In last month’s constellation tour, you explored the faint stars of Serpens Caput, the Snake’s Head. This month, you examine the bearer of this celestial snake, a star group represented by the large constellation Ophiuchus.
Ophiuchus (pronounced “Oaf-ih-YOU-kus”) lies directly opposite the constellation Orion on the celestial sphere. But Ophiuchus is no Orion. The constellation has no bright stars, and you need to expend a fair effort to imagine here a man holding a snake. But Ophiuchus is chock-a-block with globular and open star clusters, as well as dark nebulae in its southern extremes near the border with the constellation Scorpius. In an upcoming article, you’ll get the highlights of the deep-sky sights in Ophiuchus. For now, let’s explore this ancient star group itself
In ancient legend, Serpens Cauda and Serpens Caput were part of a long celestial serpent held by a serpent bearer and healer named Asclepius. According to legend, Asclepius, the son of the god Apollo, discovered the healing arts when he saw one snake lay healing herbs on the head of another, bringing it back from the brink of death.
Asclepius learned to ply his healing arts to his fellow men, but in the world of the Greek gods, few good deeds went unpunished. When the god of the underworld, Hades, complained about a potential setback in his grim business, Asclepius was struck dead by a thunderbolt from Zeus. Yet the “Rod of Asclepius”, a staff entwined by snakes, remains a symbol of the art and science of medicine to this day.
The constellation Ophiuchus falls along the ecliptic, the band of sky in which we find the Sun, Moon, and planets. The 12 constellations along the ecliptic are called the zodiacal constellations. Ophiuchus was not included in the zodiac because ancient astrologers considered the number 13 to be unlucky. But you still find solar system objects passing through Ophiuchus from time to time. The Sun itself lies among the stars of Ophiuchus from November 29 to December 17 each year, for example.
Ophiuchus is visible in the mid-northern latitudes in July and August slightly south and west of overhead. In the mid-southern latitudes, the constellation is well above the northern horizon. The constellation spans more than 20°, more than the width of two fists held at arm’s length.
The brightest star in Ophiuchus is 1st magnitude α Ophiuchi, also known as Rasalhague, a name which means “Head of the Serpent Bearer”. It’s right next to α Herculi, or Rasalgethi, which means “Head of the Kneeler”. Rasalhague is a white A5III star, which means it’s evolved off the main sequence and started to burn helium in its core. It will presumably evolve into a red giant.
The rest of the roughly pentagonal outline of the constellation is formed by the stars Cebalrai (β Oph), Sinistra (ν Oph), Sabik (η Oph), Han (ζ Oph), and the closely spaced stars Yed Prior (δ Oph) and Yed Posterior (ε Oph). These last two stars are the “hands” of the Serpent Bearer that grasp the head of the snake Serpens Caput. Yed Prior is an M0 III red giant star. Yed Posterior is a K-type red giant. The two stars are nearby in the sky, but they are physically unrelated. Prior lies about 170 light years away, while Posterior is about 100 light years away.
Ophiuchus serves as an excellent base of operations for exploring many stars and deep-sky objects (DSOs) in the night sky in July through September. In the next article, we explore a handful of the dozens of DSOs in this part of the sky…