# How to predict the radius of a Red giant

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.

• @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
Commented Nov 30, 2023 at 20:24
• 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. Commented Nov 30, 2023 at 20:53
• Did you check en.wikipedia.org/wiki/Red-giant_branch for masses, radii and luminosities? Commented Nov 30, 2023 at 21:25
• How accurate and rigorous do you need it ? Star In A Box 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 which is a general evolution code. This test suite example takes a 1 solar mass star from pre-main sequence all the way to white dwarf status. Commented Nov 30, 2023 at 22:36

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

http://user.astro.wisc.edu/~townsend/static.php?ref=ez-web

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

http://user.astro.wisc.edu/~townsend/static.php?ref=mesa-web

(full disclosure: I maintain these tools)

• This is awesome. They will come in handy in our course for kids and youths at our local observatory. Commented Dec 3, 2023 at 13:54

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.