- Solar system in astrology. For the astrological significance of the Sun, see
| Mean distance from|
| 149.6×106 km|
|Visual brightness (V)||−26.8m|
| Mean distance from|
Milky Way centre
| 2.5×1017 km |
|Galactic period||2.26×108 a|
|Diameter|| 1.392×106 km|
|Surface area|| 6.09 × 1012 km²|
|Volume|| 1.41 × 1018 km³|
|Mass|| 1.9891 × 1030 kg|
|Surface gravity|| 273.95 m s-2|
| Escape velocity|
from the surface
|Surface temperature||5780 K|
|Temperature of corona||5 MK|
|Core temperature||~13.6 MK|
|Luminosity (L☉)||3.827×1026 W|
|Mean Intensity (I☉)||2.009×107 W m-2 sr-1|
|Obliquity|| 7.25? |
(to the ecliptic)
(to the galactic plane)
| Right ascension|
of North pole 1
| 286.13? |
(19 h 4 min 31.2 s)
of North pole
| Rotation period|
| 25.3800 days |
(25 d 9 h 7 min 12?8 s) 1
| Rotation velocity|
A Sun is the star at the center of a solar system. Our sun is usually referred to as the Sun, and is occasionally referred to as Sol to distinguish it from other "suns". Planet Earth orbits the Sun, as do innumerable other bodies including other planets, asteroids, meteoroids, comets and dust.
The primary stellar body around which an object orbits is called its sun, and stars in a multiple star system are referred to as the "suns" of bodies in that system.
The Sun is a main sequence star, with a spectral class of G2, meaning that it is somewhat more massive and hotter than the average star but far smaller than a blue giant star. A G2 star is on the main sequence, and has a lifetime of about 10 billion years (10 Ga), and the Sun formed about 5 Ga (5 billion years) ago, as determined by nucleocosmochronology and computer models of stellar evolution. The Sun orbits the center of the Milky Way galaxy at a distance of about 25,000 to 28,000 light-years from the galactic centre, completing one revolution in about 226 Ma (226 million years). The orbital speed is 217 km/s, i.e. 1 light-year in ca. 1400 years, and 1 AU in 8 days.
Structure of the Sun
The Sun is a near-perfect sphere, with an oblateness estimated at about 9 millionths, which means the polar diameter differs from the equatorial by 10 km at most. This is because the centrifugal effect of the Sun's rather sedate rotation is 18 million times weaker than its surface gravity (at the equator). Tidal effects from the planets do not significantly affect the shape of the Sun, although the Sun itself orbits the barycenter of the solar system, which is offset from the center of the Sun mostly because of the large mass of Jupiter.
The Sun does not have definite boundaries as rocky planets do. Instead, the density of gases comprising the Sun drops off following an exponential relationship with distance from the centre of the Sun. Nevertheless, the Sun has well defined interior structure, described below. The Sun's radius is measured from centre to the edges of the photosphere.
The solar interior is not accessible directly and the Sun itself is opaque to electromagnetic radiation. Humanity's knowledge of the solar interior comes from a combination of analytic and computer modeling of stars, and helioseismology, the study of sound waves that travel through the Sun's interior.
At the centre of the Sun, where its density is 150 g/cm3 (that's 150 times the density of water on Earth), thermonuclear reactions (nuclear fusion) convert hydrogen into helium, producing the heat that drives the entire star. About 8.9×1037 protons (hydrogen nuclei) are converted to helium nuclei every second. This releases energy at the matter-energy conversion rate of 4.26 million tonnes per second or 383 yottawatts (9.15×1016 tons of TNT per second) which makes its way through the other layers of the Sun to escape in the form of electromagnetic radiation (sunshine) and neutrinos (and to a smaller extent as the kinetic and thermal energy of solar wind plasma and as the energy in the Sun's magnetic field). A fusion reactor, which some physicists believe may one day provide power for human use, would use a similar process to extract atomic energy.
The core is the only part of the Sun where an appreciable amount of heat is produced by fusion: the rest of the star is heated by energy that is transferred outward. All of the energy of the interior fusion must travel through the successive layers to the solar photosphere, before it escapes to space. The core extends from the center to about 0.2 solar radius.
