This is the second part of the Ophiuchus- Serpens group. The Serpent is
being grasped in the hands of Ophiuchus the Serpent Holder. Thus the
constellation wraps around Ophiuchus, and is divided into two parts:
Serpens Caput (the head) and Serpens Cauda (the tail).
The constellation Serpens is spread across a
greater part of the sky than is Ophiuchus, but it has far fewer features
of interest. Still, there are several Messier objects and some very nice
There isn't any remarkable asterism in Serpens, and it might take some
effort to decide just which stars belong to this constellation, and which
belong to Ophiuchus. That is, the Bayer stars
of Serpens compare in brilliance with those of Ophiuchus.
The brightest star, alpha Serpentis, is called Unukalhai,
meaning "Neck of the Snake". It is 67 light years away, and is
approximately ten times the size of the sun.
[NOTE: The link to the Binocular Section, below, updates the information here.]
Serpens has three visual binaries of some interest, two of which are
very attractive, and one which will test your observing skills.
Beta Serpentis (Struve 1970) is a wide visual yet difficult to
observe due to the brightness of the primary compared to the faint
companion: 3.7, 9.9; PA 263º, separation 31.3".
Theta Serpentis (Struve 2417) is a wonderful binary of two blue-white
stars: 4.6, 4.9; 104º, 22.".
Struve 2375 is a superb pair: 6.3, 6.7; 120º, 2.6".
R Serpentis is a long-period variable, 5.2-14.4, with a period of
The star is located 1.2 degrees ESE of beta Serpentis, nearly midway between
beta and gamma Serpentis and very slightly south of a line drawn between them.
Deep Sky Objects:
There are two Messier objects in Serpens: M5 and M16; the first is found
in the "head" of the serpent, the second in the "tail".
M5 (NGC 5904) is a spectacular globular cluster, containing a
half a million stars. The cluster is quite compact and rather bright; it
is about 25,000 light years away, and ten billion years old.
The cluster is found eight degrees SW of alpha Serpentis.
M16 (NGC 6611), "The Eagle Nebula", is a remarkable open star
cluster surrounded by a huge nebula, very luminous with dark streaks of
dust: a nursery of newly forming stars. Best seen in large scopes;
a nebula filter might help.
The cluster is fifteen degrees south of eta Serpentis, but an easier way
to find it may be to draw a line from eta Serpentis to xi Serpentis, to
the SW. Now midway along that line is found the bright star nu Ophiuchi.
Draw an imaginary perpendicular out from nu Ophiuchi, southeast. About
seven degrees along this line is M16.
If this seems a bit complicated, first try locating M17, The Omega
Nebula (or Swan Nebula), in Sagittarius. Two and half degrees north is
For a more detailed appreciation of Serpens visit the Binocular Section.