There’s a monster lurking in the middle of our galaxy. You might not be able to see it, but we know it’s there. Its diameter is 10 times that of the Sun, but its mass is 4 million times. It’s what we call a supermassive black hole.
OK, it’s 27,000 light years away, so it’s probably not going to get you, but still: a supermassive black hole. Let that sink in, so to speak.
‘Normal’ black holes sound pretty massive themselves. If a star is bigger than about 3 times the mass of the Sun, then eventually it reaches a point where it can no longer hold up under its own weight, and it collapses into an object with gravity so strong that even light cannot escape. These are called stellar black holes.
The biggest stellar black hole so far confirmed is about 16 solar masses, but there are indications they can get up to around 33 solar masses.
However, the black holes believed to be at the centres of most galaxies are much, much bigger: more than 100,000 times the mass of the Sun. Hence the label supermassive black holes.
So if there’s something that big in our galaxy, then why can’t we see it? Well, between it and us there’s an awful lot of stuff.
You’ve probably seen the Milky Way in the sky, a cloudy band visible at night when you’re well away from the city. That’s the main plane of our galaxy. If you could stand outside and away from it, you’d see that it’s a spiral galaxy, i.e. a sort of disc shape made of four swirling arms, with a pronounced bulge in the centre.
From the inside, you just see a cloudy band stretching across the sky, with a lot of opaque dust and gas blocking out the good bits like the dense middle. But it’s there alright, in or near the constellation Sagittarius (see the picture above).
Even though we can’t see it directly – at least not with visible light – we can detect it with radio waves. And in the radio spectrum we see a very, very powerful radio source called Sagittarius A*. The radio waves are believed to be electromagnetic radiation given off from the accretion disk of the black hole: that’s where things spin around it really, really fast before they fall in. And when charged particles spin around fast like that they give off electromagnetic radiation (which actually means they lose energy and so fall in even faster. Not a good idea perhaps, but you can’t fight physics).
So we can see the radio waves, but how do we know Sagittarius A* is a black hole? Well, we can also detect 28 other stars orbiting it. One of them, called simply S2, orbits every 15.2 years and gets as close as 122 times the distance from the Earth to the Sun.
From the speed and distance of S2, we can calculate that the object in question has a mass of about 4.1 million times the mass of the Sun. That much mass in that small a volume has to be a black hole.
Its dimensions are given by something called the Schwarzschild Radius, which tells us that the black hole’s event horizon – the point at which light is no longer able to escape – is at about 13.3 million kilometres. That’s only about 10 times the diameter of the Sun, or 9% of the distance from the Sun to the Earth.
And yet it has a mass 4 million times that of the Sun. For comparison, the Sun is 333,000 times the mass of the Earth. The difference between the black hole and the Earth is the same as that between you and a grain of pollen.
Even so, there are bigger black holes out there. Much, much bigger (you can see where this is going).
Recently, one with a mass of 17 billion suns was discovered in a galaxy only 250 million light years away (van den Bosch RCE, Gebhardt K, Gültekin K, van de Ven G, van der Wel A, Walsh JL 2012, “An over-massive black hole in the compact lenticular galaxy NGC1277”, Nature, vol. 491, no. 7426, pp. 729-731, doi:10.1038/nature11592, arXiv:1211.6429v1 [astro-ph.CO]).
I call it a superdupermassive black hole, although the authors called it ‘over-massive’.
This term is actually appropriate, because it’s much larger compared to its host galaxy than previously discovered black holes. Although small in comparison, our galaxy is in more typical proportion, with the central black hole being 0.1% the mass of all other stars. But the black hole in NGC1277 is 14% of its galaxy’s stellar mass.
The animation embedded below shows how the black hole was identified, using measurements of stars in the galaxy to calculate their orbits and hence the mass at their centre. The photo in the background was taken by the Hubble Space Telescope (NASA/ESA/Fabian/Remco C. E. van den Bosch MPIA).
But even though it’s so big, this superdupermassive black hole isn’t a unique freak. The researchers have also found five other galaxies with similar extreme proportions. Instead, it suggests we may need to rethink our theories of how galaxies form. After all, we’ve been using our own galaxy as a typical example, but there seems to be a much bigger and more complex variety.
What we can say for certain is that it shows what huge objects are out there in the universe. Much too huge for our puny human adjectives.
I spoke to Professor Rachel Webster from the University of Melbourne about this discovery, on our show that aired on 13 December 2013. You can listen to the podcast.
