Doing archaeology in the Milky Way

By dating stars and looking at how they're moving, astronomers are trying to work out where the stars of the Milky Way came from.
08 August 2013

Interview with 

Sofia Feltzing, Lund Observatory


Figure 1: The Andromeda Galaxy


Sofia Feltzing works on surveys with ground base telescopes which will NGC235 galaxycomplement the data which comes back from Gaia. She is hoping to combine the best possible estimates of how old stars are with maps of where they are in the galaxy and how they're moving to look for grouping of stars that may have a common origin. She told Dominic Ford that while Gaia is very good at measuring how fast stars are moving across the sky, it's not so good at telling us whether stars are moving towards or away from us.

Sofia - So, in order to see how the stars are actually moving in the galaxy in 3D, not just on the sky, we need that third dimension of the velocity. We can do that from the ground very well. Radial velocities today can detect planets very, very accurately circling around other the stars and the precision there is enormous. So, the requirements are easily met by ground base telescopes and instrumentation that already exists today. However, even if the technology is there, the type of spectrograph you need is not there. You need to be able to take a thousand spectra in one go or maybe up to 3,000, covering several square degrees on the sky. So, several moons in one go. You can do that by building new spectrographs.

Dominic - So, you're looking at these spectra and you're working out how fast the star is moving along the line of sight by looking for spectral features which have been red shifted.

Sofia - That's correct and we don't call it red shifted while we're looking at stars. We just say that they are shifted in the velocity. I mean, we always talk about red shift because it's the galaxies are moving away from us, but the star could be moving towards you. In the universe, the galaxies are moving apart, but in the galaxy, things are moving in many different ways. So, you have a set of spectral features that you will measure how much they changed relative to the lab wavelength.

Dominic - Once you've got this huge catalogue of stars around the sun and how they're moving, what scientifically can you learn from that catalogue?

Sofia - So, the important thing is not just around the sun we're looking anymore. That was the Hipparcos satellite. The Hipparcos satellite is looking - I mean, dwarf stars like the solar-like stars is only within 100 parsecs of the sun. Now, Gaia will see sun-like stars 20 times or more further away with equally good precision. So, a so-called G dwarf star like the sun will still be very bright and Gaia terms when it is 2,000 parsecs away which is a quarter off the distance to the galactic centre from us. So, we can see stars like the sun, almost into the galactic centre, knowing where they are in the sky and how they move. But this is not the only thing we can get from spectra.

So basically, what you do is you're splitting the light from the star up into its wavelength, just like putting a prism in front of a sunbeam and you'll see all the colours. It's what we do in a spectrograph, but you can do this with very high resolution so that the smallest elements you're looking at is really a tiny little bit of an angstrom. Perhaps you've seen out from the sun, if you split up the light in the rainbow of colours, there are dark areas in there, dark lines across. Those are absorption lines in the solar spectrum because there are elements like iron, oxygen, and carbon in the outer atmosphere of the star. Each element has its unique fingerprint of such absorption lines. From these absorption lines, we can calculate how much there is of a given element in a star.

The beauty of the dwarf stars, the solar-like star is that they live for a very long time and nothing much happens to them. They're dead boring. For people who like to study how stars behave and how they evolve and things, these stars are not very exciting. They are really actually quite dull. But they're very good because nothing happens in their outer atmospheres at all. I mean, there's nuclear burning going in the centre but you don't notice it on the surface most of the time. And therefore, if you can measure how much iron, oxygen, carbon, chromium, and titanium there is in the outer atmosphere of this star, you have a fair sample of the composition of the gas that this star formed out of.

Dominic - I guess something you really want to know though is how old those stars are and it must be quite difficult to date them if they're not changing very much.

Sofia - Right. So yes, there are many ways of dating stars, but it all has to do with us understanding how stars evolve. These stars are deemed rather good because there is clear signatures when you put them in certain diagrams; when we have the stellar evolutionary model and the temperature, and the mass of the star for example, you can see how old it is.

There can be fairly in large errors on them, it depends really a lot on the exact star. Stars like the sun can be dated within say, a billion years and the universe is 13 billion years and the oldest stars we find are at least 10 billion years or something like that. Now, there are more evolved stars, so-called red giants which you can date very well and there are two satellites, Kuru and Kepler who were studying very small pieces of the sky, not like Gaia who's going to study the whole sky. They are studying particular regions, but they get very good astro seismology data. So, they will be able to age data stars very, very accurately and combining those data with the Gaia data, they will also put very strong constraints on the stellar evolutionary models. But we would be happy with very large samples of stars that we can age date within a billion years because then we can see how the larger properties of the galaxy vary as a function of the distance from the galactic centre. Not anymore looking at things just near the sun, but how things vary at a distance from the galactic centre.

Dominic - Once you've got this catalogue of where the stars are, how they're moving and what types of star they are, I guess that's telling you about the history of the Milky Way when these stars were forming and where.

Sofia - So, the idea is that all of these are bits of a kind of jigsaw puzzle that you're trying to put together. It's a little bit like archaeology where you're getting pieces of a vase up and you dust them off and you find that they fit together. There might be bits that are missing, but then you can guess what they would look like.

So in the past, you see, it has been often like, people work on dynamics of the Milky Way or the stars, how they move and why they move like they do, or their ages and their elemental abundances. But we will really truly need to combine this much more and even if we know what it looks like very close to the sun, within a few hundred light years, and we know a little bit of what it looks like in the centre of the galaxy, and out in the halo, we know very little of what is in between and actually, Gaia is going to help.

When it comes to galactic chemical evolution or galactic evolution, a lot of what we have been inferring has historically been gleaned just from the solar neighbourhood which is a tiny, tiny piece of the Milky Way and does it really tell us about the global properties. Now, when it comes to all these ground based follow up, we would need - as I said - to build new instruments. Although there are instruments that already can do some of it.

So with around 300 other astronomers in Europe, I am part of a project called the Gaia ESO survey. So, it's Gaia for Gaia and ESO for European Southern Observatory, which owns and runs in collaborations of 15 different countries, the Paranal La Silla in Chile and the Alma observatory. There, we are using the flames spectrograph that takes about 100 plus spectra of stars in one go. It's not perfect for this purpose, but it's pretty good. It's looking at a rather small piece of the sky, so we would like something with a bigger field of view and therefore, the proposal is and ESO has actually accepted to go forward with this proposal to build a completely new machine to measure several thousands of spectra in one go. This is called Foremost and is planned to go on the Vista survey telescope. Right now, Gaia ESO survey is running. We are aiming 100,000 stars and we already have been observing for a year and a half now and it's going well. We are getting the data analysis under way and starting to write the first papers.


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