The sun is powered by the thermonuclear fusion of
hydrogen into helium, the same as an atomic bomb.
The fuel in an atomic bomb has a diameter of a few
centimeters. The sun has a diameter a 109 times
that of the Earth! Even 92 million miles away, you
probably expect it to be hot and bright.
I assume you ask how the sun stays so hot... As
you probably know, gravity pulls things together.
All the gas that makes up the sun tries to fall
into the center, making it very hot (25 million
Fahrenheit) and very dense. Under these
circumstances, hydrogen (which makes up 93% of the
sun) nuclei fuse together to form helium.
One helium turns out to weigh a little
less than the two hydrogen atoms, so where did the
extra mass go? Well, mass and energy are
conserved, but can change into each other
according to Einstein's famous formula E=mc^2
(energy=mass times the speed of light squared).
Now here's a little exercise to appreciate
how much energy a little mass can make. If I could
take one gram of matter and convert it into pure
energy, how much would it be worth? remember that
a dollar buys about 30 million watts.
turns out that a gram of hydrogen used for fusion
can produce about a small fraction of that amount,
but that's okay because the sun has about 2
octillion grams left to go :)