Some black holes are bigger than we can explain. They may have formed when supermassive stars collapsed, but we don鈥檛 know how those formed either. Now it seems the answer could be that many regular-sized stars smash together to create a bigger one, and we might be able to see this happening with the next generation of space telescopes.
Astronomers have spotted about 200 supermassive black holes 鈥� hundreds of thousands to billions of times the mass of the sun 鈥� in the early universe, which we can see by observing light that has travelled for billions of years to reach us. If these black holes formed in the usual way, via a normal star collapsing under its own mass, there is no way they could have grown so large in that little time.
One possibility is the idea that they formed from supermassive stars tens of thousands of times more massive than the sun. 鈥淎 handy workaround is to have something that鈥檚 big from the beginning and gives you a leg up,鈥� says Tyrone Woods at the Herzberg Astronomy and Astrophysics Research Centre in Canada. 鈥淭hat way you鈥檙e skipping a whole bunch of steps.鈥�
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But we don鈥檛 see any of those supermassive stars now, so we don鈥檛 know how they could form. Zoltan Haiman at Columbia University in New York and his colleagues have done modelling studies that make them think dense clusters of stars could be the answer.
Star clusters are born when huge clouds of gas fragment into pieces and each piece collapses to become a star. The early universe was much more compact than it is now, so these clouds were probably much denser in the past.
That means that when a cloud of gas formed a cluster, the stars were very close together. The biggest of the group would sink to the middle of the cluster and attract the others towards it. With each star that hits the central one, it becomes bigger and puffier, making it easier to grab the next star.
鈥淭he star is trying to collapse, and if you add stuff faster than it can collapse then you can keep building it so it can鈥檛 become a normal star and then a normal black hole,鈥� says Haiman. 鈥淚f it just keeps getting bombarded, it鈥檒l keep growing.鈥�
He and his team found that if the central star devours one of its siblings at least once every few hundred thousand years, it could quickly grow to more than 600,000 times the mass of the sun before collapsing. That鈥檚 big enough that the resulting black hole could become one of the supermassive black holes that we see in the early universe and today.
Woods and his colleagues have modelled the formation of supermassive stars in a slightly different way, via a gas cloud collapsing to coalesce into a single enormous star. These stars would have to feed on gas rather than other stars, so they wouldn鈥檛 be able to grow as fast.
Woods鈥檚 team found that once these stars reached about 150,000 times the mass of the sun, they would collapse. That isn鈥檛 quite as big as the stars that Haiman and his team looked at, but it is still enough to form the seed for a supermassive black hole.
They also found that the stars burn for a while before becoming black holes. 鈥淭hese things do live for a reasonably long time, even for a million years or more 鈥� they don鈥檛 immediately collapse,鈥� says Woods. 鈥淲hat that means is that we actually have a hope of finding them one day.鈥� The next generation of huge telescopes, like NASA鈥檚 James Webb Space Telescope, should be able to peer deep enough into the early universe to spot them, he says.
References: ; The Astrophysical Journal, DOI:
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