From about 0.2 to about 0.7 solar radii, the material is hot and dense enough that thermal radiation is sufficient to transfer the intense heat of the core outward. In this zone, there is no thermal convection: while the material grows cooler with altitude, this gradient in temperature is not strong enough to drive convection. Heat is transferred by ions of hydrogen and helium emitting photons, which travel a brief distance before being re-absorbed by other ions.
From about 0.7 solar radii to 1.0 solar radii, the material in the Sun is neither dense nor hot enough to transfer the heat energy of the interior outward. As a result, thermal convection carries the heat to the surface as thermal columns carry hot material to the surface (photosphere) of the Sun. Once the material cools off at the surface, it plunges back downward to the base of the convection zone, to receive more heat from the top of the radiative zone. There is a bit of convective overshoot at the base of the convection zone, carrying turbulent downflows into the top layers of the radiative zone.
The thermal columns in the convection zone form an imprint on the surface of the Sun, in the form of the solar granulation and supergranulation. The turbulent convection of this outer part of the solar interior gives rise to a 'small-scale' dynamo that produces magnetic north and south poles all over the surface of the Sun.
The visible surface of the Sun, the photosphere, is the layer below which the Sun becomes opaque to visible light. Above the photosphere, sunlight is free to propagate into space and its energy escapes the Sun entirely. Sunlight has a black-body spectrum that is characteristic of about 6,000 kelvins, interspersed with atomic absorption lines from the tenuous layers above the photosphere. The photosphere has a particle density of about 1023/m3 (this is about 1% of the particle density of Earth's atmosphere at sea level). The parts of the Sun above the photosphere are referred to collectively as the solar atmosphere. They can be viewed with telescopes operating across the electromagnetic spectrum, from radio through visible light to gamma rays.
The coolest layer of the Sun is the temperature minimum region about 500km above the photosphere. It is about 4,000 kelvins. It is the only part of the Sun cool enough to support simple molecules such as carbon monoxide and water; all other parts of the Sun are hot enough to break chemical bonds.
Above the visible surface of the Sun is a thin layer, about 2,000 km thick, that is dominated by a spectrum of emission and absorption lines. It is called the chromosphere from the Greek root chromos, meaning color, because the chromosphere is visible as a colored flash at the beginning and end of total eclipses of the Sun.
The corona is the extended outer atmosphere of the Sun, which is much larger than the Sun itself. The corona merges smoothly with the solar wind that fills the solar system and heliosphere. The low corona, near the surface of the Sun, has a particle density of 1011/m3.
Solar neutrino problem
For some time it was thought that the number of neutrinos produced by the nuclear reactions in the Sun was only one third of the number predicted by theory, a result that was termed the solar neutrino problem. Several neutrino observatories were created including the Sudbury Neutrino Observatory to try and measure the amount of neutrinos given off by the Sun. From these observatories and experiments it was recently found that neutrinos had rest mass, and could therefore transform into harder-to-detect varieties of neutrinos while en route from the Sun to Earth; thus measurement and theory were reconciled.
All matter in the Sun is in the form of plasma due to its extreme temperature. This makes it possible for the Sun to rotate faster at its equator (about 25 days) than it does at higher latitudes (28 days near its poles). The differential rotation of the Sun's latitudes causes its magnetic field lines to become twisted together over time, causing magnetic field loops to erupt from the Sun's surface and trigger the formation of the Sun's dramatic sunspots and solar prominences. (See magnetic reconnection) The solar activity cycle includes old magnetic fields being stripped off the Sun's surface starting from one pole and ending at the other. The magnetic field of the sun reverses once for each 11-year sunspot cycle.
Calculating the position of the Sun
Since the path of the sun across the sky varies throughout the year, a completely automatic heliostat, or sun tracker, must be guided by continuous calculations. The National Renewable Energy Laboratory has released its Solar Position Algorithm (SPA) with complete documentation. Another resource is the libnova Celestial Mechanics and Astronomical Calculation Library, which also calculates variables such as apparent position and rise, set and transit times among many others of astronomical objects.
