Star Constellation Facts: Serpens

Star Constellation Facts: Serpens
Ophiuchus and Serpens as depicted on a constellation card published in 1825

Serpens is a faint constellation that is unusual in the sense that it is divided into two parts, with both parts of “the Serpent” depending on each other, as well as a third constellation, Ophiuchus, for its mythological representation in the night sky. In terms of its shape, the constellation somewhat resembles the constellation of Hydra, on which it is loosely based, but with the major difference being that Serpens has a distinct “head”, whereas Hydra has no such discernible mark. The constellation’s brightest stellar object, Unukalhai, is a giant orange star shinning at magnitude 2.62 around 75 light years from Earth.


Out of the 88 recognized constellations, Serpens ranks at 23rd in terms of size, taking up an area of 637 square degrees of the celestial heavens. The constellation in its entirety can be seen between latitudes +80° and -80°, and from the northern hemisphere is best seen in the summer time, especially at about 9 PM (local time) during the month of July. Although Serpens is taken to be a northern hemisphere constellation, it is divided into two distinct sections by the constellation Ophiuchus (“Serpent Bearer”); with Serpens Caput, representing the head of the serpent, in the northern sky; and Serpens Cauda, the tail of the snake, lying in the southern hemisphere. The constellation can be found wrapped around Ophiuchus, just above the zodiacal constellation of Scorpius (“the Scorpion”).


In classical Greek mythology, there is a story that associates Serpens with a snake held by Asclepius, represented by the nearby constellation of Ophiuchus. One of Apollos’ many sons, Asclepius was brought up and instructed in the art of medicine by the wise centaur Chiron, and as the story goes he became so adept as a physician and healer that he could bring the dead back to life. After killing a snake, Asclepius witnessed another snake restore it to life merely by placing some unknown herbs on it, which he subsequently used himself to then bring people back to life. The constellation is generally taken to represent a huge serpent held by Asclepius, and in classical depictions, Asclepius is shown holding Serpens Caput (the top half of Serpens) in his left hand, and Serpens Cauda (the bottom half of Serpens) in his right hand, thus providing context for both halves of the constellation.

Notable Stars

Even though Serpens does not contain any stars of first magnitude, it has some spectacular stars nevertheless.

Star Constellation Facts: Serpens-Unukalhai (Alpha Serpentis) is located about 75 light years away and with an apparent magnitude of 2.62 is the most luminous star in the whole constellation. It is a binary system in which the primary component is an orange, K2 III-class giant that is roughly 12 times as big as the Sun, and at least 38 times as bright. The companion is an 11th magnitude star about 58 arc seconds away from the primary star. The star’s name derives from the Arabic phrase for “the serpent’s neck.”

– Eta Serpentis, the second most luminous star in the constellation, is located in Serpens Cauda, the tail of the snake. A K0 III-IV-class subgiant, it is well on its way to becoming a true giant, and it is currently twice as massive as the Sun, 5.8 times as big, and at least 19 times as bright. It has an apparent visual magnitude of 3.260 as seen from its distance of about 60 light years from Earth.

– Mu Serpentis, the constellation’s third brightest star, is a blue dwarf (A0V) situated 156 light years from our solar system with a visual magnitude of 3.54. It is almost three times the size of our sun, and is located at the snake’s head in Serpens Caput.

– Xi Serpentis is a triple star system located 105 light years distant with a combined apparent magnitude of 3.54. Its primary star is a yellow-white giant (F0IIIp) that is orbited by a companion every 2.29 days, while a third component found 25 arcseconds further away.

– Gliese 710 is a K7 Vk-class orange main sequence star located about 64 light years away with an apparent visual magnitude of 9.69. It is approaching the Sun, and 1.4 million years hence will be just one light year away, when it will appear as bright as the star Antares is now. This might make a pretty picture, but the close proximity of this star has the potential to dislodge a shower of comets from the Oort cloud, which could have dire consequences for the entire solar system.

