The sun is an average star with a mass equal to nearly one-third of a million Earths. It is made up of almost 80% hydrogen by mass and is entirely gaseous, although the gas near its center is under such tremendous pressure, it behaves like a fluid. Because of this gaseous state, the sun rotates unevenly. Its equatorial section turns on its axis once every 25 days while the higher latitudes on the sun take over 27 days to rotate. The sun gets its energy from the fusion reaction of two hydrogen nuclei joining together to make one helium nucleus. This is the same reaction as that of the hydrogen bomb.
The fusion reaction began a few billion years ago as a result of the great temperatures in the sun's center caused by the gravitational contraction of the huge mass of hydrogen gas from which the sun probably formed. The production of heat from the sun is quite small per unit of volume, because like all large bodies, the area of surface is relatively small compared to its volume. In solids, volume increases by the cube, surface by the square. Also, the sun has had billions of years to heat up. The sun's relatively small core is very hot (approximately 14,000,000oC) and it is in the core that the fusion reaction occurs. Even though the sun's energy output is small compared to its huge size, it turns out energy on an enormous scale by Earth's standards, some 10 million megatons of energy per second. There is estimated to be enough hydrogen in the sun convertible to helium to allow the sun to shine at its present rate for 10 billion years to come.
Man sees only the glowing surface of the sun. During solar eclipses when the main disc of the sun is blotted out, one can also see the solar atmosphere or corona. The corona is made up of ionized hydrogen, a hydrogen atom with its one electron knocked off. However, the corona also contains all of the elements common on Earth. The corona is much more diffuse than the rest of the sun and much hotter that the surface...2,000,000oC. just above the sun's surface. This corona is thought to extend in diffused form to the outer limits of the solar system. Even during quiet periods of the sun, there are coronal streamers of very hot gas out to ten solar diameters. The sun's diameter is 1,400,000 km. The surface of the sun is covered with bright granules, but the most noticeable features are the sun spots, darker and cooler regions (4,500oC). Sun spots have tremendous magnetic fields (3,000 to 4,000 Gauss compared with about one Gauss for the rest of the sun's surface).
Sun spots are believed to be places where doughnut-shaped magnetic fields emerge from the sun's surface, and they usually occur in pairs of opposite magnetic polarity. At the maximum of solar activity in 1947, a sun spot of five billion square miles was seen. The sun alternates from very quiet to very turbulent and active, then back to quiet over 11-year cycles. At the peak of a cycle, numerous solar flares occur which throw out huge masses of hot gas and energetic particles. These masses of hot, ionized hydrogen hit the Earth's magnetic field, bend it out of shape, disrupt radio communications, cause the aurora borealis, and feed the Van Allen radiation belts. Flares are 1,000 to 100,000 times more dense than the surrounding material on the solar surface; the corona around the flares gets four times hotter than normal; and gas thrown out travels around 3,600,000 kmph. The cause of flares is not known, but they are generally believed to be related to sun spot magnetic fields. In addition to flares, the sun constantly throws out ionized hydrogen in the solar wind. This material moves at nearly 1.6 million kmph and contains one to ten particles per cubic centimeter. This is still more diffuse than the "hardest" vacuum yet made on Earth. (reference 77)