Date: 16 August 2017
Satellite: Gaia
Depicts: Preliminary colour map of the sky
Copyright: ESA/Gaia/DPAC/CU5/DPCI/CU8/F. De Angeli, D.W. Evans, M. Riello, M. Fouesneau, R. Andrae, C.A.L. Bailer-Jones

This map is a preview of Gaia's measurements of the sky in colour, generated using preliminary data from the photometric instrument on board.

The image includes data from 18.6 million bright stars observed by Gaia between July 2014 and May 2016, and it shows the middle value (median) of the colours of all stars that are observed in each pixel. The colour of each star is estimated by comparing the total amount of light in the blue and red spectra provided by the photometric instrument.

Even though this map is only meant as an appetizer to the full treat of next year's release, it is already possible to spot some interesting features.

The Galactic Plane, corresponding to the most densely populated region of our Milky Way galaxy, stands out as the roughly horizontal structure that extends across the image.

The reddest regions in the map, mainly found near the Galactic Centre, correspond to dark areas in the density of stars: these are clouds of dust that obscure part of the starlight, especially at blue wavelengths, making it appear redder – a phenomenon known as reddening.

It is also possible to see the two Magellanic Clouds – small satellite galaxies of our Milky Way – in the lower part of the map.

The stars in this map have been selected randomly from all Gaia stars with magnitude brighter than 17 and for which both colour measurements (from the blue and the red channels of Gaia's photometric instrument) are available.

The map shows values of the colour index, (BP-RP), ranging from 0 (bluer stars) to 3 (redder stars). This is a subset of all (BP-RP) measurements obtained by Gaia, which range from -0.2 to 3.6.

Gaia’s first full-colour all-sky map, based on data for more than 1 billion stars, will be unleashed in its highest resolution with the second data release in April 2018.