The Constellation Sagittarius
Posted by Brian Ventrudo
The constellation Sagittarius vaults above the southern horizon for northern stargazers, and lies nearly overhead for deep-southern observers in July and August each year. This large constellation, which represents a mighty centaur shooting an arrow westward into the heart of Scorpius, holds an astonishing number of deep-sky targets for stargazers with a telescope, binoculars, or with absolutely no optics at all. Here’s a brief look at this richest of constellations…
The stars of Sagittarius were first recorded by the Sumerians some 5000 years ago. The ancient Greeks, 2000 years later, sometimes depicted this group as a satyr (half man and half goat) rather than a centaur (half man and half horse). Ancient Greek poetry associates these stars with the satyr Crotus, whose father was the god Pan, and who was said to have invented archery. But modern and renaissance star maps showthese stars as a centaur with a bow drawn and aimed westward to fire an arrow into Antares, the star at the heart of Scorpius. Some books suggest Sagittarius represents the wise and noble Centaur Chiron, but most astro-historians agree that Chiron is represented by another celestial centaur, the constellation Centaurus.
It takes a little imagination to see a centaur among these stars. But most observers instantly see the much more familiar shape of a teapot in Sagittarius. Indeed, the “Teapot” asterism is one of the most familiar in the heavens, with the handle to the east, the spout to the west, and the lid pointing northward. The pot spans about 15º of sky, about half again as wide as your fist held at arm’s length.
The bright stars and a selection of deep-sky objects in the constellation Sagittarius (click to enlarge).
Whether you see Sagittarius as a satyr, a centaur, or a pot of steaming green tea, the constellation contains a staggering array of deep-sky objects arrayed along the Sagittarius Arm, one of the thick spiral arms of the Milky Way. The center of our galaxy also lies in this direction near a point about 4º west-northwest of the Alnasl (γ Sagittarii) at the tip of the Teapot. You can’t see the galactic center because it’s obscured by dark dust, but the star fields are thicker here than anywhere else in the heavens.
Along this rich stream of the Milky Way there’s something for every stargazer, from colorful double stars to spangled blue-white open star clusters, and from ancient golden globular clusters to the silver glow of emission nebulae that churn gas and dust into newly-minted stars. The guide “What To See in a Small Telescope (July-September)” devotes two full chapters to Sagittarius and still barely scratches the surface. Here are a few suggestions of what to look for…
• The Lagoon Nebula (M8), one of the brightest emission nebula visible in our skies, and easily seen without optical aid as a small dim cloud above the spout of the Teapot
• The Trifid Nebula (M20), another glowing Chinese-lantern of an nebula right next to M8. The brightest region of the Trifid is trifurcated by fingers of dark foreground nebulosity
• The Swan Nebula (M17), another fine emission nebula that looks like a swan, a checkmark, a lobster, or the Greek letter “omega”
• M22, a loose and large globular cluster, surely one of the finest of its kind anywhere in the night sky
You can even peek through a hole in the dark interstellar dust to a tiny patch of star cloud some 9,000 light years away in the Sagittarius Arm of the Milky Way…
Yes, you need a telescope to best see the many dozens of deep sky objects in Sagittarius. But if you have no optics at all, don’t worry. Just find a good reclining chair, some dark sky, and lie back and gaze at the frosty star fields of Sagittarius and imagine you’re flying along the plane of the Milky Way, unfettered by Earthy worries, going farther and faster and seeing things that few humans will ever see…
