Early in its history the Milky Way gobbled up many tiny galaxies. The cosmic rubble it left behind is now yielding fresh clues into how our corner of the universe came to be
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Go outside on a dark, clear night, far away from the glare of city lights, and look up. You will see the glowing band of the Milky Way arching dramatically overhead. It has now been four centuries since Galileo Galilei first turned a telescope toward this awesome sight and noted that the “milk” is actually countless individual stars, too faint to be separated by the naked eye. It took another three centuries for astronomers to convince themselves that the Milky Way is just one of billions of galaxies in the universe.
In fact, the Milky Way itself is not simply one galaxy: recent work has shown that it has lured in and engulfed many smaller galaxies over time, integrating their stars into itself. At least 20 dwarf galaxies—ranging in size from one millionth to one hundredth the size of the Milky Way—are known to orbit it now, with dozens more probably still undiscovered. And the current satellites are thought to be just a tiny fraction of those that ever existed, the rest having been drawn into our galaxy by gravity and absorbed long ago. This ingestion started when the Milky Way was younger and smaller than it is now and continues today—the satellite galaxies that still exist may eventually be swallowed up.
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