It may sound intimidating to a novice stargazer to understand the fact that there are a total of 88 official constellations recognized by modern astronomers. However, if you step outside and focus your gaze towards the southern horizon, chances are that you will be staring at one in particular, the Hydra. Spotted in the southern hemisphere, Hydra is the longest constellation in the sky and is also the largest in terms of area. The constellation is so long that it takes more than six hours to rise completely. Referring to the constellation’s shape, which resembles a twisting snake, Hydra means ‘water snake’ in Latin. It was created by the Greek astronomer Ptolemy in the 2nd century. It belongs to the Hercules family of constellations and is best visible in the month of April. Let us discuss about Hydra in detail.
A few major stars in the constellation include:
- Alphard (Alpha Hydrae): Alphard is the brightest star in Hydra. With a visual magnitude of 2.0, it is about 177 light years away from the sun. The estimated age of Alphard is believed to be 420 million years old.
- Hydrae (Gamma Hydrae): It is the second brightest star in the constellation. It is also 115 times brighter than the sun and its estimated age is believed to be 372 million years. With a magnitude of about 2.993, it is about 133.8 light years away from the earth.
- Hydrae (Zeta Hydrae): It is the third brightest star with an estimated age of about 400 million years. With a visual magnitude of 3.10, it is about 167 light years away. Zeta Hydrae is about 132 times brighter than the sun.
- Hydrae (Beta Hydrae): It is basically a binary star and is about 370 light years away from the solar system.
- 27 Hydrae: It is basically an evolved yellow giant star, about 243.4 light years away from the sun. It is also believed to have a low–mass companion. It has a visual magnitude of about 4.82.
- Minchir (Sigma Hydrae): Minchir is an orange giant in Hydra. With a visual magnitude of 4.44, it is about 353 light years away from the sun.
- R Hydrae: It is basically a red pulsating variable star, which is in its last stage of evolution. R Hydrae is a red giant. It has a visual magnitude of about 3.5 to 10.9 over a period of 389 days. It is about 410 light years away.
- V Hydrae: It is a carbon star in this constellation and is one of the reddest stars in the night sky. V Hydrae has a visual magnitude of 7.0 to 11.6 over a period of 18 years and is about 1,300 light years away.
- U Hydrae: In this constellation, U Hydrae is one of the few carbon stars, which are visible without binoculars. U Hydrae is a bright giant with a visual magnitude of about 4.93 and is about 677 light years away.
The other notable stars in this constellation are Hydrae (Epsilon Hydrae), Hydrae (Nu Hydrae), π Hydrae (Pi Hydrae) and δ Hydrae (Delta Hydrae).
Hydra has some interesting deep sky objects. Some of the notable ones are:
- Messier 48 (NGC 2548): It is basically an open star cluster in the constellation and is believed to be around 300 million years old. It has a visual magnitude of 5.5 and is about 1,500 light years away from the earth. Under good conditions, it is visible without binoculars.
- Messier 68 (NGC 4590): It is basically a globular cluster in Hydra. It has a visual magnitude of about 9.67 and is about 33,600 light years away from the solar system.
- Hydra Cluster: It is a galaxy cluster in the constellation and comprises of 157 bright galaxies, all of which are 150,000 light years in diameter. It is notable as it contains a high proportion of dark matter. The cluster is about 190.1 million light years away from the solar system.
- Southern Pinwheel Galaxy (NGC 5236): It is basically a barred spiral galaxy with a visual magnitude of about 7.54 and is about 14.7 million light years away from the sun. It is also one of the brightest barred spiral galaxy, which under good conditions can be seen with binoculars.
- Ghost of Jupiter (Caldwell 59): Caldwell 59 is a planetary nebula in the constellation. It is often referred to as the Ghost of Jupiter and sometimes, the Eye Nebula. With a visual magnitude of 8.60, it is about 1,400 light years away.
- NGC 3109: It is basically a spiral or an irregular galaxy in the constellation. It is known for a large number of planetary nebulae and a considerable amount of dark matter. NGC 3109 has a visual magnitude of 10.4 and is about 4.3 million light years away.
- NGC 3621: It is basically a spiral galaxy in Hydra. With a visual magnitude of 10.0, it is about 22 million light years away from the solar system. Under good conditions, it can be easily observed with a moderate–sized telescope.
- Tombaugh’s Globular Cluster (Caldwell 66): Caldwell 66 is one of the oldest known globular clusters, which is located in the Milky Way galaxy. It is believed that it formed about 12 million years ago. Caldwell 66 has a visual magnitude of 10.2
The other notable deep sky objects in this constellation are NGC 3054, ESO 510–G13 (PGC 49473), NGC 5078, NGC 3314 and NGC 4980.
Hydra lies in the second quadrant of the southern hemisphere and can be seen at latitudes between +54° and -83°.
It occupies an area of 1303 square degrees, making it the largest constellation.
There are different myths associated to Hydra. According to one Greek mythology, Hydra is represented as the water snake that Apollo’s crow had carried with himself to the god as an excuse for returning very late after being sent to fetch some water. Anguished by the fraud, the god cast the bird, the cup and the water snake into the sky and they became the constellations Corvus, Crater and Hydra respectively. The constellation is also associated with the Lemaean Hydra, a water beast with seven heads who was killed by Heracles as one of his Twelve Labours.
Hydra is surrounded by constellations like Antlia, Canis Minor, Cancer, Centaurus, Corvus, Crater, Leo, Libra, Lupus, Monoceros, Puppis, Pyxis, Sextans and Virgo.
