<p>(I was intending this as a comment, but it might be saying just enough about an actual answer that I should place it as an answer.)</p>
<p>I think this may be a case where the wikipedia pages needs some rewriting for clarity, and possibly newer research giving different bounds. <a href="https://en.wikipedia.org/wiki/Stellar_evolution#Horizontal_branch" rel="nofollow noreferrer">The stellar evolution page</a> indicates the initial core helium flash occurs from roughly .6 to 2.0 solar masses.  Either way, .6 or .8, I think the band you're talking about is the &quot;depends heavily on the exact parameters of the star, like metallicity and helium content&quot; situation.  So below about .5, definitely no flash because no ignition.  One of .6 or .8 and up, definitely a helium flash.  Between? One of the two, but hard to say which without knowing more.</p>

CC BY-SA 4.0
2024-06-23T07:43:33.600Z
Below 0.5 there is no flash because there is no ignition at all. An important distinction I think.
ProfRob
CC BY-SA 4.0
2024-06-23T11:23:06.393Z
OK, but you are still claiming there is some in-between mass where there is He ignition, but no flash. Is that true? Because that IS what the question is about.
ProfRob
CC BY-SA 4.0
2024-06-23T22:09:54.673Z
Yes. I wasn't sure what happens between 0.5 and 0.8 solar masses.
Joe Peters
CC BY-SA 4.0
2024-06-24T00:18:36.340Z
@ProfRob That would be exactly what I supposed OP's question was getting at.  The pages are written in such a way to pretty clearly say that ignition with a non-degenerate core requires more than 2 solar masses.  But the seemingly untalked-about band of .5 to whatever solar masses makes it unclear if they have degenerate helium cores that may or may not ignite, or if they ignite without degeneracy (or, I suppose, neither ignites nor degenerate).  My not quite being certain which of those may be the case is why I was originally thinking of what I say as a comment.
zibadawa timmy
CC BY-SA 4.0
2024-06-24T13:09:47.407Z
@zibadawatimmy -- If there is no helium-flash, you get a degenerate helium core characteristic of low-mass white dwarfs. https://www.aanda.org/articles/aa/full_html/2009/28/aa11640-09/aa11640-09.html
antlersoft
CC BY-SA 4.0
2024-06-23T05:23:25.617Z
2024-06-23T08:50:12.443Z
2024-06-23T08:50:12.443Z
zibadawa timmy
<p>I don't think there is any range in low-mass objects where He is ignited but there is no flash. The only threshold will be where the core becomes almost completely degenerate <em>before</em> reaching the ignition temperature and this occurs in stars below about 0.5 solar masses (composition-dependent).</p>
<p>Above that, the He will <em>eventually</em> be ignited in a (partially) degenerate core and a runaway flash will occur. I think the source of confusion here is that stars below 0.8 solar masses have not had time to evolve to that stage in our universe and thus their post main-sequence evolution is <em>not considered</em> in most published evolutionary models.</p>

CC BY-SA 4.0
2024-10-30T21:26:08.727Z
Will stars 0.5<m<0.8 solar masses become planetary nebula?
Joe Peters
CC BY-SA 4.0
2024-06-29T06:20:23.940Z
2024-08-10T08:03:58.250Z
2024-08-10T08:03:58.250Z
ProfRob
<p>Stars between 0.8 and 2 solar masses undergo a degenerative helium flash, per <a href="https://en.wikipedia.org/wiki/Helium_flash" rel="nofollow noreferrer">Wikipedia</a>
and <a href="https://web.archive.org/web/20190520071013/https://astro.uni-bonn.de/%7Enlanger/siu_web/ssescript/new/chapter9.pdf" rel="nofollow noreferrer">these course notes</a></p>
<p>The cores of stars &lt;0.5 solar masses never heat up enough for helium fusion. In stars &gt;2 solar masses, the core is already hot enough to fuse helium without becoming degenerate first, so no degenerate helium flash takes place; instead, helium fusion begins more gradually. How about stars between 0.5 and 0.8 solar masses?</p>

CC BY-SA 4.0
2024-06-23T07:42:10.763Z
I'm not sure this claim is true. The 0.8 solar mass "limit" may just be the lowest mass stars considered that have had a chance to evolve to He burning in our universe.
ProfRob
<stellar-astrophysics>
Why don't stars 0.5<m<0.8 solar masses undergo a helium flash?
Joe Peters
# Why don't stars 0.5<m<0.8 solar masses undergo a helium flash?

