Astronomers have found one huge star eating another, but the smaller star is predicted to eventually become a black hole and go on to feed off it.
A distant star is being eaten by its companion, but it is getting ready to take its revenge. The pair make up the most massive contact binary – two stars that are so close together they are technically touching – ever found.
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| The smaller, brighter, hotter star (left) is losing mass to its bigger companion (right) UCL / J. daSilva |
The pair of stars is called SSN 7, and it is located in a star-forming region called NGC 346 in the Small Magellanic Cloud, a galaxy about 200,000 light years away. Matthew Rickard at University College London and Daniel Pauli at the University of Potsdam in Germany compiled observations of SSN 7 from six different observatories taken over the course of more than a decade to learn more about it and determine its future.
They found that the two stars – one about 32 times the mass of the sun and the other about 55 solar masses – are orbiting each other much more closely than researchers realised from previous analyses. We thought that it took more than 20 days for them to circle one another, but it turns out it takes only about three days.
The observations also suggest that the larger of the two stars is sucking matter away from the smaller one at a rate of about 13 Earth masses per year. But in about 800,000 years, we expect the situation to change. The researchers’ models suggest that at that point, the smaller star will collapse and become a black hole.
“They will be uneasy companions for a few million years, but at some point the remaining star will start to expand,” says Pauli. At that point, the tables will turn. “The black hole will get its ‘revenge’ by eating off the companion star which previously it was feeding,” he says.
“Of course there’s no emotion going on, it’s just what happens when you put two massive bodies next to each other,” adds Rickard. Eventually the other star will also collapse into a black hole and the two will slowly spiral towards one another over the course of billions of years, and then merge.
We have observed these mergers through gravitational waves, and studying their precursors could help us understand those observations. “With the advent of black hole mergers being observed through gravitational waves, there is now a need to explain how black holes in this mass range come to be so close together and merge,” says Rickard. “The evolution gets significantly interrupted and changed by the stars transferring matter – the evolutionary paths are very different from what you’d expect from a single star.”
Reference:
Astronomy & Astrophysics, in press.