IT APPEARS there is a limit to how big stars can grow in today's universe: about 200 times the mass of our sun.
Astronomers used to think the size of the biggest stars in a cluster was related to the ratio, say, of the number of small sun-like stars to the number of bigger stars with 10 times the sun's mass. Applying this ratio to R136a, which is the largest cluster of stars in our galaxy in which individual stars are discernible, the biggest star should have more than 600 times the mass of the sun. But its largest star is only about 150 solar masses and the largest known star in our galaxy is about 200 solar masses - making astronomers suspect that stars had a size limit.
Now Sally Oey of the University of Michigan at Ann Arbor and colleagues have strong support for a size limit in a survey of other clusters within our galaxy and in the nearby satellite galaxy, the Magellanic clouds. They found no star bigger than 200 solar masses (Astrophysical Journal Letters, vol 620, p L43). It is not clear whether the size is limited by the physics of star formation or by the size of the parent gas cloud. Larger stars, perhaps of up to 500 solar masses, may have existed in the early universe, Oey says.
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