(Asked by: username_0 on 2024-06-22)

Stars between 0.8 and 2 solar masses undergo a degenerative helium flash, per [Wikipedia](https://en.wikipedia.org/wiki/Helium_flash)
and [these course notes](https://web.archive.org/web/20190520071013/https://astro.uni-bonn.de/%7Enlanger/siu_web/ssescript/new/chapter9.pdf)

The cores of stars <0.5 solar masses never heat up enough for helium fusion. In stars >2 solar masses, the core is already hot enough to fuse helium without becoming degenerate first, so no degenerate helium flash takes place; instead, helium fusion begins more gradually. How about stars between 0.5 and 0.8 solar masses?

**Tags:** stellar-astrophysics
> **Comments:**
> **username_1**: I'm not sure this claim is true. The 0.8 solar mass "limit" may just be the lowest mass stars considered that have had a chance to evolve to He burning in our universe.  

---

## 2 answers

### Answer by username_2 on 2024-06-23. Score: 1

(I was intending this as a comment, but it might be saying just enough about an actual answer that I should place it as an answer.)

I think this may be a case where the wikipedia pages needs some rewriting for clarity, and possibly newer research giving different bounds. [The stellar evolution page](https://en.wikipedia.org/wiki/Stellar_evolution#Horizontal_branch) indicates the initial core helium flash occurs from roughly .6 to 2.0 solar masses. Either way, .6 or .8, I think the band you're talking about is the "depends heavily on the exact parameters of the star, like metallicity and helium content" situation. So below about .5, definitely no flash because no ignition. One of .6 or .8 and up, definitely a helium flash. Between? One of the two, but hard to say which without knowing more.

> **Comments:**
> **username_1**: Below 0.5 there is no flash because there is no ignition at all. An important distinction I think.  
> **username_1**: OK, but you are still claiming there is some in-between mass where there is He ignition, but no flash. Is that true? Because that IS what the question is about.  
> **username_0**: Yes. I wasn't sure what happens between 0.5 and 0.8 solar masses.  
> **username_2**: @ProfRob That would be exactly what I supposed OP's question was getting at.  The pages are written in such a way to pretty clearly say that ignition with a non-degenerate core requires more than 2 solar masses.  But the seemingly untalked-about band of .5 to whatever solar masses makes it unclear if they have degenerate helium cores that may or may not ignite, or if they ignite without degeneracy (or, I suppose, neither ignites nor degenerate).  My not quite being certain which of those may be the case is why I was originally thinking of what I say as a comment.  
> **username_3**: @zibadawatimmy -- If there is no helium-flash, you get a degenerate helium core characteristic of low-mass white dwarfs. https://www.aanda.org/articles/aa/full_html/2009/28/aa11640-09/aa11640-09.html  

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### Answer by username_1 on 2024-06-29. Score: 0

I don't think there is any range in low-mass objects where He is ignited but there is no flash. The only threshold will be where the core becomes almost completely degenerate *before* reaching the ignition temperature and this occurs in stars below about 0.5 solar masses (composition-dependent).

Above that, the He will *eventually* be ignited in a (partially) degenerate core and a runaway flash will occur. I think the source of confusion here is that stars below 0.8 solar masses have not had time to evolve to that stage in our universe and thus their post main-sequence evolution is *not considered* in most published evolutionary models.

> **Comments:**
> **username_0**: Will stars 0.5<m<0.8 solar masses become planetary nebula?  

---

