Mystery: What makes monster stars?
THE first signs a storm was brewing came in 2010. That was when a team of astronomers found four gargantuan stars in the Tarantula nebula, a star-forming area in our neighbouring galaxy, the Large Magellanic Cloud. The largest tipped the scales at 265 times the mass of the sun. “We were absolutely surprised by the discovery,” says team member at the University of Oxford. It has potential consequences for everything from black hole abundance to the likelihood of alien life.
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Traditional stellar theory suggests that stars above 150 solar masses shouldn’t exist. Their light, in trying to escape, would exert so much pressure that “the star’s surface layers would be peeled off and ripped apart”, says Schneider.
But the monster stars were not one-offs. This year, Schneider and his colleagues found a whole host of overweight stars in the nebula. Stars of up to 200 solar masses seem to be commonplace, with some even larger.
A local quirk? A statistical fluke? Probably not, if a result from the other end of the cosmos stands up. A team using the ALMA radio telescope array in Chile recently found a way of weighing stars in galaxies so far away that we see them as they were in the universe’s infancy. By analysing the light the stars emit, the researchers can determine the ratio of different chemical isotopes the stars must contain, giving them a handle on their masses. It seems there are theory-smashing stars in these galaxies as well.
“The universe is a battleground between two factions of cosmologists”
If so, the consequences are far-reaching. A few hundred million years after the big bang, the universe had cooled to a cold, unexciting soup of hydrogen atoms, floating around in Stygian darkness. At some point, the first stars formed, lifting the cosmos out of its dark ages. Astronomers who recently caught the signal of this event were stumped because it was much stronger than they were expecting – something that might be explained if these first stars were supersized, too.
More massive stars also means more supernova explosions at the end of stellar lives. “We’re talking about 70 per cent more,” says Schneider. Again, we may have evidence for that. Supernova SN 2007bi, which exploded over a billion light years away, seems to have erupted from a star of more than 200 solar masses. When such supernova remnants collapse back in on themselves, they might also create up to four times as many black holes as we thought, increasing the chance of spotting black hole mergers using gravitational wave detectors.
Most importantly, perhaps, having more supernovae could roughly double the quantity of heavy elements produced when massive stars die. This is how elements such as oxygen, carbon and iron get spewed out into the universe, creating a richer supply of ingredients important for life. All very hot – but as for how these massive stars come to exist, well, we still have no clue.
This article appeared in print under the headline “Object: The tarantula nebula”
