he Arches star cluster is the most massive in our Galaxy.© NASA

We'll never find a star larger than about 150 times the size of our Sun, according to observations of a star cluster at the centre of our Galaxy.

Astronomers have previously been unable to agree whether stars have a natural limit to their size, or what that limit might be. Theoretical estimates based on the turbulent dynamics of stars' guts have ranged from 10 to 1000 solar masses.

So Donald Figer, an astronomer at the Space Telescope Science Institute in Baltimore, Maryland, searched for massive stars within the Arches cluster using the Hubble Space Telescope.

The stars of the Arches cluster, discovered in the early 1990s, collectively have about 11,000 solar masses, making it the most massive star cluster in our Galaxy.

If there were no limit to how big stars can grow, you'd expect ones up to 500 times our Sun's mass to be found in this dense cluster of stars. But Figer found no stars larger than about 130 solar masses.

Mindful of limits to the accuracy of his observations, Figer says that a reasonable upper limit to a star's mass is about 150 solar masses. He adds that his results indicate there is only a 1 in 100 million chance that stars have no upper limit to their mass.

"The result will leak into many different areas of astrophysics," says Figer. For example, a galaxy's mass is estimated from the amount of light it produces, but this requires an assumption about the size distribution of its stars.

That assumption often changes with each new theoretical treatment of the problem, says Figer. "At least now there is a firm number based on evidence."

"This will heat up the debate," says Pavel Kroupa, an astronomer at the University of Bonn, Germany. "It is a clear indication that there is an upper limit."

Hunting heavyweights

Although astronomers have claimed to see more massive stars, they always turn out to be a group of stars, or have huge uncertainties about their mass, Kroupa says. He adds that a star greater than 150 solar masses could possibly exist briefly if two other stars collide. "But of course this is a very violent process, and it's not going to live very long."

Kroupa's own team of researchers have found a similar cut-off in a more distant star cluster, although that result was less statistically significant that Figer's. Theoreticians should now focus on working out why stars have this mass limit, he adds.

In the late 1910s, the English astronomer Arthur Eddington suggested that growing stars might reach a point where the pressure of radiation coming from their cores was greater than the gravity keeping the outer layers held fast. At this point, stars could accumulate no more material, putting an upper limit on their mass.

Alternatively, turbulence in the outer atmosphere of the largest stars could be enough to throw off material faster than fresh matter can accrete. The latest theoretical estimates from this model put the star's upper limit at around 120 to 150 solar masses, agreeing with Figer's observations. He is now looking to other parts of the universe to see if the same limit applies in other star clusters. 

• References

  1. Figer D. F., et al. Nature 434, 192 - 194 (2005). | Article | PubMed | ChemPort |