The Gaia space observatory, built, launched, and managed by the European Space Agency, aims to create the most ambitious star catalog to date: a map of about 1% of the estimated 100 billion–400 billion stars in the Milky Way. The first data release in September 2016, DR1, provided astrometric information for about 1.1 billion stars. The second data set, DR2, released in April 2018, includes celestial positions for almost 1.7 billion additional stars.1 

When a star is formed, it usually keeps the orbital energy and angular momentum from its parent galaxy (see the article by Joseph Silk, Physics Today, April 1987, page 28). By tracking the position and velocity of many stars, astronomers can learn whether any of them were part of a galaxy when it formed or whether a star group was accreted later in a merger. Today, it’s relatively uncommon for two big galaxies to...

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