The Sun Shines 
Nothing can make you appreciate the Sun like a cold winter day. With the Sun shining low in the sky, and remaining in view for only a few hours, those in colder climates eagerly await the warmth it will bring in spring and summer.
Most early cultures only cared about the Sun's heat and light. Some of them thought of the Sun as a burning ball, but they had little idea of what stoked its fires.
Scientists didn't begin to tackle the question until the 19th century. Early in the century, the leading idea said that the Sun burned through some chemical process -- it might be a big lump of coal, for example. And another idea said the Sun was heated by impacts from meteors.
When they calculated how long those processes could power the Sun, though, they came up with a few thousand to a few million years. Even then, geologists and biologists were providing evidence that Earth -- and by extension, the Sun -- were much older than that.
The next idea was that the Sun shines through gravitational contraction. Its gravity pulls its material inward, compressing it and heating it up. But this process would also burn out fairly quickly.
The answer became clear in the early 20th century. The Sun and other stars are powered by nuclear fusion. Atoms of hydrogen stick together to form helium. About one percent of the hydrogen is converted into energy. It doesn't sound like much, but it's kept the Sun shining brightly for four and a half billion years.
Script by Damond Benningfield, Copyright 2010