Astronomers can measure a galaxy's mass by how stars move within it: The faster they move, the more massive it is. With this knowledge, a University of California, Irvine, team found that Segue 2, discovered in 2009, weighs at most 150,000 times more than our sun -the puniest galaxy known. (Our normal-size Milky Way is 10 million times more massive.) Segue 2 is probably just one of many such astronomical miniatures orbiting the Milky Way, says UCI astronomer Evan Kirby: "Simulations predict that there should be tens of thousands of these things."
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