<p>The evolution of a star is irreversible process, so strictly speaking, it is impossible to return the star back to its earlier phase (it is impossible to collect all energy it has radiated to space and use it for conversion of helium back to hydrogen).</p>
<p>But it is possible for a star plotted on HR diagram to return to a point corresponding to main sequence phase. These stars are called <a href="https://en.wikipedia.org/wiki/Blue_straggler" rel="nofollow noreferrer">blue stragglers</a>. They are usually formed when stars merge.</p>

CC BY-SA 4.0
2020-12-11T10:45:19.317Z
really interesting. Thank you!
Dac0
CC BY-SA 4.0
2020-12-11T07:01:46.557Z
2020-12-11T18:06:39.003Z
2020-12-11T18:06:39.003Z
Leos Ondra
<p>Is it possible for a star to return to the main sequence after entered in the post-main sequence phase? I have no clue if there's a mechanism such as acquiring hydrogen from from the environment or from a substellar body might bring again the star into the main sequence. Was it ever explored as a possibility?</p>

CC BY-SA 4.0
2020-12-10T19:04:49.370Z
I don't think you can do that because the state was changed at its core. Adding hydrogen to outer layer by interaction does not change the core's condition.
Kornpob Bhirombhakdi
CC BY-SA 4.0
2020-12-10T20:38:20.117Z
What happens in older stars is a big complicated topic. Wikipedia has an extensive series of articles. See https://en.wikipedia.org/wiki/Red_giant and the other articles linked there, eg https://en.wikipedia.org/wiki/Red-giant_branch & https://en.wikipedia.org/wiki/Asymptotic_giant_branch There's a *lot* of information there, so hopefully it's not too overwhelming. ;)
PM 2Ring
CC BY-SA 4.0
2020-12-10T22:09:56.453Z
Perhaps during the subgiant stage (between red giant and main sequence) mass loss through some mechanism could cause the star to return to main sequence.
WarpPrime
CC BY-SA 4.0
2020-12-11T06:08:12.253Z
Thank you for all your comments @fasterthanlight That's exactly what I was wondering... do you have any idea on what kind of mechanism or whatever might be a candidate... or someone that studied and definitely disproofed the hypothesis?
Dac0
CC BY-SA 4.0
2020-12-11T08:13:26.610Z
Red giants lose *lots* of mass. Our Sun will lose about 50% of its mass in the form of gas and dust. But the big issue is all the helium (which is of course heavier than hydrogen) that builds up in the core.
PM 2Ring
<star><stellar-evolution><main-sequence>
Is it possible for a star to return in the main sequence?
Dac0
# Is it possible for a star to return in the main sequence?

(Asked by: username_0 on 2020-12-10)

Is it possible for a star to return to the main sequence after entered in the post-main sequence phase? I have no clue if there's a mechanism such as acquiring hydrogen from from the environment or from a substellar body might bring again the star into the main sequence. Was it ever explored as a possibility?

**Tags:** star, stellar-evolution, main-sequence
> **Comments:**
> **username_1**: I don't think you can do that because the state was changed at its core. Adding hydrogen to outer layer by interaction does not change the core's condition.  
> **username_2**: What happens in older stars is a big complicated topic. Wikipedia has an extensive series of articles. See https://en.wikipedia.org/wiki/Red_giant and the other articles linked there, eg https://en.wikipedia.org/wiki/Red-giant_branch & https://en.wikipedia.org/wiki/Asymptotic_giant_branch There's a *lot* of information there, so hopefully it's not too overwhelming. ;)  
> **username_3**: Perhaps during the subgiant stage (between red giant and main sequence) mass loss through some mechanism could cause the star to return to main sequence.  
> **username_0**: Thank you for all your comments @fasterthanlight That's exactly what I was wondering... do you have any idea on what kind of mechanism or whatever might be a candidate... or someone that studied and definitely disproofed the hypothesis?  
> **username_2**: Red giants lose *lots* of mass. Our Sun will lose about 50% of its mass in the form of gas and dust. But the big issue is all the helium (which is of course heavier than hydrogen) that builds up in the core.  

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## 1 answers

### Answer by username_4 on 2020-12-11. Score: 3 | Accepted answer

The evolution of a star is irreversible process, so strictly speaking, it is impossible to return the star back to its earlier phase (it is impossible to collect all energy it has radiated to space and use it for conversion of helium back to hydrogen).

But it is possible for a star plotted on HR diagram to return to a point corresponding to main sequence phase. These stars are called [blue stragglers](https://en.wikipedia.org/wiki/Blue_straggler). They are usually formed when stars merge.

> **Comments:**
> **username_0**: really interesting. Thank you!  

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