The European Gaia spacecraft launched about a year ago with the ambitious goal of mapping one billion years in the Milky Way. That’s 1% of all the stars in our entire galaxy, which it will monitor about 70 times over its 5-year mission. If all goes well, we’ll learn an enormous amount about the structure, movements and evolution of the stars in our galaxy. It’ll even find half a million quasars.
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Female Speaker: This episode of Astronomy Cast is brought to you by Swinburne Astronomy Online, the world’s longest running online astronomy degree program. Visit Astronomy.swin.edu.au for more information.
Fraser Cain: Astronomy Cast, Episode 365, Gaia. Welcome to Astronomy Cast, your weekly fact-based journey through the cosmos. We’ll help you understand not only what we know, but how we know what we know.
My name is Fraser Cain, I’m the publisher of Universe Today, and with me is Dr. Pamela Gay, a professor at Southern Illinois University Edwardsville, and the director of Cosmo Quest.
Hey, Pamela. How are you doing?
Pamela Gay: I’m doing well. How are you?
Fraser Cain: Doing great. And just wanted to let people know – we get a lot of questions. I don’t know – we get a lot of questions sent to me specifically, a lot of questions sent to you, I know specifically. And then we get questions sent to Astronomy Cast, and then we get questions everywhere else. And I just wanted to encourage people – like, it’s too many for us to answer. I don’t know about you – too many. We try by email and stuff.
But the best way to send us a question is actually to queue your question up for the show. And the way to do that is you go to the Astronomy Cast page on Google Plus and you’ll see what the next show is. And we always queue up the shows a week in advance. And you can go on that and you can click on the Q&A app, which is available a week in advance, or on the actual just Event page itself. And just post your question there.
And then we actually set aside half an hour of every single show that we record where we just answer questions from the public. And that is the absolute best way. And so we will look through those. And so we often run out of questions – it’s very ironic, we run out of questions for the live show, but yet we have tons and tons of questions that come in by email that we just don’t have time to deal with.
So, that is the best way to kind of get our attention and get in front of us and when we’re in a question answering kind of mood. So, if you are interested in that, just when you want to ask us a question, go to the Astronomy Cast page on Google Plus, see the next show, put your question into the Event page. I promise you I’ll be watching it every week.
So, do you agree, Pamela?
Pamela Gay: Yes. Yes, I do.
Fraser Cain: All right. And also I also highly recommend that you go to the Cosmo Quest community. And so there’s a great forum there, lots of people who will answer your questions about space and astronomy. Some of the best people we know.
Pamela Gay: Yeah.
Fraser Cain: All right, well let’s get cracking with this episode of the show.
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Fraser Cain: So the European Gaia spacecraft launched about a year ago with the ambitious goal of mapping one billion stars in the Milky Way. That’s 1 percent of all the stars in our entire galaxy, which it will monitor about 70 times over its five-year mission. If all goes well, we’ll learn an enormous amount about the structure and movements and evolution of the stars in our galaxy. We’ll even find some quasars and maybe planets.
All right. This is – this is actually I think one of the most exciting spacecraft that’s been launched fairly recently. One percent of all the stars in our galaxy. We will have an accurate map of an enormous amount of this.
So, let’s talk about the Gaia Mission.
Pamela Gay: So, this is in many ways a follow on to a mission we discussed way back in I think the first year that we did this show, and that was the Parkhurst Mission. And with the Parkhurst, they put out a catalogue of nearly a million stars out to a distance of 200 parsecs. So, about roughly 600 light years.
And the goal of that mission was to accurately use the parallax method to get both brightness measurements and distance measurements, which allowed us to calculate the luminosity of these different stars and thus be able to more effectively use things like pulsating variables – the Cepheids, to measure distances even further out in the universe.
But what we realized is first of all there weren’t any Cepheids close enough to get the high, high accuracy that we wanted, which was highly frustrating. And second of all, there were a whole lot of questions that we still couldn’t answer with that small of a sphere around the Earth – just 200 parsecs. With this mission, Gaia’s going to have 1 percent accuracy for 700,000 stars out to a few hundred parsecs. So that’s big.
Then another 20 million stars out to a few kilo parsecs. Then at distances greater than that, out to about 10 kilo parsecs, they’re still going to have 10 percent accuracy. So that’s pretty amazing.
And then on top of it this is the mission that just keeps giving. They’re also doing basic spectral work, so we’re going to be able to get at the surface gravity, the chemical composition, and a myriad of other details of all of the roughly one billion stars that this telescope’s going to look at roughly 70 times each over the course of its existence.
