<p>According to <a href="https://iopscience.iop.org/article/10.1086/304125" rel="nofollow noreferrer">Laughlin et al. (1997)</a>, (still the canonical work) it is only stars below about 0.25 solar masses that fail to become red giants.</p>
<p>Stars between 0.25 and about 0.5 solar masses <em>will</em> become red giants but won't reach the very tip of the giant branch because they will not become hot enough to ignite their degenerate helium cores. They will somehow lose what remains of their envelopes and end their lives as helium white dwarfs.</p>
<p>Anything more massive will follow a similar evolution to a solar-type star, going through a red giant phase, core helium burning and then the asymptotic red giant branch and finally a carbon/oxygen white dwarf.</p>
<p><strong>Bonus material</strong></p>
<p>AU Mic is an M1V star with a photospheric temperature of <span class="math-container">$3700 \pm 100$</span> K and an age of about <span class="math-container">$22\pm 3$</span> Myr. From this one can estimate that it is <span class="math-container">$0.50 \pm 0.03$</span> solar masses ( <a href="https://arxiv.org/abs/2006.13248" rel="nofollow noreferrer">Plavchan et al. 2020</a>) or <span class="math-container">$0.60 \pm 0.04$</span> solar masses (<a href="https://arxiv.org/abs/2304.09642" rel="nofollow noreferrer">Donati et al. 2023</a>, using different models). A star of this mass will still be a K8-M1 dwarf when it reaches the main sequence (see for example the well-used table of mass-colour-temperature relationships by <a href="https://www.pas.rochester.edu/%7Eemamajek/EEM_dwarf_UBVIJHK_colors_Teff.txt" rel="nofollow noreferrer">Eric Mamajek</a>) and it is fated to become a red giant (in perhaps 50-100 billion years), but then whether it will end its life as a helium white dwarf or a carbon/oxygen white dwarf is still uncertain.</p>
<p>Another, peripheral, point to make about your question. Stars with mass less than 0.35 solar masses are fully convective when they reach the main sequence. However, Laughlin et al. show that as hydrogen is fused into helium, that threshold moves to lower masses so that a radiative core <em>eventually</em> forms in stars of all masses, but at an increasingly later stage of main sequence evolution at lower masses.</p>

CC BY-SA 4.0
2024-09-21T15:30:11.427Z
According to https://astronomy.stackexchange.com/a/51798/16685 the lifespan of a $0.25 M\odot$ star is much larger than the current age of the universe, so I guess we haven't actually seen any stars that small which have become white dwarfs. ;)
PM 2Ring
CC BY-SA 4.0
2024-09-21T13:33:55.233Z
2024-09-22T08:58:48.530Z
2024-09-22T08:58:48.530Z
ProfRob
<p>There is an issue with regard to the statement that red dwarfs do not become red giants. This is perhaps an oversimplification. A better statement is that small red dwarfs do not become red giants in their post main sequence life. This exact point is made by Adams, Laughlin, and Graves in <a href="https://web.archive.org/web/20110716085435/http://www.astroscu.unam.mx/rmaa/RMxAC..22/PDF/RMxAC..22_adams.pdf" rel="noreferrer">&quot;Red dwarfs and the end of the main sequence.&quot; Revista Mexicana de Astronomía y Astrofísica 22 (2004): 46-49</a>: (emphasis mine)</p>
<blockquote>
<p>One of the most interesting findings of this work is that <strong>small red dwarfs do not become red giants</strong> in their post-main-sequence phases.</p>
</blockquote>
<p>There is a seperate issue with regard to spectral class and AU Microscopii. This star is very young, estimated to be only 22 million years old (with zero being the time it entered the main sequence).</p>
<p>The spectral class of a star is based on light collected now. This can change over time as the star evolves. Rather small stars increase in both luminosity and effective temperature shortly after ZAMS (zero age main sequence). See the above reference, which shows the evolution of a 0.1 <span class="math-container">$M_\odot$</span> in figure 1. I have yet to find a paper on the evolution of a 0.6 <span class="math-container">$M_\odot$</span> star, but I suspect they too increase in both luminosity and effective temperature after hydrogen ignition first starts. The increase in effective temperature means the spectral class may change.</p>
<p>In other words, AU Microscopii is a very young K-type dwarf that right now appears to be a red dwarf due to its very young age.</p>

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2024-09-21T12:31:55.627Z
2024-09-21T12:31:55.627Z
David Hammen
<p>I have a quick question. I have heard that red dwarfs that are approximately less than 0.35-0.5 solar masses are fully convective, meaning that they do not become red giants. I have also heard that red dwarfs do not become red giants. However, I know some red dwarfs, such as AU Microscopii, that are red dwarfs above 0.35-0.5 solar masses. What happens during the end of their life cycle?</p>

