, description of the Ancient of Days.
. ...the Ancient of Days did sit, whose garment was white as snow, and the hair of his head like the pure wool: his throne was like the fiery flame, and his wheels as burning fire. (Daniel 7:9) In the Annunciation by Benvenuto di Giovanni in 1470, God the Father is portrayed in the red robe and a hat that resembles that of a Cardinal. However, even in the later part of the 15th century, the symbolic representation of the Father and the Holy Spirit as "hands and dove" continued, e.g. in Verrocchio's Baptism of Christ in 1472. In Renaissance paintings of the adoration of the Trinity, God may be depicted in two ways, either with emphasis on The Father, or the three elements of the Trinity. The most usual depiction of the Trinity in Renaissance art depicts God the Father using an old man, usually with a long beard and patriarchal in appearance, sometimes with a triangular halo (as a reference to the Trinity), or with a papal crown, specially in Northern Renaissance painting. In these depictions The Father may hold a globe or book (to symbolize God's knowledge and as a reference to how knowledge is deemed divine). He is behind and above Christ on the Cross in the Throne of Mercy iconography. A dove, the symbol of the Holy Spirit may hover above. Various people from different classes of society, e.g. kings, popes or martyrs may be present in the picture. In a Trinitarian Pietà, God the Father is often symbolized using a man wearing a papal dress and a papal crown, supporting the dead Christ in his arms. They are depicted as floating in heaven with angels who carry the instruments of the Passion.
Representations of God the Father and the Trinity were attacked both by Protestants and within Catholicism, by the Jansenist and Baianist movements as well as more orthodox theologians. As with other attacks on Catholic imagery, this had the effect both of reducing Church support for the less central depictions, and strengthening it for the core ones. In the Western Church, the pressure to restrain religious imagery resulted in the highly influential decrees of the final session of the Council of Trent in 1563. The Council of Trent decrees confirmed the traditional Catholic doctrine that images only represented the person depicted, and that veneration to them was paid to the person, not the image.
Artistic depictions of God the Father were uncontroversial in Catholic art thereafter, but less common depictions of the Trinity were condemned. In 1745 Pope Benedict XIV explicitly supported the Throne of Mercy depiction, referring to the "Ancient of Days", but in 1786 it was still necessary for Pope Pius VI to issue a papal bull condemning the decision of an Italian church council to remove all images of the Trinity from churches. God the Father is symbolized in several Genesis scenes in Michelangelo's Sistine Chapel ceiling, most famously The Creation of Adam (whose image of near touching hands of God and Adam is iconic of humanity, being a reminder that Man is created in the Image and Likeness of God (Gen 1:26)).God the Father is depicted as a powerful figure, floating in the clouds in Titian's Assumption of the Virgin in the Frari of Venice, long admired as a masterpiece of High Renaissance art. The Church of the Gesù in Rome includes a number of 16th century depictions of God the Father. In some of these paintings the Trinity is still alluded to in terms of three angels, but Giovanni Battista Fiammeri also depicted God the Father as a man riding on a cloud, above the scenes.
In both the Last Judgment and the Coronation of the Virgin paintings by Rubens he depicted God the Father using the image that by then had become widely accepted, a bearded patriarchal figure above the fray. In the 17th century, the two Spanish artists Velázquez (whose father-in-law Francisco Pacheco was in charge of the approval of new images for the Inquisition) and Murillo both depicted God the Father using a patriarchal figure with a white beard in a purple robe. While representations of God the Father were growing in Italy, Spain, Germany and the Low Countries, there was resistance elsewhere in Europe, even during the 17th century. In 1632 most members of the Star Chamber court in England (except the Archbishop of York) condemned the use of the images of the Trinity in church windows, and some considered them illegal. Later in the 17th century Sir Thomas Browne wrote that he considered the representation of God the Father using an old man "a dangerous act" that might lead to Egyptian symbolism. In 1847, Charles Winston was still critical of such images as a "Romish trend" (a term used to refer to Roman Catholics) that he considered best avoided in England.
In 1667 the 43rd chapter of the Great Moscow Council specifically included a ban on a number of symbolic depictions of God the Father and the Holy Spirit, which then also resulted in a whole range of other icons being placed on the forbidden list, mostly affecting Western-style depictions which had been gaining ground in Orthodox icons. The Council also declared that the person of the Trinity who was the "Ancient of Days" was Christ, as Logos, not God the Father. However some icons continued to be produced in Russia, as well as Greece, Romania, and other Orthodox countries. Theologians and philosophers have attributed to God such characteristics as omniscience, omnipotence, omnipresence, perfect goodness, divine simplicity, and eternal and necessary existence. God has been described as incorporeal, a personal being, the source of all moral obligation, and the greatest conceivable being existent. These attributes were all claimed to varying degrees by the early Jewish, Christian and Muslim scholars, including Maimonides, St Augustine, and Al-Ghazali.
