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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8$\begingroup$ How about ignition? Or fusion ignition? $\endgroup$– PM 2RingJul 17, 2017 at 21:18
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1$\begingroup$ 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. $\endgroup$– GlacialisJul 18, 2017 at 14:37
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$\begingroup$ Yes, "stellar ignition" is a little more specific than "fusion ignition". $\endgroup$– PM 2RingJul 18, 2017 at 15:27
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3 Answers
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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
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$\begingroup$ 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. $\endgroup$ Jul 18, 2017 at 14:38
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1$\begingroup$ @StephenG there is no "event" or moment of ignition. The timescales suggested here are quite incorrect for hydrogen fusion commencement. $\endgroup$– ProfRobJul 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:
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.
answered Jul 19, 2017 at 0:47
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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. $\endgroup$– ProfRobJul 19, 2017 at 6:37
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$\begingroup$ @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
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$\begingroup$ No it does not. It refers to the point at which hydrostatic equlibrium is achieved and the star stops contracting. $\endgroup$– ProfRobJul 21, 2017 at 19:45
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$\begingroup$ From your source (1) "Accordingly, all these “Zero Age” models have already experienced a phase of nuclear burning, with time scales which largely depend on the stellar mass". Exactly. $\endgroup$– ProfRobJul 21, 2017 at 20:00