A transcript follows after the break…
Chris: I’m talking to Professor Rachel Webster from the University of Melbourne about the supermassive black hole discovery. Professor Webster, thank you for talking to me.
Prof. Webster: It’s a pleasure.
Chris: Now, just a few questions about this latest discovery, can you just tell us how did they actually find this black hole?
Prof. Webster: OK, so what they usually do is take a spectrum of the centre of the galaxy. And there’ll be a lot of stars moving around the centre of the galaxy, on orbits around the central mass. And so what they do is, they look at the velocities of those stars, which you can do by analysing the spectrum, and the velocity will tell you how big the object in the centre of the galaxy is, just by the speeds that they’re moving at.
Chris. OK, so you just calculate from the dynamics.
Prof. Webster: Yep, exactly.
Chris: And working out what the gravity would have to be.
Prof. Webster: Yes, exactly.
Chris: OK. And now this one is an unusual size, I believe, it’s something like 14% of the total mass of the galaxy, or 59% of the central part of the galaxy?
Prof. Webster: That’s right.
Chris: Yeah, so how unusual is that, compared to other galaxies we’ve seen?
Prof. Webster: OK, so this black hole is a very big black hole. It’s not the biggest one that we’ve measured, but it’s right up there. But you’ve hit on the thing that’s really unusual, and that’s the fraction of the central mass that is in the black hole. So normally there’s a very tight relationship between the size of the black hole and the matter, the amount of stuff that’s in stars. And the black hole is about one thousandth of the size of the total mass in stars. So that’s 0.1%. In this case it’s 14%, and so it’s 150 times bigger than we would expect it to be.
Chris: Wow. OK, and do we have any idea why it’s so big, why it’s so unusual?
Prof. Webster: No. Well in fact it’s exactly examples like this that’s going to help us try and understand how galaxies build up, you know, how the cores of galaxies build up, how the black holes build up. We know that they start considerably smaller than what we observe today, and then they probably build up through a sort of accretion process, sort of a couple of black holes coalesce together, form a bigger black hole and so on.
And if that is the dominant process, then we do expect the build up of the black hole to go hand-in-hand with build up of the stellar mass around the black hole. So if we’re seeing something different, there has to be a slightly different explanation for it. So that’s why astronomers are so interested.
Chris: OK. So yeah, I understand that from the age of the stars in this galaxy, that it indicates that it probably wasn’t just building up from a lot of gas, like we would normally expect.
Prof. Webster: That’s right, that’s right.
Chris: OK. So has this given us any further ideas about galaxy formation already, or is this just still a puzzle waiting to be solved?
Prof. Webster: Look, at this stage I would say it’s still a puzzle waiting to be solved. What it’s saying is that the ideas that we had, at least in this one case, don’t seem to work very well. You know, we would probably say that if there’s just one example like this, perhaps we don’t need to worry too much, but certainly if we start to find other galaxies that have much bigger black holes than we expect, then we really have to rethink how the cores of galaxies build up over time.
Chris: OK. I understand that we see some things that look fairly similar though in the very early universe, the quasars and that sort of thing. Is it possible that this has any relation to those?
Prof. Webster: It’s possible. Yeah, so quasars have supermassive black holes in their cores, and we certainly see very large black holes surprisingly early in the universe, and so one possible theory for example for this one is that it’s more or less as it was early on, and just hasn’t gone through that accretion process. But as I say, it’s still early days in terms of being sure about what’s going on.
Chris: Absolutely, and even if we could connect it to such things like quasars we’d still need an explanation for how that formed anyway.
Prof. Webster: Oh, well that’s right. Well I mean, this is connected to quasars, there’s no question about it, you know a quasar is just a black hole in the centre of a galaxy, we mostly see the black hole rather than the galaxy. So they’re the same beast, there’s no question there, yes.
Chris: Great. Well, so we can expect some more of these to show up shortly, fingers crossed?
Prof. Webster: Well I hope so! Science is always exciting when something that you hadn’t predicted comes along.
Chris: That’s right, it’s the unexpected and the unknown, is where the true interest lies.
Prof. Webster: Yeah, exactly.
Chris: Well thanks very much Professor Webster…
Prof. Webster: That’s a pleasure, Chris.
Chris: …for shedding some light on the black holes.
Prof. Webster: OK, good on you.
Prof. Webster: Bye.