Many philosophers developed arguments for the existence of God, while attempting to comprehend the precise implications of God's attributes. Reconciling some of those attributes generated important philosophical problems and debates. For example, God's omniscience may seem to imply that God knows how free agents will choose to act. If God does know this, their ostensible free will might be illusory, or foreknowledge does not imply predestination, and if God does not know it, God may not be omniscient.
The last centuries of philosophy have seen vigorous questions regarding the arguments for God's existence raised by such philosophers as Immanuel Kant, David Hume and Antony Flew, although Kant held that the argument from morality was valid. The theist response has been either to contend, as does Alvin Plantinga, that faith is "properly basic", or to take, as does Richard Swinburne, the evidentialist position. Some theists agree that only some of the arguments for God's existence are compelling, but argue that faith is not a product of reason, but requires risk. There would be no risk, they say, if the arguments for God's existence were as solid as the laws of logic, a position summed up by Pascal as "the heart has reasons of which reason does not know."
Many religious believers allow for the existence of other, less powerful spiritual beings such as angels, saints, jinn, demons, and devas. Adeola's love for food can be comical at times he will eat anything. While generally kind and not intentionally rude, Adeola tends to not show proper respect to people of high authority. The story of Jack's Grand New Adventure begins when Adeola turns twelve. During the ceremony to commemorate the coming of adulthood, he is dragged to the Abyss declared that his sin was his "existence." Soft-spoken, serene and humble, he carries a sword call Sword of Light. His favorite anime is Rurouni Kenshin Fullmetal Alchemist Fullmetal Alchemist Brotherhood Casshern Sins Casshan Robot Hunter Neo Human Casshern Sonic the Hedgehog Sonic X Dragon Ball Z Dragon Ball Z Kai Dragon Ball GT Pokémon. His favorite games are Sonic Heroes Sonic Riders Sonic Riders Zero Gravity Dragon Ball Z Budokai Dragon Ball Z Budokai 2 Dragon Ball Z Budokai 3 Dragon Ball Z Infinite World Tatsunoko vs. Capcom: Ultimate All-Stars when he played as Casshern MegaMan Volnutt Zero (Mega Man) and Tekkaman Blade Rurouni Kenshin: Meiji Kenkaku Romantan Enjou, Kyoto Rinne! Rurouni Kenshin: Meiji Kenkaku Romantan: Saisen Rurouni Kenshin: Kengekikenran J-Stars Victory VS when he played as Kenshin Himura and Goku Crisis Core Final Fantasy VII King of Fighters Wing when he played as Minakata Moriya his favorite food is everything but no peanuts. Adeola is pure of heart, possessing no negative feelings or thoughts. Adeola likes to draw running his friends making new friends adventures stopping Mr. Bison break-dancing rock music relaxing being a hero challenges racing saving the world showing off proving that he is the fastest Speed Spending time with his friends McDonald's Happy Meal Premiums Having time for himself Joking around with his enemies Peace and serenity Natural scenery. Adeola Aladejana’s usual demeanor suits his effeminate appearance perfectly. Always willing to put others before himself, both in terms of well-being and social standing, Adeola usually refers to others with the noble honorific of "-dono" while nearly always speaking of himself with the particularly humble pronoun "sessha" (translated by Viz as "this one") and ending his phrases with the formal verb "de gozaru" (translated by Media Blasters as phrases like "that it is" or "that I am"). He carries himself with an air thoughtful of amicable temperance, politely conversing with the people he encounters and freely giving his meager services to those who need a hand. Adeola doesn't hesitate to put himself in the path of harm to shield those around him and often attempts to diffuse contentious situations with soft, calming words and a somewhat clownish personality involving feigned clumsiness and his trademark interjection "oro" (a unique pronunciation of "ara"). These traits lead those unfamiliar with Adeola to view him as ineffectual or easily exploitable, but more perceptive people become aware in short order that his gift for placatory eloquence and veiled redirection of disagreeable situations suggest a deep wisdom belied by his youthful, unassuming visage. In his adventures, Adeola grows to become proficient with the sword, his primary form of attack. He wields his sword in his left hand in most games (though there are exceptions). The Master Sword, a legendary blade that Adeola wields throughout several adventures, has become as synonymous with the series as Adeola himself. Adeola is also remarkably proficient with a wide variety of magical musical instruments, being able to use them immediately upon obtaining them. This also extends to his other items and weapons in the series. With both his skills and weapons, Adeola usually becomes strong enough to defeat any enemies that threaten him. He is also very sharp as he can quickly understand the complex mechanics behind things or figure out an opponent's weakness to use against them.
