What compositional factor, absent in early models, refines modern stellar lifespan calculations?

Metallicity (the abundance of elements heavier than hydrogen and helium)

The chemical composition of the interstellar cloud from which a star forms significantly impacts its evolution and calculated lifespan, a factor refined in modern astrophysical models. This factor is termed metallicity, referring to the concentration of elements heavier than the primary components, hydrogen and helium. Stars formed very early in the universe had low metallicity, leading to different internal opacity and structure compared to later-generation stars, which incorporate heavier elements recycled from previous stellar deaths. These compositional differences subtly alter the required fusion rates and structure, necessitating adjustments to lifespan predictions.