Gaia – Teamwork For A Billion StarsOn September 12, 2019 by Raul Dinwiddie
K. Jäger: Hello everybody here from Leiden in the Netherlands. Well, Leiden is a town which has long tradition regarding astronomy and space science because of the institutes here for example. But this week in November we have additionally a very special event. We have a Gaia meeting here, a DPAC meeting about the Gaia mission. Stefan, what is so important here for this meeting here with so many scientists? S. Jordan: Here we have two hundred astronomers and engineers discussing how to process and analyse the data from the Gaia satellite. And the Gaia mission is a very important mission to measure the position, the distances and the movements of stars.
K. Jäger: And lots of many things more for very very different kinds of science in astronomy. But let’s start
just at the beginning with a little bit overview: What is the Gaia mission and what is so important about that mission? Gaia lifted off on 19 December 2013 from the launch site of the European Space Agency in French Guayana in South America. The satellites destination was a region around the Lagrange point L2 at a distance of 1.5 million kilometers from the Earth. At L2 a body follows Earth’s orbit around the Sun almost free of other forces due to the balance between the gravity of the Sun and the Earth at this region in space. Undisturbed by our home planet and the moon and equipped with a huge 10 meter diameter sunshield Gaia is able to observe any part of the sky which lies at least 45 degrees away from the Sun. And because Gaia follows the earth on its orbit this means that all parts of the sky can be observed within half year. Gaia gathers data through two identical reflecting telescopes which simultaneously observe two regions of the sky 107 degrees apart. As Gaia rotates once every six hours consecutive strips of the sky are scanned in order to precisely measure the positions of celestial objects. The primary mirror of each telescope measures 145 by 50 centimeters and focuses the light of the stars onto more than 100 CCD detectors with a total of one billion pixels. Besides position measurements Gaia also records the colors and spectra of its targets. The main objective of the Gaia mission is the high precision mapping of more than one billion stars in the Milky Way over a period of five years leading an unprecedented catalogue of a representative subset of the stellar population of our home galaxy. At the end of the mission this huge catalog will contain the three-dimensional positions and motions of the stars in space including their distances from the Earth. In addition to these astrometric data Gaia’s catalog also contain many of the physical properties of the stars derived from its onboard spectrometers. All this information will a allow very detailed exploration of the structure and dynamics of the Milky Way and a census of the properties of its stellar components. A key question of the Gaia mission is the study of the evolution of our galaxy over timescales of billions of years. Galaxies like our Milky Way undergo dramatic dynamic changes due – for example – to gravitational interactions or even merging with other mostly smaller galaxies. Tracers of such dramatic events are stellar streams orbiting or even crossing our Milky Way. Streams that can be investigated in great detail only by Gaia. And Gaia can also discover new stellar streams. The motions of the stars and galaxies are determined not only by the gravity of familiar object like stars or interstellar clouds but more significantly by the mysterious dark matter. The precise Gaia measurements of the distribution and motions of the stars will shed light on how the Milky Way gets the structure we see today and how its dark matter is distributed. However, among the billions of objects which Gaia observes are not only stars. The huge amount of data will reveal galaxies and other objects far beyond the Milky Way, such as Quasars. It would also detect many planets around other stars due to the tiny but measurable effect they have on the apparent motion of their host. Finally, Gaia will also study and discover thousands of small bodies in our own solar system such as comets and asteroids. Good to know, since some of them might be candidates for future impacts on Earth. Without any question Gaia will significantly expand our knowledge of the universe…
K.Jäger: Yes, that’s true and that is the reason why so many experts came now Leiden to this DPAC meeting to discuss the first Gaia measurements and to prepare the first official data release which can be expected in September 2016. But we may hear a bit more about the Gaia data from Stefan’s first
S. Jordan: Dr. Timo Prusti from ESA. Timo Prusti is the project scientist of the Gaia mission. Timo, what is the role of the project scientist of Gaia.
T. Prusti: It’s very easy to say, because it’s very shortly a formal responsibility of the scientific output of a mission. So the definition is very clear. But what does that mean in practice is more difficult to explain because in principle it is acting in all areas where one sees that it has some scientific importance and especially if one gets worried that the scientific outcome is – maybe – in danger. S. Jordan: Why has ESA decided to build the Gaia satellite.
