Question: Why do stars shine?

Steven Gardner answered on 17 Jun 2013:

Stars form from a cloud of Hydrogen gas called a nebula. Gravity makes this nebula contract together, and the pressure at the centre grows, it also gets hotter. Eventually the centre gets so hot and the pressure so high the star begins a process called nuclear fusion. Nuclear fusion basically turns 2 hydrogens into a helium. The thing is, the hydrogens together are heavier than the helium so some of the mass is converted into energy. The amount is according to Einstein’s famous equation E=mc^2 (E is energy, m is mass and c is the speed of light). The speed of light is huge, so it only takes a little bit of mass to make lots of energy, and trillions of these events happen every second. So lots of energy, in the form of light, is emitted.

Eventually the star will use up all it’s hydrogen and will then beginning fusing Helium to make heavier elements. When the star runs out of helium fusion of heavier elements continues until you get to Iron. It takes more energy to fuse Iron that you get out, so this is when the fuel runs out and the star stops shining.