What the Billion-Star Mapper Gaia Will Tell Us...

Artist's impression of Gaia as it begins its mission to map out the Milky Way

On 19th December 2013 astronomers around the world watched with baited breath as a Soyuz rocket blasted off from the European spaceport in French Guiana. Nineteen days later the rocket's precious cargo reached its operational orbit, 1.5 million kilometres away from the Earth. Then, last Thursday, the European Space Agency's newest flagship, Gaia, send back its first clear image.

Image of a star cluster in the Large Magellanic Cloud, taken to check that Gaia's telescopes are properly focused. 

It isn't much to look at. A simple patch of stars, a cluster in the Large Magellanic cloud. But Gaia isn't designed to take pretty pictures. In fact, its not going to take many pictures at all: This image is simple for calibration, a test to make sure that Gaia's twin telescopes are focused and working.

What Gaia will do though is rather special. Over the next few years, Gaia will return to this patch of stars and observe it again. And again. And yet again, seventy times in all. And it will do the same thing for another billion stars.

Gaia's mission is to make a map of the galaxy. For those billion stars, it will return data telling us how bright they are, what their colour spectrum is like, what their position is. And by doing this seventy times for each star, it will allow us to see how each of those things changes over time.

Measuring distances in space is very difficult. You can't do it based on how bright things are, because how do you know if the star you're looking at is dim but close, or bright and far away? Knowing the true brightness of an object, known as the absolute magnitude, is one to the key things needed to start to say more about it than "that's pretty". But to know the absolute magnitude, we need to measure the distance...

In the last post I mentioned standard candles, objects that we know have the same absolute magnitude. Comparing the apparent brightness of these objects can be used to measure distance, but it's only a relative scale: It just tells us how far away one standard candle is compared with another. If we want to know how far away an object is in actual units, like light-years,or the more commonly used parsec (3.26 light-years), we need a different technique.

How to measure distances with Stellar Parallax : By comparing the position of a star relative to background stars at different points in its orbit Gaia will be able to accurately measure the distance to one billion stars, 1% of the stars in the Milky Way.

The technique Gaia will use, stellar parallax, is simple in principle. Hold one finger out in front of you, then close one eye. Open that eye, and close the other. Your finger will appear to move against the background, a consequence of the slightly different angles that your eyes are seeing it from. If you measure how far it moved and how far apart your eyes are, a simple bit of maths will give you the distance between your face and your finger.

Instead of two eyes, Gaia will use the orbit of the Earth around the Sun to create the parallax it needs. If it observes a star twice, six months apart, it will have travelled to the other side of the Earth's orbit around the Sun in between the observations. This will allow it to pinpoint the distance to the stars that it measures, building up a map of the Galaxy.

Simple in principle, but not so in practice. Because of the vast distances between the stars, the difference in their position caused by parallax is tiny. Gaia will have to measure changes in position of just 24 microseconds, a distance in the sky 75 million times smaller than that taken up by the Moon. Gaia could see both sides of a human hair one thousand kilometres away. What's more, most of the stars it will observe are thousands of times fainter than those we can see with our eyes.

To achieve this amazing precision has required building one of the most advanced spacecraft ever flow. Inside its body, a squat 3.5 metre cylinder, Gaia carries two rectangular telescopes, with ten mirrors focusing light down onto a billion pixel camera, the largest ever put into space.

Beneath the telescope is a complex system of mircothrusters, tiny rockets that keep Gaia in a perfectly balanced, slow spin, scanning across the sky. At the base of the spacecraft is a huge sunshield, a ten metre wide circle of insulating material that keeps Gaia at a stable temperature. The sunshield had to be folded up to fit into the rocket. Had it failed to open, the mission would have been over before it began.

But unfold it did, allowing Gaia to fly to L2. a point in space 1.5 million kilometres away where the gravity of the Earth and the Sun are balanced. Here Gaia will travel around the Sun at the same pace as the Earth, allowing us to easily keep in contact, but is far enough away that the heat and light reflected off Earth will not affect its measurements.

Gaia in the clean room before launch, testing the deployment of its massive sunshield. The telescopes are housed in the main cylindrical structure, with power provided by solar panels underneath the shield.

Those measurements promise to be spectacular. As well as measuring the position of a billion stars, it will also tell how those positions change over time. Our map of the galaxy will be a moving one, allowing us to explore the dynamics of the Milky Way like never before. We'll be able to test our ideas of how it was formed and how it evolved into what we see now, as well as what will happen to it in the future. Gaia may even shed light on that most elusive of substances, Dark Matter.

Not all the movement of stars is caused by parallax. Gaia will be able to detect the tiny tugs on stars caused by the gravity of orbiting planets, many of which are completely invisible to our current methods of planet-hunting. Gaia is predicted to detect seven thousand new exoplanets, double the current number of candidates.

In addition to this, Gaia will detect hundreds of thousands of asteroids in our own solar system, thousands of previously undetected brown dwarf stars, supernova in distant galaxies, and explore the cores of active galaxies, the quasars.

Beyond its predicted results, the most significant result from Gaia will be the huge catalouge of measurements it will create, seventy billion results that will keep astronomers busy for decaeds to come. Gaia's most exciting discoveries will be things that we haven't even thought of yet.

Almost every astronomer has something they want Gaia to do. I've lost count of the number of times I've heard phrases along the lines of "Gaia will tell us..." since starting my PhD, whether its the answer to a great mystery of science or simply the exact distance to a favourite star. Personally, I'm hoping to use Gaia to find the first planet around a White Dwarf star, but it will be a few years before that is possible and I certainly wont be the only one looking!

Gaia's first batch of data will be released to the public in October next year. Expect some very interesting astronomy news soon after...

Ice cascades off Gaia's Soyuz launcher as it blasts off into a new age of astronomy

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Talking of new blogs, does anyone have any requests for space subjects they'd like me to talk about? I realised the other day that my plans for the next three posts involved two spacecraft and a rocket, which isn't a particularly wide area! I'm going to try and cover a wide variety of areas within astronomy, but if anyone asks for one subject in particular I can make that a priority. (If the answer for anyone is exoplanets, then I recommend Lost in Transits, a blog by another PhD student here at Warwick.)