Looming large in the after dark southern sky in July is the large and overlapping but often overlooked constellations of Ophiuchus and Serpens. Lacking outstandingly bright stars these “two for one” constellations still contain much of interest.
Ophiuchus is one of the thirteen constellations that cross the ecliptic, making it the 13th sign of the Zodiac lying between Scorpius and Sagittarius. It might be fun to ask an astrologer why then it is never included in the creation of horoscopes, but I suspect no answer will be forthcoming. The brightest star in Ophiuchus is second magnitude Alpha Ophiuchi, also known as Ras Alhague. This is actually a binary system about 49 light years from the Sun. The primary is an A-class star 2-4 times as massive as the Sun orbited by a K-class star about 80% as massive as our Sun. The name Ras Alhague is derived from the Arabic phrase meaning “head of the serpent charmer”.
Invisible to the unaided eye (magnitude 9.5) in Ophiuchus (near Beta and Gamma Ophiuchi) is Barnard’s Star. At 5.98 light years away, this little red dwarf is famed as one of the Sun’s nearest neighbours, only stars closer are the Alpha Centauri A and B and Proxima Centauri. Barnard’s Star was once believed to have a planetary system of at least one large gas giant planet but this has been disproven. However smaller worlds orbiting this little star are still a possibility and may yet be discovered.
Ophiuchus is home to several globular clusters, including M10, M12, M14 and M62, sadly none are as impressive as Hercules’ M13 but they are binocular objects. M62 is interesting in that it lies 35 000 light years from us, further away than the Galactic Core. There is a nice open cluster, IC4665 in Ophiuchus, observable with binoculars (some claim it to be visible to the unaided eye), which lies close to Beta Ophiuchi (Cheleb or Cebalrai). IC4665 is a young cluster, estimated to be 35-40 million years old, more youthful even than the Pleiades.
Back in October 1604 skywatchers across the globe observed a new bright star in Ophiuchus. One of them was Johannes Kepler who published his findings from a year of observations in a 1606 book titled De stella nova in pede Serpentarii (On the New Star in Ophiuchus’ Foot). This new star increasing in brightness until it outshone all other stars, rivalling Venus at its peak. This guest in the sky remained visible to the unaided eye for 18 months before it faded from visibility. Kepler and his contemporaries could not have known but they were witnessing the explosive death of a star, a Type 1a supernova, a vast distant away. Exactly how far the supernova was away is unclear, distances between 13-20 000 light years are quoted, but it must be one of the most distant stars ever observed without a telescope. Today with suitable instruments, we can see its remains, as the supernova remnant SN1604.
Kepler’s star was the last supernova we know of in our galaxy we know to have been observed from Earth even though based on indirect observations there ought to be a couple of these stellar cataclysms every century. Perhaps the Milky Way is overdue a supernova or maybe the light of exploding stars has been obscured by the lanes of dust that snake through our galaxy’s spiral arms.
Talking about snakes, Ophiuchus is usually depicted as a man holding a snake. This is the constellation of Serpens. Uniquely this constellation is bisected by Ophiuchus Serpens and is split into Serpens Caput (Serpent’s Head) to the west and Serpens Cauda (Serpent’s Tail) to the east. Why is there a great snake and its wrangler in the summer sky? The Romans had it that this was the human figure was a healer, Asclepius, who enhanced his already impressive medical skills by listening to the advice of snakes. This seems odd to us today, but to the Greeks and Romans snakes were associated not with evil but with wisdom, healing and renewal. We still see reminders of this myth in the Rod of Asclepius, a staff entwined with a serpent used as a symbol of the medical profession to this day.
Asclepius used his skill to save lives and even resurrect the dead, which seems laudable but remember this is ancient myth where no good deed goes unpunished so a sad fate lay in store for the good doctor. Worried by the possibility that Asclepius’ skill could lead to human immortality, the king of the gods, Jupiter (a rebadged version of the Greeks’ Zeus) assassinated the hapless medic with a thunderbolt (presumably the snake was a collateral casualty), afterwards Jupiter placed his victims’ bodies in the sky.
Neither segment of Serpens has any outstanding stars but part of the Milky Way passes through the tail. Although it is not very conspicuous, one famous object in Serpens Cauda cannot be left out, this is M16, the Eagle Nebula, which is low in the sky from our latitude. This cloud of gas and dust is relatively dim yet is well-known for the “Pillars of Creation” imaged by the Hubble Space Telescope. You will need a very clear sky and a good telescope to see M16. a stellar nursery and associated open cluster which lie 7000 light years coreward from our position. M16 ought to be significantly brighter in our sky but to reach us its light must pass through banks of interstellar dust a few hundred light years from us, these absorb much of the light from M16, dimming it to our eyes.
Sculpted by radiation pressure from new stars, the Pillars of Creation are dark structures are columns of cool gas, mainly hydrogen, and dust that serve as incubators for new stars. Astronomers have detected blobs of much denser gas inside the Pillars; these appear to be embryonic solar systems. Eventually the new stars will become visible as their radiation blows away the surrounding material. Each Pillar is several light years long. Possibly by now the Pillars no longer exist, the shockwave from a nearby exploding star is advancing thought the nebula. This stellar blast may already have ripped through the Pillars and we may see this happening in another millennium or so.
The duo of Ophiuchus and Serpens are easily picked out in our summer sky, why not show them to your friends?
(Article by Colin Johnston, Science Education Director)