What happens to its atoms each time a star goes through a life stage?

Answer 1

Many of the individual atoms remain unchanged throughout the star's life cycle; the hydrogen atoms fall into the proto-star as the star is forming, circulate through the star without participating in a fusion reaction, and are ejected from the star when it expands as a red giant or explodes in a nova. The hydrogen atoms may be incorporated into a new star later on, or may drift in space for ages, or may become bonded to another hydrogen atom and an oxygen atom to become a water molecule.

Many of the atoms that fall into the star during its formation will participate in a nuclear fusion reaction, and the original hydrogen atom becomes part of a helium atom, while a small portion of the mass is converted into energy. The helium may be ejected from the atom during the red giant or nova death of the star. Or, the atom may remain in the stellar core for, essentially, forever.