This process, whereby four hydrogen nuclei fuse together to form both energy and a helium nucleus, provides the major source of fuel for stars. Larger stars burn faster. As they begin to burn out, they generate within their core even heavier elements such as carbon and oxygen. Early in the universe (within the first few hundred million years) such elements appeared only in the core of these collapsing stars, but some of these stars then went through massive explosions known as supernovae, flinging heavier elements back into the gas in the galaxy.
