In Oscar Wilde’s Lady Windermere’s Fan it is said that we are all in the gutter, but some of us are looking at the stars. Taking this sentiment one step further, the European Space Agency (ESA) in 2006 commissioned a satellite called Gaia to look at áll the stars. Launched in 2013 at a cost of €740 million, Gaia has since mapped 1142 million stars.

Gaia’s mission is detecting stars, and measuring their properties in order to build up the most precise 3D map of the Milky Way. Accurately measuring the motion of each star will enable astronomers to peer back in time, which, in turn, will help in understanding the history of the Milky Way, its evolution and destiny.

As described on ESA’s website, which allows you to download Gaia’s images, Gaia registers stars, with only a selection of its data being transmitted back to Earth. However, in the densest regions of the sky, there are so many stars, so close together that the detection and processing system of Gaia can’t cope with it. This results in a less complete census of these crowded areas.

To cope with this, a scientific selection of high-density regions has been made, in order to cover them with a special imaging mode. This is the mode that is illustrated in the tweet and downloadable on the website. These kind of observations are carried out routinely, every time Gaia scans these regions. The image shown was taken on 7 February 2017, and covers part of Sagittarius.

The stellar density here is an incredible 4.6 million stars per square degree. The image covers about 0.6 square degrees, making it conceivable that there are some 2.8 million stars captured in this image sequence alone.

The image is presented in strips, with each strip representing a sky mapper Charged Coupled Devices (CCD). The image is lightly processed to bring out the contrast of the bright stars and darker traces of gas and dust. Zooming in reveals some imaging artefacts related to the CCD’s, including vertical striping and short bright streaks which indicate cosmic rays. Analysis of the images will only start once the effort required by the routine data processing allows for it. For more information about the CCD’s, ESA has made a clip on the workings of Gaia’s cameras:

Gaia first catalogue of more than a billion stars was released in September 2016, and was based on the first 14 months of data collection. Next release is targeted in April 2018, after that in 2020 and 2022.