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Date: May 15, 2010

Title: Hypervelocity Stars

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Podcaster: The Ordinary Guy from Brains Matter podcast

Organization: Brains Matter – http://www.brainsmatter.com

Description: We all know about stars in the galaxy – but did you know about some of the fastest stars observed? They are called Hypervelocity Stars. I talk to Dr Ross Church and he tells us all what they are, and how they form.

Bio: The Brains Matter podcast has been producing and communicating science stories and interviews since September 2006. The show is based out of Melbourne, Australia, and takes an everyday person’s perspective of science in easy-to-understand language.

Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored by the Physics Department at Eastern Illinois University: “Caring faculty guiding students through teaching and research” at www.eiu.edu/~physics/.

Transcript:

Hello everyone, and welcome to the 25th of May edition of 365 Days of Astronomy. I’m the Ordinary Guy from the Brains Matter podcast, based in Melbourne, Australia. For today’s episode, I talk to Dr Ross Church from Lund University in Sweden about Hypervelocity Stars.

RC: The hypervelocity stars are basically the fastest stars in the galaxy. If you look at the galaxies, you see it forms basically a disc with spiral arms, and we’re some way out along one of the spiral arms in the disc of the galaxy. Then towards the centre, there is a central bulge of stars and then there’s a diffuse halo of older red stars surrounding the galaxy. The hypervelocity stars kind of stand out of this like a sore thumb, because whereas most of the stars in the galaxy, and basically all of the younger bluer stars, are in the disc of the galaxy, the hypervelocity stars are above and below the disc.

OG: So if we think about the galaxy as a plate – if you put a lump of mashed potato on that plate, the hypervelocity stars tend to be somewhere in that mashed potato rather than in the plate itself?

RC: No, they’re much further above – they’re kind of hanging from above, the ceiling of the room.

OG: Okay, wow!

RC: They’ve generally been flung out of the galaxy at very great speeds…

OG: So what kind of speeds are we talking about? In comparison to a normal star is it twice as fast?

RC: So the stars in the galaxy are generally going at maybe a hundred, a couple of hundred kilometres a second. Hypervelocity stars get up to a couple of thousand

OG: Wow! So in the order of ten times as fast…

RC: Yes, yes, I mean they’re really very obviously wrong, if you see what I mean. They’re very obviously different objects to the rest of the stars in the galaxy.

OG: So it’s kind of like watching the cars on the freeway and then a formula one car goes past

RC: Yeah

OG: Except much faster! So do we now what causes these hypervelocity stars to go at such speeds?

RC: Of the things we have that accelerate stars, there are two basic possibilities. When a supernova goes off somewhere in a set of stars, then the resulting object will get a velocity kick, that is it will move off at a faster speed – so that’s one possible way they could be formed. A second possibility is through some sort of interaction with a very massive, dense object – and there’s an obvious candidate for that which is that there’s very good evidence that there’s what’s known as a supermassive black hole at the centre of the galaxy.

OG: So most people assume that the majority of galaxies have a supermassive black hole at their centre.

RC: Yup, that’s probably true. And these things are of the order of millions to billions of times the mass of the sun, the sun being a fairly ordinary, average mass star. The supermassive black hole at the centre of our galaxy is about five million times the mass of the sun.

OG: Okay, wow… don’t get too close to that!

RC: Well indeed! And these things – it is thought that the hypervelocity stars form when they do get too close to the supermassive black hole because what is thought to happen is that you somehow get a binary star – that’s a pair of stars – that have some sort of interaction with the supermassive black hole. The reason you need a binary star for this to work is the very basic physical concept that is the conservation of energy.

OG: So the binary star, I’m assuming, would be rotating around a centre of gravity between the two objects

RC: Yeah, exactly

OG: .. and – this is just me thinking out loud – if they get too close to a supermassive black hole, which has a huge amount of gravitational attraction, then does that mean it flings one of them off?

