Mapping the Milky Way

The Gaia spacecraft will soon begin mapping out the 3D positions of the Milky Way's stars
03 December 2013

Interview with 

Gerry Gilmore, Institute of Astronomy, Cambridge


Gerry -   Hi.  I'm Gerry from Cambridge.  I'm one of those very rare astronomers who actually tries to understand things that you can actually go out and look at at night.  There's not many of my colleagues who do this and most of the time, I don't either.  But in particular, we've got a huge project that's coming into fruition right now.  We're trying to understand the origins of the Milky Way.  So, it's the most basic question you can go outside and answer or ask at least.  Every culture that we know of has its own answer called mythology or gods or whatever name you like, at to, how to break the 'what came first' problem.  Here we are, what was there before us?  And so, the challenge is to say, look up at the sky and you say well, I can see what I can see.  But what's really out there?  What's the Milky Way really made of?  How big is it?  How fast is it moving?  What is there out there that's important that doesn't shine, that we can see?  So, how much does it weigh?  A few basic little home-grown questions like, where did the oxygen that you are breathing right now get created and how did it get here from wherever it was formed?  How is the stuff you're made of got to be around so that you can be made of it?

Chris -   Can you give us a few stats on the Milky Way?  In other words, where we are in it, how big it is, how many planets and stars it may have, that kind of thing.

Gerry -   Well, that's really what this project I'm going to talk about, Gaia, is going to answer, but our approximate number at present, the Milky Way is made of roughly 100 billion stars of which the sun is pretty typical.  The sun is about 30,000 light years from the centre of the Milky Way and the far outer reaches of the Milky Way that we still think of as the Milky Way are about a couple of hundred thousand light years from the centre.  So, the whole shows 300 or 400,000 light years across.  It's a big thing.  It's made of a hundred million stars, but mostly, it's made of dark matter, about 90% of the sun is stuff that's not the same as what you're made of.  It's something mysterious that we know nothing about except that we can weigh it.  And we know that a star like the sun which is made of the same sort of chemistry that you're made of is pretty typical.  And so, most of the stars and planets are made of the same sort of chemistry that you are.  But some parts are made of quite different things, really primordial stuff that's left over from the very early stages of the Big Bang.  And so, that also tells us how old the Milky Way is.  The whole universe is about 13 billion years old and it's pretty clear that the first structures that eventually came to form what we now call the Milky Way were already in place a few hundred million years after the very beginnings of the universe.  So of course, it's even older than that.  The hydrogen in the water that you're made of was created in the Big Bang.  And so, in a very real sense, you are a fossil of the Big Bang itself.

Chris -   Do you think the water companies will kick in and try charge us for that? Bills are going up!  So, one question though.  If you've got all these matter which is coalescing into stars in this galaxy, why does it spread out into separate little stars?  Why doesn't it just form one massive blob of matter?

Gerry -   Well, it's because the universe is a big place actually.  The challenge in astronomy is not to stop everything forming one giant thing which will be a giant big black hole.  It's quite the reverse.  It's how to get it in close enough that had actually has stars so that you can see them in the sky.  In the very early universe, the whole universe was very uniform, but it was noisy because of the Big Bang.  And the sound waves, there really were sound waves.  There really was a bang.  The sound waves pushed the matter around and allowed to accumulate and then fall together.  So, good old gravity, the dominant force in the entire universe eventually brings stuff together.  But ordinary matter, ordinary gas and so on doesn't like being in a small place.  It's really hard for it to cool down and collapse into a small place and then form stars.  So, to do that, first of all, you need chemistry to cool the matter.  So, you get this chicken and egg problem.  If I don't have any chemistry to form the first stars, how do I form the first stars?  That's a question that we're thinking about right now.  But then other things do form stars, but the thing that stops it all going crazy is again, the dark matter.  The dark matter dominates the weight and the mass of everything.  And there's so much of it that all the stuff in the universe is whizzing around quite fast.  It's whizzing around so fast that it never gets a chance to accumulate into very large pieces.  Largely because one side of a large blob is moving quite fast relative to the other side and it just shears itself apart.  So, if I try to put all Milky Way together into a single thing, it would rapidly fragment again into what we see.

