A visualisation of how Gaia scanned the sky during its first 14 months of operations, between July 2014 and September 2015.The oval represents a projection of the celestial sphere, with different portions of the sky gradually appearing, according to when and how frequently they were scanned by Gaia.
The satellite scans great circles on the sky, with each scan lasting about 6 hours. During the first month, the scanning procedure was such that the ecliptic poles were always included. This meant that Gaia observed the stars in those regions many times, providing an invaluable database for the initial calibration of the observations.
Then, starting on 21 August 2014, the satellite started its main survey, employing a scanning ‘law’ designed to achieve the best possible coverage of the whole sky.
Data collected over Gaia's first 14 months make up the mission’s first data release. Based on these observations, scientists produced a map of the sky, showing the density of stars observed by Gaia. Brighter regions indicate denser concentrations of stars, while darker regions correspond to patches of the sky where fewer stars are observed.
The Galactic Plane, where most of the Milky Way’s stars reside, is visible as the brightest portion of the map: a horizontal strip that is especially bright at the centre. Darker regions across the Galactic Plane correspond to dense clouds of interstellar gas and dust that absorb starlight along the line of sight.
Stellar clusters also appear, sprinkled across the image alongside a handful of nearby galaxies – most notably the Large and Small Magellanic Clouds in the lower right.
A number of artefacts, reflecting Gaia's scanning law, are also visible on the all-sky map. As this map is based on observations performed during the mission’s first year, the survey is not yet uniform across the sky. These artefacts will gradually disappear as more data are gathered during the five-year mission.
Acknowledgement: B. Holl (University of Geneva, Switzerland), A. Moitinho & M. Barros (CENTRA – University of Lisbon), on behalf of DPAC