T. Prusti: Actually it is not ESA who is deciding it. Because who decides in ESA the science program are the member states. And based on what the member states do this definition is that we use in ESA advisory bodies. So there is first a community which explains why we want to have Gaia and then there are other astronomers and scientists who are evaluating. Does it make sense? And gets a high recommendation. And only after that sort of ESA decides. But the vote is done by our member states. S. Jordan: Gaia has now performed one year of scientific measurements. How well is the Gaia satellite doing? T. Prusti: This is a question coming very often and it is sort of funny that this comes even from professional astronomers who know that the basic thing what we measure with Gaia are the parallaxes. And we need a lot of time to get the parallaxes. But somehow people forget this every now and then. So we can see only partial result at the moment. But what we have seen so far looks very good. S. Jordan: It is not a very long time to the first release of Gaia data. In summer 2016 it is planned to have the first catalogue. What will the first catalogue for Gaia contain.
T. Prusti: So we have just made a decision what are the production runs that we are going to do for the first release. So the decision is that we will do the positions of about one billion stars and get also their broadband magnitudes. But we also managed to get some parallaxes with a trick to use the Tycho catalog in order to get the degeneracy between proper motions and parallaxes solved. And finally we are also looking for some variability data of selected stars. All these results, we have to first produce it and then validate it. I’m afraid, the final decision much later.
S. Jordan: And how long will the full Gaia mission last?
T. Prusti: Here I have some good news. Because all the mission lengths are always dependent on consumables. Of course, you never know in space what will happen. But the consumables you know. If you are running out of it then it’s the end. And the consumable which is most critical in Gaia is called gas in the micro propulsion system. That is something what we need for the high accuracy astrometry. And we seem to have plenty. So the originally planned one-year extension, I dare to say that as far as that aspect is concerned at least four more years after the five years is possible.
S. Jordan: We will now have the first data release in 2016. What will be the further data releases contain additionally to the data from 2016.
T. Prusti: So we have the photometer on board and we have the spectrometer on board. So what one can really expect is the photometric part. So that we can actually tell what the stars look like. The spectroscopy part is really to get the radial velocities out. So those are something which are eagerly waited. But beyond then when we have more data we are doing further processing in order to be able to provide the community: astrophysical parameters, more variability information. So the whole spectrum will come in the course of years.
S. Jordan: What do you think is the most interesting aspect of the Gaia mission? What does fascinates you about Gaia?
T. Prusti: So if I leave a little the astronomy apart, because that’s of course the basic driver in a way to get the research done and the the information. If I really look just for my work, I think the nice thing is that even though its many many years in the same project there’s a big change like at the beginning, my work was really dominated working together with engineers trying them to understand the scientists but also me trying to understand the engineers. So that I can explain the astronomers, why we are doing certain things. And now we are in a totally new phase. Now it’s dominated by data processing, scientists. So it is the variability of the work environment which I really enjoy.
