Café Sci November 2015

Richard Alexander, from Leicester University, comes to Café Sci to talk about super-massive black holes.

However, before we get onto the really big ones, Richard gives us some context as to what black holes are. They are predicted by General Relativity. If you drop something into a black hole, by the time it reaches it, that thing will be travelling at the speed of light. A regular black hole has the same mass as our sun but in a space that is only one and a half miles across.

But how can we see something that swallows light? Well, we can see the energy given off by the black hole. Accretion coverts up to 40% of mass to energy. In contrast, nuclear fusion converts just 0.7%.

At the end of its life, our sun will become a white dwarf. Larger stars will become neutron stars while even bigger ones will collapse in on themselves to form black holes. Most stars are actually binary stars and so when one collapses, the matter from the other star becomes an accretion disc. This energy then “shines” in the x-ray spectrum.

Quasars, such as the exotically named 3C273 and M87, have been detected at the centre of galaxies. However, they are so bright that we can’t actually see the galaxies. So far the only things that we know about in the universe that are this energetic are black holes. Could this mean that quasars are in fact, black holes?

In the centre of our own galaxy, the Milky Way, we can see clearly enough to see that we do have a super-massive black hole. Looking at the orbits of stars around this black hole (orbits than take between 10 and 15 years), we can measure its size. It is around 4 million times the size of our sun and is the size of the orbit of Neptune.

In 2017, one of the stars in orbit, S2, will make another really close pass to the black hole. On Earth the race is now on to build better instruments so that we can observe this happening. This will allow us to test some parts of General Relativity that we haven’t been able to so far. In fact Newton predicts that one thing will happen to the star, Einstein something else.

The super-massive black holes at the centre of the galaxies grow with the galaxies that they are part of. There are no small black holes at the middle of large galaxies and there are no big ones in small galaxies. Which regulates the other? No-one is certain but it seems that the black hole regulates the size of the galaxy due to the way that it blows away gasses as it “feeds” thus preventing the creation of new stars.

So, what happens when galaxies merge? We’ve never seen two super-massive black holes so do the black holes merge as well? The calculations that predict this fall down when the black holes are about a parsec apart – the so-called “last parsec problem”

Once they get very close to each other, gravitational waves take over. How do you see a gravitational wave? Whoever solves this problem will earn themselves a Nobel Prize.

While we think that every galaxy has a super-massive black hole at the centre, we’re yet to see one in dwarf galaxies. However, this could just be due to the fact that we haven’t found one yet. In fact it could just be that there is a black hole but it has yet to grow to super-massive size yet.

If you were to drop an object from the Earth to the black hole at the centre of the Milky Way it would take the equivalent of half of the lifetime of the universe so far before it actually fell in. That’s before you even considered how accurately you’d have to drop it – imagine getting a hole in one on a golf course on Earth from the moon.

Black holes are always a fascinating subject if only because even the experts don’t really know much about them. Richard was a good speaker and it was clear why he’d been invited back by the Café Sci team to talk again.

Café Sci will return to The Vat and Fiddle on December the 14th where Kimberley Wade from Warwick University will talk on “The Science of False Memories”

Author’s notes

Talk from 09/11/2015 at The Vat & Fiddle – Richard Alexander: Super-massive Black Holes – Cosmic Giants At The Hearts Of Galaxies

Originally published on the On Nottingham website


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