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The Serpent

Right Ascension15h08 to 16h20 (Serpens Caput)
17h14 to 18h56 (Serpens Cauda)
Declination-3.4° to +25.7° (Serpens Caput)
-16.0° to +6.3° (Serpens Cauda)
Area (sq deg)636
Brightest StarUnukalhai
Optimum VisibilityMay/June
NotesThe constellation of Serpens is closely connected with that of Ophiuchus, the Serpent Holder, which both seperates and connects the Serpent's two segments. Serpens is lacking bright stars (its brightest, Unukalhai, is only third magnitude) but rich in star clusters, notably the globular cluster M5.
Map of Serpens Map of Serpens

Serpens, or the Serpent, is held by the Serpent Holder, Ophiuchus, and is the only constellation to consist of two separate parts - Serpens Caput (the Head) and Serpens Cauda (the Tail).


Though the stars of this region have been seen as a serpent for thousands of years, their symbolic meaning remains uncertain. It is thought likely that the Serpent and its bearer Ophiuchus are related to the Greek healer-god Asclepius. The Greeks believed that the serpents of Asclepius could cure diseases, and so belong a long association between serpents and medicine that continues to our own time.


The stars of Serpens form the shape of a long writhing snake that starts near the Northern Crown, passes southward through Serpens Caput, then through the body of Ophiuchus. When the shape emerges into Serpens Cauda, it turns northward again, and carries on to reach its end into the Milky Way. The tip of the Serpent's nose is the faint star Rho Serpentis, while the tip of its tail is marked by the similarly faint Alya. A number of the stars that form part of the Serpent's body actually belong to the constellation Ophiuchus. Yed Prior, Yed Posterior, Upsilon Ophiuchi, Zeta Ophiuchi and Sabik are all Ophiuchan stars that lie along the star-snake's body.

Though its shape is well-defined, Serpens is lacking in bright stars. The brightest is Unukalhai (the 'serpent's neck') an orange giant about seventy light years away, with a magnitude of +2.6. Eta Serpentis, the next brightest star, is nearly a magnitude fainter at +3.3.

Star Clusters and Nebulae

Serpens contains no deep-sky objects that are visible to the naked eye, but it is not without points of interest. One of these is the very distant Globular Cluster M5, in the southern parts of Serpens Caput. About 25,000 light years away, this gigantic ball of stars is close to the Nucleus of the Milky Way Galaxy. Much closer to Earth, at approximately 1,300 light years, is the open cluster IC 4756. This is close to the tip of the Serpent's Tail, in Serpens Cauda.

Also to be found within the Serpent's Tail is the well-known nebula M16, better known as the Eagle Nebula because it forms the shape of a flying eagle with outstretched wings. In the heart of this nebula are three towering columns of gas and dust, the subject of one of the Hubble Space Telescope's most famous images.


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