The two most massive stars ever discovered are circling each other 20,000 light years away near the centre of the Milky Way. The heavier of the two is 114 times as massive as the Sun – so big that according to some astronomers it should have blown itself to pieces.
Anthony Moffat of the University of Montreal announced the discovery of the two stars, labelled “A1”, today at a meeting of the Canadian Astronomical Society in Kingston, Ontario.
There are already a handful of stars known that are thought to weigh more than 100 times the Sun, but this is the first time any star has actually been measured to exceed that landmark.
The measurement is possible because the two stars, in star cluster NGC 3603, are in orbit around one another. This allows astronomers to observe the stretching and squashing of light, due to the Doppler effect, as the stars race around each other, completing one orbit every 3.8 days.
Moffat and colleagues collected the Doppler data using the European Southern Observatory’s Very Large Telescope (VLT) in Chile. Knowing the speed of the stars, and their distance apart, astronomers can calculate how great their masses must be in order for gravity to keep them in that orbit.
The two previous record holders also form a binary star system. WR20a, as they are collectively called, weigh in at around 80 solar masses each. So even the smaller partner in this new system, at 84 solar masses, just beats them – and the larger one smashes the record. For astronomers, it’s like hitting the jackpot twice in as many goes.
On the brink
“I’m astonished and very curious,” says astrophysicist John Brown of the University of Glasgow, who is not a member of the team. “I didn’t think stars that big could exist, they should blow themselves to pieces,” he says.
The more massive a star becomes, the more radiation it generates inside its core. That increases the pressure exerted on the outer layers of the star as the radiation fights to escape outwards into space.
Estimates of the absolute upper mass limit vary, but the heavier member of A1 must be close to self-destruction.
Stars on this scale are extremely rare in the Galaxy, not least because they are expected to race through their stock of fuel and explode in a few million years. Luckily, when these new heavyweights do decide to blow, they are far enough away not to pose a danger to Earth.