PSR B1919+21

PS B1919+21 is a pulsar with a period of 1.3373 seconds and a pulse width of 0.04 seconds. Discovered by Jocelyn Bell Burnell and Antony Hewish on November 28, 1967, it is the first discovered radio pulsar. The power and regularity of the signals was thought to resemble a beacon, so for a time the source was nicknamed "LGM-1" (for "little green men").

The original designation of this pulsar was CP 1919, which stands for Cambridge Pulsar at RA 19^h 19^m. It is also known as PSR J1921+2153 and is located in the constellation of Vulpecula.

In 1967, a radio signal was detected using the Interplanetary Scintillation Array of the Mullard Radio Astronomy Observatory in Cambridge, UK, by Jocelyn Bell and Antony Hewish. The signal had a 7000133730208833100♠1.337302088331-second period and 0.04-second pulsewidth. It originated at celestial coordinates 19^h 19^mright ascension, +21° declination. It was detected by individual observation of miles of graphical data traces. Due to its almost perfect regularity, it was at first assumed to be spurious noise, but this hypothesis was promptly discarded. After that, the discoverers proposed an alternative explanation that the signal might be a beacon or a communication from an intelligent extraterrestrial civilization and named it little green men 1 (LGM-1).