On 14 September, the team behind the European Space Agency satellite Gaia released their first data set, spanning almost one year of observations. After imaging more than one billion stars, Gaia focused on the brightest two million objects, obtaining both their distance and motion in three dimensions. The all-sky map (pictured) shows that the Milky Way running across the centre of the image is larger than we thought, as are the Large and Small Magellanic Clouds — two dwarf galaxies appearing as bright spots in the lower-right region of the map. (The stripes are a measurement artefact that will disappear with more data.)

The reason for the increased sizes is Gaia's incredible sensitivity. Its billion-pixel CCD camera, orbiting 1.5 million kilometres from Earth, can measure the properties of stars that are a million (106) times fainter than the unaided human eye can observe. This is 10,000 times more stars and down to an impressive 1,000 times fainter objects than its predecessor, the Hipparcos mission.

Credit: © ESA / GAIA / DPAC / A. MOITINHO & M. BARROS CENTRA – UNIVERSITY OF LISBON

Over five years of operations, Gaia will observe each of the one billion objects (about 1% of the Milky Way) on average seventy times. This kind of sampling will allow astronomers to measure the celestial motion of stars and quantify their brightness variations. The combination of astrometry, photometry and spectroscopy provided by Gaia will be used to map in detail the baryons and dark matter distribution in our Galaxy; to refine distance measurements as far as the Magellanic Clouds, based on observations of 'standard candles' such as Cepheid stars; and to accurately determine the initial mass distribution of young stars in star-forming regions in our Galaxy.

The first data release boasts sub-milliarcsecond resolution in stellar positions and a sensitivity limit matching the pre-launch expectations. So far, Gaia has measured nearly 4,000 variable stars (12% of them previously unknown to be variable) and 400 star clusters, and resolved dark clouds in Orion, which were unknown before. It will also be able to image tens of thousands of exoplanets (Jupiter-type). These are impressive feats, considering Gaia has suffered a series of unexpected technical setbacks — these include excess water freezing on the optics of the telescope and stray light from the sunshield protecting the satellite — and is a testimony to the work of 450 people from 24 countries contributing to the Gaia consortium.