<p>Further to the comment by astrosnapper, you might want to play around with EZ-Web (easier):</p>
<p><a href="http://user.astro.wisc.edu/%7Etownsend/static.php?ref=ez-web" rel="nofollow noreferrer">http://user.astro.wisc.edu/~townsend/static.php?ref=ez-web</a></p>
<p>...or MESA-Web (more sophisticated):</p>
<p><a href="http://user.astro.wisc.edu/%7Etownsend/static.php?ref=mesa-web" rel="nofollow noreferrer">http://user.astro.wisc.edu/~townsend/static.php?ref=mesa-web</a></p>
<p>(full disclosure: I maintain these tools)</p>

CC BY-SA 4.0
2023-12-03T13:54:28.910Z
This is awesome. They will come in handy in our course for kids and youths at our local observatory.
planetmaker
CC BY-SA 4.0
2023-12-03T06:00:28.697Z
2023-12-03T06:00:28.697Z
Richard Townsend
<p>In general we calculate a Red Giant will have ten or more times the radius of the main sequence star (depending which scientists you want to believe) I don't think we have the math yet to work it out to a more precise figure. So ten times the radius is the lower limit, and the upper limit is unknown until you hit Super Red Giant size which is 100 million to a billion km wide, which obviously isn't particularly precise either.</p>
<p>A Red giant is what our Sun will become. A Super Red Giant is what a star 8+ masses of our sun will become.</p>
<p>Which means you can happily use anything within those ranges for your stories.</p>

CC BY-SA 4.0
2023-11-30T23:34:19.947Z
2023-11-30T23:40:47.803Z
2023-11-30T23:40:47.803Z
Kilisi
<p>I am working on a sci-fi that goes over such a large length of time that I will be having stars dying out, and this raises a question that I would need to find an answer to.</p>
<p>How can one predict the radius and luminosity of a star in its Red Giant phase compared its qualities (mass, radius, luminosity, metallicity) during the main-sequence phase. As I definitely need to find a way to find reasonable specs for a star after it leaves main-sequence. I don't want to make numbers that are completely unreasonable and unrealistic.</p>
<p>I tried finding information on this, but I am having difficulty finding anything that describes how the size of a red giant is predicted, despite the fact that methods of prediction obviously do exist given we have predicted the size the sun would be as a red giant. Information seems pretty scarse on this topic.</p>

CC BY-SA 4.0
2023-11-30T20:24:41.063Z
@SurpriseDog I agree, but on second thought I believe the correct procedure for that is to Flag the question for moderator attention (which I have done).
Tom
CC BY-SA 4.0
2023-11-30T20:53:53.963Z
Do you just want pointing to a website that contains stellar evolutionary models? The question of describing how the size is predicted is a large textbook on stellar physics.
ProfRob
CC BY-SA 4.0
2023-11-30T21:25:28.627Z
Did you check https://en.wikipedia.org/wiki/Red-giant_branch for masses, radii and luminosities?
planetmaker
CC BY-SA 4.0
2023-11-30T22:36:11.680Z
How accurate and rigorous do you need it ? [Star In A Box](https://starinabox.lco.global/) in Advanced mode, click Data Table, will give you radius, luminosity, temperature and duration for major evolutionary stages such as Red Giant Branch, Core He burning and Asymptotic Red Giant Branch. Or there's [MESA](https://docs.mesastar.org/en/release-r23.05.1/) which is a general evolution code. [This](https://docs.mesastar.org/en/release-r23.05.1/test_suite/1M_pre_ms_to_wd.html#m-pre-ms-to-wd) test suite example takes a 1 solar mass star from pre-main sequence all the way to white dwarf status.
astrosnapper
<star><stellar-evolution><red-giant>
How to predict the radius of a Red giant
DanceroftheStars
# How to predict the radius of a Red giant

(Asked by: username_0 on 2023-11-30)

I am working on a sci-fi that goes over such a large length of time that I will be having stars dying out, and this raises a question that I would need to find an answer to.

How can one predict the radius and luminosity of a star in its Red Giant phase compared its qualities (mass, radius, luminosity, metallicity) during the main-sequence phase. As I definitely need to find a way to find reasonable specs for a star after it leaves main-sequence. I don't want to make numbers that are completely unreasonable and unrealistic.

I tried finding information on this, but I am having difficulty finding anything that describes how the size of a red giant is predicted, despite the fact that methods of prediction obviously do exist given we have predicted the size the sun would be as a red giant. Information seems pretty scarse on this topic.

**Tags:** star, stellar-evolution, red-giant

> **Comments:**
> **username_1**: @SurpriseDog I agree, but on second thought I believe the correct procedure for that is to Flag the question for moderator attention (which I have done).  
> **username_2**: Do you just want pointing to a website that contains stellar evolutionary models? The question of describing how the size is predicted is a large textbook on stellar physics.  
> **username_3**: Did you check https://en.wikipedia.org/wiki/Red-giant_branch for masses, radii and luminosities?  
> **username_4**: How accurate and rigorous do you need it ? [Star In A Box](https://starinabox.lco.global/) in Advanced mode, click Data Table, will give you radius, luminosity, temperature and duration for major evolutionary stages such as Red Giant Branch, Core He burning and Asymptotic Red Giant Branch. Or there's [MESA](https://docs.mesastar.org/en/release-r23.05.1/) which is a general evolution code. [This](https://docs.mesastar.org/en/release-r23.05.1/test_suite/1M_pre_ms_to_wd.html#m-pre-ms-to-wd) test suite example takes a 1 solar mass star from pre-main sequence all the way to white dwarf status.  

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## 2 answers

### Answer by username_5 on 2023-12-03. Score: 4

Further to the comment by username_4, you might want to play around with EZ-Web (easier):

[http://user.astro.wisc.edu/~username_5/static.php?ref=ez-web](http://user.astro.wisc.edu/%7Etownsend/static.php?ref=ez-web)

...or MESA-Web (more sophisticated):

[http://user.astro.wisc.edu/~username_5/static.php?ref=mesa-web](http://user.astro.wisc.edu/%7Etownsend/static.php?ref=mesa-web)

(full disclosure: I maintain these tools)

> **Comments:**
> **username_3**: This is awesome. They will come in handy in our course for kids and youths at our local observatory.  

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### Answer by username_6 on 2023-11-30. Score: 0

In general we calculate a Red Giant will have ten or more times the radius of the main sequence star (depending which scientists you want to believe) I don't think we have the math yet to work it out to a more precise figure. So ten times the radius is the lower limit, and the upper limit is unknown until you hit Super Red Giant size which is 100 million to a billion km wide, which obviously isn't particularly precise either.

A Red giant is what our Sun will become. A Super Red Giant is what a star 8+ masses of our sun will become.

Which means you can happily use anything within those ranges for your stories.


---