Chris -   One physicist put it to me that the only reason that dark matter got invented is so you could put the word dark in front of it.  It sounds sexy and you get more grant money.  But there we are.

Gerry -   That's true for dark energy.  It's not true for dark matter.

Chris -   Tell us about actually what this Gaia project is.  How are you trying to map out the Milky Way and understand it?

Gerry -   So, Gaia is the name of the satellite that gets launched on December the 19th about quarter past 9 in the morning.  You'll read about it in the news that day hopefully as a wild success and triumph for 25 years of my life.

Chris -   We noticed we were recording the programme before the launch of Gaia, just in case because otherwise, we wouldn't be able to use Gerry on the show because he would've been talking about the satellite that wasn't.

Gerry -   And otherwise, you'd be hearing sobbing noises, but it's going to work.  It's astonishing technology actually- just blow your socks off.  But the real thing that Gaia is going to do is take the first ever census of the Milky Way.  At present, we can't do that.  We can take pictures of the sky and count the stars, get this 100 billion number.  By eye, you see about 6,000, so that's a teeny fraction of what's really out there.  But we don't know where they are.  We don't know how far away they are.  The hard thing in astronomy is measuring distances.  There's only one way we can do it and that's by using a triangulation technique called parallax that - here's an experiment that all your listeners should be doing right now.  Hold your arm out in front of your face.  Come on audience, hold your arm out.  That's it.  Now close one eye and then close the other eye and you'll see your thumb.  Hold your thumb still and you'll see it apparently jumping from side to side.  Now that's parallax and that tells you- the amount it moves, tells you how far, how long your arm.  All animals have evolved that way so you can find out where the food is, so you can go and catch it.  And that's the same trick we use in astronomy except in astronomy, the distances are rather larger than they are to your nearest dinner.  And so, we need to measure much smaller shifts and that's where we need fancy technology.  And in fact, the scale is such that the distances in the universe are so large that Gaia measures angles who has silly names - nanoradians and things that only a nerd would understand.  But to give you a useful analogy, the accuracy with which Gaia will measure the positions of each star is equivalent to locating a shirt button on the moon.  So, that's equivalent to measuring the thickness of a human hair when you are sitting here in Cambridge and the hair is on somebody's head in Paris.  So, that's the level of precision.  It just blows your socks off, doesn't it?  I mean, it's awesome, this thing.  And Gaia is going to do that for a billion stars.  And so, it's going to provide this 3D map of where these billion stars are.  But it's going to do even better than that because we're going to carry on doing that for 5 or 6 years.  So, we'll see how they're all moving.  So, we not only get a 3D map of where stuff is now, we'll also know how it's moving, so we'll know where it's going to be in the future and we'll know where it came from in the past.  And we're going to do even more than that.  Why stop there?  So, while we're at it, we're going to actually deduce the properties of those stars and work out what chemical elements they're made of.  So, we'll be able to track back the history of the oxygen, the carbon, the nitrogen that you're made of and find out what stars made this stuff and when and where, and how it got to be in our bit of the universe, and how the Milky Way is still forming today.  The Milky Way is still growing.  It's like some people in this room.  Most of us are actually probably either growing or putting on weight. And the Milky Way does it too.  It's getting heavier every day.  It's gobbling up its neighbours, which I hope you're not doing! And so, it gets bigger and heavier, and Gaia will find these.  No matter how cleverly you try and eat your neighbour, you'll always leave a few crumbs around.  Gaia will do the same thing.  At least it'll find the crumbs and the debris of these little satellites that have been gobbled up.  And so, we'll be able to count the things that used to be alive and have now been gobbled and work out just when and where the Milky Way put itself together.

Chris -   Let's take some questions.  So, get your thinking caps on, but in the meantime, what have you got on email there, Ginny?

Ginny -   So, I've got an email from Patrick Monde who says, he's heard about a planet which was inventively named planet X approaching Earth and he wants to know if a new planet did enter our solar system, how would it affect us here?

