Serpens (the Serpent) is one of the most unusual constellations in the night sky. It is the only one of the 88 officially recognized constellations divided into two separate sections, with the constellation Ophiuchus lying between them. Despite its relatively faint stars, Serpens contains some remarkable objects, including an ancient globular cluster, the famous Pillars of Creation, and one of the most distinctive ring galaxies ever discovered.
Where Is Serpens and When Can You See It?
Serpens covers approximately 637 square degrees, making it the 23rd-largest constellation. It lies near the celestial equator and is best observed from the Northern Hemisphere during summer, particularly on July evenings.
The constellation consists of Serpens Caput (the Serpent’s Head) to the west of Ophiuchus and Serpens Cauda (the Serpent’s Tail) to the east. Although separated by Ophiuchus, the two sections are officially considered a single constellation.
Serpens is bordered by several recognizable constellations, including Hercules, Libra, Scorpius, Sagittarius, and Aquila. Its brightest stars are relatively modest, so a dark sky helps when tracing the complete serpent.
Why Is Serpens Divided Into Two Parts?
Serpens is unique because no other modern constellation consists of two disconnected regions of sky. The division reflects the traditional depiction of Ophiuchus, the Serpent Bearer, holding a snake in both hands. The serpent’s head extends to one side of his body, while its tail stretches to the other. Although the two sections are physically separated in the sky, astronomers treat them as one constellation, with a single set of star designations.
Serpens in Greek Mythology
In Greek mythology, Serpens is closely associated with Asclepius, the legendary physician represented by Ophiuchus. Asclepius was said to have learned the art of healing from the wise centaur Chiron. In one story, he witnessed a snake revive another snake using medicinal herbs. By learning its secret, Asclepius gained the ability to restore the dead to life. Snakes became enduring symbols of healing and renewal, partly because they shed their skin. The serpent-entwined staff of Asclepius remains associated with medicine today.
Notable Stars in Serpens
Although Serpens contains no first-magnitude stars, several are interesting in their own right.
- Unukalhai (Alpha Serpentis) is the constellation’s brightest star, shining at magnitude 2.6. Located approximately 74 light-years away, it is an orange giant in Serpens Caput. Its traditional Arabic-derived name refers to the serpent’s neck.
- Eta Serpentis, in Serpens Cauda, is another orange giant with an apparent magnitude of approximately 3.3. It lies around 60 light-years from Earth.
- Mu Serpentis is a blue-white main-sequence star of approximately magnitude 3.5, situated in the serpent’s head.
- Gliese 710 is a faint orange dwarf with a particularly interesting future. Calculations indicate that it will pass through the outer reaches of the Sun’s Oort Cloud in roughly 1.3 million years, potentially disturbing the orbits of some distant comets. It is far too faint to see without a telescope today.
Deep-Sky Objects in Serpens
Despite its faint stars, Serpens contains some extraordinary deep-sky objects.
Messier 5 — An Ancient Globular Cluster
Messier 5 (M5) is a magnificent globular cluster in Serpens Caput, approximately 25,000 light-years away. It contains hundreds of thousands of stars, many of them more than 12 billion years old. At around magnitude 5.7, M5 can sometimes be detected with the naked eye under exceptionally dark skies. Binoculars reveal a fuzzy patch, while a moderate telescope begins to resolve its individual stars.
Eagle Nebula and the Pillars of Creation
The Eagle Nebula (M16) lies in Serpens Cauda, approximately 6,500–7,000 light-years away. It is a vast region of gas and dust containing the young open star cluster NGC 6611.
Within the nebula are the famous Pillars of Creation, towering columns of gas and dust where new stars are forming. Images taken by the Hubble Space Telescope and James Webb Space Telescope have revealed their intricate structure.
The pillars are being eroded by radiation from nearby massive stars, making them a remarkable example of the processes that shape stellar nurseries.
Hoag’s Object — A Remarkable Ring Galaxy
Hoag’s Object is an unusual galaxy approximately 600 million light-years away in Serpens Caput.
It has a nearly circular ring of young, blue stars surrounding a central region dominated by older, yellowish stars, with an apparent gap between them. Its striking appearance has made it one of the best-known examples of a ring galaxy.
Astronomers have proposed several explanations for its formation, but its precise origin remains uncertain.
Red Square Nebula
The Red Square Nebula, associated with the star MWC 922, is another unusual object in Serpens. Its remarkable symmetry is thought to result from material expelled by the central star and shaped by the surrounding environment. Seen from Earth, the glowing structure has a distinctive square or X-shaped appearance.
How to Find Serpens in the Night Sky
The easiest way to locate Serpens is to first identify Ophiuchus, which lies above Scorpius in the summer sky. Serpens Caput extends westward toward Corona Borealis, while Serpens Cauda stretches eastward toward Aquila and Sagittarius. Unukalhai is a useful starting point for identifying the serpent’s head.
The constellation’s stars are not particularly bright, but its location near the rich star fields of the Milky Way makes the surrounding region rewarding to explore with binoculars or a telescope.