This artist's impression shows how the Arches star cluster would look from the center of our galaxy. Credit: NASA, ESA and A. Schaller (for STScI).

Astronomers announced today that stars can be born big, but only so big.

Theories over the years have predicted that an upper limit on stellar mass existed somewhere between 10 and 1,000 times the mass of our Sun. Recent observations of a massive cluster of stars in the center of our galaxy has found that this mass cutoff is about 150 times the Sun.

"The primary result is that there is a cutoff. There is little uncertainty about that," said Don Figer from the Space Telescope Science Institute. "The secondary result is that it is about 150 solar masses."

Figer used the Hubble Space Telescope to measure the masses of stars in the Arches Cluster, which is 25,000 light-years away and ten times the size of typical clusters in our galaxy. A cluster is a conglomeration of stars that all formed pretty much at the same time.

"The Arches Cluster contains many more smaller mass stars than larger mass stars. This is typical for a cluster," Figer said during a NASA teleconference today.

The rarity of behemoth stars is one of the reasons it has been so hard to nail down an upper limit for stellar mass. But the best place to find the biggest stars is in the biggest clusters. Arches, which contains 10,000 solar masses - is our galaxy's largest known open cluster. (Note that there are larger globular clusters.)

"The richer the cluster, the more likely you are to find extremely large stars," said Sally Oey from the University of Michigan, who was not involved in Figer's research.

Oey and her collaborators have looked for the most massive stars in nine clusters of a smaller, more typical size. They were able to estimate an upper limit that agrees with Figer's more statistically robust result.

Live fast and die young

It has long been known that a ball of gas weighing less than about a tenth of our Sun's mass will not be able to light the fusion reactor in its core. Once turned on, though, bigger stars burn brighter. A star 100 times more massive than the Sun will be a million times brighter, said Stan Woosley from University of California, Santa Cruz.

But these big stars burn their nuclear fuel more quickly. Stars greater than 100 solar masses will only live about 3 million years - compared to our Sun which is expected to live for more than 10 billion years. Therefore, to catch a glimpse of a giant star, one has to look not only at a big cluster, but a young one as well.

Fortunately, the Arches Cluster is between 2 and 2.5 million years old. With Hubble's acute eye, Figer found stars there ranging in size from 2 to 130 solar masses.

"We expected to find 20 to 30 stars with masses between 130 and 1,000 solar masses," Figer said. "But we found none. If they could have formed, we would have seen them."

If the prediction had been smaller - say, one or two behemoth stars - then Figer could explain their absence as just bad luck. But this big of a drop-off can only be explained by a physical barrier to more massive stars.

Limiting formation

No one is yet sure what this physical barrier is. Woosley thought there were two likely scenarios: either something stops the star's growth early on, or the star forms, but its intense radiation causes it to blow apart.

Still, there may be stars that circumvent the 150 solar mass limit. For example, the mass-spewing Pistol star may weigh as much as 200 solar masses. One way to explain this apparent paradox, Figer said, is that the Pistol star could be a "born-again" star, formed from the merger of two smaller stars. This sort of formation mechanism would be very unusual, however.

"There is no star in the galaxy that credibly breaks the 150 solar mass limit," Figer said.