Hydra
Hydra, the largest constellation of all, sprawling across a full quarter of the sky. It is the nine-headed monster Hydra that Heracles faced.
Science
The scienceHydra is the largest and longest of all 88 constellations, winding across more than 100 degrees of sky (about a quarter of the whole). So long is it that hours pass between the rising of its western head and the appearance of its eastern tail. For its size it has few bright stars; its only 2nd-magnitude star, the chief star Alphard (meaning "the solitary one"), glows alone and red in the southern spring sky. Near its head lie the beautiful spiral galaxy M83 and the open cluster M48.
Mythology
How it became a constellationHydra is the monstrous water serpent that appears in Heracles' second labor. This beast, dwelling in the swamp of Lerna, had nine heads, and it was immortal: cut off one and two new heads sprang up in its place. With the help of his nephew Iolaus, Heracles seared each neck with fire as he struck off its head, so that no new head could grow, and he sealed the last, immortal head beneath a great boulder. The crab Hera sent to hinder the fight became Cancer, and the fallen Hydra became this constellation, the two hung side by side in the spring sky.
Observing Guide
how to observe🔭 How to find it
The longest constellation of all, Hydra sprawls more than 100 degrees across the spring southern sky, and the easiest part to find first is the head. Come down to the southwest from Regulus (the lead star of Leo) and you'll find the serpent's head, a small group of stars, leading on to the lone reddish 2nd-magnitude star Alphard ("the solitary one"). From Alphard, faint stars wind eastward in a long, undulating body and tail reaching below Virgo.
✨ What to look for
As long as it is, Hydra offers a variety of deep-sky objects. Toward the tail lies the beautiful face-on spiral galaxy M83 (the Southern Pinwheel), and near the head sits the open cluster M48, visible to the naked eye. There's also the globular cluster M68 and the "Ghost of Jupiter" (NGC 3242), a bright planetary nebula formed by a dying star — so you can take in clusters, galaxies, and nebulae alike.
📍 Where to see it (US · Canada · UK · Australia)
The sky's longest constellation, Hydra sprawls from about +6° at the head down to −30° at the tail. From the US, Canada, and the UK it stretches across the southern sky on spring nights (February–May); the head sits at a comfortable altitude, but the tail and M83 near −30° transit low and want a clear southern horizon. From Australia the whole serpent rides high overhead during the southern autumn, with its southern end far better placed than in the north.