Gerry -   People spent ages looking for planet X and then they eventually found Pluto and realised they had found plutino X as we heard just before.  And there will be more Pluto-like objects in the far outer solar system.  The way we find them is the way we find anything else.  It's by weighing them.  It's the only way we can find things that we can't see is by weighing them.  And so you say, how can I weigh something if I can't see it?  Well, you didn't say, "Let me tell you about dark matter" but that's a different question.  But the key to the planet X thing is that there are lots and lots of asteroids.  In fact, Gaia will measure very carefully the orbits around the sun of about 40 million asteroids.  And some of these go a long way out.  And so, if there is an extra planet out there then Gaia will notice that all the asteroids coming from that direction in the sky have slightly funny orbits compared to the ones coming from other directions in the sky.  And so, by looking for patterns in the asteroid orbits, we'll be able to tell you what is there or equally, what is not there.

Chris -   That's how a spaceman keeps his trousers up of course, asteroid belt.

Victoria -   Hello.  It's Victoria from Cambridge.  When you talk about the size of the Milky Way, are you talking about it as in like a 2D frisbee or more like a 3D sphere, like how do you picture the Milky Way in terms of size?

Gerry -   All of the above is the answer to that one, aren't I annoying!  The Frisbee picture of the Milky Way is a very good picture of how the stars are distributed.  That is actually very realistic.  We all take it for granted but that's only because we were told that was the answer.  Newton failed to deduce that.  He tried very hard and it was one of his great failures in his life was to try and work out the structure of the Milky Way so he gave up astronomy and moved off to run the Mint.  But about 100 years later, a guy named William Herschel working in the exotic astronomical centre of Slough did actually produce a proper star map of the sky and deduced this Frisbee structure.  And so, the appreciation that the Milky Way is a Frisbee is actually quite modern.  But that's only the stars, the dark stuff, and some of the very oldest stars are actually distributed in a - so, it's not quite a sphere, it's more like a rugby ball shape.  You can tell from my accent I'd go for a rugby balls as an analogy.  So, most of the mass, the real stuff that's out there, reality, is in a big rugby ball shape.  Most of the stars, the uninteresting bits that we can see and you and I are made of, that's in the big Frisbee.

Chris -   You didn't say anything about cricket though, did he?  Who's next?

Jeff -   Jeff from Cambridge.  Can you tell us a bit more how Gaia is actually going to see these things?  Is it a camera?  What is it doing when it's out there for these 5 years, discovering these billions and billions of stars?

Gerry -   Well, there's a famous quote from an American sports coach.  He says, "The best way to find out what there is is look."  And so, that's what we do.  The first simple thing, the only basic step you take at any experiment but especially in astronomy is going to take a picture.  And so, that's what Gaia does.  Gaia is just a gigantic video camera.  There's two telescopes mounted on a big ceramic ring.  It's got two telescopes and these two telescopes feed light onto a gigantic video camera which is the largest video camera ever built.  And so, it's made of CCDs, just like the ones you have in your mobile phone.  Except the ones in your mobile phone have a CCD in them which is about the size of the nail on your little finger.  The Gaia camera is about the size of a large desktop.  It's over a meter long, half a meter wide.  It's got a billion pixels.  So, it's the biggest camera ever built.  This billion pixel camera is just going to be taking pictures.  Doesn't it sound easy?  And beaming the information down to us for 5 or 6 years.  Now, from these pictures, we can measure how bright the star is and where it is.  If we keep doing that for 5 years, we'll see everything moving. And the dominant movement that we see, actually, the second most important movement that we see to be technically correct, is this parallax which is the distance to the star.  So, the second thing we measure is distance to a star.  The first thing we measure, the dominant thing, the most important thing is actually general relativistic light bending by the sun which is a really big effect compared to measuring the distance to a star which is a long way away.

Kyle -   Hello.  It's Kyle from Cambridge.  How long will it take for it to analyse one star?  And before the Big Bang, what was there and how did it get there?

Gerry -   Okay, I think that's two questions.  The first question is that Gaia will be measuring stars at the rate of about 40 million stars per second and it'll carry on doing that for 5 or 6 years.  So, Gaia is going to measure about a billion stars.  It's going to measure each one of them about 100 times.  So, it's going to make 100 billion measurements.  That's a big number.  It's quite interesting to think about that number and you think, how long would it take me to get to 100 billion if I clapped my once a second.  You can work that one out.  You'd be quite old and your hands would be very tired by the time you finished.

Chris -   Gerry, how many hard disks is that?

Gerry -   If the data were compressed into minimal format, it's about 35,000 DVDs.

Chris -   So, how are you going to store it?

