A black hole's spin could finally be measured by a high-speed star

**Charmaine Bundell** (0:04)
Nature Podcast In experiment, I don't know yet, why it's like so far.

**Dan Fox** (0:08)
Like it sounds so simple.

**Enrica Tricomi** (0:09)
They had no idea.

**Stefan Gillesen** (0:10)
But now the data's...

**SPEAKER_5** (0:12)
I find this not only refreshing, but at some level astounding.

**Benjamin Thompson** (0:21)
Nature Welcome back to the Nature Podcast. This week, the star that could reveal a black hole spin, and will exoskeletons change the way we move?

**Nick Howe** (0:33)
I'm Nick Paltrychow.

**Benjamin Thompson** (0:35)
And I'm Benjamin Thompson.
A star zooming around the centre of our galaxy could help physicists unravel a long-standing mystery. Namely, how much is our neighbourhood supermassive black hole spinning? This week in Nature, researchers describe a star that orbits around the supermassive black hole at the heart of the Milky Way, called Sagittarius A star, or Sag A star. While researchers know a lot about this black hole, where it is, how far away it is, its likely mass and so on, how much it's spinning has been hard to pin down.
In fact, whether Sag A star is spinning at all is unknown, but given that everything in the universe spins, planets, stars, galaxies, physicists are pretty confident that it is. In fact, spin is built into the models of how supermassive black holes work. Things like the enormous jets of matter they fire out require spin in order to occur. But gauging precisely what an enormous black hole spin is, is tough. You can't see these objects, and so researchers have had to infer and estimate spin rates for them by other means. And so, working out how much our closest supermassive black hole is spinning should help confirm that black hole models are correct, and explain more about how the wider universe functions. And this brings us back to the star the researchers are writing about this week, which they call S301. To find out more about it and how it could help in the search for Sag A star's spin, I called up one of the authors, Stefan Gillesen, from the Max Planck Institute for Extraterrestrial Physics in Germany. Stefan laid out a bit more about what makes S301 useful in this quest.

**Stefan Gillesen** (2:39)
The star S301 is on a very eccentric orbit, and it has an orbital period of just below nine years. It's also the fastest and the quickest orbital period we know for any of those stars going around Sag A star. And it's not only the orbital period which is short, but in particular, it's also the star which comes closest to the black hole of all the stars which we know. And by the way, that will also make it, at least for that moment, when it's at that closest point, it will be the fastest star which we know. It will actually achieve a speed of 25,000 kilometers per second.
That's a lot, right? I mean, that's going more than half way around Earth in a second. I mean, the speed of light is 300,000. So that's like a 12th of the speed of light or something like that. That's quite impressive.

**Benjamin Thompson** (3:25)
So we have this star then with an elliptical orbit. And at one end of the ellipse, it's very close to Sagittarius A star. How close to this star gets to the black hole? Maybe in units that I can comprehend.

**Stefan Gillesen** (3:38)
Yeah, of course, you can express it in kilometers. That would be a stupid number. But I think it's not that further away than Saturn from the Sun. So it's solar system scale. And it essentially gets attracted by the black hole, falls almost into it, but just almost. And so in that process, it actually gains a lot of angular momentum, and that actually makes it then swing out again. So the total orbital time scale is nine years, and it's only a week or so in which the star passes by the black hole.

**Benjamin Thompson** (4:08)
And how might the orbit of this star help researchers figure out the spin of Sagittarius A star? How do these two things go together?

**Stefan Gillesen** (4:15)
Think about a plane which is flying across the ocean and doesn't quite know that there's a hurricane. And all of a sudden, the plane will sort of feel a different force than what the pilot might expect. So the total direction this plane is flying will be a combination of its own speed and the one which is imposed on it by the air in which it's moving. The closer the plane is to the hurricane, the stronger the effect is. And very similar, S301 will feel the spin of the black hole, which is affecting the space around the black hole. And thereby it's not traversing through, let's say, flat normal space, but it's actually passing through a region where the space itself gives the object an extra kick. And thereby you get a different direction of flight. And that is essentially what we would like to measure.