Gaia, ESA’s billion-star surveyor, is detecting stars and measuring their properties to build up the most precise 3D map of the Milky Way. By measuring the motion of each star, astronomers will be able to look back in time to understand the Milky Way’s history, its evolution and its destiny.
In general, as Gaia registers stars, only data covering the object of interest is transmitted to the ground. However, in the densest regions on the sky there are more stars close to each other than Gaia’s detection and processing system can cope with, which could result in a less complete census in these crowded areas.
To make life easier, a scientific selection is made and these high-density regions are covered in a special imaging mode, as illustrated here.
The image, taken on 7 February 2017, covers part of the Sagittarius I Window (Sgr-I) located only two degrees below the Galactic Centre. Sgr-I has a relatively low amount of interstellar dust along the line of sight from Earth, making it easier to see the Galactic Centre.
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 appears in strips, each representing a sky mapper CCD. The image has been lightly processed to bring out the contrast of the bright stars and darker traces of gas and dust. Zooming in reveals some imaging artifacts relating to the CCDs, including some vertical striping, as well as short bright streaks indicating cosmic rays.
http://www.esa.int/spaceinimages/Images/2017/08/Gaia_sky_mapper_image_near_the_Galactic_centre
