The sun is a medium sized star, about 4.5 billion years old, and is mostly composed of hydrogen and helium. The sun is a later generation star, so there are also traces of the heavier elements in proportions found on earth. The sun's radius is about 700,000 kilometers, its mass is about 2 x 10^30 kilograms, and it is about 150,000,000 kilometers from the earth.
The sun's energy source is hydrogen fusion at the core. The surface of the sun, the photosphere, is the solar disk we see. The chromosphere, is a colored layer above the photosphere which was first observed during a solar eclipse. Also seen during a solar eclipse is the rarefied solar atmosphere, known as the corona, which bathes the solar system in a solar wind.
Other features can be observed. Sunspots dot the solar disk. Solar activity, related to sunspots, also influences radio transmissions, produces aurora, and influences weather patterns. Radio transmission is improved with solar activity. Tree rings for thousands of years show wet and dry years related to the solar cycle, the periodic change in solar activity.