Fraser Cain: And in addition to looking at the stars, it’s going to be looking at quasi-stellar objects. It’s going to be looking at quasars; it’s going to find half a million quasars.
Pamela Gay: And it’s going to look at near Earth objects that go through its field, far Earth objects basically if it’s brighter than the 20th magnitude, which is about as faint as you want to go with a one-meter telescope to get good resolution. If it’s brighter than 20th magnitude, it will see it. So we’re going to be getting a bazillion stars – a billion stars, quasars, asteroids, everything else that goes through its field of view.
Fraser Cain: So let’s talk a bit about, you know you mentioned the Parkhurst and the parallax method, so do you want to give a little bit more information about exactly how it’s going to be doing its science?
Pamela Gay: So, this is where we ask all of you to do a simple, yet a humiliating science demonstration. Hold up your thumb. Close one eye. Move your thumb so it’s blocking some fairly nearby object and compare the location of the nearby object, in my case the webcam, and some far away object. Now, switch eyes and you’ll see that your thumb relative to that nearby object bounces around. And depending on how near that object is, it will bounce even more relative to further away objects.
With cartoons, if you’ve ever noticed, the way they fake motion is they’ll have multiple layers that move across the screen at multiple rates faking the distance of nearby shrubs, further away trees, with further away mountains. This is because objects that are nearby, compared to our two eyes, appear to jump more. Whereas things that are further away require a great deal more motion before they appear to move. Quasars? They just don’t appear to move, they’re too far away.
Now, we know the distance between our eyes, or at least we can figure it out using a mirror and a ruler. We can measure the distance to our thumb. If we wanted to, we could use all of this in the angular jump to start to calculate with basic trigonometry how far away things are.
Fraser Cain: So I could do that and I could actually figure out how far away a tree is, or how far away that car is –
Pamela Gay: Exactly.
Fraser Cain: – just by measuring the angles of my eyes, my thumb, the angle to the car, and how things change.
Pamela Gay: And usually people start using different instruments to do this more accurately, but it’s not hard calculations. And with the planet Earth, it’s not like we can look at a star – close your left eye, close your right eye, and see the star jump. But what we can do is wait 12 months for the planet Earth to go from one extreme side of the sun over to the other extreme side of the sun.
Fraser Cain: I get six months. You said 12 months.
Pamela Gay: Yeah, six months. Twelve months gets you all the way to the beginning, doesn’t it?
Fraser Cain: Yes. Yes.
Pamela Gay: So you wait six months to get to the other side of the sun and make your measurements and nearby stars will appear to jump compared to background galaxies, and quasars are about as background as the galaxy can get.
Fraser Cain: And so you make this one precise measurement of the position of the star compared to a background object. Six months later you make another precise measurement of the position of the star compared to a background object, and you then – that gives you some positions, some angles, and then break out your trigonometry –
Pamela Gay: Calculate, calculate, calculate.
Fraser Cain: Calculate, calculate, calculate. And that tells you how far away it is and even potentially if it’s moving – speed.
Pamela Gay: Now, no. You see there you start to need three measurements. So the problem is if you only make two measurements you don’t know how much of the motion is due to the sucker chugging along in its orbit, and how much of the motion is due strictly to its nearby, therefore it appeared to jump. Now admittedly, unless you’re very unlucky, the direction that it’s chugging through the sky and the direction that it appears to jump due to the Earth’s motion aren’t going to be exactly lined up. But you are going to get unlucky now and then.
So what you actually want to do is start taking multiple measurements. You measure it over multiple years, and you can see December to December, here the 12 months comes into play – oh, it moved that far. That means the star has actual motion in the plane of the sky of however many fractions of an arc second per year. Then due to its distance it appears to jump with that six-month.
Now at the same time there’s one more motion that you have to get at, and that’s the motion along our line of sight. So it could be moving towards us, it could be moving away from us. We can get at that information using a Doppler shift. And this telescope, it’s – calling a telescope is really an understatement.
Gaia is the optical bench that tried to take over science as near as I can tell. It has more CCDs on it than you can shake a stick at, it has multiple mirrors, multiple systems, and this amazing design that has the light coming on, getting mapped out by one instrument, and then sliding across the field and getting imaged and spectrally analyzed as it goes.
Fraser Cain: Yeah. And to think about how they’re going to be imaging a billion stars 70 times. So that means that they’re just going to take an enormous amount of data nonstop. That they don’t just stare at one star at a time, they have to stare at a whole pile of stars all at the same time in different fields, and then move onto the next one, right?
