zero-age main sequence

The zero-age main sequence refers to the initial stage of a star's life on the Hertzsprung-Russell diagram, where stars are primarily fusing hydrogen into helium in their cores. This is a stable phase where stars spend the majority of their active lifetimes. Stars on the zero-age main sequence are chemically homogeneous. As they evolve, the hydrogen in their cores is converted to helium, causing them to gradually move away from this initial line. The rate of evolution on the main sequence is directly proportional to a star's mass; more massive stars evolve much faster than less massive ones. This stage is fundamental to understanding stellar evolution and the life cycles of stars in the universe.123