The constellation Serpens Caput, the Snake’s Head, lies well off the band of the Milky Way and holds relatively few deep-sky sights. But it’s not completely barren. Let’s have a look at three targets in this ancient constellation for stargazers equipped with modest optics and an urge to see something good.
Let’s start off with an easy sight: the very top of the Snake’s head. This triangular arrangement of stars just south of the semicircular constellation Corona Borealis consists of the stars iota (ι), kappa (κ), beta (β), and gamma (γ) Serpentis. The brightest of these stars is close to magnitude 3.5, so they’re easily seen in all but the most light-polluted skies. The zigzag of the snake’s neck moves south from the little head. Grab your binoculars and inspect the lovely cloud of 5th and 6th-magnitude stars that surround the head in a sparse halo about 5º across. The halo sharpens slightly to a wedge of stars to the west and south of the head that comes to an apex at the 5th-magnitude red-orange star tau1 (τ1) Serpentis, one of eight stars west of the snake’s head to have the tau designation. To my knowledge, the stars in the halo around the snake’s head are unrelated. But they present a pleasing view in an otherwise sparse region.
Now let’s go deeper to see the showpiece of Serpens Caput, the spectacular globular cluster Messier 5 (M5). The 5th-magnitude cluster is located about 7.5o southwest of Unukalhai, α Serpentis, the brightest star in Serpens Caput. M5 is easy to spot in a finder scope or binoculars as a round fuzzy glow.
This cluster is a splendid object in a telescope, the bigger the scope the better. Many stargazers, me included, find M5 more spectacular than the better-known globular cluster M13 in Hercules. In an 8″ scope at 100x, you see tight sparkling core and innumerable tiny stars spraying throughout the halo in lines and arcs. Many see in this cluster an “electric spark” that seems to emanate from the core. Most observer’s agree the cluster is perceptibly non-circular, or at least non-symmetric, with the core appearing somewhat brighter to the north (see sketch at the top of this page).
At 13 billion years of age, M5 is one of the oldest globular clusters. And it’s one of the largest, at 130 light years across. The cluster is 24,000 light years away.
Now to the last object on the tour, the double star 5 Serpentis, a slightly more challenging sight in a small telescope. Look for this bright star just south of M5 within the same low-power field of view. The star is also known as Σ1930. The primary is magnitude 5, easily visible to the eye in dark sky. The companion shines at magnitude 10 and lies about 11” to the northeast. That makes for a widely-split star, and it would be an easy star to split if the primary didn’t vastly outshine the secondary. The difference in brightness makes this a good challenge of your observing skills. A small scope can split the stars at 70-80x or more, but again, the brighter star tends to overwhelm the fainter. So you have to look carefully.