Our sun’s core is made up of hydrogen which is using fusion to create helium with an outer layer of hydrogen. This will continue for a total of about 10 billion years. When there is enough helium in the core the helium in the core will then start fusing into carbon but there will still be a layer of hydrogen fusing into helium above that and an outer layer of hydrogen. This faze will continue for another 1 billion years when the core will be completely carbon and the fusing will cease. In stars with 8 times the sun’s mass the fusing process can continue and the carbon core will fuse into oxygen. In large enough stars this process of fusing will continue with the oxygen fusing into neon, neon fusing into magnesium, magnesium into silicon and finally the silicon into iron which is the last element which can created by natural fusion. These large stars core will then collapse from gravity and the electrons and protons will combine and create neutrons with the result being an extremely dense neutron star since there is no electrical repulsion to keep the particles apart. Water has a density of about 1 gram per cubic centimeter but a neutron star has a density of about 100 trillion grams per cubic centimeter and has a size of about 10 kilometers in diameter.
(The Great Courses – The Life and Death of Stars)