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I have read of this process many times, but I don't think I know the term specifically for the moment when hydrogen fusion begins. What is this moment called?

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    $\begingroup$ How about ignition? Or fusion ignition? $\endgroup$
    – PM 2Ring
    Jul 17, 2017 at 21:18
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    $\begingroup$ James got it below, I think. "Stellar ignition" is a search term that brings up many more relevant hits than I was seeing before. Stellar ignition is technically fusion ignition, but there are other consequences to stellar ignition specific to star and star system formation. $\endgroup$
    – Glacialis
    Jul 18, 2017 at 14:37
  • $\begingroup$ Yes, "stellar ignition" is a little more specific than "fusion ignition". $\endgroup$
    – PM 2Ring
    Jul 18, 2017 at 15:27

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Ignition or more fully Stellar ignition is the term you want.

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  • $\begingroup$ I presume this has never been witnessed, but no harm asking if you know of such an event been recorded. $\endgroup$ Jul 18, 2017 at 3:43
  • $\begingroup$ Yes indeed, that's what I was after. :) Thank you! If anyone comes across a more specific term, I'd love to see it. For now stellar ignition is sufficient. $\endgroup$
    – Glacialis
    Jul 18, 2017 at 14:38
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    $\begingroup$ @StephenG there is no "event" or moment of ignition. The timescales suggested here are quite incorrect for hydrogen fusion commencement. $\endgroup$
    – ProfRob
    Jul 18, 2017 at 23:45
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It is known as the nuclear turn-on in the Hertzsprung-Russell diagram; the opposite of the nuclear turn-off when hydrogen is exhausted in the core.

In actual fact there is no "moment" when the star begins to fuse hydrogen, it is a gradual process.

For example, the models of Siess et al. (2001) suggest that a solar mass pre main sequence star gets 0.01% of its luminosity from hydrogen fusion at 2 million years old, 1% after 16 million years, but it is not until it is 56 million years old that 99% of the luminosity comes from hydrogen fusion.

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The onset of hydrogen fusion for a star (i.e. when it enters the main sequence) is called the zero-age main sequence (ZAMS). Conversely, the terminal-age main sequence (TAMS) refers to the point a star stops fusing hydrogen (and by definition leaves the main sequence); the time between the ZAMS and TAMS is called the main sequence lifetime. On the Hurtzsprung-Russell diagram, several stars at their ZAMS are grouped into a line:

enter image description here

Picture from Evolution from the Main Sequence to Red Giants, courtesy of Rice University under a Creative Commons Attribution License 4.0.

Some astronomers consider the ZAMS to be the "birth" of a star, in the sense that the object is no longer a protostar and meets the criteria of a star (source). These astronomers thus consider a star's age to be the time between now and its ZAMS. However, this is simply a matter of terminology, as others consider the formation of the star to be the moment when its molecular cloud collapses.

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  • $\begingroup$ This is a definition, but the star is still contracting for some period of time, whilst generating energy from fusion, before it reaches the ZAMS. $\endgroup$
    – ProfRob
    Jul 19, 2017 at 6:37
  • $\begingroup$ @RobJeffries As far as I know, ZAMS only refers to when hydrogen fusion begins, which is what the OP is asking about. $\endgroup$ Jul 21, 2017 at 16:26
  • $\begingroup$ No it does not. It refers to the point at which hydrostatic equlibrium is achieved and the star stops contracting. $\endgroup$
    – ProfRob
    Jul 21, 2017 at 19:45
  • $\begingroup$ @RobJeffries Several sources apparently say it refers to when hydrogen burning begins, as well. (1, 2, 3, 4) $\endgroup$ Jul 21, 2017 at 19:50
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    $\begingroup$ Read the rest of the section. The quote I supplied comes from a few lines further on. Are you attempting to argue that a star stops contracting and only then does H fusion begin? I have given the numbers from a published stellar evolutionary code in my answer. The figures are not even vaguely controversial. $\endgroup$
    – ProfRob
    Jul 21, 2017 at 21:43

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