The mass-radius relation is a relationship between the radius, R, of a main-sequence star and its mass, M. If R and M are both in solar units, then R = M ^0.8.

 

For red giants (stars that have exhausted their supply of hydrogen in their cores and are burning helium), the mass-radius relation is more complex, and depends on the star's composition and temperature.

 

It is important to note that the mass-radius relation is not a precise prediction, and there can be significant variations from star to star. However, it can provide a rough estimate of a star's size based on its mass.