Last time we mentioned the time when Soviet radio astronomers went public about listening in on radio signals from an alien civilization. As it turned out, what they were actually listening to was a quasar: the blazing core of a galaxy that can emit radio waves that fluctuate in a regular pattern. So regular, in fact, that people thought it was an alien intelligence.
Well, it wasn't. However, in 1967, British radio astronomers spotted a signal much closer to our planet that was turning on and off with unbelievable regularity. Sure, the Soviets had been duped by the same kind of thing, but the exactitude with which this signal kept time was astounding, it was accurate to ten figures! It had to be a beacon of some kind. Aliens!
A little more cautious than the Soviets, though, the British astronomers didn't go public. They named the mysterious radio source LGM-1, for Little Green Men, and continued to study it.
It turned out what they had was the first pulsar to be detected. A pulsar is the final state of a massive star that has collapsed down to about the size of New York City. As it rotates it can send out radio beams that seem, from our vantage here on earth, to be turning on and off rapidly, like a code, but it's an entirely natural phenomenon.
Not that aliens sending out radio signals wouldn't be a natural phenomenon as well. After all, there is one planet at least where indigenous life beams out radio signals all the time. You're listening to one.