Pamela Gay: Well, and the awesome thing is, the way they’re doing it, it’s not so much that they move onto the next one as they’re constantly moving. The way the system is designed is it has a pair of mirrors that are looking at two different sections of the sky, and the mission – the spacecraft is sitting out at roughly – it’s orbiting around the L2 Lagrange Point. This is the same place that we’re going to put the James Webb Space Telescope if we’re lucky.
And while it’s sitting out there in its position it’s slowly drifting across the sky with a bit of procession in that. And the angles work out just right that as it drifts across the sky, stars that it’s detecting move exactly along the CCD such that they track down the columns as the CCD reads out. So the CCD’s reading out the data at the same rate as the star’s going along, it moves all the way across one field, gets detected by the read-out instrumentation, moves into the next set.
And the way it’s set up is it starts out going across – drift skimming across the SkyMapper CCDs. And these are CCDs that basically go light into light, okay, I’ve got something. And they decide if there’s a star that the rest of the sensors need to look at, and they decide how much of the chips need to get read out in order to make the needed measurements of those stars. Once something has been detected by the SkyMapper it moves on to the astrometric CCD.
These CCDs very precisely measure the centroids of the stars. This is what gets used to make the parallax measurements, gets used to look at all of the different things that can happen down to the level of super Jupiter’s tugging the stars around creating slight deviations over time. Once they’re done with the astrometric CCDs, then they get passed onto two different spectral analyzers, one that looks at the blue, one that looks at the red. And this is to get at the temperature and the composition.
And then finally they get passed off to some radial velocity CCDs, look strictly at the calcium H and K lines, and very, very precisely measure how fast is that star moving towards us or away from us.
Fraser Cain: So I think knowing the position is important, and then knowing the movement, I guess in 3-dimensions we’ll see – we’re going to know whether – how far it’s moving side-to-side, and how far it’s moving forward and back. And that will tell us really where that star is moving in comparison to us in the Milky Way. And so you can imagine them putting together this simulation of all the stars and mapping it forward and backwards and knowing where all these stars are going and how they’re interacting, and it’s mind bending.
So you talked briefly about being able to tell a bit about sort of like what the stars are made of, right?
Pamela Gay: Right. So when we’re trying to figure out a star’s composition, what we look at is the bright and dark patches – stripy bits in this case, that appear in a dispersion of the lights. You take the light from the star, in this case you put it through a prism, and instead of seeing a point, what you see is a streak. And within that rainbow of light there’ll be places that are darker. This is where gases inside the star’s atmosphere are pulling the light out and instead of letting the light continue on its merry way to be observed, are taking that light, absorbing it in as part of a transition.
Now in other places you might see a place where that rainbow is super bright. This is an emission line. It’s someplace where the atoms in the star are radiating light in a specific transition as the electrons jump to a lower energy level. By measuring this fingerprint of bright and dark lines – mostly dark lines – we’re able to get at how much of the different elements are in the atmosphere of the star. The depths of the lines change with temperature, with amount of compositions – you have to look at a lot of different things.
But what’s kind of awesome is this spectra gets you at what it’s made of, what the surface gravity is by how much those lines get broadened out, once you take into account the fact that the star is rotating. So you know, how fast it’s rotating, what’s it made of, and how much gravity is pulling down on the surface of the star.
Fraser Cain: And doesn’t knowing what the star is made of tell you all kinds of stuff about it as well? That can tell you maybe how old it is, that can tell you maybe –
Pamela Gay: It’s doesn’t tell you how old it is, it tells you what generation it is.
Fraser Cain: Right. So, how many populations of supernova, you know how much heavy metallic elements are in the star, and maybe how long – and so that – you know, maybe how long it’s been around. But I understand what you’re saying.
Yeah, and so I mean one of the analogies that I always like with this is that it’s sort of like a polling station. Like up until this point astronomers have done this work in a very manual process with the Parkhurst, and with different instruments and different satellites and different observations, and gotten a real fraction of this number of stars. And that’s told them sort of overall some things they know about the galaxy.
And it’s like you’re watching the first polls come in on an election. You know, oh, it looks my candidate is winning. But there’s only a small number of polls that have actually reported in so far. But then over the course of the night as all the polls come in, then you get a much better understanding.
And so the assumptions that astronomers have made about the composition of the galaxy, about the motions, about, you know what kinds of things – processes are going on in the galaxy, when you take it to this level, you know 1 percent of the entire galaxy, it just tells you so much about – with a very high degree of accuracy – about what’s going on here.
