Free Astronomy Magazine January-February 2019

50 JANUARY-FEBRUARY 2019 SPACE CHRONICLES A ll-sky distribution of an ‘odd collection’ of stars detected in the second data release of ESA’s Gaia mission. These stars move along elongated trajectories in the opposite direction to the majority of our Milky Way’s other hundred billion stars and have a markedly different chemical composition, in- dicating that they belong to a clearly distinct stellar population. [ESA/Gaia/DPAC; A. Helmi et al 2018] C omputer simulation of the merger between a galaxy like the young Milky Way, whose stars are shown in cyan, and a smaller galaxy, indicated in red. [Koppelman, Villalobos & Helmi, Kapteyn Astronomical Institute, University of Groningen, The Netherlands] under Mount Etna, in Sicily, and re- sponsible for local earthquakes. Sim- ilarly, the stars of Gaia-Enceladus were deeply buried in the Gaia data, and they have shaken the Milky Way, leading to the formation of its thick disc,” explains Amina. Even though no more evidence was really needed, the team also found hundreds of variable stars and 13 globular clusters in the Milky Way that follow similar trajectories as the stars from Gaia-Enceladus, indicat- ing that they were originally part of that system. Globular clusters are groups of up to millions of stars, held ! Way. Ten billion years ago, however, when the merger with Gaia-Ence- ladus took place, the Milky Way itself was much smaller, so the ratio be- tween the two was more like four to one. It was therefore clearly a major blow to our Galaxy. “Seeing that we are now starting to unravel the formation history of the Milky Way is very exciting,” says An- thony Brown, Leiden University, The Netherlands, who is a co-author of the paper and also chair of the Gaia Data Processing and Analysis Con- sortium Executive. Since the very first discussions about building Gaia 25 years ago, one of the mission’s key objectives was to examine the various stellar streams in the Milky Way, and reconstruct its early history. That vision is paying off. “Gaia was built to answer such questions,” says Amina. “We can now say this is the way the Galaxy formed in those early epochs. It’s fantastic. It’s just so beautiful and makes you feel so big and so small at the same time.” “By reading the motions of stars scattered across the sky, we are now able to rewind the history of the Milky Way and discover a major milestone in its formation, and this is possible thanks to Gaia,” con- cludes Timo Prusti, Gaia project sci- entist at ESA. together by their mutual gravity and orbiting the centre of a galaxy. The fact that so many clusters could be linked to Gaia-Enceladus is another indication that this must have once been a big galaxy in its own right, with its own entourage of globular clusters. Further analysis revealed that this galaxy was about the size of one of the Magellanic Clouds – two satellite galaxies roughly ten times smaller than the current size of the Milky