Ophiuchus

In combination with its neighbour Sagittarius, the constellation of Ophiuchus encloses the central regions of our Milky Way Galaxy.

Derivation

The group of stars has been known by the name Ophiuchus for two thousand years or more. What is not known, though, is why the name 'Serpent Holder' was assigned to them. Some suggest that the constellation represents Asclepius or Aesculapius, the healer of Greek myth whose cult was closely associated with serpents. This identification is by no means certain.

Stars

Ophiuchus is a large constellation (the eleventh largest in the sky), and contains a number of bright stars. The star designated Alpha Ophiuchi is Rasalhague, a fairly nearby star with a name that means 'Head of the Serpent Holder'. There are four other stars above third magnitude: in descending order of brightness, these are Sabik, Zeta Ophiuchi, Yed Prior and Cebalrai.

More famous than any of these is Barnard's Star, which lies just under six light years from Earth, and is one of the Sun's closest neighbours. It is a very small and faint red star of the type known as subdwarfs, so that despite being so nearby, it is invisible to the naked eye.

Star Clusters

Ophiuchus abounds with Globular Clusters, and though none can be seen with the naked eye, Messier 10 is quite easy to make out with even a small telescope. Other similar star clusters in this constellation are M12, M19, M62 and M107.

These Globular Clusters are very distant objects, with the nearest (M10) being more than 14,000 light years from Earth. Nearer to home, at just 1,000 light years, a bright open cluster can be seen near Ophiuchus' border with Serpens Cauda. This is NGC 6633.

The Thirteenth Sign of the Zodiac

When the zodiac was first delineated by the ancients, it consisted of twelve 'houses', each associated with one of twelve constellations that lay along the Ecliptic: Aries, Taurus, Gemini, Cancer, and so on. In fact, the Sun passes through thirteen constellations on its annual journey around the sky, with Ophiuchus being omitted from the traditional sequence of twelve. Between the points where the Sun passes out of Scorpius towards Sagittarius, it spends an intervening period of nineteen days each year, from 30 November to 18 December, in the 'house' of the Serpent Holder.