Sun

The Sun, also known as Sol, is a G2-type dwarf star located at the centre of the solar system. It contains more than 98% of the mass of the solar system; Jupiter contains most of the rest. The Sun is the most important star to humanity; it provides our home, Earth, with light and heat. The sun is at its main sequence stage, burning its hydrogen for fuel. Nearly 75% of the Sun consists of hydrogen; the majority of the rest is helium.

The Sun is often considered to be an "ordinary" star, due to the fact that many other stars are similar to it. However, the fact remains that the Sun is one of the smallest stars known, with little energy compared to most of the other stars in the galaxy.

Description
The Sun is an average main-sequence star. As stated above, in consists of 98% of the mass of the solar system. The Sun, like most stars, is not a perfect sphere or elongated, but is slightly elliptical. The Sun is orbited by eight planets, namely Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. When seen from Earth, the Sun appears white due to the way our visual processing has developed; however, it is actually a G-type star with a peak in the green portion of the visible spectrum.

Atmosphere (Corona)
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, and comprise five principal zones: the temperature minimum, the chromosphere, the transition region, the corona, and the heliosphere. The heliosphere, which may be considered the tenuous outer atmosphere of the Sun, extends outward past the orbit of Pluto to the heliopause, where it forms a sharp shock front boundary with the interstellar medium. The chromosphere, transition region, and corona are much hotter than the surface of the Sun. The reason why has not been conclusively proven; evidence suggests that Alfvén waves may have enough energy to heat the corona.

The coolest layer of the Sun is a temperature minimum region about 500 km above the photosphere, with a temperature of about 4,100 K. This part of the Sun is cool enough to support simple molecules such as carbon monoxide and water, which can be detected by their absorption spectra.

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 eclipses of the Sun. The temperature in the chromosphere increases gradually with altitude, ranging up to around 20,000 K near the top. In the upper part of chromosphere helium becomes partially ionized.

Taken by Hinode's Solar Optical Telescope on January 12, 2007, this image of the Sun reveals the filamentary nature of the plasma connecting regions of different magnetic polarity. Above the chromosphere there is a thin (about 200 km) transition region in which the temperature rises rapidly from around 20,000 K in the upper chromosphere to coronal temperatures closer to one million 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 extended outer atmosphere of the Sun, which is much larger in volume than the Sun itself. The corona continuously expands into the space forming the solar wind, which fills all the Solar System. The low corona, which is very near the surface of the Sun, has a particle density around 1015–1016 m−3. The average temperature of the corona and solar wind is about 1–2 million kelvins, however, in the hottest regions it is 8–20 million kelvins. While 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 heliosphere, which is the cavity around the Sun filled with the solar wind plasma, extends from approximately 20 solar radii (0.1 AU) to the outer fringes of the Solar System. Its inner boundary is defined as the layer in which the flow of the solar wind becomes superalfvénic—that is, where the flow becomes faster than the speed of Alfvén waves. Turbulence and dynamic forces outside this boundary 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. Both of the Voyager probes have recorded higher levels of energetic particles as they approach the boundary.

Photosphere
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 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 actually tens to hundreds of kilometers thick, being 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.

Sunspots
Occasionally, an area of the Sun will cool, forming a dark spot, surrounded by regions of the photosphere that appear as normal colour. These dark spots, known as ‘’sunspots’’, are much cooler, about 12,000 degrees Celsius, and as a result they appear black. Sunspots become more and less common every 11 Earth years; the next peak is due around 2011.