The position on the Hertzsprung-Russell diagram where stars first arrive when they begin stable hydrogen burning in their cores, representing the start of a star's main sequence lifetime.
From 'zero age' (indicating the beginning of main sequence life) and 'main sequence'. The term was established in stellar evolution models of the 1960s to mark this crucial evolutionary milestone.
The zero-age main sequence is like a cosmic starting line where all stars begin their adult lives! Despite having different masses, all stars converge to this sequence when they first achieve the perfect balance between gravity and nuclear fusion.
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