The Arecibo message, to which Sagan contributed, was aimed at which star cluster? It was aimed at Messier 13, or M13, the Great Hercules Cluster.
The message was transmitted from Puerto Rico’s Arecibo Observatory on November 16, 1974, during a ceremony marking improvements to the telescope. It carried 1,679 binary digits encoding information about numbers, chemical elements, DNA, a human figure, Earth’s population, the Solar System, and the transmitting telescope.
M13 was selected because it is a large, relatively nearby globular cluster visible from Arecibo at the time of transmission. It lies roughly 25,000 light-years away in the constellation Hercules, so the signal will take about 25,000 years to reach the cluster. Because M13 is moving through the Milky Way, the message is aimed toward its present direction rather than a fixed point.
The broadcast was mainly a demonstration of radio-astronomy and human signaling technology, not an expectation of an immediate alien conversation. The message’s designers included Frank Drake, James C. G. Walker, Linda French, Richard Isaacman, Carl Sagan, and others.