By Phil Plait
Black holes are the Universe’s ultimate garbage disposals: Stuff falls in, and never gets back out. It can’t. To get out, you’d have to travel faster than the speed of light, which (as far as we know) is impossible.
Black holes grow by consuming matter, and in the centers of galaxies they can grow to huge size. In the gorgeous barred spiral galaxy NGC 1365, there’s one lurking in the core that has about 2 million times as much mass as our Sun. Not only that, it is gobbling down matter, and that allows us to measure some interesting properties of this cosmic monster, including its spin. Astronomers observed NGC 1365’s black hole using the NuSTAR and XMM-Newton observatories, and were surprised to find out it’s spinning so fast that the outer edge is moving at very nearly the speed of light.
Black holes are confusing, but the bottom line is that they are such highly concentrated massive objects that their escape velocity is faster than light. Once something falls in, it cannot get out, but some of the properties of that material remain: specifically mass, spin and charge. That last bit is literally electrical charge, like how an electron has a negative charge.
Mass is the critical property, because the more mass a black hole has, the bigger it gets and the stronger its gravity is as well. But spin is important too. Look at, for example, a black hole forming via the collapse of a star’s core when the outer layers explode in a supernova. The core is spinning since the star rotates. As the core collapses, that spin rate increases, in much the same way a skater can increase his or her spin by bringing their arms in close to their body. This is called conservation of angular momentum; objects spinning tend to stay spinning due to momentum, just like any object in motion tends to stay in motion due to momentum. The total angular momentum depends on the object’s size and rate of spin. Increase one and the other must decrease; if you make something smaller it’ll spin faster.
So by the time the core of our doomed star collapses all the way down to a back hole, the spin can be ferociously large.
But there’s more. If there is material around the black hole falling in it can change the spin as well. If material fell straight into the black hole, the spin wouldn’t change much.
But if that material comes in at a slight angle, it can actually add to the spin of the black hole, increasing its angular momentum. That gives a kick to the spin rate, bumping it up.
And that brings us back to NGC 1365, located about 60 million light years from Earth. Astronomers used NuSTAR to look at X-rays pouring out of material falling into the black hole there. As that material falls in it heats up to millions of degrees, blasting out X-rays that are easily bright enough to see from Earth with the right equipment. Careful observations allowed astronomers to see these X-rays coming from matter just before it reached the Point Of No Return, at a position called the Innermost Stable Circular Orbit. If it gets any closer, blooop! It falls in, and it’s gone.
This is a pretty amazing finding by the NuSTAR astronomers. It shows that extremely detailed X-rays observations are possible; something that’s very difficult and painstaking to do. It also demonstrates that we can take a pretty close look at black holes and tease out details that were previously not possible to see. By themselves, black holes are invisible, dark, and nearly impossible to observe. But they’re sloppy eaters, and this betrays many of their secrets. Even from 600 million trillion kilometers away.
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