Really big black holes

Dying stars can form black holes up to five times larger than once thought possible, scientists have discovered.

Black holes form when the cores of dying stars explode. The power of the blast blows away the stars’ outer layers in supernova explosions that briefly emit more light than entire galaxies. But most of the material generally remains in the star, collapsing into a black hole.

The largest such black holes in our own Milky Way galaxy measure about 10 to 20 times the mass of the Sun, Jorick Vink, a research astronomer at Armagh Observatory and Planetarium, Belfast, Northern Ireland, said yesterday at a virtual meeting of the American Astronomical Society.

And until recently, he said, it was thought that was as large as was possible, because the blast from the death of bigger stars would be so it would simply blow all the extra material away. “So, you can start with a very high mass,” he says, “but you end up with only a 10 to 20 solar mass black hole.”

Then, about five years ago, astronomers detected gravitational waves from the merger of two black holes in a distant galaxy, each about 30 to 40 times the mass of the Sun. Clearly something was wrong. “Why do we suddenly have these heavy black holes?” Vink asks.

The answer, he says, turns out to lie in the fact that these celestial giants were in a galaxy so far away that the gravitational waves from their merger had been travelling to us for billions of years. That meant the merger had occurred billions of years ago, when the Universe was much younger than today.

At that time, he says, stars formed from gas clouds that contained less iron and other metals than today’s. That turns out to be important, because metals more easily absorb the radiation created by the blast, translating its energy into the impulse that blows off the star’s outer layers.

And this, he says, is why large black holes can’t form in metal-rich galaxies like the Milky Way.

But in metal-poor galaxies, that’s no longer a constraint. There, he says, it turns out they can be up to 50 stellar masses.

So far, so good.

Then last summer, astronomers detected gravitational waves from the merger of two “impossibly” large black holes: one 85 times the mass of the Sun, the other at 66 solar masses. How could that be?

One theory is that, by coincidence, these were both second-generation black holes, each created by the merger of smaller ones.

But there’s another explanation, Vink says. Giant stars can sometimes be surprisingly compact. This keeps them smaller and hotter (bluer) than less-dense red giants, making them less prone to blowing off massive amounts of matter when they die.

There’s still a point where a big-enough star will be blown to smithereens. But with these dense blue stars, Vink says, ones as large as 90 to 100 solar masses can remain largely intact, forming 80 to 90 solar mass black holes, “which were supposed to be impossible.”

 Stellar black holes, of course, aren’t the largest black holes in the Universe. That honour belongs to the supermassive black holes that lie at the centres of galaxies, where they can grow to masses millions of times that of the Sun.

Traditionally, it was thought that these formed by a cascade of mergers among smaller black holes comparable to the ones observed today. But, Vink says, astronomers have detected supermassive black holes up to 100,000 times the mass of the Sun from very early in the Universe, and it is difficult to figure out how they could have grown that large, that fast, by merger.

Something else must have happened, he says, probably involving the deaths of even larger stars, perhaps themselves 100,000 times the mass of the Sun.

Vink’s new model doesn’t explain how these could produce supermassive black holes, but it does suggest that astrophysicists might need to think more carefully about whatever processes might occur in truly enormous stars.

“It doesn’t solve the problem,” he says, “but it gives us more insight.”

---

See more:

• Astronomers declare no further research required
• Astronomers find ‘Goldilocks’ black hole
• Our galaxy probably isn’t that special