How do the gasses and densities of a star change of its life time?

A star is primarily composed of hydrogen at birth, with trace amounts of helium and heavier elements.

A star's main sequence is characterized by the fusion of hydrogen into helium, which causes the star's helium content to rise and its density to slightly decrease.

After the hydrogen is used up, helium fusion begins, producing carbon, nitrogen, and oxygen. As heavier elements replace lighter ones, the star once more becomes denser.

White dwarfs, which are primarily composed of carbon and oxygen, are the result of smaller stars not heating up enough to initiate carbon fusion.

Bigger stars have temperatures high enough to fuse carbon and heavier elements together; they keep fusing, becoming denser and denser, until the core is mostly iron and no more fusion reactions are possible because they absorb energy instead of releasing it.

Neutron stars, which are extremely dense, are created when a typical iron core collapses due to gravity; if the core is large enough, the neutron star will collapse into a black hole.

A star's composition varies with age; at first, it is mostly made of hydrogen and helium. Later, nuclear fusion in the star's core transforms hydrogen into helium, releasing energy that causes the star to expand and its outer layers to become less dense. Later still, heavier elements are created through fusion, which further modifies the star's composition and density. Finally, the star may explode as a supernova, releasing its material into space.