Pamela Gay: And getting at the how this is doing a survey, it goes beyond just the compositions to it. It’s for the first time allowing us to sample periods of stellar evolution that are very brief. To allow us to get at an accurate stellar luminosity function. This is that distribution that tells us that for every one giant OB star there are 500 little tiny red stars. I’m just making up numbers; we don’t know the actual numbers. That’s why we need this mission.
And it also allows us to understand in our region of our galaxy, what stars are on crazy orbits that are plunging through the disc versus well, perhaps part of that cluster of stars that we formed with, or just other every day disc stars. We’re going to be able to get at that consensus of orbital mechanics, of compositions, of – well, baby stars to big stars and everything in between.
And most exciting to me is the fact that we are going to finally be able to examine – because we’re going to have a large enough sample – the hard to observe rapid stages of stellar evolution.
Fraser Cain: Right. So we’re going to see things like the T Tauri stars, and some of the [inaudible] [00:19:28] invariables and some of the things that are in some of those final stages, red giants, younger, hot stars, things like that.
Pamela Gay: And older stars and temporary phases. For instance between when the hydrogen stops getting burned in the core of a star to when you get that flash of a shell burning. There are so many brief transitory phases as stars evolve, and we haven’t well studied pretty much any of them.
Fraser Cain: If we’re really lucky, maybe like a precursor to a supernova.
Pamela Gay: Yeah.
Fraser Cain: That would be amazing.
Pamela Gay: And there are so many different things. And Gaia – the calibration phase for this mission took what felt like forever. But it’s now mostly calibrated, and they’ve actually already detected their first supernova and they’re starting to get science results.
Fraser Cain: So, we talked a little bit about the stars, and that really is the main goal of this mission is what they’re going to be doing for the stars in the Milky Way. But because they’re looking at bright stuff in all directions, there’s just a pile of other science data that they’re going to be gathering. So let’s talk about a few of them.
Pamela Gay: So my favorite rouge science result I didn’t know they were going to get until I started reading up on the mission, is they’re actually going to be able to get more accurate measurements of how the sun gravitationally bends the light background stars as they compare looking over, under, beside the sun – pick a direction – around the sun. And then looking at those same stars with the sun behind the spacecraft and seeing how the astrometry changes whether or not you have gravity from the sun affecting the path of that light. This is going to be very precise confirmation of the theory of relatively, not that it needs to be more precise, right?
Fraser Cain: Yeah. Will Einstein be right?
Pamela Gay: Yes. Yes, he will. But it’s still cool. And the fact that they have to like, start taking that into account. I mean what blew my mind is a lot of this ancillary science that they’re getting; they have to get to fix their astrometry. One of the reasons that they have to do such high resolution – or it’s actually low resolution – but one of the reasons they have to do the low resolution spectral thermometry that they’re doing is their positions are actually sensitive to if the star is more red or more blue. It’s centroid would be in a slightly different place due to chromatic aberration.
So they need to get at the color distribution of the stars so they can correct for the chromatic aberration. So, all of this work to get at the very precise color of the star is also getting at its position more accurately. Getting at the bending caused by the sun is getting at the position more accurately.
And then along the way, they’re going to actually – just as extra science that comes at trying to get at these precise positions, they’re going to be able to see how massive nearby planets – planets that are massive and nearby to the star being observed, are actually able to sway the motion of those stars. And this will give us precise measurements of the inclinations of those orbits that we couldn’t otherwise get.
Fraser Cain: Right. So when we observe planets right now, we get the transit method where the planets pass in front of the star, and we’ve got the radio velocity method where the giant – a heavy planet is yanking its parent star back and forth, forwards and backwards towards us. But are you saying that we might get a chance to be able to see the wobble of those stars in the sky because of the planet – side-to-side?
Pamela Gay: Yes. Yes. The side-to-side, up-to-down motion. What people had looked so hard for in the Bernard Star, which is one of the more nearby stars, it was hoped that if it had planets we’d see it this way. Nope, not there. But this is a new telescope with new instrumentation that’s capable of so much more than we’ve been able to do with previous space-based missions. And this is just work that you can’t do from the ground because our atmosphere distorts the light just far too much.
Fraser Cain: But that’s the Holy Grail because I mean people don’t realize when we find all these planets that we’re discovering, they’re all total flukes that the planet happens to be passing through the disc of the star from our perspective. Or with the radio velocity method that – again that they’re lined up enough that the star is being yanked back and forth. But that’s – when you think about the number of possible configurations of stars and planets from our perspective, that’s a tiny fraction of the actual number of planets, you know.
Patient: [Inaudible]. [00:24:25]
Fraser Cain: What’s it gonna take?
