Ophiuchus

The Serpent Bearer

Ophiuchus, whose Greek name means 'serpent holder', shows a man gripping a large snake. The snake is a separate constellation, Serpens, and it is split into two halves, one on each side of Ophiuchus, as if the serpent were wrapped around his body.

In Greek myth the figure is Asclepius, the god of medicine. He was said to have made a potion that could give humans immortality. To keep that power from mortals, Zeus took the potion, removed Asclepius from Earth and set him among the stars.

Ophiuchus is one of the larger constellations, but apart from Rasalhague its stars are modest, so the figure takes some patience to trace.

The Sun, the zodiac and Ophiuchus

The ecliptic is the yearly path of the Sun against the background stars. Under the official constellation boundaries used by astronomers, this path runs through 13 constellations, not 12, and Ophiuchus is the extra one. The Sun stands in front of it from about November 30 to December 18, between its visits to Scorpius and Sagittarius.

Astrological signs are a different system. They divide the ecliptic into 12 equal parts of 30 degrees each, while the real constellations have very unequal sizes. Earth's slow wobble, called precession, has also shifted the stars against the calendar over the centuries. Astronomy only describes where the Sun is among the constellations; this guide makes no claims about astrology.

How to find it

At mid-northern latitudes, Ophiuchus is best on July, August and September evenings, when it stands in the south after dark. Start from Antares, the red heart of Scorpius low in the south, and look upward: the wide, dim figure of Ophiuchus fills the sky between Scorpius below and Hercules above.

Its brightest star, Rasalhague, marks the top of the figure. Look for it not far from Rasalgethi, the head of Hercules. From the Southern Hemisphere, Ophiuchus passes nearly overhead and is much easier to trace.

The main stars and their names

Rasalhague (Alpha Ophiuchi) shines at magnitude about 2.1 and lies about 49 light-years away. Its Arabic name means 'head of the snake handler'. It spins very fast, at about 537,000 mph at its equator, almost fast enough to tear itself apart, and a smaller companion star orbits it.

Barnard's Star is far fainter, at magnitude about 9.5, so you need a telescope to see it. It is a small red dwarf with about 14 percent of the Sun's mass, about 6 light-years away. If you count the Alpha Centauri system as one, it is the second nearest star system to the Sun.

What makes it famous is its motion. In 1916 the American astronomer E. E. Barnard compared photographs taken years apart and found that this star shifts against the background faster than nearly any other: about 10.3 arcseconds a year, or a full Moon's width in about 174 years.

Star clusters and dark clouds

Ophiuchus holds several globular clusters, dense balls of very old stars. M10 and M12 are the easiest to see in ordinary binoculars, where both look like round, fuzzy spots. M10 is about 14,000 to 15,000 light-years away and about 80 light-years across; M12 lies a little farther.

Near the border with Scorpius, the Rho Ophiuchi region is one of the closest places where new stars are forming, about 400 to 460 light-years away. Its colourful clouds of gas and dust show up well in long-exposure photographs, but not to the eye. It was also the first place where astronomers detected hydrogen peroxide molecules in interstellar space.

In southern Ophiuchus, the Pipe Nebula is a dark cloud of dust about 7 degrees across, roughly 600 to 700 light-years away. Under a very dark sky it can show up as a dark shape against the glow of the Milky Way.

Kepler's supernova

In October 1604 a new star blazed up in Ophiuchus. It was bright enough to see in daylight for about three weeks and stayed visible to the naked eye for about 18 months. Johannes Kepler followed it closely, and it now carries his name.

Astronomers now know it was a Type Ia supernova, the explosion of a white dwarf, a dense, burnt-out star core. Its remnant is roughly 20,000 light-years away, although estimates vary. No supernova in our own galaxy has been seen since.

Practical observing tips

Choose a clear summer evening away from city lights, ideally without the Moon. Find Antares first, then work upward toward Hercules. A planisphere or a star chart app helps you connect the faint stars of the figure.

Sweep with binoculars for M10 and M12, using a star chart to locate them. To see Barnard's Star, use a telescope and a detailed finder chart for the current year, since the star is moving; sketch its position and look again a few years later to notice the change.