Hydra Constellation


Located along the equator of the celestial sphere, the constellation of Hydra occupies a prominent position. This ancient constellation, one of 48 described by Claudius Ptolemy in the Almagest during the XNUMXnd century, has the distinction of being the largest and longest constellation in the night sky.

In this article we are going to tell you everything you need to know about the Hydra constellation and their characteristics.

Hydra Constellation

hydra constellation

With more than six hours of right ascension and an impressive angle of more than 100°, it captivates observers with its serpentine, multi-headed shape. However, some atlases may depict him with only one head. It is important to note that the constellation of Hydra It should not be confused with the male Hydra, which resides in the southern hemisphere.

This particular constellation, covering a vast expanse of 1302,8 square degrees, encompasses a remarkable collection of celestial objects. Among them are 3 Messier objects, 237 NGC objects and 2 Caldwell objects. Shining brightly within the Hydra is the star known as Alfard. Regarding its nomenclature, it is commonly abbreviated as Hya, called Hydra in Latin, and has the genitive form Hydrae. The Hydra constellation has a total of 238 stars.

The head of the Hydra constellation, made up of six small stars, is the most easily identifiable feature, located approximately 20° southwest of Regulus, the brightest star in the nearby Lion constellation.

Located in the posterior region of the constellation Hydra are the celestial formations known as the Raven, the Cup, and the Sextant. Located just over 15° west of the star Alpha Sextantis within the constellation Sextantus is the head of this legendary creature.


The constellation known as Hydra can be seen during a significant part of the year due to its wide range in right ascension, which It ranges from 8 hours and 10 minutes to 15 hours. This celestial formation extends along the celestial equator, making it visible, in whole or in part, from anywhere on Earth. However, it is important to note that while Hydra's head rises at positive declinations, most of its stars are located at negative declinations, specifically between 7° N and 35° S.

Celestial bodies of the constellation Hydra

hydra constellation

At the center of the Hydra constellation is Alfard, also known as Alfa Hydrae, which shines as the main and most radiant star. With a magnitude of 2, Alfard, derived from the Arabic language, means "the solitary one." Located approximately 90 light years from our own solar system, this orange-hued star illuminates the celestial Hydra.

In the Epsilon Hydrae system there is a fascinating arrangement of stars that form a multiple star system. This system consists of two primary stars, each of which shows a different color: one yellow and the other blue. The magnitudes of these components are measured at 3,8 and 4,7 respectively. Surprisingly, it takes a period of 15 years for these stars to complete one full revolution of each other. However, The unfortunate drawback is that their separation distance is only 0,2 arc seconds.

It is a binary star system made up of four stars; The third component has a magnitude of 7,8 and is located about 4,5 arcseconds from the others. Besides, there is a fifth component with a magnitude of 10, visually separated by 19 arcseconds, which orbits the other four stars at a distance of approximately 800 AU.

Within the constellation Hydra, there is a variable star that shares similarities with Mira, a star found in the constellation Whale. Known as R Hydrae, this red giant star experiences brightness fluctuations ranging from magnitude 3,5 to magnitude 10,9 over a period of 389 days, a period that gradually decreases. Located approximately 22 arcseconds away, R Hydrae has a tiny companion star with an apparent magnitude of 12.

Objects of importance

Within the constellation of Hydra you can find several celestial objects that are part of Messier's renowned catalogue. M48, an open cluster located at the threshold of naked-eye visibility, consists of several dozen stars and is most effectively observed with binoculars. This cluster was first identified by Charles Messier in 1771 and is believed to be approximately 300 million years old.

Located approximately 33.000 light-years from our solar system, the globular cluster M68 presents an observational challenge due to its apparent magnitude of 9,7. Composed of approximately 250 colossal stars, this cluster is a captivating celestial formation.

Southern Pinwheel, also known as M83, is a galaxy noted for its exceptionally high rate of star formation. With an apparent magnitude of 7,6, this galaxy is located approximately 15 million light years from our own.

Among the various celestial entities encompassed by the constellation, a multitude of objects listed in the NGC catalog can be found, including a large number of galaxies. Of particular interest is the planetary nebula NGC 3242, affectionately known as the Ghost of Jupiter, which It deserves close observation through a telescope. This remarkable phenomenon appears as a radiant disk, with a magnitude of nine and spanning approximately forty arcseconds in diameter, emitting a distinctive green hue. It is worth noting that telescopes with an aperture larger than 10 cm are capable of capturing this spectacle.

At the heart of this celestial cloud resides a tiny star of magnitude 11, responsible for the creation of this phenomenon by expelling its outer layers. As a result, The star we observe is the burning core of what it was before.

Another object worth mentioning is the remote globular cluster NGC 5694, with an apparent magnitude of 11. Located approximately 120.000 light-years from our position, this cluster serves as a reminder of the immense scale of the universe. To understand the magnitude of this distance, we must consider that the diameter of our own galaxy, the Milky Way, covers about 100.000 light years.

Hydra constellation mythology

hydra mythology

There are two ancient myths associated with the constellation Hydra. One of them involves a raven sent by the god Apollo to retrieve a glass of water. The Raven, however, arrived late because he was waiting for the figs to ripen and falsely blamed the delay on a sea monster. Apollo, aware of the deception, He punished the characters involved by transforming them into constellations in the night sky.

During one of his twelve labors, Hercules encountered a formidable many-headed hydra in the second myth. This fearsome creature had the extraordinary power of growing two new heads for each one that was cut off, which made it even more dangerous. However, Hercules successfully defeated the Hydra by decapitating it and cauterizing its neck, effectively thwarting the regeneration of its heads.

I hope that with this information you can learn more about the Hydra constellation and its characteristics.

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