RC: Yup, so it captures one of them and that one is then very tightly bound. You need a large amount of energy to extract that star back to where it was previously. And that energy goes into the other star and accelerates it such that it is kicked out of the galaxy and forms one of these hypervelocity stars.

OG: So if we had a normal star which wasn’t in a binary, would that mean it would be accelerated but not nearly as much because it doesn’t have that tight bound?

RC: Yes, because you don’t have this – you get quite a complicated interaction which leads to this pair of a tightly bound star and an ejected star. And for a single star it would go rather quickly when it approached close to the black holes, assuming it wasn’t swallowed by it. But it comes back out again and at a given distance, the same speed it came in at.

OG: So conservation of momentum

RC: Exactly, yeah.

OG: How does the conservation of momentum relate to the binary star then? So for people who aren’t aware of the conservation of momentum, the laws of physics basically say that momentum which is mass times velocity doesn’t change over time. It can be transferred, but the total amount doesn’t change. Assuming the two stars in a binary system are equivalent in weight and one gets flung out, does that mean that the one that gets flung out has got half the mass now, so it’s got twice the velocity, or is there something a bit more complicated than that?

RC: It’s a bit trickier to try and think in terms of momentum and where it goes. Partially because the black hole itself has such a large mass, that just a very small change in its speed takes up a lot of momentum. It’s actually easier for once to think about the energy. And effectively the star that remains bound to the black hold in the centre has a very large negative energy, that is, it’s captured very tightly – you’d have to put in a lot of energy to get it out again, whereas the hypervelocity star that is kicked out is a very large positive energy. It’s escaping at a very large speed. What we looked at is some of these stars that are – they’re evicted from the galaxy at very high speeds, but there’s a range of speeds, depending basically on how close the binary gets to the black hole – or rather how close the star that’s captured ends up. So you get a range of speeds of the hypervelocity stars and some of them – they’re still moving exceedingly fast, but the gravitational pull of the whole galaxy is sufficient that they will eventually come back again.

OG: Okay – how long will that take? That’s probably in the order of millions or billions of years though, isn’t it?

RC: It’s in the order of billions of years, the orbital time, yes.

OG: So going back to that analogy we were talking about – the plate and the mashed potato and the room and the ejected hypervelocity star being on the ceiling, that’s a quite a distance away so that given that it’s coming back, is there the likelihood that of the ejected star being attracted by another galaxy for example?

RC: No – because although it’s a very large distance away compared to the size of the galaxy, the space in between galaxies is in general rather large, there are some local galactic companions, the Magellenic Clouds, and the Andromeda Galaxy, but the chances are that it would remain bound to our galaxy.

OG: When it comes back, will it be going at the speeds it was ejected at as well?

RC: Yes, basically so because – and again that’s pretty much the conservation of energy that it’s gravitational energy depends almost only on the distance away it is. There’s a bit about what angle it’s moving at, but, its gravitational energy is basically just on the distance from the centre of the galaxy. So at a given distance away it will move at the same speed. So it will – currently zooming away at a great speed, they’ll slow down towards the furthest away point of their orbit and then they’ll come back again accelerating back towards this very high speed as they come back to the centre of the galaxy again.

OG: At the moment we’re talking about ejected stars or ejected hypervelocity stars that tend to go – roughly speaking perpendicular to the orbit of the galaxy. Are there ones that eject along the plane of the galaxy, so likely to hit other stars or…

RC: Yes, there is no particular reason why they should go in any particular direction. The main reason we see them above the plane of the galaxy is that that’s where they’re easy to find, because there aren’t any other blue stars above the plane of the galaxy, and there are plenty of blue stars in the disc of the galaxy – and also there’s all sorts of other stuff in the disc of the galaxy – big clouds of gas and dust and lots and lots of other stars which make it very hard to …

OG: … to observe them.

Well I hope you enjoyed listening to that discussion with Dr Church. For the full interview, go check out www.brainsmatter.com. I’ll leave you with a quote from Thomas Edison – “We don’t know a millionth of one percent about anything.”

Bye for now.

End of podcast:

365 Days of Astronomy
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