Solar System -
Description of the Sun
I have often heard the Sun described as an insignificant star, in a non-descript part of an average galaxy. This is far from the truth. In fact, it is brighter than at least 90% of the stars in the Milky Way, and is in the top 5% or so in terms of mass. It is only a little over half way out from the center of the galaxy, which is itself quite large for a spiral or, to be more accurate, barred-spiral galaxy. So not too insignificant!
The Sun is nearly 1.4 million kilometers (~865,000 miles) across, about 109 times the diameter of the Earth, and its interior could accommodate more than 1,300,000 Earths. It represents about 99.86% of the entire mass present in the Solar System including all the planets, moons, asteroids, comets etc. At its surface, the Sun's temperature is around 5,800 K, but at the core it is approximately 15 million K.
The sun's equator takes about 25 days to rotate once about its axis. As the Sun is made from gas, or a plasma, rather than being a solid body like the Earth, different parts rotate differently; for example, gas at latitudes towards the poles take more than 28 days to rotate once.
The Sun, as a main sequence Population I star, spends its time converting hydrogen nuclei into helium nuclei through proton-proton chain. It has been doing this for the last 4.5 billion years, and should continue doing it for at least as long again. Every second, between 600 and 700 million tons of hydrogen are converted into helium. About 0.7% of the original mass of hydrogen nuclei is released as energy because the total energy of a helium nucleus is less than the energy of the four protons used to create it. Thus, up to five million tons of hydrogen is converted into energy every second; the equivalent of more than 9 × 1010 megatons of TNT. Having said that, it is interesting to note that the core of the sun only produces about 8 watts per cubic foot, so its huge power output is due to its huge size and not to efficiency.
For a great many years, the source of the Sun's energy was unknown. A number of scientists put forward a number of suggestions, none of which were able to explain the time scale over which the Sun had been radiating. In the early 20th century, Ernest Rutherford suggested radioactive decay as the energy source. Einstein's famous formula provided an explanation of the amount of heat that could be produced, and in 1920, Sir Arthur Eddington suggested that the source could be a fusion reaction converting hydrogen into helium. The conversion of the small loss of mass, a helium nucleus is less than twice the mass of a hydrogen nucleus, would be the heat source. Over the years, the theory was refined, and observations prove that the heavy elements, up to iron, are produced by nuclear reactions in stars.
Here is a really good, basic introduction to the Sun.
Views of the Solar System has a lot of information about the sun, with some excellent photographs, particularly some of sunspots.