Hydrus is a small constellation in the deep southern sky. It was one of twelve constellations created by Petrus Plancius from the observations of Pieter Dirkszoon Keyser and Frederick de Houtman, and it first appeared on a 35-cm (14 in) diameter celestial globe published in late 1597 (or early 1598) in Amsterdam by Plancius and Jodocus Hondius. The French explorer and astronomer Nicolas Louis de Lacaille charted the brighter stars and gave their Bayer designations in 1756. Its name means “male water snake,” as opposed to Hydra, a much larger constellation that represents a female water snake. It remains below the horizon for most Northern Hemisphere observers. Irregular in shape, Hydrus is bordered by Mensa to the southeast, Eridanus to the east, Horologium and Reticulum to the northeast, Phoenix to the north, Tucana to the northwest and west, and Octans to the south. Covering 243 square degrees and 0.589% of the night sky, it ranks 61st of the 88 constellations in size. In the equatorial coordinate system, the ascension coordinates of these borders lie between 00h 06.1m and 04h 35.1m, while the declination coordinates are between −57.85° and −82.06°.
As one of the deep southern constellations, Keyzer and de Houtman assigned fifteen stars to the constellation in their Malay and Madagascan vocabulary, with a star that would be later designated as Alpha Hydri marking the head, Gamma the chest and a number of stars that were later allocated to Tucana, Reticulum, Mensa and Horologium marking the body and tail. Beta Hydri, the brightest star in Hydrus, is a yellow star of apparent magnitude 2.8, lying 24 light-years from Earth. It has about 104% of the mass of the Sun and 181% of the Sun’s radius, with more than three times the Sun’s luminosity. The spectrum of this star matches a stellar classification of G2 IV, with the luminosity class of ‘IV’ indicating this is a subgiant star. As such, it is a slightly more evolved star than the Sun, with the supply of hydrogen fuel at its core becoming exhausted. It is the nearest subgiant star to the Sun and one of the oldest stars in the solar neighbourhood. Thought to be between 6.4 and 7.1 billion years old, this star bears some resemblance to what the Sun may look like in the far distant future, making it an object of interest to astronomers. It is also the closest bright star to the south celestial pole. Located at the northern edge of the constellation and just southwest of Achernar is Alpha Hydri, a white sub-giant star of magnitude 2.9, situated 72 light-years from Earth. Of spectral type F0IV, it is beginning to cool and enlarge as it uses up its supply of hydrogen. It is twice as massive and 3.3 times as wide as our sun and 26 times more luminous. In the southeastern corner of the constellation is Gamma Hydri, a red giant of spectral type M2III located 214 light-years from Earth. It is a semi-regular variable star, pulsating between magnitudes 3.26 and 3.33. It is around 1.5 to 2 times as massive as our Sun and has expanded to about 60 times the Sun’s diameter. It shines with about 655 times the luminosity of the Sun. Located 3° northeast of Gamma is the VW Hydri, a dwarf nova of the SU Ursae Majoris type. It is a close binary system that consists of a white dwarf and another star, the former drawing off matter from the latter into a bright accretion disk. These systems are characterised by frequent eruptions and less frequent supereruptions. One of the brightest dwarf novae in the sky, it has a baseline magnitude of 14.4 and can brighten to magnitude 8.4 during peak activity.
Three systems have been found to have planets, most notably the Sun-like star HD 10180, which has seven planets, plus possibly an additional two for a total of nine—as of 2012 more than any other system to date, including the Solar System. Lying around 127 light-years from the Earth, it has an apparent magnitude of 7.33. GJ 3021 is a solar twin—a star very like our own Sun—around 57 light-years distant with a spectral type G8V and magnitude of 6.7. It has a Jovian planet companion (GJ 3021 b). Orbiting about 0.5 AU from its sun, it has a minimum mass 3.37 times that of Jupiter and around 133 days. The system is a complex one as the faint star GJ 3021B orbits at a distance of 68 AU; it is a red dwarf of spectral type M4V. HD 20003 is a star of magnitude 8.37. It is a yellow main sequence star of spectral type G8V a little cooler and smaller than our Sun around 143 light-years away. It has two planets that are around 12 and 13.5 times as massive as the Earth with periods of just under 12 and 34 days respectively.
Hydrus contains only faint deep-sky objects. IC 1717 was a deep-sky object discovered by the Danish astronomer John Louis Emil Dreyer in the late 19th century. The object at the coordinate Dreyer observed is no longer there and is now a mystery. It was very likely to have been a faint comet. PGC 6240, known as the White Rose Galaxy, is a giant spiral galaxy surrounded by shells resembling rose petals, located around 345 million light years from the Solar System. Unusually, it has cohorts of globular clusters of three distinct ages suggesting bouts of post-starburst formation following a merger with another galaxy. The constellation also contains a spiral galaxy, NGC 1511. Located mostly in Dorado, the Large Magellanic Cloud extends into Hydrus. The globular cluster NGC 1466 is an outlying component of the galaxy and contains many RR Lyrae-type variable stars. It has a magnitude of 11.59 and is thought to be over 12 billion years old. Two stars, HD 24188 of magnitude 6.3 and HD 24115 of magnitude 9.0, lie nearby in its foreground. NGC 602 is composed of an emission nebula and a young, bright open cluster of stars that is an outlying component on the eastern edge of the Small Magellanic Cloud, a satellite galaxy to the Milky Way. Most of the cloud is located in the neighbouring constellation Tucana. Credit: Wikipedia.