How to map the Milky Way

25 October 2016

Interview with

Anna Hourihane, University of Cambridge


Surprisingly, scientists are using GCSE maths to figure out where the stars are inThe Milky Way our galaxy. Anna Hourihane put Graihagh Jackson through her paces...

Graihagh - Anna - I'm going to have to ask you how you pronounce your second name because when I can in here at reception I was like Anna Hari-hane, Hower-hane?

Anna - Almost - Harra-hen.

Graihagh - Have other people called you all sorts of really funny surnames? I'm just thinking Graihagh - I get a lot of these things.

Anna - Yes, I actually wasn't sure how to pronounce your name either at first? But my last name it can also be pronounced Hourahan, so depending on which part of Ireland you're from, so I'm used to different pronunciations and it doesn't bother me too much.

Graihagh - Thank goodness for that! Let's jump in - why do we want to measure the distance between stars and how do we do it?

Anna - Well, one thing that we don't know very well are distances to the stars and that's because they're very difficult to measure in astronomy. So Gaia's goal is to get accurate distancing for a very large sample of stars in our galaxy, actually more than one billion stars.

Graihagh - One billion - wow!

Anna - Yeah, so it's still less than probably one percent of the total stellar content of our galaxy but it will be the largest sample we have so far of accurate distances and it will be fundamental to lots of other areas of astronomy as well.

Graihagh - I'm just thinking - Gaia's got to have some incredible accuracy on it to be able to map, even if it is just one percent of the galaxy?

Anna - Yeah, the actual precision needed for the measurements is equivalent to resolving say a pound coin on the moon if you measured it from Earth. It needs very, very precise measurements to be able to measure these tiny movements that translate in being able to measure distances to stars.

Graihagh - How is it measuring where the stars are?

Anna - Well, it has two telescopes on board and collects light through two viewing windows in the satellite housing. So the starlight comes in through those viewing windows and passes through the telescopes and then gets focused onto a camera inside Gaia. So they're kind of continually transiting across the camera almost like a continuous video of the sky.

Graihagh - A bit like a time-lapse I'm thinking or something like that?

Anna - Yes exactly - it's like a time-lapse. And that data then gets down linked to Earth every day from the satellite.

Graihagh - And it was on the fourteenth of September that all this data was released. It was the first Gaia release of data.

Anna - Yes. The data that was released on the fourteenth of September  was coming from almost the first year of observations by Gaia. And that's the first big release of data that is really useful to see how everything is going with the data processing and checking the quality of the data. The precision of the data is probably only about one tenth of what we will get at the end of the mission.

Graihagh - And that's because it's going to map these stars lots of times?

Anna - Exactly, yes.

Graihagh - And when we say data and things transiting across the camera - this data release was it pictures or was it numbers? I'm trying to envisage what was released.

Anna - No, we don't get pretty pictures like the Hubble space telescope images that we're used to seeing. With Gaia it's more about very precise measurements. So the raw data in terms of the times of transit of the stars across the camera and then the brightness of each star as measured.

Graihagh - Once you have this time code, if you like, and how bright this star is - how does that then enable you to measure how far away it is?

Anna - Well, there's a lot of complex mathematics involved but you can simplify it down into the concept of measuring distances by parallax. Parallax is the effect of the apparent change in the position of an object depending on which angle you view it from.

So, as a demonstration, why don't you hold out your thumb at arm's length.

Graihagh - It will have to be left thumb okay.

Anna - So now close your left eye and look at it with your right eye and notice the background behind your thumb. And then switch eyes closing your right and looking at it with your left and do you notice how the apparent position of your thumb appears to change against the backgrounds?

Graihagh - Yeah - it moves by about an inch or so.

Anna - Okay. Now bring your thumb closer, so about halfway, and try the experiment again. Do you notice anything different about the change in position?

Graihagh - It moves further.

Anna - Exactly. So by measuring the change in position of an object, say a star, against a background, we can figure out how far away the star is. And the parallels in this with thumb experiment are imagining that your thumb is a foreground star so a star that's closer to us, and the background is a backdrop of more distant stars. So you can see how one star appears to move against more distant stars. And, in this case, instead of the distance between your two eyes you're looking at the foreground star from two distant positions in the Earth's orbit around the Sun.

Graihagh - Ah, okay. I thought you were going to say the two telescopes but no, that makes more sense, two different positions around the Sun.

Anna - Exactly.

Graihagh - Okay - you've Gaia orbiting and capturing snaps from the stars at different angles. But then you've also got to take into consideration that the stars are also moving.

Anna - Yes well, actually, like the Earth and the other planets orbit around the Sun, the stars in our galaxy all orbit around the centre of the galaxy as well. But the motions of the stars around the centre of the galaxy, what that translates to in what we can observe is, over time, a slow track - kind of straight line of the stars across the sky. And over Gaia's lifetime, over the five years or more that it's observing, it will cause the stars to move a small distance across the sky. But then, when we combine that with the apparent motion due to the Earth's and Gaia's orbit around the Sun,that introduces kind of an ellipse into the motion.

Graihagh - It sounds quite complicated then to look at their distance and stuff?

Anna - Yeah, so quite complex algorithms, quite complex software, has had to be developed.  But it actually relates back to basic trigonometry where if you have an angle, for example in the experiment where you're looking at the star or you're looking at your thumb with your two different eyes, so the angle between the two measurements is the angle in a triangle where the baseline is the distance between your eyes. And then basic trigonometry can be used to work out the parallax angle and then the distance to the star.

Graihagh - But there's something quite nice about it all boiling down to something that we all learn at GCSE maths.

Anna - Exactly. So you can make a complex calculation and you can relate it back to simple geometry and simple trigonometry, so that's nice.

Graihagh - How does this then relate then to Gaia's missions, answering some of these fundamental questions in physics?

Anna - Well, by measuring exactly where the stars are in the galaxy, we can associate stars together. Are they part of one structure in the galaxy or do they appear to be moving in different clumps or groups? So, for example, our galaxy (the Milky Way) is a spiral shaped galaxy with a central bulge or bur and then a disk of stars orbiting around that, and then a halo of stars around that again. So there are different sub-structures within these bigger structures and these give clues to the formation of our galaxy. And by looking at both the motion of stars in these sub-structures and also what they're made of we can compare them, for example, to stars in other parts of the galaxy and we can compare them to stars outside our galaxy, and we can figure out if stars in different parts of the galaxy have a common origin or if they appear to have formed at different times. So actually getting ages for the stars is important as well.


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