<p>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.</p>

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

<p>For example, the models of <a href="http://www.astro.ulb.ac.be/~siess/pmwiki/pmwiki.php/StellarModels/Z02" rel="nofollow noreferrer">Siess et al. (2001)</a> 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.</p>

CC BY-SA 3.0
2017-07-18T23:40:34.470Z
2017-07-18T23:51:50.077Z
2017-07-18T23:51:50.077Z
ProfRob
<p>The onset of hydrogen fusion for a star (i.e. when it enters the main sequence) is called the <a href="https://en.mimi.hu/astronomy/zero-age_main_sequence.html" rel="nofollow noreferrer">zero-age main sequence</a> (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:</p>

<p><a href="https://i.sstatic.net/ckAzH.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/ckAzH.jpg" alt="enter image description here"></a></p>

<p><sup> Picture from <a href="http://cnx.org/contents/im49RUTc@4/Evolution-from-the-Main-Sequen" rel="nofollow noreferrer">Evolution from the Main Sequence to Red Giants</a>, courtesy of Rice University under a <a href="https://creativecommons.org/licenses/by/4.0/" rel="nofollow noreferrer">Creative Commons Attribution License 4.0</a>.</sup></p>

<p>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 (<a href="https://astronomy.swin.edu.au/cosmos/Z/Zero+Age+Main+Sequence" rel="nofollow noreferrer">source</a>). 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.</p>

CC BY-SA 3.0
2017-07-19T06:37:06.083Z
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.
ProfRob
CC BY-SA 3.0
2017-07-21T16:26:31.883Z
@RobJeffries As far as I know, ZAMS only refers to when hydrogen fusion begins, which is what the OP is asking about.
Sir Cumference
CC BY-SA 3.0
2017-07-21T19:45:14.180Z
No it does not. It refers to the point at which hydrostatic equlibrium is achieved and the star stops contracting.
ProfRob
CC BY-SA 3.0
2017-07-21T19:50:36.937Z
@RobJeffries Several sources apparently say it refers to when hydrogen burning begins, as well. ([1](https://arxiv.org/pdf/astro-ph/9908291.pdf), [2](http://cnx.org/contents/im49RUTc@4/Evolution-from-the-Main-Sequen),
 [3](https://astronomy.swin.edu.au/cosmos/Z/Zero+Age+Main+Sequence), [4](https://en.mimi.hu/astronomy/zero-age_main_sequence.html))
Sir Cumference
CC BY-SA 3.0
2017-07-21T20:00:40.447Z
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.
ProfRob
CC BY-SA 3.0
2017-07-21T20:02:04.653Z
@RobJeffries I might be mistaken, but couldn't nuclear burning refer to deuterium or lithium burning?
Sir Cumference
CC BY-SA 3.0
2017-07-21T20:08:29.637Z
And Siess's isochrone server defines the ZAMS "as the time, after deuterium burning, when the nuclear luminosity provides at least 99% of the total stellar luminosity, i.e. when Lnucl > 0.99 L". The ZAMS is the minimum luminosity point in the HR diagram when contraction ceases. It is when H-burning becomes utterly dominant, not when it commences. The difference is perhaps <1% of the main sequence lifetime, but that is still of order 50 million years for a star like the Sun.
ProfRob
CC BY-SA 3.0
2017-07-21T20:14:41.467Z
@RobJeffries Hmm...the paper says "stellar models which are just starting central H burning with the timescale of H consumption." The source from [Rice University](http://cnx.org/contents/im49RUTc@4/Evolution-from-the-Main-Sequen) says "We use the term zero-age to mark the time when a star stops contracting, settles onto the main sequence, and begins to fuse hydrogen in its core." The one from [Swinburne](https://astronomy.swin.edu.au/cosmos/Z/Zero+Age+Main+Sequence) says 
 "when a star first...[burns] hydrogen in its core". Is it possible that the term is ambiguous, or just misused?
Sir Cumference
CC BY-SA 3.0
2017-07-21T21:43:11.137Z
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.
ProfRob
CC BY-SA 3.0
2017-07-19T00:47:33.960Z
2017-07-19T01:01:51.950Z
2017-07-19T01:01:51.950Z
Sir Cumference
<p><strong>Ignition</strong> or more fully <a href="http://www.schoolsobservatory.org.uk/learn/astro/stars/formation" rel="nofollow noreferrer">Stellar ignition</a> is the term you want. </p>

CC BY-SA 3.0
2017-07-18T03:43:28.023Z
I presume this has never been witnessed, but no harm asking if you know of such an event been recorded.
StephenG - Help Ukraine
CC BY-SA 3.0
2017-07-18T14:38:47.110Z
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.
Glacialis
CC BY-SA 3.0
2017-07-18T23:45:07.040Z
@StephenG there is no "event" or moment of ignition. The timescales suggested here are quite incorrect for hydrogen fusion commencement.
ProfRob
CC BY-SA 3.0
2017-07-17T21:41:19.813Z
2017-07-19T08:00:27.030Z
2017-07-19T08:00:27.030Z
James K
<p>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?</p>

