Hya · Hydrae
Hydra Constellation
IAU constellation mapHyaHydra (Hya) is one of the 88 constellations recognised by the International Astronomical Union. It covers 1303 square degrees of the equatorial sky in quadrant SQ2. Its brightest star is Alphard. The Latin genitive of the name is Hydrae, used to form star names within the constellation. In Greek mythology, Hydra is associated with greek tradition. The IAU standardised all constellation boundaries in 1930.
Quick facts
- IAU name
- Hydra
- Abbreviation
- Hya
- Latin genitive
- Hydrae
- Hemisphere
- equatorial
- Area
- 1,303 sq deg
- Brightest star
- Alphard
- Quadrant
- SQ2
- Family
- Hercules
- Mythology origin
- Greek
- Discovery era
- Ptolemy's 48 (Almagest, c. 150 CE)
Mythology
Hydra, the Water Snake, is the largest of all 88 IAU constellations, covering 1303 square degrees — it stretches across more than a quarter of the sky from the celestial equator southward. In Greek mythology Hydra was the monstrous multi-headed serpent of the Lernaean swamp, the Second Labour of Heracles. The Hydra's peculiarity was that when one head was cut off, two more grew in its place; additionally, Hera placed the Crab (Cancer) in Heracles's path to distract him. Heracles eventually resolved the problem by having his companion Iolaus cauterise each neck stump with a burning torch immediately after decapitation, preventing regrowth; the immortal central head was buried under a rock. The Corvus (Crow) and Crater (Cup) constellations ride on Hydra's back, connected to a myth about the crow who was sent to fetch water in a cup for Apollo, lingered to eat figs, and then blamed a water snake for the delay. Alphard (Alpha Hydrae), the constellation's only bright star, is an orange giant 177 light-years from Earth with magnitude 1.98 — its popular name means 'the solitary one' in Arabic, reflecting how it stands alone in an otherwise faint region of sky.
Sources: Apollodorus Bibliotheca 2.5.2; Aratus Phaenomena 443-449; Hyginus Astronomica 2.11.
Overview
Hydra is one of the 88 constellations formally recognised by the International Astronomical Union (IAU). It occupies 1303 square degrees of sky in the equatorial sky region, crossing both hemispheres, best visible at latitudes between +45 and -90 degrees in March. The constellation belongs to the Hercules family of constellations. Its Latin genitive is Hydrae, the form used when naming stars within the constellation — for example, the brightest star Alphard may appear in catalogues as Alpha Hydrae or similar. The IAU standardised the boundaries of all 88 constellations in 1930 under the direction of Eugène Delporte, using straight lines of right ascension and declination to eliminate the ambiguities of earlier variable-boundary systems.
Notable stars
The brightest star in Hydra is Alphard. Individual stars within Hydra are conventionally named using Bayer designation — Greek letters followed by the Latin genitive of the constellation name (Hydrae) — introduced by Johann Bayer in his 1603 star atlas Uranometria. Flamsteed numbers (numeric designators followed by Hydra) were added by John Flamsteed in his 1712 catalogue. Variable stars, double stars, and deep-sky objects within the Hydra boundary are catalogued by the IAU with the abbreviation Hya.
History and mythology
The figure of Hydra appears in the classical astronomical tradition. Ptolemy included this constellation in the Almagest (c. 150 CE), the definitive ancient catalogue of 48 constellations, which formed the foundation for Islamic, medieval European, and Renaissance astronomy. The constellation's figure and mythology were transmitted through works such as Aratus's Phaenomena, Eratosthenes's Catasterismi, and Hyginus's Astronomica. See the Mythology section above for the full narrative.
Sources & further reading (2)
- official-iau — accessed 2026-05-06
- encyclopedia — accessed 2026-05-06
Frequently asked questions
What does the name Hydra mean?
Hydra is the Latin name used by the IAU to designate this constellation. The name comes from classical Latin and Greek astronomical tradition. In star catalogues, stars within Hydra are designated using the genitive form of the name (e.g. Alpha Hydra or similar Latin genitive forms) — a naming convention introduced by Johann Bayer in his 1603 atlas Uranometria and still in use today.
How large is Hydra compared to other constellations?
Hydra covers 1303 square degrees of sky. The full sphere of the sky contains 41,253 square degrees, so Hydra occupies about 3.2% of the total sky. For comparison, the largest constellation is Hydra at 1303 square degrees; the smallest is Crux at 68 square degrees.
When and where is Hydra visible?
Hydra straddles the celestial equator and is visible from both hemispheres, though part of it may be low on the horizon depending on the observer's latitude. The brightest star, Alphard, serves as the main visual anchor for locating the constellation. As with all constellations, the best viewing conditions are a dark sky away from artificial light pollution, with the constellation high enough above the horizon to minimise atmospheric absorption.
