Stars 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. Lacaille charted and designated 20 stars with the
Bayer designations Alpha through to Tau in 1756. Of these, he used the designations Eta, Pi and Tau twice each, for three sets of two stars close together, and omitted Omicron and Xi. He assigned Rho to a star that subsequent astronomers were unable to find. 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. 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 the Sun and 26 times more luminous. It is a
semi-regular variable star, pulsating between magnitudes 3.26 and 3.33. Observations over five years were not able to establish its
periodicity. It is around 1.5 to 2 times as massive as the 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. The former are smooth, while the latter exhibit short "superhumps" of heightened activity. 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. There are two notable optical double stars in Hydrus. Pi Hydri, composed of
Pi1 Hydri and
Pi2 Hydri, is divisible in binoculars. Pi1 is a red giant of spectral type M1III that varies between magnitudes 5.52 and 5.58. Pi2 is an orange giant of spectral type K2III and shining with a magnitude of 5.7, around 488 light-years from Earth. Eta Hydri is the other optical double, composed of
Eta1 and
Eta2. and is located just over 700 light-years away. Eta2 has a magnitude of 4.7 and is a yellow giant star of spectral type G8.5III around 218 light-years distant, which has evolved off the main sequence and is expanding and cooling on its way to becoming a red giant. Calculations of its mass indicate it was most likely a white A-type main sequence star for most of its existence, around twice the mass of the Sun. A planet,
Eta2 Hydri b, greater than 6.5 times the mass of
Jupiter was discovered in 2005, orbiting around Eta2 every 711 days at a distance of 1.93
astronomical units (AU). Three other 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 from the Earth, it has an apparent magnitude of 7.33.
GJ 3021 is a
solar twin—a star very like the 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 star, it has a minimum mass 3.37 times that of Jupiter and a period of 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 the 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.
Deep-sky objects 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, which lies edge on to observers on Earth and is readily viewed in amateur telescopes. 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. ==See also==