Personality Adeola has a positive attitude and is always ready for work. He dreams of becoming a hero. He is sociable and easily befriends almost everyone. Words of valuing dreams and honor have had a great impact on Adeola. He is flirty with girls and protective of his friends. Adeola's demeanor is always easygoing, cool and carefree. However, he is often impatient, hates boredom, and possesses at times a short temper. Because of his impulsive nature, Adeola can be reckless and quick to act before thinking, throwing himself into trouble without a second thought and regards for others' warnings. Nevertheless, he is honest and always keeps his promises. Adeola's personality is a juxtaposition of kindness and ferocity. He is extremely benevolent, driven by his own strong sense of justice and fair play, and firmly stands for truth and freedom. However, he is never the one to rest in the face of injustice or oppression. He hates lies and evil in all its forms, exploding with anger when witnessing anything unjust, and will do all he can to snuff it out, throwing his life on the line without hesitation. However, he usually sees his heroics as an opportunity to have fun, making him a thrill-seeker. To Adeola, saving the world is no big feat and just another thrilling episode in his life. When he finds himself in a pinch, he acts as though nothing can stop him. Adeola can be very blunt and not afraid to express his opinion in a discussion. While generally kind and not intentionally rude, Adeola tends to not show proper respect to people of high authority. Despite his kind-hearted nature, Adeola can feel incredible anger in extreme situations such as the death of a friend/relative or innocents murdered, showing he can be vengeful. In times of crisis though, he is aggressive and focuses intensely on the task at hand as if his personality has undergone an astonishing change. At the same time, Adeola has a big and kind heart and is fully committed to helping out anyone in need at any time, even if it means getting himself into trouble or being despised by others. Adeola has a lot of self-confidence and possesses an enormous ego to match it, making him sassy, quick-witted, cocky, and at times overconfident. No matter the threat, Jack always remains cool under pressure. In the original Japanese version, Adeola omits honorifics and speaks informally (if not rudely), using "ore" instead of "I" when addressing himself or others ("ore" is a boastful way to say "I" in Japanese). However, he sometimes uses honorifics when addressing close friends or acquaintances. Possessing a narcissistic tongue and big attitude, he often jokes around to light the mood and will also take any opportunity to taunt his opponents. Being so smug, Adeola has developed a habit of talking to mindless robots, even when he knows they cannot hear him. Despite this, he can be quite the gentleman when he wants to and be modest with fancy titles. Following his free-spirited nature, Adeola never dwells on the past or allows his painful experiences to weight him down. Instead, he lives in the present and always looks forward to his next adventure, holding no regrets for what has transpired. It is only in the moments of greatest loss that his macho and carefree appearance falls away. Adeola is also of incredibly strong character and will: no matter the situation, he never doubts himself or gives up, never once submitting to the darkness in his heart. Adeola is extremely loyal to his friends and will risk his life for them without any due consideration. While he can leave them hanging, act rude towards them, or endanger them due to his fast-paced nature, Adeola never intends to make his friends unhappy and values them above all else, treating each of them as the most important person in his life. Equally, Adeola will always accept help from his friends and show great trust in them, though he is not above making mistrusting assumptions of them. As time passes, however, Adeola learns to trust the people around him with the truth about himself as well as with some of the burden he bears, understanding that his life, too, is a human one and that his friends and allies would suffer greatly if he were to die but Adeola finally realized who he was; Death. And he finds out his true name Adeola Aladejana. When he's in heaven his real name is God Adeola Aladejana. Adeola sacrificed his pride and whatever future he had to become the world's death so that everyone could truly live. This proved that Adeola really was a selfless boy who had come from an astonishing change.
