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The Sun is the star at the centre of our Solar system. It is occasionally referred to by its Latin name, Sol, to distinguish it from other "suns". Planet Earth orbits the Sun, as do many other bodies, including other planets, asteroids, meteoroids, comets and dust. Its heat and light support almost all life on Earth.

The Sun is a ball of plasma with a mass of about 2×10³⁰kg, which is somewhat higher than that of an average star. About 74% of its mass is hydrogen, with 25% helium and the rest made up of trace quantities of heavier elements. It is thought that the Sun is about 5 billion years old, and is about half way through its main sequence evolution, during which nuclear fusion reactions in its core fuse hydrogen into helium. In about 5 billion years time the Sun will become a planetary nebula.

Although it is the nearest star to Earth and has been intensively studied by scientists, many questions about the Sun remain unanswered, such as why its outer atmosphere has a temperature of over 10⁶ K when its visible surface (the photosphere) has a temperature of just 6,000 K.

General Information

The Sun is classified as a main sequence star, which means it is in a state of "hydrostatic balance", neither contracting or expanding, and is generating its energy through nuclear fusion of hydrogen nuclei into helium. The Sun has a spectral class of G2V, with the G2 meaning that its color is yellow and its spectrum contains spectral lines of ionized and neutral metals as well as very weak hydrogen lines, and the V signifying that it, like most stars, is a "dwarf" star on the main sequence.

The Sun has a predicted main sequence lifetime of about 10 billion years. Its current age is thought to be about 4.5 billion years, a figure which is determined using computer models of stellar evolution, and nucleocosmochronology. 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 million years. The orbital speed is 217 km/s, equivalent to one light year every 1400 years, and one AU every 8 days.

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Structure

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 about 10 km. This is because the centrifugal effect of the Sun's slow 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 center of mass of the solar system, which is offset from the Sun's center mostly because of the large mass of Jupiter. The mass of the Sun is so comparatively great that the center of mass of the solar system is generally within the bounds of the Sun itself.

The Sun does not have a definite boundary as rocky planets do, as the density of its gases drops off following an approximately 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 directly observable and the Sun itself is opaque to electromagnetic radiation. However, just as the study of the waves generated by earthquakes (seismology) can be used to study the interior structure of the Earth, helioseismology, the study of sound waves that travel through the Sun's interior, has also contributed greatly to our understanding of the Sun's structure. Computer modeling of the Sun is also used as a theoretical tool to investigate its deep layers.

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Core

At the center of the Sun, where its density reaches up to 150,000 kg/m³ (150 times the density of water on Earth), thermonuclear reactions (nuclear fusion) convert hydrogen into helium, producing the energy that keeps the Sun in a state of equilibrium. About 8.9×10³⁷ protons (hydrogen nuclei) are converted to helium nuclei every second, releasing 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). Models predict that the high-energy photons released in fusion reactions take about a million years to reach the Sun's surface, where they escape as visible light. Neutrinos are also released in the fusion reactions in the core, but unlike photons they very rarely interact with matter, and so almost all are able to escape the Sun immediately.

The core extends from the center of the Sun to about 0.2 solar radii, and 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.

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Radiative Zone

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 temperature gradient 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.

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Convention Zone

From about 0.7 solar radii to 1.0 solar radii, the material in the Sun is not dense enough or hot enough to transfer the heat energy of the interior outward via radiation. As a result, thermal convection occurs 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. Convective overshoot is thought to occur at the base of the convection zone, carrying turbulent downflows into the outer 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.

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Photosphere

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 approximately a black-body spectrum that indicates its temperature is about 6,000 K, 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.

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Temperature Minimum

The coolest layer of the Sun is the temperature minimum region about 500 km above the photosphere. It is about 4,000 K. 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.

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Chromosphere

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.

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Corona

The corona is the extended outer atmosphere of the Sun, which is much larger in volume than the Sun itself. The corona merges smoothly with the solar wind that fills the solar system and heliosphere. The low corona, which is very near the surface of the Sun, has a particle density of 10¹¹/m³ (Earth's atmosphere near sea level has a particle density of about 2x10²⁵/m³). The temperature of the corona is several million degrees K.

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Theoretical Problems

Solar Neutrino problem

For some time it was thought that the number of neutrinos produced by the nuclear reactions in the Sun was only a third of the number predicted by theory, a result that was termed the solar neutrino problem. Several neutrino observatories were constructed, including the Sudbury Neutrino Observatory and Kamiokande to try to measure the solar neutrino flux. It has recently been found that neutrinos have rest mass, and can therefore transform into harder-to-detect varieties of neutrinos while en route from the Sun to Earth in a process known as neutrino oscillation. Thus, measurement and theory have been reconciled.

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Coronal Heating Problem

The optical surface of the Sun (the photosphere) is known to have a temperature of about 6,000 K. Above it lies the solar corona with a temperatures of one million degrees. The high temperature of the corona suggests that it is heated by something other than 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. Two main mechanisms have been proposed to explain coronal heating: Wave heating, in which sound, gravitational and 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 gas in the form of heat. The other proposed mechanism is flare heating, in which magnetic energy is continuously built up by photospheric motion and released through magnetic reconnection in the form of solar flares and waves.

Currently, it is unclear whether waves are an efficient heating mechanism. All waves except Alfven waves have been found to dissipate or refract before reaching the corona. In addition, Alfven waves do not easily dissipate in the corona. Current research focus has therefore shifted towards flare heating mechanisms. One possible candidate to explain coronal heating is continuous flaring at small scales, but this is still an open topic of investigation.

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Faint young Sun problem

Theoretical models of the sun's development suggest that 3.8 to 2.5 billion years ago, during the Archean period, the Sun was only about 75 percent as bright as it is today. Such a weak star would not have been able to sustain liquid water on the Earth's surface, and thus life should not have been able to develop.

However, the geologic record shows that the Earth has remained at a fairly constant temperature throughout its history. In fact, the young Earth was actually warmer than it is today. Some scientists have suggested that the young Earth's atmosphere contained much larger quantities of greenhouse gases such as carbon dioxide and/or ammonia than are present today. Others suggest that cosmic rays might strongly influence the Earth's climate, and that their flux was much higher in the early history of the solar system.

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Solar Space Missions

To obtain an uninterrupted view of the Sun, the European Space Agency and NASA cooperatively launched the Solar and Heliospheric Observatory (SOHO) on December 2, 1995.

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.

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History and Future of Sun

The Sun is thought to be a second-generation star, whose formation may have been triggered by shockwaves from a nearby supernova. This is suggested by a high abundance of heavy elements such as iron, gold and uranium in the solar system: the most plausible ways that these elements could be produced are by endothermic nuclear reactions during a supernova or by transmutation via neutron absorption inside a massive first generation star.

Our Sun does not have enough mass to explode as a supernova, and its mass is below the Chandrasekhar limit. Instead, in 4-5 billion years it will enter its red giant phase, its outer layers expanding as the hydrogen fuel in the core is consumed and the core contracts and heats up. Helium fusion will begin when the core temperature reaches about 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 evolve into a white dwarf, slowly cooling over eons. This stellar evolution scenario is typical of low to medium mass stars.

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