The Earth-Sun distance is defined as the astronomical unit, and is equal to 149,597,870km (IAU 1976). The Sun's luminosity is 3.826 x 10 to the power of 26 J s-1, the solar mass is 1.989 x 10 to the power of 30 kg, and the solar radius is 6.960 x 10 to the power of 8 m. The sun is approximately 4.6 billion years old, and will remain on the main sequence for another 4.4 billion years.
The sun's atmosphere consists of the chromosphere, corona, and photosphere. The photosphere is the bright visible surface of the sun, with a surface temperature of 5,800 K. The chromosphere consists of the bright gases just above the photosphere of the sun and is 10,000-15,000 km thick. The corona is the sun's faint (and hot) outer atmosphere from 1.03R .o (about 20,000 km above the photosphere) to at least several solar radii. The temperature is about 1.5 MK. The surface of the sun consists of convection cells termed granules. The granules have diameters of 1000 km and lifetimes of 5-10 min and are observable in visible light. Superimposed upon the granules are supergranules (observed by Doppler shift measurements) which have diameters of 30,000 km and lifetimes of 20-25 h. The Doppler measurements show that the motion is entirely horizontal, with negligible vertical velocity. Why the sun should have only two convective length scales is not well understood. Magnetically heated filamentary regions which appear near the limb of the sun are called faculae. The continuum opacity of the sun is provided by bound-free and free-free transitions of the hydrogen ion . This was proposed by R. Wildt in 1940, and shown to be quantum mechanically consistent with observations by Chandrasekhar in 1945.
The sun's magnetic field has a surface strength of 2 gauss. It reverses with a period of 22 y (twice the period of the sunspot cycle). The sun rotates with a period of 24 days at the equator (corresponding to 27.2753 days as seen from the Earth) and 28 days near the poles. The number of rotations of the sun is counted using the Carrington rotation number. The sun's rotation appears to be constant on radial cones. Helioseismology, the study of oscillations of the sun indicates that the rotation persists at least 200,000 km down.
Dark, cool sunspots appear on the sun's surface often in pairs. They have magnetic field strengths of 100-1000 gauss and lifetimes of 2 months. The number of sunspots varies with a period of 11 y. During each cycle, sunspots migrate from high latitudes to the equator. Sunspots may affect Earth's climate, as deduced from the low number present during the little ice age of 1640-1710. During a solar cycle, all sunspot pairs in the northern hemisphere have the leading spot north and the trailing spot south (it is reversed in the southern hemisphere). Polarities are then reversed during the next solar cycle. An apparent 80-90 year cycle of long-term variations in solar activity is called the Gleisberg cycle.
The sun's output changes over the period of several days, as measured by the Solar Max satellite. The variation over the solar cycle was observed to be 0.04% between 1980 and 1986, when the flux at the Earth (known as the solar constant declined from 1368.5 to 1367 W m-2. The flux may be influenced by sunspots crossing the disk. Large sunspots groups moving across the sun's equator temporarily decrease the flux. This occurred April 3-10, 1980 when the Solar Maximum Mission on Nimbus 7 revealed a decrease of about 1.5 W m-2 during passage of a large sunspot group across the sun. However, faculae surrounding sunspots cause a general small solar flux increase!
An interplanetary dust ring was observed around the sun in the mid-1960s.
Source:
scienceworld.wolfram.com/astronomy/Sun.html
