Just how humungous can a star be? One more massive than any other we know has been has been identified in a nearby galaxy. At more than 250 times the mass of the sun, it may one day explode in an exotic blast that involves the creation of antimatter.
Named R136a1, the star sits toward the centre of RMC 137a, a crowded cluster of hot young stars some 165,000 light years away in the Large Magellenic Cloud, one of the Milky Way’s nearest galactic neighbours.
Paul Crowther of the University of Sheffield, UK, and colleagues used the European Southern Observatory‘s Very Large Telescope on the mountain Paranal in Chile to study stars in the cluster, which is so tightly packed it was once thought to be a single, ultra-massive star.
The team estimate that R136a1, the brightest of the stars they studied, is about 265 times the mass of the sun, making it the most massive star ever measured. In its infancy, roughly a million years ago, the star was probably even more massive – some 320 times the mass of the sun: it will since have shed much material in hot, violent winds.
No one is sure how massive a star can be, but until now, the most massive ones seen have had about 150 times the mass of the sun. Such stars must be born in big clusters and shine only briefly before exploding, so they are expected to be rare.
Mass production
Measuring the mass of R136a1 was not straightforward. The best measurements of star masses come from observations of stars orbiting one another. But Crowther and colleagues suspect R136a1 is a single star, so they had to rely on models that relate the brightness of stars to their mass, taking account of how stars are thought to evolve with time.
That means we can’t be sure just how massive R136a1 is, or how massive it was when it was born. “I think they’ve gotten a very believable answer,” says Philip Massey of the Lowell Observatory in Flagstaff, Arizona. “It’s certainly a very significant find,” he adds, though he says many astronomers already suspected that 150 solar masses was not a hard limit. “I think most people will view this will glee and say ‘I told you so’.”
If there are more supermassive stars out there than we thought, we may have to revise estimates of how quickly galaxies form stars. That’s because the light we see from galaxies could be produced by fewer, brighter stars than we thought possible, says Mark Krumholz of the University of California, Santa Cruz.
Exotic death
R136a1 and a handful of other massive stars identified by the team could also be stars to watch, as they are candidates for an exotic stellar death
triggered by the creation of electrons and their antimatter counterparts. These explosions – called pair-instability supernovae – are expected to rip apart stars and could explain the properties of some recent bright supernovae.
Finding more of these massive stars may require new telescopes that can see individual stars in more distant clusters. “My suspicion is this is about as big as they get in places where we can pick out individual stars with our telescopes,” Crowther says.
But such discoveries may not be far off. A new generation of telescopes is coming, such as NASA’s James Webb Space Telescope, set to launch in 2014, and the planned European Extremely Large Telescope, a 42-metre telescope astronomers hope will be ready by 2018.