# What increases or decreases the mass and density of a fixed radius star?

Assuming a relatively even proportion of mass and radius, a 0.25 solar masses and radii star would have a density of 22.5003 g/cm³, or about 16 times our Sun's density.

Keeping the radius of 0.25 solar radii, what influences would cause different 0.25 solar radii stars in the main sequence to have higher or lower masses/densities than the "normal" 0.25 solar masses?

• Mass loss or mass accretion change the mass. There is no one to one relationship between mass and radius on the main sequence because it changes as the star burns through its hydrogen. Feb 26, 2021 at 8:05
• You can not keep the radius constant and change mass or vice versa. If you increase/decrease one, the other increases/decreases as well. Look up the mass-radiius relation personal.psu.edu/rbc3/A534/lec18.pdf Nov 23, 2021 at 21:27

Magnetic Activity

There is a well-known "radius inflation" problem for low-mass main-sequence stars - especially those that are magnetically active and rapidly rotating (either because they are young or because they are in tidally-locked short-period binary systems).

This "radius inflation" means that magnetically active low-mass stars are bigger than they ought to be at the same mass. Thus, in principle, you could vary the levels of rotation and magnetic activity in low mass stars and this would produce a different locus in the mass-radius plot.

The result is that for stars of a given radius, the more magnetically active examples would have a higher mass. This can be a 10% effect for reasonable levels of magnetic activity.

Some reading: Jackson et al. (2018); Garrido et al. (2019)

Other factors

The radius of a main sequence star is age dependent, they get bigger as they get older. This is a very slow increase for stars with $$R \sim 0.25R_\odot$$, because their main sequence lifetimes are long. Nevertheless an older, slightly lower mass star could have the same radius as a younger, higher mass star. The effect is small ($$<1$$%) between ages of 1 and 10 billion years.

However, there is a larger effect in the opposite direction at younger ages because lower mass "stars" are in fact still contracting towards the main sequence at ages <1 billion years.

The radius of a star is also affected by composition. A lower metallicity star (at these low masses) will have a slightly larger radius at the same mass and thus a slightly lower mass at the same radius. The size of the effect is perhaps a few percent for a factor of 10 change in metallicity.

Stars are broadly divided into 3 types. These 3 types are divided on the basis of their composition of gases and elements other than H and He. So even if these stars are of same radius, their composition is different.

I am not sure but I think that as different stars have different composition and their rate of fusion is different, their density is different and so is their mass. Also while some stars are just forming and are at that radius, their gas composition is different than what a dying star of that radius is and so their masses is different.

I don't know if my version of the answer is right, but I hope this helped.