S. Jordan: Thank you very much, Timo, for this interview. Timo Prusti, the project scientist of the Gaia mission.
T. Prusti: Thank you. It was a pleasure. K. Jäger: Many thanks also from here. Well, after this interesting information we should have a closer look on one important keyword: The parallax. Well, if I say one important keyword this is almost a bit of an understatement. Regarding Gaia one might even say with good reason: It’s the most important keyword. The Milky Way – like millions of other galaxies – consists of about 200 billion stars, many star clusters, and clouds of gas and dust from which new stars and planets are born. The rotating, disc-shaped structure measures about 100,000 light years in diameter and is a typical example of a large spiral galaxy. To investigate its structure and dynamics we need to know the distances and motions of as many stars as possible – and we need such data up to the largest distances possible. The key to this is the measurement of accurate stellar parallaxes. Parallaxes are familiar to each of us from everyday life. We are able to estimate distances to nearby objects because each of our eyes sees an object from a slightly different perspective. We also notice such a perspective shift depending on the distance to an object when we move, for example in a train through the countryside. Thus as the Earth moves around the Sun – by about 300 million kilometers in half a year – we should observe such a parallax effect also for stars. But due to the large distances of even the nearest stars, these perspective shifts are extremely small, making this annual parallax very, very difficult to measure. After many attempts and premature claims, it took until 1838 before the German astronomer Friedrich Wilhelm Bessel finally announced the first reliable reliable stellar parallax. He deduced a distance of 10.4 light-years to the star number 61 in the constellation Cygnus – this is more than 10000 billion kilometers. Even with modern ground-based telescopes it is difficult to measure parallaxes of stars more than about 100 light years away. Going beyond this requires observing from the much stabler environment in in outer space. Therefore, a major step forward in distance measurement accuracy occurred in 1989 with the launch of the astrometric satellite Hipparcos, which measured very very accurate parallaxes of over 100,000 stars. Gaia will extend this much further, by observing a billion stars with even higher precision and out to much larger distances. Gaia’s best astrometric measurements can reach an accuracy of about 20 micro arc seconds. This is roughly equivalent to measuring the motion across a coin at the distance of the Moon – as seen from Earth. This incredible precision is at the limit of what is technically possible. For this purpose the internal geometry of Gaia’s instruments have to be under control with an accuracy of a few dozen atoms – continuously over the five years of observing. Gaia is a marvel of technology and of observational astronomy. K. Jäger: No question!To measure parallaxes and many other properties of celestial objects with the precision of Gaia is really a challenge. And I think we will hear now a bit more about this challenge, Stefan. S. Jordan: Yes, we now have an interview with Dr. Uli Bastian from the Center of Astronomy of the University of Heidelberg. Uli, you are the head of the so-called coordination unit 3 for core processing. Can you please explain what core processing means?
U. Bastian: Well, core processing is that part of the Gaia data reduction, that – it’s a complex part – going from the raw telemetry – the bits that are send down by the satellite – processing them to a complex, complicated and long chain of steps, going to two main products: The first is the Gaia source list, the list of stars and other astronomical objects actually seen by Gaia, which is complicated because Gaia observes the sky blindly and records everything that gets in front of its lenses, so to speak. And the second is to calibrate Gaia astrometrically, to turn the raw measurements into actual star positions, motions, and distances.
S. Jordan: Gaia is a satellite. Why do we need a satellite to measure the positions of the stars? Why can’t we use just ground-based telescopes to perform these measurements?
U. Bastian: In fact can also use ground-based measurements. Astronomers have recorded and measured star positions from the ground for almost three thousand years. But, it is not possible to do that to the precision that is desired by present-day astronomers. And it is the precision which is the point that makes people go out into the quiet space environment, where a lot’s of disturbing effects on the ground are absent.
S. Jordan: And why do we need this high precision in order to measure for instance the distance of the stars.
U. Bastian: That is because both the distances of the stars as well as their motions through our Milky Way express themselves in very very tiny time variations of their positions on the sky. And in order to measure these very tiny variations that we are after we have to measure the positions today and tomorrow and next year very very precisely.
S. Jordan: And what other major steps that you have to do for analyzing and processing the data in order to get a final catalog out of these data?
U. Bastian: Oh well, there are about a thousand or so of major steps. But to summarize these many major steps you could say: First of all calibrate Gaia. Calibrate the instrument, the optics, the detectors. Check everything, validate what you have seen and what you have derived. And calibrate again, and again, and again. Mind you, to get to the precision that we want we need in – in a way – to determine the relative locations of the Gaia mirrors – 1.5 meters big – and the CCD chips on which the light is recorded, to the order of a few atom diameters. S. Jordan: You and the astrometry team of Gaia are already now in the process of producing the first catalogue for Gaia. Are you happy with the first year of measurements that we have now for Gaia and what is the approximately the precision that we can expect from the first Gaia catalogue.
U. Bastian: Well, by and large we are happy. The are a number of unexpected problems with the data which caused a lot of complications and extra work – not a time delay yet. The initial position that we get now will be about a tenth of the final one, or in other words, the uncertainty of the star positions and motions and distances in this first release will be of the order of about 10 times as big as in the final catalog to be produced from the full five or six years of mission and with even much more work. S. Jordan: The first Gaia catalogue is expected to come in the summer of 2016. But when will the final catalogue come, and what what precision position will be expect for the final catalog.
