When you look at the night sky (preferably from a site with minimal light pollution), you’ll see hundreds of dots sprinkled above you. Those are nearly all stars — some just a few light-years away, while others make up galaxies millions of light-years distant. (One light-year is the amount of distance light travels in a year. It’s about 6 trillion miles.) All stars form from clouds of gas. Slightly denser regions pull in material from less-dense areas due to gravity. Once the object’s center becomes dense enough — and thus hot enough (some 10 million degrees) due to increasing pressure — hydrogen begins to fuse to form helium. And this process emits energy.