As babies, obese stars go on a forced diet that stunts their growth, a new study shows. A census of our galaxy's richest stellar cluster reveals that no stars surpass 150 times the mass of our sun. Some astronomers thought that stellar titans up to 1000 times our sun's mass might exist, but big stars appear to become too active as newborns to gather gas beyond the weight limit.
Most stars are dim dwarfs less than half our sun's size. Defined as having one "solar mass," our sun is a long-lived object that rations its nuclear fuel for billions of years. Rare giants with dozens of solar masses burn their fuel furiously and blow up as supernovas in just a few million years. Theorists have a poor grasp of how huge stars arise, so they didn't know whether there was an upper mass limit. And our Milky Way contains more than 100 billion stars, so astronomers couldn't be sure they had found the largest ones.
The new research examines the Arches cluster, a stunning nest of bright stars near the galaxy's core. The cluster contains thousands of stars less than 2.5 million years old, making it the best place to find the biggest young stars that have not yet exploded. Astronomer Donald Figer of the Space Telescope Science Institute in Baltimore, Maryland, used an infrared camera on the Hubble Space Telescope to peer through dust and resolve the cluster's individual stars. Stars shine brighter if they have more mass, so Figer "weighed" every star by gauging its brightness.
According to his report in the 10 March Nature, Figer found no stars with more than 130 solar masses--even though an extrapolation of trends from lower-mass stars predicted that 18 such giants should exist. After allowing for possible errors, Figer concludes that no stars bigger than 150 solar masses arose. "It's a steep cutoff," Figer says. "It's the first measurement in our galaxy to give us such a hard limit." Newborn giant suns produce fierce radiation that blows away gas and dust and keeps stars from violating the weight limit, Figer believes.
"His result looks very convincing," says astronomer M. Sally Oey of the University of Michigan, Ann Arbor. "It would be hard to hide anything bigger than that." The limit matches Oey's own review of massive stars in numerous other clusters, published in the 10 February Astrophysical Journal Letters.