U. Bastian: The final catalogue should be ready about three years after the end of the mission. Now, the mission started in 2014, plus five years is 2019 maybe one year more if ESA funds and if Gaia functions long enough. So 2023 or 2024 will be the date of the final catalog. And the precision we are aiming at, is of the order of – well that’s not easy number for a layman, for a non astronomer – of the order of 20 to 30 micro seconds. That is 20 to 30 millionth of an arc second. An arc second again in turn is the 2000th of the of the diameter of the seeming diameter of the moon in the sky. In other words it is the size of a coin as seen from the distance of the Moon that we are aiming at. So we want to distinguish whether a light beam comes from the left rim or from the right rim of an euro coin based on the moon and seen from the earth. That is at the at the brightness of the stars that are most important for the Gaia science. And at the very faint and the faintest stars recorded by Gaia it’s about a fifteenth of this precision.
S. Jordan: This is an incredibly high precision that we aim for. Do you think, after seeing the first Gaia data, that this ambitious goal can be reached?
U. Bastian: By and large yes. These little problems and surprises that I mentioned they give some restrictions at the faint end, which are already, so to speak counted in, in what I said. But at again at the intermediate magnitudes, the intermediate brightness of these stars which are most important for the Gaia science, we will reach it to at least a few percent.
S. Jordan: Speaking of science and astronomy. So, if we reach the precision that the Gaia catalogue is aiming for, what will we learn about astronomy and what will be the impact of Gaia for it.
U. Bastian: The main impact will be on the Milky Way. This is the stellar system of a few hundred billion stars to which our Sun belongs as one member. The workings, the structure, the history, the forces inside the system derived from the motions and from the then existing three-dimensional knowledge of its structure in space and in velocity of the stars. This is the one thing. And the other main thing is knowing that distances of the stars. We then we’ll know for many types of stars how big they really are, how bright they really are, and this will lead to a deepened and physical understanding of how the stars work internally. But that is the two major point. Even more interesting kind of about Gaia is, that the Gaia data will shine new light into practically every corner of astronomy. And astronomy is a wide field. S. Jordan: Yeah, that sounds very fascinating! What is it what fascinates you most about the Gaia mission? U. Bastian: The variety of things, that you are facing, that you are challenged with, and the variety, the width the broadness of the science aspects that will in the end be able to be attacked using the Gaia results.
S. Jordan: Thank you very much, Uli, for this interview!
U. Bastian: You are welcome! K. Jäger: We now have heard several times that almost every field of astronomy benefits from the Gaia data and here are some further statements to highlight this fact: E. Antiche: I am currently working with the validation team – specifically with the validation team that is devoted to verify the consistency of the catalogue. We are doing sanity checks and are making sure that all the parameters that are inside the catalogue are correct. This is a challenge because there is no other mission like Gaia, and there is no other mission with that precision that Gaia will have. P. Tanga: It is quite impressive to think that Gaia was built for studying the stars in the Galaxy. But in fact it is a very precious tool to study the solar system, too. Because Gaia, as it observes systematically the whole sky, will also see, of course, all these asteroids. And for all of them Gaia would be able to provide the magnitudes, so the brightness, information that are useful to derive the shape, the colors for the composition of those objects. We will determine precise orbits so much better, trajectories in space, and all of these data will be collected in a homogeneous way for five years. Nothing like that existed before Gaia. Gaia will also detect asteroids that are considered to be potentially dangerous for the Earth. So Earth crossers that come closer to the Earth and few of them could be in a future on a trajectory that brings them close to the Earth. S. Klioner: The measurement principles of Gaia are really very similar from the physical point of view to this first or second classical test of general relativity, namely the light deflection tests measured for the first time in 1919 by Eddington and his team. So we measure the light deflection in the optical. Light reflection of sources. But Gaia so sensitive