Stars

Stars usually seem very simple, but thats far from the truth. Stars are fueled by nuclear fusion. Stars are massive enough that hydrogen is changed (by nuclear fusion) into helium. This change creats heat and light energy. Some stars are larger that others because there is more Hydrogen inside of them. Supermassive stars have very short lives of only a few million years because they burn their fuel faster. People only recently discovered what fueled a star.

Neutron Star in center of a SuperNovae

Stars of all shapes and sizes are in a constant battle against their own gravity which is trying to pull the star into itself. The only thing stopping this from happening is the star heat and light output is pushing against the gravity. So, naturally, when a supergiant star runs out of fuel it collapses into itself. The result is a massive explosion known as a supenovae. In the center of the debris is the colapsed core of the star it is a neutron star (for more info on neutron stars, I have an entry on them as well). If the star was massive enough it would be a black hole. I will have information on black holes very soon.