Gerry -   Well, we don't put it on DVDs.

Chris -   It would be quite interesting to watch.

Gerry -   It would be, yeah.  Modern technology is just amazing actually.  I have a super computer at home near my office which is going to process all the stuff.  And we have petabyte scale storage already.  But the billion pixel camera is actually equivalent to a high definition film and so, what we're doing is just getting a high definition movie running for 6 years.  So when you say it like that, it's not so bad.  I mean, you'd have a hell of a 4G phone bill, but...

Chris -   Do you ever get tempted while you've got the thing on Earth just to take a few snapshots of things?

Gerry -   We did.

Chris -   What did you take a picture of?  Did you do a selfie with it?

Gerry -   Unfortunately, it's kind of hard to get an image when the thing is that big.  So, they're really dull test images, but we know the camera works.  It's also ultrasensitive, so it's a pretty cool piece.  So basically, all this is video and from that, we have to deduce everything.  But we can, we can deduce distances, we can deduce speeds, we can deduce from the colours, we can deduce the chemistry.  And so, we can just setup a clock and say, which bit of the universe formed when and how did it get where it is today?  With orders of magnitude more accuracy than we could do before and including things like finding planets.  Didier finds planets by measuring how the speed changes, or how the brightness changes.  Gaia is going to find planets by actually watching how the sun moves.

Chris -   Well hopefully, on the 19th of December, everything is going to go well.  So, we thought we'd reveal to you some of the technology is going to be involved in getting it up there.

Ginny -   Yeah.  So, we are going to launch our very own rocket.  We're not going to try and send it into space just yet but it works on the same principles as a real rocket.

Dave -   So, sticking to our high technology theme, the basis of our rocket is a standard lemonade bottle.  I've pre-filled this with one of the fuels which are used in the very high spec rockets, which is hydrogen.  So, the top of this bottle at the moment is filled with hydrogen.  We're getting comments from the front which may be working out what's about to happen next.  So, hydrogen is a very flammable gas and it will burn, releasing a huge amount of energy.  When you heat up a gas, it gets bigger and so, we have a bottle with a load of gas which suddenly got maybe 20 times bigger inside it.  We're going to have taken the lid off at this point, otherwise it wouldget very messy.  There's only a hole at one end so all that gas which is expanding, it can only get out in one direction and if I push you, you actually push me back.  If you push anything, it pushes you back, not necessarily the kind of fight way.  If you lean against the wall, it pushes you back.  Otherwise, you'd fall through the wall.

Ginny -   Or if you're sitting on a wheely-chair and you push someone, you'll move.

Dave -   Indeed.  It's a very, very fundamental piece of physics which Newton worked out.  And so, if the bottle is pushing lots of gas out one way, the gas should be pushing the bottle the other way and we should have an interesting effect.

Ginny -   So, there will be a bit of a bang.  So, we're going to need people to put their fingers in their ears.  Don't do it just yet.  I will tell you when to do it.  Dave is going to open up the bottle and let out the water.  What that'll do is it'll let in some air because hydrogen is quite explosive, but it gets a lot more explosive when you mix it with air.  So, we're going to let in a nice amount of air and I'm going to now put my ear defenders on.  And then we're going to put it into our rocket launcher and Dave is going to light the gases and everyone, put their fingers in their ears now...


Dave -   So, that's exactly the same principle which a space rocket works on.  You burn actually hydrogen and oxygen, the same thing we're burning here.  You liquefy it so you get more in the rocket.  It expands, pushes downwards and the rocket goes upwards.  And it works even in space with nothing else to push against.

Chris -   Anymore questions from you guys for any of our panellists before we finish?

Paul -   It's Paul from Cambridge.  In the earlier discussion about planets, it was mentioned that there are some planets we can't see because it's dark and they're not illuminated.  Is dark matter as simple as that or you make it sound very mysterious?

