Hercules

Hercules's own article rightly gives pride of place to M13, the Great Globular Cluster whose stars are packed inside our own galaxy and which carried the 1974 Arecibo message toward it. A second cluster, much farther away and a different kind of object entirely, sits in the same figure and is easy to confuse with the first by name alone.

Abell 2151, the Hercules Cluster of galaxies, is a rich concentration of several hundred galaxies roughly 500 million light years away, far beyond anything in our own Milky Way, notable among galaxy clusters for the unusually large number of its members caught in the act of merging or interacting with one another rather than sitting apart as isolated systems. Where M13 is a ball of stars bound together by gravity within a single galaxy, Abell 2151 is an entire cluster of separate galaxies, each itself containing billions of stars, a difference of scale that the shared word "cluster" tends to hide rather than reveal.

The two objects have nothing to do with each other beyond sharing the constellation's outline as their address in the sky as seen from Earth, one a nearby star cluster suitable for a symbolic message to any possible listener, the other a distant laboratory for watching how galaxies collide and merge over time.

Hercules, known first simply as the Kneeler before it was fixed as the hero, holds one famous cluster and one considerably less famous one, at wildly different distances, both content to share his name.

Hercules is a large northern constellation, one of Ptolemy's forty-eight, representing the greatest of the Greek heroes, though anciently it was known simply as the Kneeler, a figure kneeling with one foot raised, before it was firmly identified with Heracles. It has no first-magnitude stars, but its central four stars form a distinctive shape called the Keystone, and within it lies the Great Globular Cluster of Hercules, one of the finest such clusters in the northern sky. The hero is shown amid the sites of his labours, near the dragon Draco and the constellations he overcame. In astronomical tradition the constellation is a pattern of stars rather than a zodiac sign. These are the features of Hercules as the classical and modern traditions describe it, distinct from any astrological reading of the sky it occupies.

Between Hercules and Cygnus, Hevelius in 1687 introduced Vulpecula, the little fox, originally depicted grasping a goose in its jaws; the goose was later dropped from the figure, leaving only the fox, a reminder that even the officially adopted modern constellations were not always fixed in their final form on first introduction. The absence of any first-magnitude star has not diminished Hercules's importance to observers, since its globular cluster, catalogued by Messier as M13, is among the finest deep-sky objects visible from northern latitudes and was the target of the famous 1974 Arecibo radio message aimed at any potential audience among the stars.