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Prof Gerry Gilmore of Cambridge’s Institute of Astronomy on Gaia’s astonishing 3D map of the galaxy


By Paul Brackley


Professor Gerry Gilmore, one of the original proposers of the Gaia project, with a model of the spacecraft. Picture: Keith Heppell
Professor Gerry Gilmore, one of the original proposers of the Gaia project, with a model of the spacecraft. Picture: Keith Heppell

Stunning videos as one of the original proposers of the mission explains how spacecraft’s second data release changes astrophysics forever

The Gaia Science team at the Institute of Astronomy in Cambridge after winning the Sir Arthur Clarke Centenary Awards
The Gaia Science team at the Institute of Astronomy in Cambridge after winning the Sir Arthur Clarke Centenary Awards

It has opened a new window on the universe and will revolutionise our understanding of the Milky Way.

The Gaia spacecraft has created an astonishing three-dimensional map of our galaxy, charting with great precision the positions of nearly 1.7 billion stars.

The data – the second release from the Gaia mission – was unveiled to the world last Wednesday, and since then researchers around the world have been racing to uncover its secrets.

Gaia will help us understand the formation and evolution of the Milky Way and its information will be an invaluable resource for astrophysicists for decades to come.

A team at the Institute of Astronomy in Cambridge is one of six worldwide processing the complex data sent back to Earth from the space observatory.

Prof Gerry Gilmore, who is based at the Institute off Madingley Road, explains: “This all started 25 years ago when a couple of us here were among the original people who proposed the mission. I’ve been involved in it very actively ever since. Gaia is a very big European-scale mission with a one billion euro budget, a thousand people involved and a lot of industrial partners, including Airbus and e2v in the East of England.

“What Gaia is doing is running a high definition video of the Milky Way, continuously, with sufficient precision that one can measure the position of stars to an accuracy equivalent to measuring the thickness of a fingernail across the Earth, or the thickness of a human hair at 1,000km…crazy numbers, but because space is big you need to measure to that level of precision.”

Gaia has dual telescopes, exploring two areas of the sky at once, taking in about two million stars an hour. Its ‘eyes’, created at e2v in Chelmsford, are sensors called charge-coupled devices (CCDs), like those in cameras and mobile phones, but on a different scale. Its array of 106 CCDs – the largest ever sent into space – gives Gaia one billion pixels with which to scan the universe and generate 50Gb of data daily.

An image from the Gaia data of the Milky Way
An image from the Gaia data of the Milky Way

“It allows us to measure the distances to stars by measuring the slight change in angle to each star as the Earth goes around the Sun,” continues Prof Gilmore. “If you look in summer and look in winter, everything appears to have moved very slightly. If you measure that very slight movement, or parallax, you can measure the distance. It’s the same way our eyes work – it’s why we have two.

“Once the data comes to the ground from this huge mission, there are five groups around Europe who are processing it and we are one. Our responsibility and lead is to measure the brightnesses of the sources and their colours, and also to calibrate how the spacecraft is working.”

Launched in December 2013, Gaia’s first data release was published in 2016 based on just over a year of observations. It contained distances and motions of two million stars, and led to numerous discoveries.

The new data release – freely available for all to download and work on – covers observations from July 25, 2014, to May 23, 2016, and reveals the positions of nearly 1.7 billion stars with much greater precision. For some of the brightest stars, this level of precision can be compared to an observer on Earth being able to spot a coin on the surface of the Moon. It also reveals the colour, parallax and proper motion of about 1.3 million stars, the surface temperature of 100 million and the positions of about 14,000 asteroids.

“Gaia has recently reached one trillion observations. It’s by far the biggest scale of operation like that mankind has ever done,” says Prof Gilmore, who is principal investigator for the UK participation in the Gaia Data Processing and Analysis Consortium.

“The data release has enormous, fundamental applications to everything we do in astronomy. These are the precision measurements and calibration from which all of astrophysics and space science are derived. So it’s really exciting to be a part of.

“It leads to an understanding of how our Milky Way came to be assembled and how it’s evolving, where and when all the chemical elements of which we are made were created and how they got to where they are now, how stars evolve, right down to how asteroids in the Solar System are orbiting, which tells us about the debris left over from forming Earth. Some of these orbits of these asteroids are changing in ways that are a test for how general relativity works. Others are changing in ways that tell us whether we’re going to be killed by one hitting us soon… all useful information.”

The release was highly anticipated by the astronomical community.

The Gaia Extinction Map
The Gaia Extinction Map

Günther Hasinger, director of science at the European Space Agency (ESA), which is in charge of the mission, said: “The observations collected by Gaia are redefining the foundations of astronomy.”

Prof Gilmore says it will be impossible to read every paper that results from Gaia’s treasure trove of data.

“There were two independent scientific results within an hour of the release. There are teams all around the world busy putting photos of themselves on Twitter saying ‘Here I am, working on Gaia data at 5am…’ There’s a big race going on worldwide.

“Fundamentally, this is one of a handful of transitions in the history of mankind. This is the first time that we’ve had a genuine three-dimensional picture of our local universe. Mankind has wanted this since ancient times…”

Gaia will unlock the secrets to the life cycle of stars.

“Most stars spend most of their lives being rather dull and not changing very much. But when they do start to change, they change really fast,” says Prof Gilmore. “Those complicated evolutionary stages are not well understood but we’ll be watching them. Understanding stars will be a fundamental advance.

“One of the very obvious things is to look at places where stars are forming, like Orion or Ophiuchus – those constellations in the sky where you can see very, very young stars. We know that most stars are spread fairly uniformly across the sky so clearly they get out from those dense clusters to spread around.

