CP 1919

PSR 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 were briefly thought to resemble an extraterrestrial beacon, leading the source to be 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. The discoverers jokingly named it little green men 1 (LGM-1), considering that it may have originated from an extraterrestrial civilization, but Bell soon ruled out extraterrestrial life as a source after discovering a similar signal from another part of the sky.