What fundamental factor dictates a star's lifespan and the types of fusion reactions it can sustain?

Answer

Its initial mass.

A star's entire life and subsequent elemental output are determined by its initial mass, as greater mass leads to stronger gravitational compression, higher core temperatures, and faster fusion rates.

What fundamental factor dictates a star's lifespan and the types of fusion reactions it can sustain?
ElementstellarstarFusionnucleosynthesis