CC BY-SA 4.0
2024-09-23T15:16:57.857Z
A nitpick, but "*I have heard that red dwarfs [..] do not become red giants. I have also heard that red dwarfs do not become red giants.*" reads a bit redun**dun**dant :).
Astrogator
CC BY-SA 4.0
2024-09-24T21:14:42.970Z
@Astrogator Whoops! XD
Anonymous
<star><red-giant><red-dwarf>
Red dwarfs and red giants
Anonymous
# Red dwarfs and red giants

(Asked by: username_0 on 2024-09-20)

I have a quick question. I have heard that red dwarfs that are approximately less than 0.35-0.5 solar masses are fully convective, meaning that they do not become red giants. I have also heard that red dwarfs do not become red giants. However, I know some red dwarfs, such as AU Microscopii, that are red dwarfs above 0.35-0.5 solar masses. What happens during the end of their life cycle?

**Tags:** star, red-giant, red-dwarf

> **Comments:**
> **username_1**: A nitpick, but "*I have heard that red dwarfs [..] do not become red giants. I have also heard that red dwarfs do not become red giants.*" reads a bit redun**dun**dant :).  
> **username_0**: @username_1 Whoops! XD  

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## 2 answers

### Answer by username_2 on 2024-09-21. Score: 11

According to [Laughlin et al. (1997)](https://iopscience.iop.org/article/10.1086/304125), (still the canonical work) it is only stars below about 0.25 solar masses that fail to become red giants.

Stars between 0.25 and about 0.5 solar masses *will* become red giants but won't reach the very tip of the giant branch because they will not become hot enough to ignite their degenerate helium cores. They will somehow lose what remains of their envelopes and end their lives as helium white dwarfs.

Anything more massive will follow a similar evolution to a solar-type star, going through a red giant phase, core helium burning and then the asymptotic red giant branch and finally a carbon/oxygen white dwarf.

**Bonus material**

AU Mic is an M1V star with a photospheric temperature of $3700 \pm 100$ K and an age of about $22\pm 3$ Myr. From this one can estimate that it is $0.50 \pm 0.03$ solar masses ( [Plavchan et al. 2020](https://arxiv.org/abs/2006.13248)) or $0.60 \pm 0.04$ solar masses ([Donati et al. 2023](https://arxiv.org/abs/2304.09642), using different models). A star of this mass will still be a K8-M1 dwarf when it reaches the main sequence (see for example the well-used table of mass-colour-temperature relationships by [Eric Mamajek](https://www.pas.rochester.edu/%7Eemamajek/EEM_dwarf_UBVIJHK_colors_Teff.txt)) and it is fated to become a red giant (in perhaps 50-100 billion years), but then whether it will end its life as a helium white dwarf or a carbon/oxygen white dwarf is still uncertain.

Another, peripheral, point to make about your question. Stars with mass less than 0.35 solar masses are fully convective when they reach the main sequence. However, Laughlin et al. show that as hydrogen is fused into helium, that threshold moves to lower masses so that a radiative core *eventually* forms in stars of all masses, but at an increasingly later stage of main sequence evolution at lower masses.

> **Comments:**
> **username_3**: According to https://astronomy.stackexchange.com/a/51798/16685 the lifespan of a $0.25 M\odot$ star is much larger than the current age of the universe, so I guess we haven't actually seen any stars that small which have become white dwarfs. ;)  

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### Answer by username_4 on 2024-09-21. Score: 7

There is an issue with regard to the statement that red dwarfs do not become red giants. This is perhaps an oversimplification. A better statement is that small red dwarfs do not become red giants in their post main sequence life. This exact point is made by Adams, Laughlin, and Graves in ["Red dwarfs and the end of the main sequence." Revista Mexicana de Astronomía y Astrofísica 22 (2004): 46-49](https://web.archive.org/web/20110716085435/http://www.astroscu.unam.mx/rmaa/RMxAC..22/PDF/RMxAC..22_adams.pdf): (emphasis mine)

> One of the most interesting findings of this work is that **small red dwarfs do not become red giants** in their post-main-sequence phases.

There is a seperate issue with regard to spectral class and AU Microscopii. This star is very young, estimated to be only 22 million years old (with zero being the time it entered the main sequence).

The spectral class of a star is based on light collected now. This can change over time as the star evolves. Rather small stars increase in both luminosity and effective temperature shortly after ZAMS (zero age main sequence). See the above reference, which shows the evolution of a 0.1 $M\_\odot$ in figure 1. I have yet to find a paper on the evolution of a 0.6 $M\_\odot$ star, but I suspect they too increase in both luminosity and effective temperature after hydrogen ignition first starts. The increase in effective temperature means the spectral class may change.

In other words, AU Microscopii is a very young K-type dwarf that right now appears to be a red dwarf due to its very young age.


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