that we see these effects all over the sky. Not only for sources very close to the Sun as it was 1919, but also sources which are almost 180 degrees away from the Sun. D. Pourbaix: Binaries are very important because they are the only way we can measure the weight of the stars. So it is really the scale – based on the Keplerian third law – that gives you a direct relation between the separation of the of the two stars and the masses of the components. So with Gaia we will get an instrument that will give us a lot of binaries of different types: Resolved binaries where you see the two components. Astrometric binaries where you see just a wobble of one object, one photocenter with respect to the usual single-star solution. But we also have spectroscopic binaries where we see that the radial velocity is changing and not just a constant as it should. And we also have eclipsing binaries where we see that the light curve shows special features, typically from eclipses. L. Eyer: The point is, that if you look at the catalogue of variable objects, you have maybe hundreds of groups and subgroups. And Gaia, thanks to the performance in its photometry, will be able to detect most of these known phenomena, and we hope that will detect also other unknown phenomena. There are some groups of variable stars which allow to measure the distance – thats called the standard candles – and the most famous ones are the Cepheids which were discovered at the beginning of 20th century. There is a remarkable relation between the luminosity of the star and the period. The longer the period the more luminous is the object. And then it is very easy to determine the distance of these objects because by measuring the period of pulsation of this object which is independent of the distance, you can infer – because they are calibrated – the luminosity. And you measure the flux here on Earth and you determine the distance of these objects. And the Cepheids are the first step to calibrate the distance scale in the universe. There is, however, a problem: In order to have a good zero point of this calibration, you need to know what are the distance of the nearest Cepheids. And so HIPPARCOS provided some distances, but Cepheids are quite far away. So Gaia will be able really to pinpoint these Cepheids and calibrate extremely well the distance and the period-luminosity relation of Cepheids. There are other standard candles and there again Gaia will provide enormous contribution to that field. The supernova explosion, so Gaia found already, there is a group called “Gaia Science Alerts System”, it is coordinated by the University of Cambridge. And they discovered already hundreds of Supernovae. K. Jäger: So, there are reasons enough that scientists have to discuss about the Gaia data and about the science with Gaia. And one opportunity is such a meeting like here in Leiden. Its a DPAC meeting. And I think, Stefan has now another interview partner who might explain us what DPAC is. S. Jordan: Dr. Anthony Brown from the Leiden observatory is with us now. Dr. Brown is the head of DPAC. Anthony, can you please tell us what DPAC means and what DPAC is? A. Brown: Well, let me just first give you the meaning of the acronym DPAC: It stands for Data Processing and Analysis Consortium. And Gaia, of course, sends down a lot of data every data, and a lots of bits and bytes, that we receive and extend on to our data processing centers. But these data are not useful as such. The raw form of the data cannot be interpreted directly by astronomers or by anyone for that matter, and they need to be turned into the eventual science products. The distance to the stars, their properties etc. And this is the work that DPAC does. DPAC takes the raw information from Gaia, processes that through a complicated big software system, involving lots of data processing centers and lots of people, and turns that eventually into the results that scientists can then actually interpret and use and also the general public of course.
S. Jordan: How many members does DPAC have and from how many countries do the members come?
A. Brown: There are about 25 countries that participate in the DPAC effort. And from those countries there are about 450 members which consist of a mix of people from academic institutions such as myself, but also people working at the Space Agencies who are working at one of these dedicated data processing centers. A lot of them are actually software engineers from a non astronomical background, but usually with a strong interest in running such astronomy projects. S. Jordan: 200 members of DPAC are here now for this conference in Leiden. Why do they meet here and why don’t they use the usual communication techniques like video conferences, email, and so on? Why do you personally have to meet here?
