Stars that collide after being sped up by orbiting close to supermassive black holes may be so bright they rival the most powerful supernovae, some of the brightest phenomena in the cosmos. Studying these cosmic crashes could help us learn about the universe’s most massive black holes and their environments.
Near the most massive black holes, stars are expected to move close to the speed of light without getting shredded by their enormous hosts’ gravity. But that doesn’t mean they couldn’t be destroyed in a head-on collision. and at Harvard University simulated what would happen if such a smash-up were to occur. “The vicinity of a black hole is an accelerator like the Large Hadron Collider, except it’s a large star collider,” says Loeb.
They found that these collisions could release as much energy as supernovae – which can reach up to the lifetime power output of the sun – and maybe more if the black holes are big enough. Not only that, but after the collision there might be a second, even more powerful flare of energy as the resulting debris falls into the black hole.
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However, because of the extreme circumstances required for a pair of stars to be aligned perfectly so that they could smash together at such high speeds, these flares are expected to be rare. Depending on the size of the host black hole, these collisions could occur anywhere from once every 50 years to once every five years in the small proportion of galaxies where circumstances for collisions are right – and only the brightest of those events, created by the fastest stars, would be visible to us.
“If we were to look for these collisions around the universe, we might see these powerful explosions at the centres of other galaxies,” says Loeb. “It’s possible that we have seen some and attributed them to some other phenomenon, but they would be a small minority of the events that we have seen.” The Vera C. Rubin Observatory, a huge telescope currently under construction in Chile, should be able to spot several of them after it turns on in 2022, he says.
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