Half a century ago, astronomers observed their first pulsar: a dead, distant, ludicrously dense star that emitted pulses of radiation with remarkable regularity. So consistent was the object's signal that astronomers jokingly nicknamed it LGM-1, short for "little green men." It wasn't long before scientists detected more signals like LGM-1. That decreased the odds that these pulses of radiation were the work of intelligent extraterrestrials.

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