figuring out mass of a star based on diameter

[NOBARTHOLEM]

How do you find the mass of a star based on its diameter? are there other factors that play into it and other things like that or is it just an equation???

[Janus]

For main sequence stars you can use

R=M^{.8}

with R being the diameter. For non-main sequence stars, this doesn't hold however. For instance, when our Sun expands into a red giant, its mass will not increase but its radius will do so greatly. Another Example is Betelgeuse, a super giant with a mass 14 times that of the Sun and a radius 630 times as much.

[NOBARTHOLEM]

i can figure out that non main sequence stars hold the same mass because its just less matter per square mile or sqaure yoctometer or whatever measure u want to use.