CC BY-SA 3.0
2017-07-17T21:18:02.127Z
How about ignition? Or [fusion ignition](https://en.wikipedia.org/wiki/Fusion_ignition)?
PM 2Ring
CC BY-SA 3.0
2017-07-18T14:37:47.733Z
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.
Glacialis
CC BY-SA 3.0
2017-07-18T15:27:23.750Z
Yes, "stellar ignition" is a little more specific than "fusion ignition".
PM 2Ring
<star-formation><terminology>
Term for the moment when hydrogen fusion begins in a star
Glacialis
# Term for the moment when hydrogen fusion begins in a star

(Asked by: username_0 on 2017-07-17)

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?

**Tags:** star-formation, terminology
> **Comments:**
> **username_1**: How about ignition? Or [fusion ignition](https://en.wikipedia.org/wiki/Fusion_ignition)?  
> **username_0**: 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.  
> **username_1**: Yes, "stellar ignition" is a little more specific than "fusion ignition".  

---

## 3 answers

### Answer by username_4 on 2017-07-17. Score: 5 | Accepted answer

**Ignition** or more fully [Stellar ignition](http://www.schoolsobservatory.org.uk/learn/astro/stars/formation) is the term you want.

> **Comments:**
> **username_5**: I presume this has never been witnessed, but no harm asking if you know of such an event been recorded.  
> **username_0**: 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.  
> **username_2**: @StephenG there is no "event" or moment of ignition. The timescales suggested here are quite incorrect for hydrogen fusion commencement.  

---

### Answer by username_2 on 2017-07-18. Score: 5

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)](http://www.astro.ulb.ac.be/~siess/pmwiki/pmwiki.php/StellarModels/Z02) 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.


---

### Answer by username_3 on 2017-07-19. Score: 2

The onset of hydrogen fusion for a star (i.e. when it enters the main sequence) is called the [zero-age main sequence](https://en.mimi.hu/astronomy/zero-age_main_sequence.html) (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](https://i.sstatic.net/ckAzH.jpg)](https://i.sstatic.net/ckAzH.jpg)

Picture from [Evolution from the Main Sequence to Red Giants](http://cnx.org/contents/im49RUTc@4/Evolution-from-the-Main-Sequen), courtesy of Rice University under a [Creative Commons Attribution License 4.0](https://creativecommons.org/licenses/by/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](https://astronomy.swin.edu.au/cosmos/Z/Zero+Age+Main+Sequence)). 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.

> **Comments:**
> **username_2**: 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.  
> **username_3**: @RobJeffries As far as I know, ZAMS only refers to when hydrogen fusion begins, which is what the OP is asking about.  
> **username_2**: No it does not. It refers to the point at which hydrostatic equlibrium is achieved and the star stops contracting.  
> **username_3**: @RobJeffries Several sources apparently say it refers to when hydrogen burning begins, as well. ([1](https://arxiv.org/pdf/astro-ph/9908291.pdf), [2](http://cnx.org/contents/im49RUTc@4/Evolution-from-the-Main-Sequen),  [3](https://astronomy.swin.edu.au/cosmos/Z/Zero+Age+Main+Sequence), [4](https://en.mimi.hu/astronomy/zero-age_main_sequence.html))  
> **username_2**: 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.  
> **username_3**: @RobJeffries I might be mistaken, but couldn't nuclear burning refer to deuterium or lithium burning?  
> **username_2**: And Siess's isochrone server defines the ZAMS "as the time, after deuterium burning, when the nuclear luminosity provides at least 99% of the total stellar luminosity, i.e. when Lnucl > 0.99 L". The ZAMS is the minimum luminosity point in the HR diagram when contraction ceases. It is when H-burning becomes utterly dominant, not when it commences. The difference is perhaps <1% of the main sequence lifetime, but that is still of order 50 million years for a star like the Sun.  
> **username_3**: @RobJeffries Hmm...the paper says "stellar models which are just starting central H burning with the timescale of H consumption." The source from [Rice University](http://cnx.org/contents/im49RUTc@4/Evolution-from-the-Main-Sequen) says "We use the term zero-age to mark the time when a star stops contracting, settles onto the main sequence, and begins to fuse hydrogen in its core." The one from [Swinburne](https://astronomy.swin.edu.au/cosmos/Z/Zero+Age+Main+Sequence) says   "when a star first...[burns] hydrogen in its core". Is it possible that the term is ambiguous, or just misused?  
> **username_2**: 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.  

---

