Hydra is the largest of the 88 modern constellations, measuring 1303 square degrees. Also one of the longest at over 100 degrees, its southern end abuts Libra and Centaurus, and its northern end borders Cancer. It has a long history, having been included among the 48 constellations listed by the 2nd-century astronomer Ptolemy. It is commonly represented as a water snake. It should not be confused with the similarly named constellation of Hydrus. The constellation lies in the second quadrant of the southern hemisphere and can be seen at latitudes between +54° and -83°. The neighbouring constellations are Antlia, Cancer, Canis Minor, Centaurus, Corvus, Crater, Leo, Libra, Lupus, Monoceros, Puppis, Pyxis, Sextans and Virgo.
Despite its size, Hydra contains only one reasonably bright star, Alphard, designated Alpha Hydrae. It is an orange giant of magnitude 2.0, 177 light-years from Earth. Its traditional name means “the solitary one.” Beta Hydrae is a blue-white star of magnitude 4.3, 365 light-years from Earth. Gamma Hydrae is a yellow giant of magnitude 3.0, 132 light-years from Earth. Hydra has one bright binary star, Epsilon Hydrae; it has a period of 1000 years and is 135 light-years from Earth. The primary is a yellow star of magnitude 3.4, and the secondary is a blue star of magnitude 6.7. However, there are several dimmer double stars and binary stars in Hydra. 27 Hydrae is a triple star; the primary is a white star of magnitude 4.8, 244 light-years from Earth. The secondary, a binary star at magnitude 7.0 but is composed of a magnitude 7 and a magnitude 11 star; it is 202 light-years from Earth. 54 Hydrae is a binary star 99 light-years from Earth. The primary is a yellow star of magnitude 5.3, and the secondary is a purple star of magnitude 7.4. N Hydrae (N Hya) is a pair of stars of magnitudes 5.8 and 5.9. Struve 1270 (Σ1270) consists of a pair of stars, magnitudes 6.4 and 7.4. Hydra is also home to several variable stars. R Hydrae is a Mira variable star 2000 light-years from Earth; it is one of the brightest Mira variables at its maximum of magnitude 3.5. It has a minimum magnitude of 10 and a period of 390 days. V Hydrae is an unusually vivid red variable star 20,000 light-years from Earth. It varies in magnitude from a minimum of 9.0 to a maximum of 6.6. Along with its notable colour, V Hydrae is also home to at least two exoplanets. U Hydrae is a semi-regular variable star with a deep red colour, 528 light-years from Earth. It has a minimum magnitude of 6.6 and a maximum magnitude of 4.2; its period is 115 days. The constellation also contains the radio source Hydra A.
Hydra contains three Messier objects. M83, also known as the high Southern Pinwheel Galaxy, is located on the border of Hydra and Centaurus, M68 is a globular cluster near M83, and M48 is an open star cluster in the western end of the serpent. NGC 3242 is a planetary nebula of magnitude 7.5, 1400 light-years from Earth. Discovered in 1785 by William Herschel, it has earned the nickname “Ghost of Jupiter” because of its striking resemblance to the giant planet. M48 (NGC 2548) is an open cluster that is visible to the naked eye under dark skies. Its shape has been described as “triangular.” This 80-star cluster is unusually large, more than half a degree in diameter, larger than the diameter of the full Moon. There are several globular clusters in Hydra. M68 (NGC 4590) is 31,000 light-years from Earth and of the 8th magnitude. NGC 5694 is a globular cluster of magnitude 10.2, 105,000 light-years from Earth. Also called “Tombaugh’s Globular Cluster,” it is a Shapley class VII cluster; the classification indicates that it has an intermediate concentration at its nucleus. M83 (NGC 5236), the Southern Pinwheel Galaxy, is an 8th magnitude face-on spiral galaxy. It is easily observed in skies south of 40°N latitude, found by using 1, 2, 3, and 4 Centauri as guide stars. It has been host to six supernovae, more than any Messier object. NGC 3314, usually delineated as NGC 3314a and NGC 3314b, is a pair of galaxies that appear superimposed, despite the fact that they are not related or interacting in any way. The foreground galaxy, NGC 3314a, is at a distance of 140 million light-years and is a face-on spiral galaxy. The background galaxy, NGC 3314b, is an oblique spiral galaxy and has a nucleus that appears reddened because of NGC 3314a’s dusty disk. ESO 510-G13 is a warped spiral galaxy located 150 million light-years from Earth. Though most galactic disks are flat because of their rate of rotation, their conformation can be changed, as is the case with this galaxy. Astronomers speculate that this is due to interactions with other galaxies. NGC 5068 may be a member of the M83 group, but its identity is disputed. It has a low surface brightness and has a diameter of 4.5 arcminutes. It is not perfectly circular, rather, it is elliptical and oriented on a west-northwest/east-southeast axis. However, it is of fairly uniform brightness throughout. Another notable galaxy is NGC 4993, an elliptical galaxy, where it hosts GW170817, GRB 170817A and SSS17a events from two neutron stars merger
The Sigma Hydrids peak on December 6 and are a very active shower with an unknown parent body. The Alpha Hydrids are a minor shower that peaks between January 1 and 7. Credit: Wikipedia.