Going for Gaia

20 September 2016

Interview with

Professor Gerry Gilmore, The University of Camrbidge

Gaia's five year mission is to map out the millions of stars in our Milky Way galaxyVST Snaps Gaia en Route to a Billion Stars and it's being heralded as a revolution in astronomy, but why? Cambridge scientist Gerry Gilmore first pitched this idea to the European Space Agency, and he's been shepherding the project along over its lifetime.

Gerry - OK. The Milky Way is the name of the band of stars one sees across the sky on a dark night. It's a rather narrow band; it's very long and rather thin and this is the fundamental information about the structure of the Milky Way, the galaxy in which we live. It's flatish, but we know from looking at other spiral galaxies that it is a spiral so it has a big bar in the middle and maybe two, maybe three, maybe four spiral arms coming off from that. And there's a huge supermassive black hole at the very centre and a very large number of stars are densely packed around that supermassive black hole.

Chris - So the central black hole provides a source of gravitational attraction that holds our galaxy and those stars together?

Gerry - No, no. Not at all. No, the massive black hole is pretty massive but, actually, compared to the hole of the Milky Way it's an insignificant little thing. So the Milky Way is made up of maybe a hundred billion stars like our own Sun and they make up most of the mass in the disc of the Milky Way itself, in the actual plane. But the galaxy itself is maybe a hundred times more massive than that again and that is made up of the famous dark matter. So it's that mass that's distributed through the Milky Way that holds it all together and that, of course, was one of the reasons for Gaia is to try and weigh that mass and find out where it is and, hopefully, determine where it's come from.

Chris - I was just going to say, if you know all this about the Milky Way already, what are you seeking to learn from this Gaia mission?

Gerry - Well what I told you was a bit top level maybe, a bit vague; we'd like to put some numbers on things and be a bit more specific. We know that from studies from many, many wavelengths, many telescopes that Milky Way, and so far as we know what it is, is probably just a dead average galaxy just like one of the one hundred billion other Milky Way-like galaxies in the whole universe. And so the real bottom line ambition for the Gaia mission was to set our own galaxy up as a rosetta stone. Let's do something that we actually know in detail, we can put real hard numbers on it, we can really work it out. Everything that we can measure about it and then we can do differential tests for everything else across the history of the universe and watch where these things came from and even where they're going to turn into.

Chris - And what exactly are you measuring?

Gerry - The key parameter for Gaia is where the stars are. You'd think the history of astronomy -  people have been trying to measure where the stars are since the Mesopotamians. Actually, astronomers are not very good at this sort of thing and so, in the whole history of science up until last weekend, we had reasonably accurate measurements of the positions and distances from us of about maybe a hundred thousand stars and how those stars were moving. Now out of a hundred billion that's pretty pitiful really and they're all very local.

The reason for that is that space is big so it's really hard to measure the distances of stars. You need exquisite precision which Gaia provides. So Gaia is the first ever exquisite measuring system which is designed to measure not only where stars are. A large sample one per cent of them all - a billion or so but, also, how those stars are moving. So we get three dimensions in space of where the stars are and then three little speeds for how they're moving in each of those three directions.

Chris - Why does that matter knowing how they move?

Gerry - The key information from motions is, in fact, being able to weigh things, if you know where something is and how fast it's moving, you know how much force, how much weight there is holding it where it is. If it's moving fast you need more weight than if it's moving slowly. And so that simple measurement (it's not simple at all); in principle straightforward measurement, if you can do it with enough stars in enough different places will tell us how the dark matter is distributed and, hopefully, give us the first clues as to what the stuff actually is.

But there's another even more fundamental thing about the way stars are moving, is that many stars are relatively and from where they're moving now we can track them back in time. If we can measure it accurately and find out where they came from. You can't do that for very far back just because of chaotic things and spiral arms sloshing us about and so on.

But one of the really interesting questions we'd like to know is where are all the Sun's brothers and sisters? The average star, like the Sun, forms in groups of tens of thousands of other stars so, somewhere, there's ten thousand siblings. Do they have earth's? It would be kind of nice to know.

Chris - And you can find that?

Gerry - We can get towards finding that, yes. Whether we can actually answer all these questions, we won't know until we get the data but that's the sort of question that people are trying to ask. So, for example, about four hours after we published the data on Wednesday, the first science result came out from the United States team studying planets in which the used Gaia data to very substantially improve our knowledge of all the planets that have been found up till now.

They did this by using Gaia data to know the distances and, therefore the true brightnesses of these stars and from that you can calculate the radius of the star. And then when a planet goes across the front of the star, if you know the size of the star and how long the planet is in front of it, you can calculate the size of the planet.  And, therefore, the density of the planet work out is it rocky, is it gaseous, what is it, has it got an atmosphere? So that really fundamental result was improved by an accuracy of five time improvement on Wednesday afternoon. And that's just one of thousands of things that will come out of Gaia.

Chris - Now some people are also saying that this offers us an opportunity to do things like test Einstein because we can look at general relativity, for example and we can also look at how the galaxy is changing, how it's evolving, how it's growing or what it's past was because we can look back in time by looking across the galaxy and how things are moving.

Gerry - Yes, the range of science goals for Gaia is enormous; it goes everything from studying the structure of the very early universe, right through to fundamental physics down to finding out whether there are potential killer asteroids lurking up in the afternoon sun. And through that range, pretty well the whole of basic astrophysics will be fundamentally rewritten and produce a lot more significant numbers with Gaia.

Chris - How does the data get from the spacecraft back to the Earth?

Gerry - Well the spacecraft is a really simple thing - that's why it's so accurate. It's a billion pixel camera and so it's basically sending down a high definition movie continuously down to the ground, and those images are what we turn into the positions and brightnesses of the stars. And because the camera keeps observing all the time we can not only see where things are moving slowly, but also we can find things that weren't there yesterday.

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