All stars are first born of mostly hydrogen, the simplest form of matter in space. Throughout their life, stars convert more and more hydrogen to helium, the next simplest form of matter. Helium can then be converted to oxygen or carbon if the star is large enough. Star size matters to how long it lives and how strong its gravity is. It also determines whether it can fuse together some kinds of elements or not.
Small stars can only convert hydrogen to helium.
Medium stars, about the size of our Sun, can fuse together helium into oxygen and carbon later on in their lives. Finally, the larger stars, more than five times the larger than the Sun, can convert oxygen and carbon to elements such as neon, sodium, magnesium, sulfur and silicon. If the star is large enough, other reactions can transform those elements into calcium and iron. To find out what happens in to the star as it dies, click here . Click here for a printable version of this page.