Celestial bodies in the universe like the stars, gain their energy by nuclear fusion. This is a nuclear reaction that emits radiation by joining subatomic particles together to yield another new element. This cause by instability of certain elements due to their high neutron-to-proton ratio. The most stable element there is, is Fe-26. Elements lighter than Fe-26 are most likely to undergo nuclear fusion (combining), while elements heavier than Fe-26 are most likely to undergo nuclear fission (breaking).
So that is how the Sun gains its energy. It is very abundant in hydrogen, such that hydrogen undergoes nuclear fusion. Two protons from two hydrogen atoms combine at very very high temperatures to form a Helium atom. Therefore, a high-mass star life is very abundant in Hydrogen, while a low-mass star life is very abundant in Helium.