Gerry -   Dark matter is not as simple as that, no.  Some of it is certainly made up from things we can't see, but are like things we know.  In fact, the original experiments to measure the weight of the galaxy and to deduce dark matter took place just over 100 years ago, and they were deliberately designed to count or deduce the number of very faint stars and planets that must be out there that with the technology at the time, they couldn't find.  Gaia will find lots of these planets that we can't see just by weighing them.  But dark matter is different.  We know from a variety of evidence partly from just weighing things like the Milky Way, weighing galaxies, but also from weighing the universe.  From detailed studies of the sound waves and how they propagate through the early universe, that dark matter can't be made of the same stuff as ordinary matter is.  So, ordinary matter, baryonic matter as we call it, the stuff we are made of, makes up at most a few percent of the total mass in the universe.  We don't know what this other stuff is.  The best guess is that it's a variety of families of elementary particles, sort of new Higgs Boson-y sorts of things.  But it might not be.  It might be our theory of gravity is wrong.  There's lots of possibilities going on.  We just don't know what it is and that's one of the key challenges for Gaia, is to do precision weighing.

Wendy -   Hello.  I'm Wendy from Perth, Australia.  Can you tell me how far away from Earth the Gaia satellite will be positioned and once it's completed its mission in 5 or 6 years time, what will happen to it?

Gerry -   The key thing with Gaia is, it has to be maintained ultra precision and stability.  To measure these tiny angles we're talking about, everything has to be absolutely no varying.  So, it's got no moving parts.  It's got to be kept sufficiently cold and stable that the temperature across the whole thing which is about 3 metres across, changes by less than 1 millionth of 1 degree over 5 years.  The way to do that is to keep well away from the Earth and the moon.  Partly to avoid eclipses, partly to avoid gravity as the Earth and moon change, from shaking you around.  And so, real precision satellites and Gaia will be the fourth of these that's done this, go out to a point 1 ½ million km beyond the Earth which is where the gravity from the sun and the Earth and the moon basically more or less cancel out.  And so, it's a nice stable place.  You're far away from the Earth.  You can step off to one side a bit and avoid eclipses so you get no temperature changes.  The sun is always illuminating your solar panels.  You can always see the Earth to communicate with the Earth, but it's nice and cold and stable.  So, there have been 3 previous satellites, two cosmology ones and one infrared one called Herschel that have used this L2 point.  Gaia will do the same, but this L2 point is not actually stable.  It's only semi-stable.  So, if you leave something there for long enough, it'll come and fall on your head.  And these days,space agencies are responsible.  So, when satellites die, they get thrown into an orbit, such that either you know what that orbit is very accurately or that orbit is so far away that you never have to worry about them coming back to Earth.  So, just about 3 weeks ago, the previous occupant of the spot which was the Planck Microwave Background Satellite got thrown out of this place and is now in its own orbit around the sun.  The same with Herschel and the same with WMap, an American one which was there before that and Gaia will do the same thing.  So, in 5 years time, Gaia will become its own satellite of the solar system.  In fact, will be with these other things, artefacts that survive longer than the Earth does.  So, when we come to the end of the solar system, the Earth will be burned up to a cinder, but anyone who came to look would find Gaia out there.

Chris -   Comforting thought isn't it?  One last question.

Mira -   Hi.  It's Mira from Cambridge.  What theory do you believe in like how the Big Bang theory started and how it created everything?

Gerry -   It's a really interesting question actually because it underlies the whole way one approaches science.  The key idea is not to believe in anything.  My whole approach to doing science and one that I would recommend to anybody is never to say, "Aha!  I think this is the answer.  I wonder if it's true."  The key approach is to look at something and say, "Why did that happen?"  And then work it out.  You should do that for everything.  An amazing number of people had no clue what happens when they turn on the light switch because they've never stopped to think, "Why does that happen?"  This happens because it always happens.  That's not the real answer.  It doesn't happen because it's always happened.  So, if you just keep asking yourself "why is it so?" then one day, we'll get to answer these questions.  Now none of us knows the answer to your question.  It's quite possible that you might answer that question one day.  Someone of your generation is more likely to answer that question than any of us today.  But you'll only do it by keep asking, "why is it so?"

Chris -   Please thank our panels this evening, Didier Queloz, Alan Tunacliffe, Gerry Gilmore.  Thanks also to dangerous Dave and Ginny Smith over on the experimental side.  So, this has been a special episode of the Naked Scientists from the Cambridge Science Centre.  We'll be back doing more of this in and New Year.  I hope you've enjoyed yourself.  I'm Chris Smith.  Thank you very much for listening have a wonderful Christmas and goodbye.


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