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Most massive star ever

<img alt=“cluster_corr.jpg” src=“http://blogs.nature.com/news/thegreatbeyond/cluster_corr.jpg” width=“224” height=“225” /align="right">

Astronomers have discovered the most massive, brightest stars to date. One of them weighs in at about 265 solar masses and is almost a staggering 10 million times more luminous than the Sun. The observation is surprising, since stars are not thought to grow heavier than 150 solar masses—see the 2005 Nature News article about the solar-mass limit.

The discovery was made using ESO’s Very Large Telescope (VLT), examining two star clusters dubbed ’NGC 3603’ and ’RMC 136a’. It is the first time that individual stars in such dense clusters have been resolved. In addition to the VLT, the team used archival data from the Hubble Space Telescope to study the clusters.

The new-found stars are not only more massive and luminous than the Sun—they’re also hotter. In fact, several stars in the clusters have a surface temperature of 40,000 degrees—about seven times hotter than the Sun.

Paul Crowther, from the University of Sheffield, who led the research team of says: ”Owing to the rarities of these monsters, I think it is unlikely that this new record will be broken any time soon”. (press release)

In cluster RMC 136a—located 165,000 light years away in the Large Magellanic Cloud—the team found that four of its 100,000 stars are above the 150 solar-mass limit. One of them, called ’R136a1’, is the most massive and luminous star ever found. Although it currently weighs about 265 solar masses, it is thought to have been as heavy as 320 solar masses at its birth.

Crowther explains that the stars are born heavy and lose weight as they age. “Being a little over a million years old, the most extreme star R136a1 is already ‘middle-aged’ and has undergone an intense weight loss programme, shedding a fifth of its initial mass over that time, or more than fifty solar masses,” he says. (press release)

But understanding how stars can get so massive is unfortunately not easy. There are massive stars known to contain about 150 solar masses, but there is no satisfactory explanation as to how they have become so big. Crowther thinks that the stars were either born big, or result from the merging together of several smaller stars. (press release)

When stars between 8 and 150 solar masses die, they usually go as big supernovae explosions, collapsing into black holes or neutrons stars. However, some researchers believe that stars between 150-300 solar masses could explode without leaving behind a trace, in so-called ’pair-instability’ supernovae. As such a supermassive star ages, its photons would start converting into electrons and positrons (so-called ’pair production), which would decrease the outward pressure on the star. It is this decrease in pressure that would make the giant collapse and finally explode. To read more about ’pair-instability’ supernovae, see the 2009 Nature News article about a giant star explosion.

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Images: ESO/P. Crowther/C.J. Evans

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