<p>One day, while researching on <a href="https://en.wikipedia.org/wiki/Blue_dwarf_(red-dwarf_stage)" rel="nofollow noreferrer">blue dwarfs in Wikipedia</a>, I stumbled across something that I cannot understand:</p>
<blockquote>
<p>Rather than expanding, however, red dwarfs with less than 0.25 solar
masses are predicted to increase their radiative rate by increasing
their surface temperatures and becoming &quot;bluer&quot;. This is because the
surface layers of red dwarfs do not become significantly more opaque
with increasing temperature.</p>
</blockquote>
<p>But I could not find more data on this topic.</p>
<p>However, this also means that the Sun and massive stars expand into red giants and supergiants, as the star becomes &quot;opaque&quot; with increasing temperature and cannot radiate heat fast enough to cool itself down. So it swells up in size to increase the surface area to radiate excess heat away.</p>
<p>However, red dwarfs are many, many times more dense than the Sun. This should mean that a red dwarf would be incredibly opaque, so it blocks heat and light from the core. This should mean that a red dwarf should theoretically swell up to radiate heat away. But, a red dwarf does none of these. It simply gets hotter.</p>
<p><strong>Why do red dwarfs remain &quot;transparent&quot; to internal heat, whereas the Sun and other stars remain &quot;opaque&quot; despite them being many times lighter than red dwarfs?</strong></p>

CC BY-SA 4.0
2022-11-05T08:05:43.790Z
Note that the photospheric radius of a star is where it stops being opaque, by definition. The text you quote does not say that the Sun expands to become a giant because it becomes opaque. All stars are opaque.
ProfRob
CC BY-SA 4.0
2022-11-05T10:28:05.143Z
Can you provide a link to the article you're quoting from? Contextless quotes like that are harder to work with.
Peter Erwin
CC BY-SA 4.0
2022-11-05T10:34:42.150Z
Question needs rewriting to omit all the supposition and assumptions that you are making. The bold face question cannot be answered because it isn't true.
ProfRob
CC BY-SA 4.0
2022-11-05T12:52:34.177Z
@PeterErwin The quote appears to be from the [wikipedia article](https://en.wikipedia.org/wiki/Blue_dwarf_(red-dwarf_stage)) on blue dwarfs, and the OP has edited the link into the question.
David Hammen
CC BY-SA 4.0
2022-11-05T12:55:29.927Z
Re *However, red dwarfs are many, many times more dense than the Sun.* That isn't true, at least not in the core where fusion takes place. The cores of red dwarfs have a much lower temperature and a much lower density than do larger stars.
David Hammen
CC BY-SA 4.0
2022-11-05T15:03:51.400Z
Opacity of various elements under stellar conditions is an active area of research. The higher the Z of an element the more possible transitions there are possible and the higher the probability of absorbing and re-emitting photons. This slows down radiative heat transport.
Jon Custer
CC BY-SA 4.0
2022-11-06T08:30:51.490Z
@DavidHammen actually, the core density of a 0.1 solar mass main sequence star is about 5 times that of a 1 solar mass star. The trend is that lower mass stars have higher densities.
ProfRob
<star><red-dwarf><opacity>
"Opacity" in red dwarfs
Arcturus
# "Opacity" in red dwarfs

(Asked by: username_0 on 2022-11-04)

One day, while researching on [blue dwarfs in Wikipedia](https://en.wikipedia.org/wiki/Blue_dwarf_(red-dwarf_stage)), I stumbled across something that I cannot understand:

> Rather than expanding, however, red dwarfs with less than 0.25 solar
> masses are predicted to increase their radiative rate by increasing
> their surface temperatures and becoming "bluer". This is because the
> surface layers of red dwarfs do not become significantly more opaque
> with increasing temperature.

But I could not find more data on this topic.

However, this also means that the Sun and massive stars expand into red giants and supergiants, as the star becomes "opaque" with increasing temperature and cannot radiate heat fast enough to cool itself down. So it swells up in size to increase the surface area to radiate excess heat away.

However, red dwarfs are many, many times more dense than the Sun. This should mean that a red dwarf would be incredibly opaque, so it blocks heat and light from the core. This should mean that a red dwarf should theoretically swell up to radiate heat away. But, a red dwarf does none of these. It simply gets hotter.

**Why do red dwarfs remain "transparent" to internal heat, whereas the Sun and other stars remain "opaque" despite them being many times lighter than red dwarfs?**

**Tags:** star, red-dwarf, opacity
> **Comments:**
> **username_1**: Note that the photospheric radius of a star is where it stops being opaque, by definition. The text you quote does not say that the Sun expands to become a giant because it becomes opaque. All stars are opaque.  
> **username_2**: Can you provide a link to the article you're quoting from? Contextless quotes like that are harder to work with.  
> **username_1**: Question needs rewriting to omit all the supposition and assumptions that you are making. The bold face question cannot be answered because it isn't true.  
> **username_3**: @PeterErwin The quote appears to be from the [wikipedia article](https://en.wikipedia.org/wiki/Blue_dwarf_(red-dwarf_stage)) on blue dwarfs, and the OP has edited the link into the question.  
> **username_3**: Re *However, red dwarfs are many, many times more dense than the Sun.* That isn't true, at least not in the core where fusion takes place. The cores of red dwarfs have a much lower temperature and a much lower density than do larger stars.  
> **username_4**: Opacity of various elements under stellar conditions is an active area of research. The higher the Z of an element the more possible transitions there are possible and the higher the probability of absorbing and re-emitting photons. This slows down radiative heat transport.  
> **username_1**: @DavidHammen actually, the core density of a 0.1 solar mass main sequence star is about 5 times that of a 1 solar mass star. The trend is that lower mass stars have higher densities.  

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## 0 answers

