Gaia’s first sky map

The first results from the Gaia satellite, which is completing an unprecedented census of more than one billion stars in the Milky Way, are being released today to astronomers and the public.

Gaia’s first major data release is both a wonderful achievement in its own right, and a taster of the truly dramatic advances to come in future years.

Gerry Gilmore

Detailed information about more than a billion stars in the Milky Way has been published in the first data release from the Gaia satellite, which is conducting the first-ever ‘galactic census.’ The release marks the first chance astronomers and the public have had to get their hands on the most detailed map ever made of the sky.

Gaia, which orbits the sun at a distance of 1.5 million kilometres from the earth, was launched by the European Space Agency in December 2013 with the aim of observing a billion stars and revolutionising our understanding of the Milky Way. During its expected five-year lifespan, Gaia will observe each of a billion stars about 70 times.

The unique mission is reliant on the work of Cambridge researchers who collect the vast quantities of data transmitted by Gaia to a data processing centre at the University, overseen by a team at the Institute of Astronomy.

“Gaia’s first major data release is both a wonderful achievement in its own right, and a taster of the truly dramatic advances to come in future years,” said Professor Gerry Gilmore from the Institute of Astronomy, who is also the UK Principal Investigator for Gaia. “Several UK teams have leading roles in Gaia’s Data Processing and Analysis efforts, which convert the huge raw data streams from the satellite into the beautiful science-ready information now made available for the global scientific and public communities. UK industry made critical contributions to the Gaia spacecraft. The UK public, including school students, as well as scientists, are sharing the excitement of this first ever galactic census.”

In addition to the work taking place at Cambridge, teams from Edinburgh, the Mullard Space Science Laboratory (MSSL) at UCL London, Leicester, Bristol and the Science and Technology Facilities Council’s Rutherford Appleton Lab are all contributing to the processing of the vast amounts of data from Gaia, in collaboration with industrial and academic partners from across Europe.

The team in Cambridge, led by Dr Floor van Leeuwen, Dr Dafydd Wyn Evans and Dr Francesca De Angeli, processed the flux information – the amount of energy that crosses a unit area per unit time – providing the calibrated magnitudes of around 1.6 billion stars, 1.1 billion of which are now published as part of the first data release.

The Cambridge team also check the daily photometric data for unexpected large outliers, which led to the regular publication of photometric science alerts that were ready for immediate follow-up observations from the ground.

“The sheer volume of data processed for this first release is beyond imagination: around 120 billion images were analysed, and most of these more than once, as all the processing is iterative,” said van Leeuwen, who is Gaia photometric data processing lead. “Many problems had to be overcome, and a few small ones still remain. Calibrations have not yet reached their full potential. Nevertheless, we are already reaching accuracies that are significantly better than expected before the mission, and which can challenge most ground-based photometric data in accuracy.”

“This first Gaia data release has been an important exercise for the Gaia data reduction teams, getting them to focus on deliverable products and their description,” said Evans. “But it is only the first small step towards much more substantial results.”

While today marks the first major data release from Gaia, in the two years since its launch, the satellite has been producing scientific results in the form of Gaia Alerts.

Dr Simon Hodgkin, lead of the Cambridge Alerts team said, “The Gaia Alerts project takes advantage of the fact that the Gaia satellite scans each part of the sky repeatedly throughout the mission. By comparing successive observations of the same patch of sky, scientists can search for transients – astronomical objects which brighten, fade, change or move. These transients are then announced to the world each day as Gaia Alerts for both professional and amateur astronomers to observe with telescopes from the ground.”

The range of Gaia’s discoveries from Science Alerts is large – supernovae of various types, cataclysmic variable stars, novae, flaring stars, gravitational microlensing events, active galactic nuclei and quasars, and many sources whose nature remains a mystery.

Gaia has discovered many supernovae, the brilliant explosions of stars at the end of their lives. Many of these have been ‘Type Ia’ supernovae, which can be used to measure the accelerating expansion of the Universe. But among these apparently typical supernovae there have been some rarer events. Gaia16ada was spotted by Gaia in the nearby galaxy NGC4559, and appears to be an eruption of a very massive, unstable star. The Hubble Space Telescope observed this galaxy some years ago, allowing astronomers to pinpoint the precise star which erupted.

Another lucky catch for Gaia was the discovery of Gaia16apd – a supernova which is nearly a hundred times brighter than normal. Astronomers still don't know what the missing ingredient in these ultra-bright supernovae is, and candidates include exotic rapidly spinning neutron stars, or jets from a black hole. Cambridge astronomer Dr Morgan Fraser is trying to understand these events, saying, “We have only found a handful of these exceptionally bright supernovae, compared to thousands of normal supernovae. For Gaia to spot one so nearby is a fantastic result.”

Many of the Gaia Alerts found so far are bright enough to be observable with a small telescope. Amateur astronomers have taken images of supernovae found by Gaia, while schoolchildren have used robotic telescopes including the Faulkes Telescopes in Australia and Hawaii to do real science with transients.

Dr Hodgkin said: “Since the announcement of the first transients discovered with Gaia in September 2014, over one thousand alerts have been released. With Gaia continually relaying new observations to ground, and our team working on finding transients continually improving their software, the discoveries look set to continue well into the future!”

For the UK teams the future means providing improvements in the pre-processing of the data and extending the processing to cover the photometric Blue and Red prism data. Also data from the Radial Velocity Spectrometer, with major involvement from MSSL, will be included in future releases. The photometric science alerts will continue to operate throughout the mission, and summaries of the results will be included in future releases. “Despite the considerable amount of data, the first Gaia data release provides just a taste of the accuracy and completeness of the final catalogue,” said De Angeli.

The Cambridge Gaia team has also released a dedicated smartphone app, which will allow anyone worldwide to follow the Gaia Alerts discoveries as they happen. Real spacecraft data will be available to the world as soon as it is processed, with users able to follow the discoveries and see what they are. Information to access the app is available at

The Gaia data processing teams in the UK have been and are being supported by the UK Space Agency and the STFC. STFC helped the set-up of the data applications centre and STFC’s current support involves the UK exploitation of the scientific data to be yielded from the mission. Industrial partners include Airbus, MSSL and e2v Technologies.

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