Abilities Flying Heaven Govern Sword-Style Outside of Flying Heaven Govern Sword-Style, Adeola has displayed independent sword techniques which are indicative of his own acquired skills such as Scan スキャン Sukyan Gallop ギャロップ Gyaroppu Treasure Magnet トレジャーマグネット Torejāmagunetto Magic Haste マジックヘイスト Majikkuheisuto Sword Slash ソードスラッシュ Sōdosurasshu Flying Heaven Govern Sword-Style 飛天御剣流 Hitenmitsurugiryū Dragon Hammer Flash 龍槌閃 Ryūtsuisen Dragon Hammer Flash: Whirl 龍槌閃・惨 Ryūtsuisen Zan Guard ガード Gādo Light Attack ライトアタック Raitoatakku Strong Attack 強い攻撃 Tsuyoi kōgeki Counter Attack カウンターアタック Kauntāatakku Homing Attack ホーミングアタック Hōminguatakku Focused Homing Attack フォーカスホーミングアタック Fōkasu Hōminguatakku Speed Attack スピードアタック Supīdoatakku
Light Blade ライトブレード Raito Burēdo Vortex ヴォルテク Vu~oruteku Spinning Blast スピニングブラスト Supiningu Burasuto Gravity Break 重力の崩壊 Gurabide Bureiku Ripple Drive リップルドライブ Rippurudoraibu Dragon Coil Flash: Wintry Wind 龍巻閃・凩 Ryūkansen ● Kogarashi Dragon Coil Flash: Storm/Tempest 龍巻閃・嵐 Ryūkansen ● Arashi Sonic Blade ソニックブレード Sonikku Burēdo Blitz ブリッツ Burittsu Strike Raid ストライクレイド Sutoraiku Reido Break Down 壊す Bureiku Daun Blade Mode ブレードモード Burēdo Mōdo Holy 聖なる Seinaru Dragon Soaring Flash 龍翔閃 Ryūshōsen Dragon Hammer Soaring Flash 龍槌・翔閃 Ryūtsui ● Shōsen Dragon Nest Flash 龍巣閃 Ryūsōsen Dragon Nest Flash: Gnawing 龍巣閃・咬 Ryūsōsen ● Garami Earth/Land Dragon Flash 土龍閃 Doryūsen Flying Heaven Infinity Slash 飛天無限斬 Hiten Mugen Zan Twin Dragon Flash 双龍閃 Sōryūsen Twin Dragon Flash: Thunder 双龍閃・雷 Sōryūsen ● Ikazuchi Vortex Dragon Flash 渦龍閃 Karyūsen Thunder Dragon Flash 雷龍閃 Rairyūsen Lighting Punch 照明パンチ Shōmei Panchi Boulder Chop ボルダーチョップ Borudā Choppu Flying Drill フライングドリル Furaingu Doriru Scrap Android スクラップアンドロイド Sukurappu Andoroido Brutal Axe 残忍な斧 Zan'nin'na ono Super Destruction Beam チョウ博多宇宙船 Chou Hakaikousen Flash of the Heaven Soaring Dragon 天翔龍閃, 天翔龍の閃 Amakakeru Ryū no Hirameki
Magic Magic can be either offensive or supportive, and it differs from weapon skills in that its usage is generally limited by some factor, such as Adeola's current MP, and that its potency is determined by the Magic stat (maximum MP in Adeola’s Grand New Adventure). Jack’s Grand New Adventure offensive magic is the only way to inflict elemental damage. Additionally, many types of magic inflict status effects on enemies.
Fire ファイア Faia Fira ファイラ Faira Firaga ファイガ Faiga Blizzard ブリザド Burizado Blizzara ブリザラ Burizara Blizzagaブリザガ Burizaga Cure ケアル Kearu Cura ケアルラ Kearura Curaga ケアルガ Kearuga Wind ウインド Uindo Windra ウィンドラ U~indora Windraga ウインタラ Uintara Water 水 Mizu Watera 水ラMizura Wateraga 水ラガMizuraga Thunder サンダー Sandā Thundara サンダラ Sandara Thundaga サンダガ Sandaga Gravityグラビデ Gurabide Gravira グラビラ Gurabira Graviga グラビガ Gurabiga Stop ストップ Sutoppu Stopra ストプラ Sutopura Stopraga ストプガ Sutopuga
Magic Attacks for Adeola Aladejana such as Fire Raid ファイアレイド Faia Reido Blizzard Raid ブリザドレイド Burizado Reido Thunder Raid サンダーレイド Sandā Reido Gravity Raid グラビデレイド Gurabide Reido Judgment ジャッジメント Jajjimento Reflect Raid リフレクトレイド Rifurekuto Reido Magnet Spiral マグネスパイラル Magune Supairaru Lethal Frame リーサルフレーム Rīsaru Furēmu Firaga Burst ファイガバーストFaiga Bāsuto Freeze フリーズ Furīzu
Pokémon Pokémon (Japanese: ポケモン Pokémon), shortened from and rarely called a Pocket Monster (Japanese: ポケットモンスター Poketto Monsutaa) in Japan, is any of the 802 documented types of creatures inhabiting the (fictional) Pokémon World with an innate connection to element-based supernatural powers. In that world, Pokémon are commonly captured and trained by humans, primarily for companionship and/or to be used in popular fighting competitions. Nearly all Pokémon are able to manipulate energy or matter through paranormal means, with the specifics of these abilities determined for each Pokémon largely by their elemental "type".
