The Andromeda Nebula
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A century ago, universe and galaxy were interchangeable words. And most scientists thought the entire universe was only 30,000 light years across. Then an astronomer noticed something everyone else had missed in photographs of a particular nebula. When he calculated how far away it is, minds were blown. And it turns out, what he saw is even farther away than he thought. And today, a telescope named after him enables us to see many times farther than that.
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