Hydra

Hydra is already the longest constellation in the sky, its faint length carrying the myth of Corvus and Crater across a wide stretch of stars. Far beyond that length, in the same direction as seen from Earth, lies an entire cluster of galaxies bearing the serpent's name.

The Hydra Cluster, catalogued as Abell 1060, is a rich concentration of galaxies roughly 150 million light years away, its individual members far too faint and far too numerous to have meant anything to any tradition that named the visible water snake overhead. Clusters of this kind are bound together by their own mutual gravity, hundreds of galaxies orbiting a shared centre of mass over timescales of billions of years, and the Hydra Cluster is one of several such structures used by astronomers to study how galaxies behave when crowded together rather than sitting in relative isolation the way the Milky Way mostly does.

The name is a modern astronomer's convenience, borrowed from the constellation the cluster happens to sit within as seen from Earth, exactly as the Hercules Cluster of galaxies borrows its name from the hero rather than from any mythological connection between the two.

A serpent stretched across a quarter of the visible sky turns out, at far greater distance than any star in its outline, to have an entire society of galaxies looking back along the same line of sight.

Hydra is the largest of all the constellations, one of Ptolemy's forty-eight, a long water snake winding for more than a quarter of the way around the sky south of the ecliptic. Despite its great length it is faint, its brightest star Alphard, the solitary one, marking the serpent's heart. In Greek myth it is associated with the many-headed Lernaean Hydra slain by Heracles, or with the water snake of the tale of the crow and the cup, which places the small constellations Corvus and Crater upon its back. In astronomical tradition the constellation is a star pattern rather than an astrological sign, and its watery identity groups it among the water-associated figures. These are the features of Hydra as the classical and modern traditions describe it, distinct from any astrological reading of the wide band of sky it crosses.

Hydra's great length made it a natural quarry for later astronomers wanting to carve out new figures: Sextans, the sextant, was cut from its faint northern stretch by Hevelius in 1687, commemorating the astronomical instrument he used in his own observations after a fire destroyed his observatory, while Lacaille's eighteenth-century Antlia, the air pump, and Pyxis, the compass, were drawn from stars near the constellation's southern reach, close to the old figure of Argo.