Hydrus is a modern constellation created by the Dutch astronomer Petrus Plancius, who based his view of the constellation on measurements taken by Pieter Dirkszoon Keyser and Frederick de Houtman, who were both prominent Dutch sailors during the early 16th century. Meaning “the male water snake” in Latin, the first depiction of the constellation in a celestial atlas was in Johann Bayer’s “Uranometria”, which was published in 1603.
Hydrus is located in the southern celestial hemisphere, where it takes up an area of only 243 square degrees between latitudes +8° and -90°. The constellation can be seen in the northern hemisphere from late autumn to early winter, although it is best viewed at around 9 PM during much of November. Look for Hydrus between the two Magellanic Clouds; more precisely, between the constellation Eridanus and the South Celestial Pole, and to the east of Tucana, and the west of Reticulum.
Hydrus is too far south of the Equator for it to have been visible to the ancient Greeks and Romans. The reference to sea, or water snakes, in the constellation’s name is thought to derive from the sea snakes the Dutch sailors would have encountered on their voyages around much of the world in the 16th century. An alternate view holds that the constellation was named by the French astronomer Nicolas Louis de Lacaille in 1756, who named it “l’Hydre Mâle”, to distinguish it from Hydra, which is generally taken to represent a female water snake.
In terms of its shape, it is difficult to discern a serpent, but the constellation is nevertheless easy to find due to the large triangle formed by the three brightest stars in the constellation proper. While Hydrus has no first magnitude stars, it does have some stars that are considered to be solar analogues, and one star with a large retinue of planets.
– HD 10180 is a GIV-class star, which makes it very close to the Sun (G2V) in terms of composition, size and mass. It is only 20% bigger than the Sun, 6% more massive, and 50% brighter. With an apparent visual magnitude of 7.33, the star is located about 127 light years away, and is estimated to be about 7.3 billion years old, which is slightly older than the Sun. It’s most remarkable aspect is, however, that it has the largest known number of planets of any star orbiting it- some estimates put the total number of planets around HD 10180 as high as 9, although this number is yet to be confirmed.
– Beta Hydri is another Sun-like star and the most luminous star in Hydrus, with an apparent visual magnitude of 2.80. It has a classification of G2IV, and is only 8% more massive than the Sun. However, it is 80% bigger than the Sun, and about 3.5 times as bright. Beta Hydri is also one of the oldest stars close to the Sun, and the closest sub-giant to us. In about the year 150 BC, the star was only two degrees away from the South Celestial Pole, and is now the brightest and closest star to the Pole.
– Alpha Hydri is a young, 800-million-year-old F0IV-class star located about 72 light years away, and the second most luminous star in the constellation, with an apparent visual magnitude of 2.90. The star is a yellow-white sub-giant; about twice as heavy as the Sun, and about 80% bigger, which makes it 32 times as luminous as the Sun in absolute terms.
– Gamma Hydri is a very bright M2III-class red giant, and with an apparent visual magnitude of 3.24, it is the third most luminous star in the constellation. It is perhaps the only truly remarkable star in Hydrus, since it is 60 times as big as the Sun, and at least 655 times as bright. Look for Gamma Hydri at the southeastern apex of the triangle that defines the constellation.
Notable Deep Sky Objects
Hydrus is not known for its spectacular deep sky objects but it used to have a mysterious object , known as IC 1717, that has since disappeared, which is a rare event by any standard.
– IC 1717 was a mysterious object even when Danish astronomer John Louis Emil Dreyer first recorded it at coordinates RA 01h: 32m: 30s, and DEC 67°32’12”, and entered it into the Index Catalogue of Nebulae (IC), in the late 19th century. Dreyer, who was widely credited as a highly skilled observer, described the object as “…very faint, very small, and very extended with a stellar-like nucleus”, but whatever this object was, it has since disappeared. What is known is that Eta2 Hydri, a yellow giant star is very close to the location IC 1717 had once held, and that this star has one gas planet about six times as big as Jupiter. Current theories hold that the object recorded by Dreyer was another gas planet in orbit around this star, but that it somehow got destroyed; which would explain the “extended” aspect of the object, since it would likely have become stretched out in its final few orbits around its parent star.
As of 2016, Hydrus has had four stars with confirmed planets discovered, with one star having at least 6 planets, making this the most complex system of exo-planets discovered so far.