“Our Sun must have done that. We must have had a lot of neighbours when we formed. If you’ve got enough stars in enough different places you can find groups in all the intermediary stages. We can map out the history of how they evolve.

The Gaia data. Image: ESA
The Gaia data. Image: ESA

“We can also find out how the Sun came to be where it is, because we are probably a long way from where we formed, and whether we have any siblings still around.”

Gaia will help answer questions about our own galaxy’s formation.

“One of the things that a lot of people are working on right now is mapping the early assembly history of the Milky Way,” says Prof Gilmore. “If you look in the very outer parts of the Milky Way, our galaxy is still merging with other galaxies. These galaxies are being torn apart and spread across the sky. Gaia can see them because they are all moving in the same way. Gaia can identify these orbits and put them back together. We can then run the clock backwards and look at the assembly history and accretion history of the Milky Way.

“Just by looking locally at the complex motion of stars as they orbit around, we can map the way spiral arms work. We know the Milky Way is a spiral galaxy. We can look at the dynamics of it and ask questions like ‘What does a spiral arm weigh?’

Gaia’s telescopes also see beyond our galaxy to others.

“Neighbouring galaxies to us, like Andromeda, can be studied in fairly considerable detail. As you get further away, you get less detail.

“But one of the things Gaia does really well is to set up the universal scale of distances. There are pulsating stars and supernovae that are used to calibrate distances, and those calibrations depend on Gaia.

“For the first time ever we can build a uniform measuring stick for the universe.”

Gaia has also tracked the positions of half a million quasars – the astonishingly bright nuclei of galaxies with supermassive black holes at their cores.

“One of the things people are finding as we speak in Gaia is quasars at huge distances that are gravitationally lensed – a quasar with a massive galaxy between us that distorts the light. Those gravitational lenses allow us to weigh the galaxy we can’t even see.

“That means we can map out how dark matter is distributed and independently work out how far that quasar and galaxy are, and that gives us another measure of how far the universe is expanding. It’s another test of fundamental cosmology.”

Incredibly, the mission that Prof Gilmore and others proposed at the turn of the century is working out much as envisaged.

Professor Gerry Gilmore at the Institute of Astronomy in Cambridge. Picture: Keith Heppell
Professor Gerry Gilmore at the Institute of Astronomy in Cambridge. Picture: Keith Heppell

“We published an article in 2000 and the very detailed proposal was in a book, and it corresponds exactly to what people are doing today and that’s very, very unusual because normally technology advances so fast. But fundamental astronomy from space is something that can’t be done in any other way.”

And Gaia’s work is not done, with new data releases expected every two or three years until it runs out of fuel around the end of 2024.

“About a quarter of the information from Gaia is still yet to be published because it’s so technically complex that we’re still learning how to process it. The colours of everything have been published in rather crude form but the high-precision colours are going to be available next time around.”

Processing this data is an enormous task, and requires a cutting-edge data centre on the University of Cambridge’s West Cambridge site.

Prof Gilmore says: “That’s part of the interest and benefit of doing projects like this. It’s state-of the-art – as hard as it gets. But when we learn to do this, everyone benefits. Other projects and subjects can learn from what we have learned. There’s lots of medical physics and medical diagnostics that can benefit. The Human Genome Project, for example, is learning from how we do stuff. This is at the forefront of big data and big data processing is at the forefront of most of society’s challenges.”

Cambridge astronomers are also using the data for their own research. Prof Gilmore is leading a large related project, called the Gaia-ESO spectroscopic survey.

“It’s a 500-person consortium combining the detailed measurements of what the stars are made of and their age with the Gaia data to get a fuller picture of the evolution of the Milky Way,” he says.

“There is hardly a branch of astrophysics which will not be revolutionised by Gaia data.”

1.7 billion sources viewed 200 times

Dr Francesca De Angeli is head of the Cambridge processing centre and says: “This data release has proven an exciting challenge to process from spacecraft camera images to science-ready catalogues. More sophisticated strategies and updated models will be applied to the Gaia data to achieve even more precise and accurate photometric and spectrophotometric information, which will enable even more exciting scientific investigations.”

Another member of the Cambridge team, Dr Dafydd Wyn Evans, adds: “Gaia has so far observed each of its more than 1.7 billion sources on average about 200 times. This very large data set has to have all the changing satellite and sky responses removed, and everything converted on to a well-defined scale of brightness and colour. While a huge challenge, it is worth it.”

Dr Nicholas Walton, a member of the ESA Gaia Science Team also based at the Institute, adds: “The Gaia data will be a globally accessible resource for astronomical research for decades to come, enabling the future research of today’s young astronomers in the UK, Europe and the world.”

Data yielding new insights already

Gaia project
Gaia project

Gaia’s data has already revealed new insights, as Cambridge’s Dr Floor van Leeuwen, project manager for the UK and European photometric processing work, explains: “Groups of dwarf galaxies, including the Magellanic Clouds, can now be observed to be moving around in very similar orbits, hinting at a shared formation history.

“Similarly, a pair of globular clusters has been observed with very similar orbital characteristics and chemical composition, again pointing towards a shared history of formation.

“The accurate observed motions and positions of the globular clusters and dwarf galaxies provide tracers of the overall mass distribution of our galaxy in a way that has not been possible with this level of accuracy before.”

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One of sensors from the Gaia project sensor array - each sensor cost 250,000. Picture: Keith Heppell
One of sensors from the Gaia project sensor array - each sensor cost 250,000. Picture: Keith Heppell

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