Question

Explain why most of the mass of stars with M_i > 20M_ʘ is in the form of Carbon and Oxygen.

Answer 1

when is stars formed it's made up of many of hydrogen like sun and hence so many stars are made up of hydrogen and oxygen upto iron.

For instance using carbon required very high temperature to fuse show formation of hydrogen and oxygen takes place because the oxygen is the second known and founding element in the universe after hydrogen and Helium there after we can say that the stars greater than that mass usually made up of Oxygen and Carbon

Star like Sun makes hydrogen and carbon while the heavier stars make other element too.