– Tau Serpentis is without doubt one of the most impressive star systems known, and consists of 8 gravitationally bound stars, the primary component of which is a red giant (M1III) around 54 times as big as the Sun, with an absolute magnitude of -2.1. The distances from Earth vary in this system from 900 light years away for Tau Serpentis, to 160 light years distant for Tau-5 Serpentis, the closest star.

Other notable stars in the constellation includes the triple star systems Beta Serpentis and Alya; the binary system Delta Serpentis, Nu Serpentis, and Iota Serpentis; the white dwarf Epsilon Serpentis; the yellow-white dwarf
Gamma Serpentis; the yellow dwarf Lambda Serpentis; and the red giant star Kappa Serpentis.

Notable Deep Sky Objects

The constellation of Serpens contains a number of deep-sky objects, including two Messier objects.

– Messier 5 (M5, NGC 5904) is one of the most ancient globular clusters associated with the Milky Way, with the oldest stars in the 13 billion years old group as old as the Milky Way itself. It is also amongst the largest with an estimated 100,000 to 500,000 stars, a number of which are young, hot blue stragglers, which are thought to have formed through star mergers. This 6th magnitude cluster is about 165 light years in diameter, and 24,500 light years away, making it an easy naked-eye object in good seeing conditions.

Pillars of Creation– The Eagle Nebula (M16, NGC 6611) is a bit of a misnomer as it is actually an open star cluster situated 7000 light-years away of magnitude 8.24. It is about 9.5 light-years across, and contains a cluster with roughly 8,100 stars.

Pillars of Creation (IC 4703 ) is part of the M16 globular cluster, and consists of 8 diffuse emission nebulae that surrounds it proper. Several stars can be seen forming at the tips of light-year-long pillars of gas and dust, but new research suggests that the pillars might have been destroyed in a supernova explosion that occurred between 8,000 and 9,000 years ago. Confirmation of this should reach Earth in about 1,000 years or so, since we are seeing this part of the nebula as it appeared several thousand years in the past.

– Hoag’s Object is a ring galaxy located about 600 million light years distant with a visual magnitude of 16. Together with the few other known examples of this type of structure, it remains one of the most puzzling types of galaxies ever discovered. Current theories hold that the ring structure is the result of galactic interactions, but this has yet to be proven. The nucleus of Hoag’s Object spans about 17 light years, the outer ring is about 75,000 light years in diameter, while the whole structure is about 121,000 light years across, or about 20% bigger than the Milky Way.

Red Square Nebula– Red Square Nebula (MWC 922) is actually a square nebula, although it is not yet known how the progenitor star, MWC 922, manages to arrange the bi-polar magnetic lines in a square shape. Investigations into the process of formation are ongoing, but so far, all attempts to model the process have failed.

Other objects of interest in Serpens includes the emission nebula IC 4703 and Sh2-54; the star cluster Serpens South; the open clusters NGC 6604 and IC 4756; and the globular star clusters NGC 6539, NGC 6535, and Palomar 5. It also contains numerous types of galaxies, including a group of six galaxies called Seyfert’s Sextet; the Ultraluminous Infrared Galaxy Arp 220; the spiral galaxies NGC 5964, NGC 5962, NGC 5972, and the Blinking Galaxy (NGC 6118); the barred spiral galaxy NGC 5970 and NGC 5921; and a pair of galaxies called the Death Star Galaxy (3C 321).


As of 2016, there have been discovered 12 stars with 15 planets in Serpens. One star, HD 168443, has two planets, one being a gas giant 7.6 times as massive as Jupiter, and another a gas giant 17.3 times as massive as Jupiter, which makes this object a brown dwarf star, rather than a regular planet.

Meteor Showers

There are two meteor showers associated with Serpens, the Omega, and Sigma Serpentids, both of which peak between December 15th and December 25th. However, being daytime showers, all but the biggest and brightest fireballs are rendered invisible by bright sunlight.

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