Referring to the moment when a star first begins stable hydrogen fusion in its core, marking the start of its main sequence lifetime. This defines the zero-age main sequence (ZAMS) on the Hertzsprung-Russell diagram.
From Arabic 'sifr' (empty, which became 'zero') + 'age' (from Old French 'aage'). The term was coined in stellar evolution theory to mark the beginning of a star's nuclear-powered lifetime, distinct from its formation period.
Zero-age is like a star's official birthday - not when it started forming from gas and dust, but when it finally 'turned on' nuclear fusion and became a true star! This moment ends millions of years of gravitational contraction and begins billions of years of stable energy production.
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