Ophiuchus (pronounced off-ih-YOU-kuss ) is a large constellation lying just above Scorpius and below Hercules. The main stars are in the rough shape of a large pentagon, but it covers a much larger area of sky than most other constellations, so think big when you are looking for it.
Nowadays, Ophiuchus is the unofficial thirteenth member of the zodiac. In the past, Scorpius – or what astrologers call Scorpio – was reagarded as covering a larger area. But when the boundaries of the constellations were pinned down in the early 20th century, it so happened that more of the ecliptic ran through Ophiuchus than though Scorpius. So if you were born between 30 November and 18 December you can proudly claim that you are born under the sign of Ophiuchus and therefore none of the horoscopes apply to you!
The constellation represents a serpent-bearer – not an everyday profession, you have to admit. But maybe you’ve seen medical companies with a snake coiled round a rod as part of their symbol. This is not because they got rich selling snake oil, but because of the legendary Greek healer Asclepius.
One legend says that Ophiuchus represents Asclepius, who learned how to defeat death by watching one snake bringing another healing herbs. Zeus, worried that humans might learn to become immortal, killed Asclepius but placed him in the night sky to honour his good works.
Another legend relates to the god Apollo who wrestled the Python to win control of the Oracle at Delphi. And there, up in the sky, Ophiuchus is sandwiched between the two halves of the constellation of Serpens, the Serpent.
Things to look for in Ophiuchus
The brightest star in the constellation is Rasalhague, just south of Hercules. If you draw a line between the bright stars Altair in Aquila and Arcturus in Boötes, Ophiuchus can be found about halfway between them. Slightly below and to the left of Rasalhague is the star Cebalrai. The other bright stars in the constellation are called Sabik, Yed Prior and Yed Posterior. The last two odd names are a mixture of Arabic and Latin, Yed coming from the Arabic for ‘hand’, prior being Latin for ‘leading’ and posterior meaning ‘following’. If you have trouble remembering this, bear in mind that your own posterior follows you around wherever you go.
There are quite a few deep-sky objects in Ophiuchus. Binoculars will reveal about 20 stars in the open cluster IC4665 (right), which is close to Cebalrai. NGC6633, on the border with Serpens Cauda, is a much larger open cluster the apparent size of the full Moon.
Two interesting globular clusters, M10 and M12, are a few degrees to the left of the star Yed Prior. They should be both visible in the same binocular field and under the right conditions you might be able to tell that M12 is slightly brighter. However, the stars in M10 appear more closely packed together.
Globular clusters are groups of stars tightly bound together by gravity giving them the spherical shapes that we can see. There are over 150 in our Milky Way and many more in other galaxies. Depending on the cluster they can contain up to a million or more stars. They orbit in the halo or fringes of a galaxy and are some of the oldest stars in the Universe. It’s estimated that they are at least twelve billion years old, which is nearly as old as the Universe itself.
Although you won’t be able to see it easily in binoculars, one faint star in Ophiuchus known as Barnard’s Star at six light years is one of the nearest stars to the Sun – and it’s getting nearer. If you can wait until the year 11,800 it will only be about 3 light years away so keep those binoculars handy! Because it is so close, it is one of the stars being investigated for evidence of Earth-like planets. Only Proxima Centauri and Alpha Centauri are closer.
For a real challenge, try locating Barnard’s Star for yourself using the finder chart below. The inset shows the general location compared with Rasalhague and Cebalrai. Find the pattern of fifth magnitude stars to the east of Cebalrai, then compare the area with the large map, which shows stars to magnitude 11. Barnard’s Star is shown by the small circle. At magnitude 9.5 it should be within the reach of binoculars, and certainly small telescopes.
Here’s a printer-friendly version of the main map:
And here is a photo of the area in the central rectangle in the above map, taken June 2018 (photo by Robin Scagell):
Text by Geoff Knight and Robin Scagell