Pamela Gay: By looking just at stars with something like the HARPS spectrograph, which we’ll be talking about I believe in the next episode, we’re able to get at a lot of planets with a variety of different orientations. But what we can’t get at is the planet that goes around and around its star completely in the plane of the sky. So this would be the perfect candidate for trying to image the orbit because it never passes in front of its star, it never gets, well closer to its star than whatever it happens to be at its peri stellar location.
But what we can’t do is find this with a Doppler shift; we can find it with astrometry. So we can finally start to find those with this awesome mission. And for all of those ones that we can find that have catawampus orbits of some sort, by combining the Doppler shift from highly precise instruments like HARPS with the highly precise astrometry, we can now get at the inclination, so we can get the full 3-dimensional solution for the orbits for the first time.
Fraser Cain: Yeah. And so we may – and this again is a prototype; if this methodology seems successful, then you can imagine a future version that is just going after this way of trying to find planets. It’s going to be stunning. And it’s just a side benefit of the mission.
Pamela Gay: And what’s really kind of weird to think about is this is a spacecraft that – it’s name Gaia actually comes from back when they thought that it was going to be the Global Astrometric Interferometer for Astrophysics. It was originally planned as multiple telescopes that were feeding their light together and doing interferometry to get the most precise locations possible. As it went through its design phases, as budgets became realized – this is a billion dollar mission – it no longer stayed an interferometer.
They kept the name Gaia changing the capitalizations, since it was no longer an acronym. But there’s a lot of room to expand what we do with the instrumentation to keep pushing this idea further out and finding systems where smaller planets are creating those observed protobations.
Fraser Cain: Yeah. And so where are we now in the status? I know the mission launched about a year and a half ago?
Pamela Gay: Last fall – Northern Hemisphere –
Fraser Cain: Yeah.
Pamela Gay: Fall, it finished its calibrations and now we’re into the science mode. Like I said, its first supernova has been found and we’re all waiting with nervous energy to start getting those first astrometric results, hopefully of some awesome Cepheids will allow us to know hey, do we have this whole distance ladder thing calibrated vaguely correctly.
What gets me at a fundamental level is this mission has the potential to expand or contract the entire universe in terms of the numbers we put down on pieces of paper by several percent. Because we just don’t have highly accurate measurements of the actual luminosity and distance of a Cepheid, and that’s what the key project used to calibrate distances to the nearest galaxies.
Fraser Cain: And the mission is planned for five years, but assuming there’s enough gyros on board, it can go further than that.
Pamela Gay: We’ll see. It’s always a matter of what the budgets allow. And with it out at the Lagrange point, you have to know ahead of time this sucker can’t be fixed. But it’s out there now and it’s doing awesome science.
Fraser Cain: But is it in a stable – what’s it – it’s the L – to the L2 Lagrange?
Pamela Gay: It’s the L2.
Fraser Cain: Yeah.
Pamela Gay: So this is the one that’s on a line between the sun-Earth-moon system and nothing.
Fraser Cain: Right. So it’s not super stable the way you say the Trojan ones are, like the L4 and the L5?
Pamela Gay: It’s stable enough. Like I said, this is also where we’re going to stick the James Webb space telescope because it’s where the sun doesn’t shine. And it’s just going to sit there doing awesome science.
Fraser Cain: Oh, this is so exciting. Cool. Well, of course we’ll be publishing lots of news about it as the science starts to roll out. But this is going to be one of those sleepers. Just, you know, mark my words; it was sort of like Rosetta. Like remember, we knew Rosetta was going to be huge when most people had no idea that this was all about to happen. So mark my words right now – Gaia. You’re going to hear so much about Gaia.
Pamela Gay: And if you want to marvel at a true fete of optical engineering, no telescope has really impressed me with its design in terms of the simplicity and grace of its optical bench as this one. It’s a really sweet design.
Fraser Cain: Super. Cool
All right. Well, thanks, Pamela. And we’ll continue our coalitions next week. What was it again?
Pamela Gay: HARPS.
Fraser Cain: HARPS. Awesome. All right. We’ll talk to you all next week.
Pamela Gay: Talk to you later.
Fraser Cain: Thanks for listening to Astronomy Cast, a nonprofit resource provided by Astrosphere New Media Association, Fraser Cain, and Dr. Pamela Gay. You can find show notes and transcripts for every episode at astronomycast.com. You can email us at firstname.lastname@example.org, tweet us #astronomycast, like us on Facebook, or circle us on Google Plus.
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Duration: 32 minutes