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Last Updated: Wednesday, 9 March, 2005, 19:19 GMT
Upper limit placed on star growth
Artist's impression of the Arches cluster, STSCI
The Arches cluster lies near the centre of our galaxy
An upper limit has been placed on how heavy stars can get: they are constrained to about 150 times the mass of our own Sun, new research claims.

It had previously not been possible to determine this limit or even to say whether stars were constrained in mass.

But Donald Figer used the Hubble Space Telescope to look for massive stars in the Arches star cluster, which is close to the centre of our own galaxy.

Details of the research are published in the latest issue of Nature.

There is a heavy burden in making this kind of claim, because it means that you have to go out and potentially look at every star in the Universe
Donald Figer, STSCI

Dr Figer, of the Space Telescope Science Institute (STSCI) in Baltimore, Maryland, chose the Arches cluster because it is young enough to expect massive stars but old enough that the cloud of material in which it was "born" has disappeared, giving astronomers a clear view.

The dense cluster is sufficiently large that, statistically speaking, stars up to 500 times our Sun's mass should be found there. But the astronomer turned up no stars bigger than about 130 solar masses.

The researcher concludes, on probability, that 150 solar masses is the upper limit for stars.

And the observations further indicate that there is only a one in 100 million probability that stars have no upper limit to their mass.

Scrutinising claims

"There is a heavy burden in making this kind of claim, because it means that you have to go out and potentially look at every star in the Universe," Dr Figer told the BBC News website.

"That's not going to happen. But the least we can do is be aware of any claim of a star being above the limit."

Dr Figer added that previous claims for stars above 150 solar masses had not stood up to scrutiny.

"For instance, there was an object called R136. We used to think it was a few thousand solar masses. Now we know it's composed of hundreds of stars," he explained.

Professor Janet Drew, of Imperial College in London, UK, commented that the research was a "nice piece of work" with a very interesting interpretation.

"Yes, there are loopholes, but I think that the loopholes are not that large," she explained.

Other researchers have pointed out that the cluster could once have hosted more massive stars that have since exploded, and whose remnants could be lurking as black holes.

Further confirmation

If confirmed by further work, the finding of an upper limit for star mass would have important implications for our understanding of the structure of the Universe.

The upper limit of stars was thought to be related to how enriched they were with heavy elements such as iron. But the new research suggests this may not be the case.

It also questions whether a quantity called radiation pressure is the physical mechanism that stops stars from becoming too heavy. When radiation pressure exerted by a star reaches a certain level, it was thought to stop the incorporation of any new matter from the surrounding stellar "nursery" cloud.

Dr Figer said he wanted to look at whether some massive stars were actually binary stars (composed of two stars).

"If all those stars in the Arches cluster that are 130 solar masses are binary, it would mean the upper limit might actually be less. But that's highly unlikely," he explained.

The researcher has just been given a grant by the US space agency (Nasa) to expand the work. But he admits the decision to de-orbit the Hubble Space Telescope means the investigation will probably have to be conducted with haste.

Infographic, BBC

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