Asked by: Stephen Brown, Huddersfield
Astronomers reckon that stars probably can’t survive above a mass of about 150 solar masses. This is because the enormous radiation pressure and mass loss from the star would disrupt its gravitational stability. Although more massive stars have been discovered, such as the 265-solar-mass star R136a1, these are likely to form by the merger of two or more stars.
But mass and physical size are not simply related, particularly for giant stars. The best candidate for the largest known star is UY Scuti which, although it is only about 30 solar masses, has a radius 1,700 times that of the Sun. Giant stars such as UY Scuti are known to swell up due to changes in their internal structure as they evolve.
But there is no clear-cut equation that determines how big a star can get since it depends not only on mass, but composition, evolutionary history and the strength of its stellar wind. For giant stars, such as UY Scuti, the outer atmosphere can continue to puff up and grow essentially without limit. But at some point the diffuse gas of its outer layers become merged with the interstellar medium and cannot really be regarded as the stellar surface.