A. Brown: So they, of course, in the course of their work meet very regularly by videocon, they communicate with email, through all kinds of systems that we have to keep track of issues with our processing , but from time to time you need to sit together to talk face-to-face. Communications over email work but they can break down because of the way people might misinterpret how things are said, misunderstanding can easily occur even if you think you’ve written down something carefully. And with direct communication your really pick up all other things that are important when you’re talking to someone, which is body language, so that he not understands or she not understands something, which is something you miss when you’re doing a telecon or communicate by email. And the other important thing it simply getting together, also socializing after meeting, getting to know each other, having a beer together, having a dinner together, talking about other things. And that creates a better team feeling for DPAC. So I think team building is an important part of this meeting. Now we are here together with two hundred people which, is of course a lot – about half the consortium. We usually have smaller meetings of the various individual units within DPAC, and we thought it would be good to bring everyone together at a big meeting, because then you can also really discuss all the issues that are between the units, all the interface problems that we usually have, which you don’t cover very well always meet with your own unit. So this is a very important aspect of this meeting. S. Jordan: As you said, the data processing of Gaia has so many aspects so that DPAC is divided into units. Can you explain us what kind of coordination units are there in Gaia and what different aspects are covered by the data processing in the Gaia mission? A. Brown: Gaia has three instruments on board, one instrument dedicated to astrometry, so collecting distances and motions of stars, one instrument dedicating to measuring colors and also the properties of stars, such as their age and their chemical composition. And we have one instrument which is collecting high-resolution spectra and these are dedicated to measuring the radial velocity of the stars and also, actually, to look at their atmospheric properties. Now all these three instruments are in fact treated by three different coordination units, one dedicated to astrometry, one dedicated to photometry, and one to spectroscopy. So that is a natural sort of division of the tasks. But also what needs to be done, for example, is turning the raw telemetry into data that is actually useful for the subsequent processing. This is another unit within DPAC, which is responsible for that. And then once we have the astrometry, the colours of stars, as well as the results from the spectra, we do further analysis, we look, for example for variable stars by looking at the multiple observations of the stars. So, this is another unit. There is a unit also, that is in charge of doing all the characterisations of the stars based on their colours. What are their properties? What are their ages? What are their chemical compositions? And we have a unit that looks at the more complex objects, such as binary stars, planets. We also observe lots of galaxies, which are slightly extended, requiring a little bit of different treatment. And again that is a separate unit. And, of course, lets not forget the solar-system objects that Gaia also looks at. And again that requires a slightly separate treatment. And that’s why we have subdivided the huge task of this data processing into these various units to make it manageable. S. Jordan: We have now more than one year of scientific measurements from the Gaia satellite. How happy are you with the quality of the data and – very important for the DPAC head – how efficient is the process of communicating between these different units and make the best out of the Gaia data? A. Brown: So I think the Gaia data quality itself is actually excellent. There is a lot of potential in there to do precisely the science that we
want to do and to deliver the accuracies that were promised before the mission. That’s why I am very happy. Of course there’s a lot of issues with the data, lots of little problems that we have to deal with, but this is not unexpected. But I think by and large the quality of the data and the instruments themselves are really very very good. Now the work of DPAC I think so far so good. We’ve managed really quite well, given the enormous amount of data that we have to process, the various spacecraft problems that we’ve had to deal with. But of course there are always coordination problems, there are always issues between people, misunderstandings, miscommunications etc., that can always be improved. But it is also exactly the reason for us to get together at a meeting like this, to iron out all these little problems that we have. But I think in general, for such a large and spread out consortium, we are doing actually quite well.
S. Jordan: So the Gaia mission is a very good teamwork. Anthony, what is what fascinates you most about a the Gaia mission? What is the reason why you like to work on Gaia? A. Brown: The reason is that I actually find it very interesting the way data is collected and then turned into measured distances, and motions of stars. And it is in fact the only direct way we have of doing so. But what is also fascinating is that one is leaving a real legacy for astronomy. So these are observations that are going to be the standard in astronomy for the next 50 years and will be valid in a hundred years time. We are still, in fact, using astrometric measurements from a hundred years ago, and the Gaia measurements themselves will still be valid also a hundred years from now and still be used by people. So leaving such a fundamental legacy is also one of the really fascinating aspects working on this mission. S. Jordan: Thank you very much, Anthony, for this interview. Anthony Brown, the head of DPAC. K. Jäger: Thank you. Let’s finally talk about other measurements with Gaia. Besides the determination of positions and motions of stars in space, from which distances can be calculated, Gaia gains also photometric and