The Arecibo message is an interstellar radio message carrying basic information about humanity and Earth that was sent to globular star cluster M13 in 1974.

It was meant as a demonstration of human technological achievement, rather than a real attempt to enter into a conversation with extraterrestrials. It has been noted that the low resolution of the image makes it infeasible for any extraterrestrial recipients to attach the intended meaning to most of its elements. The message was broadcast into space a single time via frequency modulated radio waves at a ceremony to mark the remodeling of the Arecibo radio telescope in Puerto Rico on 16 November 1974. The message was aimed at the current location of M13, about 25,000 light years from Earth, because M13 was a large and relatively close collection of stars that was available in the sky at the time and place of the ceremony.

Links:
1st intentional signal to space sent by Arecibo