Solar space missions
Elemental abundances in the photosphere are well known from spectroscopic studies, but the composition of the interior of the Sun is much less well known. A solar wind sample return mission, Genesis, was designed to allow astronomers to directly measure the composition of solar material. It returned to Earth in 2004 and is undergoing analysis, but it was damaged by crash-landing when its parachute failed to deploy on reentry to Earth's atmosphere.
History and future of the Sun
The Sun is thought to be a second-generation star, perhaps formed from some of the remains of a previous supernova. The evidence is mainly a high abundance of heavy elements such as iron, gold, and even uranium in the solar system: the most plausible ways that these elements could be produced is by nucleosynthesis inside a large, hot star.
Our Sun does not have enough mass to explode as a supernova. Instead, in 4–5 billion years it will enter its red giant phase, expanding as the hydrogen fuel in the core is consumed. Then it will start to fuse helium and the core temperature will rise to 3×108 K. While it is likely that the expansion of the outer layers of the Sun will reach the current position of Earth's orbit, recent research suggests that mass lost from the Sun earlier in its red giant phase will cause the Earth's orbit to move further out, preventing it from being engulfed. Following the red giant phase, giant thermal pulsations will cause the Sun to throw off its outer layers forming a planetary nebula. The Sun will then become a white dwarf, slowly cooling over eons. This scenario is typical of small stars: our Sun appears to be a fairly run-of-the-mill star.
Human understanding of the Sun
In many prehistoric and ancient cultures, the Sun was thought to be a deity or other supernatural phenomenon. One of the first people in the Western world to offer a scientific explanation for the sun was the Greek philosopher Anaxagoras, who reasoned that it was a giant flaming ball of rock or metal, and not the chariot of Apollo. For teaching this heresy he was imprisoned by the authorities and sentenced to death.
Sun and eye damage
Sunlight is quite bright and looking directly at the Sun is painful to the eyes. Looking directly at the Sun when it is high in the sky causes temporary bleaching of the photosensitive pigments in the retina, which makes phosphene visual artifacts and temporary partial blindness. Direct viewing of the Sun with the naked eye delivers about 4 milliwatts of sunlight to the retina that is in the solar image, heating it up and potentially (though not normally) damaging it. Brief viewing of the direct Sun with the naked eye is unpleasant but generally safe. Long-term exposure of the eyes to direct sunlight contributes to the normal UV-induced yellowing of the lens and cornea over periods of decades, and could play a part in the formation of cataracts.
Viewing the Sun through light-concentrating optics such as binoculars is hazardous without an attenuating filter to dim the sunlight. Suitable filters are available at welding supply shops and camera stores. Viewing the Sun through unfiltered 7x50mm binoculars can deliver as much as 2.5 watts of sunlight into each eye, over 300 times more power than naked eye viewing. Even brief glances at the Sun through binoculars can cause permanent blindness.
During partial eclipses of the Sun, another hazardous condition exists because of the way that the eye responds to bright light. The pupil is controlled by the total amount of light in the visual field, not by the brightest object in the field. During partial eclipses, most sunlight is blocked by the Moon passing directly in front of the Sun, but the uncovered parts of the photosphere have the same surface brightness as during a normal day. In the dim overall light, the pupil tends to dilate from ~2mm to perhaps 6mm diameter, increasing the eye's collecting area by a factor of nearly 10. Each retinal cell that is exposed to the partially-eclipsed solar image thus receives about ten times as much light as it would looking at the normal, non-eclipsed Sun. This can cause permanent localized damage to the retina, resulting in small, permanent blind spots for the viewer. This is an especially insidious hazard for inexperienced observers and for children, because there is no immediate perception of pain and it is tempting to stare at the spectacle of the eclipsing Sun.
- Current SOHO snapshots
- Far-Side Helioseismic Holography from Stanford
- NASA Eclipse homepage
- Nasa SOHO (Solar & Heliospheric Observatory) satelliteFAQ
- Solar Sounds from Stanford
- Eric Weisstein's World of Astronomy – Sun
- The Position of the Sun
- A collection of solar movies
- The Institute for Solar Physics- Movies of Sunspots and spicules