spectroscopic data. So photometry means that we measure the brightness of objects as a function of colour, and the amount and the colour of the light of the stars tells us a lot about their physical properties. For example about the surface temperature of a star, or if a star is variable or not. Spectroscopy means that the light of a source is dispersed and analysed in detail. Depending on the wavelength, for example by using a prism. So, a rainbow, for example, is a spectrum with extremely low resolution, due to the raindrops in the air, which are working like a prism in that case. Much better astronomical spectra, gained with professionals spectrographs, or also with instrumentation onboard of Gaia, which are able to detect faint details – we are talking about spectral lines, which are characteristic for special atomic or molecular processes, for example. So we learn a lot about the intrinsic physics of stars and other objects. And in our last interview, Stefan, we might hear more about such other kind of Gaia date besides the classical astrometry, isn’t it? S. Jordan: Dr. Carme Jordi is with us now. Carme is working at the University of Barcelona on the Gaia project. Carme, what is your role in the Gaia project, what are you working on? C. Jordi: I have been in Gaia involved since the very beginning. I was contributing to the definition of the photometric instrument, coordinating the photometric working group, and since the formation of the DPAC consortium, I have been coordinating the activities around photometry in Barcelona. Since 2002, I am also member of the Gaia Science Team, which is the body advisory for ESA in scientific terms. We have heard that Gaia is an astrometric satellite, but you are talking about photometry and other measurements. What kind of other measurements are taken by Gaia? C. Jordi: As your said, Gaia is mainly astrometry, but we want to characterize the objects that we observed. Then we need information about the luminosity, about the temperatures. About which kind of objects we are measuring. We have designed the photometric instrument and the spectroscopic instrument. Both intend to determine the electromagnetic distribution of the light coming from the objects. One, the photometric instrument, is for the low-dispersion, two prisms that disperse the light in the blue and red. And they intend to determine a broad classification and parameterisation of the objects. And the spectroscopic instrument spreads the light in the infrared, in a short range of wavelengths in the infrared. And the aim of this instrument is to determine the chemical compositions, more precise classification, and also the velocity of the objects that maybe be approaching us or moving away from us. And this third component of the velocity allows the 3D determination of the speed of the stars on the sky. S. Jordan: If we take together these astrometric measurements and the measurements your are talking about in photometry and spectroscopy. What will we learn about the stars in our Milky Way? C. Jordi: The goal of Gaia is the archeology of the Galaxy. How the Galaxy has evolved, how it has formed? So, if we know how the stars move, but we do not know what is the age of the stars, which is the chemical composition of those stars, we are missing a lot. So the full picture, of how the Galaxy has evolved, can come only joining together all the information: the kinematics, the dynamics of the objects but also the properties, the physical properties of the objects, ages and chemical compositions, basically. S. Jordan: These are the main aims of the Gaia mission. But are there any other topics in science, in physics or astronomy, that Gaia will contribute to? C. Jordi: Well, actually, these spectra of the objects that we observe, allow, as I said, the determination of the ages and the chemical compositions. Basically with the combination of all Gaia data we have the absolute luminosities of all kinds of stars in all stages of stellar evolutions. So, from the very beginning, the pre-main sequence, the main sequence, the giants, and when the stars are dead as white dwarfs, supernovae, etc. So, the important models for the characterisation, for the modeling of the stars, structure and evolution, will be improved much with the Gaia data. And also Gaia is observing quasars and solar system objects, and exoplanets, and many, many, many kinds of objects. Then all astrophysics will be impacted by Gaia. And also relativity. Because, when the light comes to us from the object, the path maybe bended because we have other massive objects along the way, and then these bend the path of the light. This allows to test relativity. S. Jordan: This sounds very interesting.
C. Jordi: This is. S. Jordan: What do you find most interesing about the Gaia mission? What fascinates you most? C. Jordi: To me the combination of all the data together. Because from ground we focus on astrometry, or photometry or spectroscopy. And here we have everything together with an extremely large precision which was never reached before. And for a billion objects or more, which means that actually we are reaching steps that nobody else has done. Contributing to such a big project, challenging project, European leadership, everything is exciting I think. S. Jordan: Thank you very much, Dr. Carme Jordi from the Science Team of Gaia.
C. Jordi: Thank you. K. Jäger: Hello everybody. We are now here in the townhall at the reception of all the people which were coming to Leiden to the meeting. And, Stefan, what do you think? S. Jordan: I think it was a very successful meeting. And we have seen that Gaia is a very complicated satellite especially in the processing and the analysis of the data. But I have seen that there is large progress in understanding what we get from Gaia and how to deal with the complicated situation. I am very optimistic that the first catalogue will be a very very good catalogue. K. Jäger: And after all that already doing the first two days – the people have worked so hard and discussed in already many meetings about these different things which are related to Gaia, I think it’s a good point now to have a little bit a glass of wine, and cheers! And let’s see how it will work during the next days. S. Jordan: Yes!