Though relatively few earthlings are aware of it, they are embedded in a huge, disk-shaped spiral galaxy. Earth's astronomers have a hard time seeing much else; every star visible in the sky is part of it.* They have an even harder time seeing into its heart (located roughly in the direction of the constellation Sagittarius) because it is obscured by the close-packed stars and cosmic dust that comprise the Milky Way.
But radio waves pass through cosmic dust clouds. By tracking the 21-cm. waves given off by hydrogen, radio astronomers have been able to probe...
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