1. Introduction

Hydrus, the Water Snake, is a minor constellation located in the southern celestial hemisphere. Spanning an area of approximately 243 square degrees, it is among the smaller constellations in the night sky. Hydrus is best observed from latitudes in the Southern Hemisphere, particularly during the months of October and November, when it reaches its highest point in the sky. Hydrus is situated between the constellations of Eridanus and Octans, with its boundaries defined by celestial coordinates. Its right ascension extends from approximately 0 hours to 4 hours, while its declination ranges from about -60 degrees to -85 degrees (Figure 1). This positioning places Hydrus in an optimal viewing area for observers in the Southern Hemisphere, where it can be observed throughout the year, particularly during the southern spring and summer months. The constellation is characterized by its faintness and lack of prominent stars, with its brightest star, Alpha Hydri, having a visual magnitude of only 2.8. The neighboring constellations of Hydrus include Eridanus, Horologium, Octans, Mensa, Phoenix, and Tucana.

Figure 1. IAU chart of Hydrus. Source: https://www.iau.org/static/archives/images/screen/hyi.jpg. Credit: IAU and Sky & Telescope. Reproduced under CC BY 4.0 license.

2. Historical Background

The constellation Hydrus has a relatively short history compared to many other constellations, as it was introduced by European explorers and astronomers during the Age of Exploration in the 16th and 17th centuries.

Hydrus was first introduced by Dutch astronomer Petrus Plancius in the late 16th century. Plancius was known for creating several new constellations based on observations made by early navigators and explorers during their expeditions to the Southern Hemisphere. Hydrus was one of these new constellations, and it was depicted as a water snake, likely inspired by the Latin term for "male water snake." Plancius's constellation designs were later incorporated into celestial maps created by prominent cartographers of the time, such as Willem Blaeu and Johannes Bayer. These maps helped disseminate knowledge of the southern skies to European astronomers and navigators, aiding in navigation during the Age of Exploration. Over time, as astronomers conducted more detailed observations of the southern skies, the constellation boundaries and star patterns of Hydrus were refined. In 1752, French astronomer Nicolas Louis de Lacaille conducted an extensive survey of the southern skies from the Cape of Good Hope in South Africa. Lacaille introduced several new constellations and cataloged numerous stars, including those in Hydrus.

Lacaille's work was instrumental in defining the modern boundaries and configurations of Hydrus, as well as other southern constellations. His catalog, known as "Coelum Australe Stelliferum," provided a comprehensive overview of the southern celestial hemisphere and remains an important resource for astronomers studying the southern skies. Today, Hydrus is recognized as one of the 88 modern constellations officially defined by the International Astronomical Union (IAU). While it may lack the mythological associations of many classical constellations, Hydrus holds historical significance as one of the constellations introduced during the Age of Exploration and remains an essential feature of the southern night sky.

3. Notable Stars

Alpha Hydri is the second brightest star in the southern circumpolar constellation of Hydrus. It is readily visible to the naked eye with an apparent visual magnitude of +2.9. It is sometimes informally known as the Head of Hydrus. It is one of only three stars in the constellation Hydrus that are above the fourth visual magnitude. This star can be readily located as it lies to the south and east of the prominent star Achernar in the constellation Eridanus.

Beta Hydri, also known as β Hydri, is the brightest star in the constellation Hydrus. It is a yellow-white dwarf star located approximately 24.33 light-years away from Earth. With a visual magnitude of about 2.8, Beta Hydri is easily visible to the naked eye from the Southern Hemisphere. This star is similar in spectral type to our Sun.  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.

Gamma Hydri, is a solitary, red-hued star, marks the southeastern apex of a large triangle of stars that dominates the modern constellation of Hydrus. It has an apparent visual magnitude of 3.26. Based upon an annual parallax shift of 15.24 mas as measured from Earth, the system is located about 214 light-years from the Sun. 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. Alpha and Beta Hydri are both yellow to white mid-temperature stars, the latter a dwarf, the former a subgiant, that lie relatively nearby. In contrast, Gamma is a luminous class M (M2) red giant with a temperature of about 3800 Kelvin.

4. Deep-Sky Objects

NGC 1466 is a globular cluster in the deep southern constellation of Hydrus. It is located in the outskirts of the Large Magellanic Cloud, which is a satellite galaxy of the Milky Way. The object was discovered in 1834 by English astronomer John Herschel. John Dreyer described it as "pF, pS, iR, glbM, *7 f", meaning "pretty faint, pretty small, irregular round, gradually a little brighter middle, with a 7th magnitude star nearby". When using a small telescope, this is a "faint, small, unresolved and difficult" target with an angular size of 1.9 arc minutes. It has an integrated visual magnitude of 11.4.

Although not entirely within the boundaries of Hydrus, the Large Magellanic Cloud is a prominent deep-sky object visible from the constellation. The LMC is a satellite galaxy of the Milky Way, located approximately 160,000 light-years away from Earth. The LMC is the second- or third-closest galaxy to the Milky Way, after the Sagittarius Dwarf Spheroidal. The LMC has a wide range of galactic objects and phenomena that make it known as an "astronomical treasure-house".