Free Astronomy Magazine January-February 2019

48 JANUARY-FEBRUARY 2019 SPACE CHRONICLES Gaia uncovers major event in the formation of the Milky Way by ESA E SA’s Gaia mission has made a major breakthrough in unravel- ling the formation history of the Milky Way. Instead of forming alone, our Galaxy merged with another large galaxy early in its life, around 10 billion years ago. The evidence is littered across the sky all around us, but it has taken Gaia and its extraordinary precision to show us what has been hiding in plain sight all along. Gaia measures the position, move- ment and brightness of stars to un- precedented levels of accuracy. Using the first 22 months of observa- tions, a team of astronomers led by Amina Helmi, University of Gronin- gen, The Netherlands, looked at seven million stars – those for which the full 3D positions and velocities are available – and found that some 30,000 of them were part of an ‘odd collection’ moving through the Milky Way. The observed stars in par- ticular are currently passing by our solar neighbourhood. We are so deeply embedded in this collection that its stars surround us almost completely, and so can be seen across most of the sky. Even though they are interspersed with other stars, the stars in the col- lection stood out in the Gaia data