The position on the Hertzsprung-Russell diagram where stars end their main sequence phase as they exhaust hydrogen in their cores and begin to evolve into giants, representing the end of stable hydrogen burning.
From Latin 'terminalis' meaning 'of boundaries' and 'main sequence'. This term emerged alongside zero-age main sequence to bracket the hydrogen-burning lifetime of stars.
The terminal-age main sequence marks the end of a star's longest and most stable life phase! For our Sun, this moment is still 5 billion years away, but when it arrives, the Sun will begin its dramatic transformation into a red giant.
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