On a simple level, most stars fuse hydrogen to form helium. But things are obviously more complex than that. Most of the hydrogen in our Sun is the lightest form, with just a single proton as its nucleus. The helium produced in stars has two protons and two neutrons. Obviously, making helium from only protons requires a series of nuclear reactions, each with distinct probabilities of occurring that depend in part on the conditions inside the star. Complicating matters further, there are some other possible reactions that don't lead directly to helium but can still occur inside a star, producing things like heavier isotopes of hydrogen. Read more.