HYDRA "The Water Snake" pronounced HIGH-druh

Chart showing the constellations.

Hydra The Water Snake, is the largest and longest constellation in the sky, but is not easy to identify because most of the stars are faint. Apart from the brightest star, Alphard, marking the heart of the Water Snake, Hydra's only recognisable feature is its head, which is made up of an attractive group of six stars. Hydra winds its way irregularly from the head near Procyon in Canis Minor, to the tail near Libra and Centaurus.

Hydra is one of the old groups, to which Ptolemy assigned twenty-five stars in all. Hydra is usually identified with the multi-headed monster slain by Hercules, but another legend links it with Corvus the Crow and Crater the Cup, which are found on its (northern hemisphere view) back, the bird having returned to the god Apollo with Hydra in its claws as an excuse for its delayed mission to fetch water in the cup.

Chart showing the constellation.

Hydra and surrounding constellations

Constellation Crux Constellation Centaurus Constellation Lupus Constellation Libra Constellation Virgo Constellation Corvus Constellation Crater Constellation Sextans Constellation Leo Constellation Cancer Constellation Canis Minor Constellation Monoceros Constellation Canis Major Constellation Puppis Constellation Pyxis Constellation Antlia Constellation Vela

Details of some of the objects shown in the chart.

α Hydrae (Alphard, the solitary one) is a magnitude 2.0 orange giant star 130 light years away.

β Hya, is a magnitude 4.3 blue-white star 270 light years away.

γ Hya, is a magnitude 3.0 yellow giant star 105 light years away.

δ Hya, is a magnitude 4.2 blue-white star 140 light years away.

ε Hya, is a beautiful multiple star of contrasting colours. The visible but difficult double stars are yellow magnitude 3.5, and blue magnitude 6.9, but in reality this is a system of five stars.

54 Hya is an easy double star for small telescopes, consisting of yellow and purple stars of magnitudes 5.2 and 7.1, 150 light years away. 54 Hya is in the tail of Hydra, near gamma and R (see next).

R Hya is a crimson-red giant variable star with a period of 387 days. It fluctuates between magnitudes 4 and 10.

U Hya is a deep red carbon star that fluctuates irregularly between magnitudes 4.8 and 5.8.

M 48 (NGC 2548) is a large bright open cluster of about 80 stars, 3000 light years away, just visible to the unaided eye under clear dark skies, but easily seen in binoculars.

M 83 (NGC 5236) is a large face on fine spiral galaxy of 8th magnitude, visible in small telescopes. It has a bright nucleus.

NGC 3242 is a 9th magnitude planetary nebula appearing similar in size to the disk of Jupiter, giving it its popular name "The Ghost of Jupiter". This interesting object lies about 1900 light years away.


The great length of Hydra spreads the constellation across nearly 7 hours of right ascension. As a result the head of the constellation, containing eg δ and ε Hya, is due north (for the southern hemisphere) and at its highest nearly 7 hours before the tail, beyond γ and R Hya. By the time the tail is due north, the head of the constellation, which lies nearer the equator, has already set.

In terms of dates and time, α Hya is due north at 9 pm NZST, in early April, as seen from New Zealand, with the tail due north at 2.30 am. It is not until late June that the tail of Hydra is due north at 9 pm. By then α Hya transits at 3.30 pm.