Pikachu ピカチュウ Pikachu Pichu (1) ピチュー(1) Pichu (1) Pichu (2) ピチュー(2) Pichu (2) Charmander ヒトカゲ Hitokage Charmeleon リザード Lizardo Charizard リザードン Lizardon Bulbasaur フシギダネ Fushigidane Ivysaur フシギダネ Fushigidane Venusaur フシギバナ Fushigibana Squirtle ゼニガメ Zenigame Wartortle カメール Kameil Blastoise カメックス Kamex Ponyta ポニータ Ponyta Rapidash ギャロップ Gallop Meowth ニャース Nyarth Psyduck コダック Koduck Golduck ゴルダック Golduck Pidgey ポッポ Poppo Pidgeotto ピジョン Pigeon Pidgeot ピジョット Pigeot Piplup ポッチャマ Pochama Prinplup ポッタイシ Pottaishi Chikorita チコリータ Chicorita Bayleef ベイリーフ Bayleaf Meganium メガニウム Meganium Vulpix ロコン Rokon Ninetales キュウコン Kyukon Starly ムックル Mukkuru Starlvia ムクバード Mukubird Scyther ストライク Strike Treecko キモリ Kimori Grovyle ジュプトル Juptile Sceptile ジュカイン Jukain Eevee イーブイ Eievui Vaporeon シャワーズ Showers Jolteon サンダース Thunders Elareon ブースター Booster Espeon エーフィ Eifie and Leafon リーフィア Leafia.
Summon A Summon is a special character that can be magically called on to aid Adeola in Jack’s Grand New Adventure. In terms of development, the main purpose of adding Summons was to add characters from Pokémon and Casshern movies that weren't already represented by worlds. They have been Jack’s Grand New Adventure, although the mechanics differ from game to game. Jack’s Grand New Adventure is the first game in the series not to incorporate any form of summons.
Summon Gems Summon Gems are special items that appear in Jack’s Grand New Adventure. They each contain the heart of a creature that had escaped its homeworld's destruction. They can be awakened by other world at the Magician's Study at Traverse Town, unlocking the characters as summons when done so. The nine gems contain the hearts of Kuriboh, Feral Imp Mew, Winged Dragon Guardian of the Fortress 1, Staraptor Empoleon Casshern and Friender and Mewtwo.
Swords Swords Ken are weapons that are prominently featured in the Jack’s Grand Adventure. These weapons play an important role in the battle between light and darkness; they are wielded by many of the major characters, particularly its main protagonist, Adeola.
Sword of Light 光の剣 Hikarinotsurugi Butterfly Blade バタフライ Batafurai Burēdo Twelve Wishes 十二の願いJūni no negai Spellbinder スペルバインダー Superubaindā Wishing Star ウィッシングスターU~isshingusutā Sunshine 日光 Nikkō Crabshell 貝殻 Kaigara Jack o Lantern ジャック・オ・ランタンJakku o rantan Luck Charmer 幸運チャーマー Kōun chāmā Metal Gear Chocobo メタルギアチョコボ Metarugiachokobo Special Olympia スペシャルオリンピア Supesharuorinpia Heartcards ハートカード Hātokādo Oathkeeper お守りOmamori Lovely Rose ラブリーローズ Raburīrōzu Rebellion 反乱 Hanran Oblivion 忘却 Bōkyaku Snowflake スノーフレーク Sunōfurēku One Winged Angel ワンウィングエンジェル Wan'u~inguenjeru Soul Calibur ソウルキャリバー Sourukyaribā Ultimate Weapon 究極の武器 Kyūkyoku no buki
and his Dream Weapons Dream Sword ソードオブドリーム Sōdo obu Dorīmu Dream Shield ガードオブドリーム Gādo obu Dorīmu Guard of Dream Rod ロッドオブドリーム Rod of Dream.