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 to 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 called 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.
– Beta Hydri, the constellation’s most luminous star, is a yellow subgiant (G2IV) located 24 light-years distant with an apparent visual magnitude of 2.80. This Sun-like star has 180% the Sun’s radius, 108% its mass, and is around 3.5 times brighter. At 6.7 billion years old, Beta Hydri is one of the oldest stars situated close to our sun, and the closest sub-giant. Around 150 BC, the star was only two degrees away from the South Celestial Pole, and is currently the brightest star close to the South Pole.
– Alpha Hydri, the second brightest star in Hydrus, is a yellow-white subgiant (F0IV) found 72 light years distant that shines with a visual magnitude of 2.90. This young 800-million-year-old star is about 80% bigger than the Sun, twice as massive, and 32 times more luminous in absolute terms. Alpha Hydri is also referred to as the Head of Hydrus.
– Gamma Hydri, the constellation’s third brightest star, is a red giant (M2III) situated 214 light years from our solar system with an apparent visual magnitude of 3.24. 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.
– HD 10180 is a yellow dwarf (G1V) found about 127 light years away with an apparent visual magnitude of 7.33. It is very close to the Sun in terms of composition, size and mass, being only 120% bigger, 6% more massive, and 150% brighter. The star is estimated to be about 7.3 billion years old, which is slightly older than the Sun, but it’s most remarkable aspect is that it has the largest number of known planets of any star orbiting it, with some estimates putting the total as high as 9, although this number is yet to be confirmed.
Other stars of interest in Hydrus includes the blue-white giant Epsilon Hydri; the white subgiant Zeta Hydri; the white dwarf Delta Hydri; the yellow giant Eta-2 Hydri; and the orange giant star Nu Hydri.
Notable Deep Sky Objects
Hydrus is not known for its spectacular deep-sky objects but it does contain the globular cluster NGC 1466; the irregular galaxy NGC 1473; and the spiral galaxy NGC 1511. It also used to have a mysterious object, known as IC 1717, that has since disappeared, which is a rare event in itself 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 is orbited by one gas planet about six times the size of 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 during 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.