The Constellation Serpens Caput, the ‘Snake’s Head’

The region of Ophiuchus and Serpens in a map circa 1825.

The region of Ophiuchus and Serpens in a map circa 1825.

In last month’s constellation tour, we examined the dazzling stars of the constellation Corona Borealis, the Northern Crown. This month, we move due south to the northern section of the constellation Serpens, the only constellation split into two parts. These are the stars of Serpens Caput, the ‘head of the snake’, which zigzags vertically along the celestial sphere, just north of the celestial equator, and just west of the much larger constellation Ophiuchus, the ‘serpent bearer’. The stars of Serpens Caput are visible in the northern and southern hemispheres.

Like many prominent northern constellations, Serpens Caput dates back to the star maps of the 2nd-century astronomer Ptolemy which listed 48 constellations in all. In legend, Serpens Caput and its other half, Serpens Cauda (the ‘snake’s tail’) represent the snake that was held by the ancient healer Asclepius, who is now represented by the large adjacent constellation Ophiuchus. Asclepius once killed a snake, but it was resurrected by a second snake which placed a magic herb on its head before death. Snakes shed their skin every year and were a symbol of rebirth in ancient Greece. Legend tells that Asclepius learned to revive dead or dying humans using the same herbs used by the legendary snakes.

The stars of Serpens Caput, due south of Corona Borealis. In this image, north is up and east is to the left.

The stars of Serpens Caput, due south of Corona Borealis. In this image, north is up and east is to the left.

Stars in Serpens Caput include α, β, γ, δ, ε, ι, κ, λ, μ, π, ρ, σ, τ, χ and ω Serpentis. Stars in the tail, Serpens Cauda, include ζ, η, θ, ν, ξ, and ο Serpentis.  The brightest star in the constellation is Unukalhai (“uh-NOO-kul-lye”), Arabic for “Serpent’s Neck”.  Older star maps list the star as Cor Serpentis, the “Heart of the Serpent”.  Call it what you will, but 3rd-magnitude α Serpentis is a fine orange K2 III red giant star that’s swollen to 15x the diameter of our Sun and 70x the brightness of our Sun.

Hoag's object, a ring galaxy in the constellation Serpens Caput.

Hoag’s object, a ring galaxy in the constellation Serpens Caput.

The constellation holds a number of very faint galaxies of interest to professional astronomers. The most beautiful is surely Hoag’s object, a rare ring galaxy formed perhaps when the arms of a barred spiral galaxy were accelerated and detached from the dense core. The object is some 600 million light years away.

Serpens Caput has two must-see objects for a modest telescope, and we will turn to those in the next article.

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