How 2 millions stars move. ESA’s Gaia mission reveals the changing face of our Galaxy

ESA’s Gaia mission traced the motion of two million stars 5 million years into the future. The data will enable scientists to investigate the formation history of our Galaxy.

Edmond Halley was the one who first discovered, in the eighteenth century, the changes in stars’ positions on the sky, even if the motion is too small and slow to be appreciated with the naked eye over human timescales. Nowadays, stellar motions can be detected with a few years’ worth of high-precision astrometric observations, and ESA’s Gaia satellite is currently leading the effort to pin them down at unprecedented accuracy.

The changing face of our Galaxy is revealed in a new video from ESA’s Gaia mission.

Launched in 2013, Gaia started scientific operations in July 2014, scanning the sky repeatedly to obtain the most detailed 3D map of our Galaxy ever made. The first data release, published in September 2016, was based on data collected during Gaia’s first 14 months of observations and comprised a list of 2D positions – on the plane of the sky – for more than one billion stars, as well as distances and proper motions for a subset of more than two million stars in the combined Tycho–Gaia Astrometric Solution, or TGAS.

The TGAS dataset consists of stars in common between Gaia’s first year and the earlier Hipparcos and Tycho-2 Catalogues, both derived from ESA’s Hipparcos mission, which charted the sky more than two decades ago.

According to an ESA press release, this video shows the 2,057,050 stars from the TGAS sample, with the addition of 24,320 bright stars from the Hipparcos Catalogue that are not included in Gaia’s first data release. The stars are plotted in Galactic coordinates and using a rectangular projection: in this, the plane of the Milky Way stands out as the horizontal band with greater density of stars. Brighter stars are shown as larger circles, and an indication of the true colour of each star is also provided; information about brightness and colour is based on the Tycho-2 catalogue from the Hipparcos mission.

The video starts from the positions of stars as measured by Gaia between 2014 and 2015, and shows how these positions are expected to evolve in the future, based on the proper motions from TGAS. The frames in the video are separated by 750 years, and the overall sequence covers 5 million years. The stripes visible in the early frames reflect the way Gaia scans the sky and the preliminary nature of the first data release; these artifacts are gradually washed out in the video as stars move across the sky.