Six Globulars Near Antares

Scorpius and its adjoining constellations Ophiuchus and Sagittarius must have driven 18th-century comet hunter Charles Messier and his contemporaries to exasperation. These three constellations are filled with "false comets" in the form of globular star clusters. Through a small telescope a globular cluster mimics a comet very well — it's small, dim, round, and fuzzy, usually with a bright core. Most of them are bunched around the center of our Milky Way Galaxy, so from our vantage point many are seen toward the galactic center in Sagittarius and in neighboring constellations.

If you can find bright Scorpius, highlighted by orange-red Antares, you can use the map above to scout out six of Messier's globular clusters in the vicinity. Print out the map and bring it with you. (Click on the map to get the full-resolution version.) For easiest use outdoors with a flashlight, enlarge it to the full size of your sheet of paper. You can reverse it black-for-white in an image-editing program to save ink.

The black circle around M80 on the map is 5° in diameter, about the size of a typical good finderscope's field of view. This is also about the size of a 10x binocular's field. If you're not familiar with how to use a star map with a telescope (you gotta know the tricks), brush up with this article.

M4, Antares's Pal

Get your binoculars or finderscope on Antares, include Sigma (σ) Scorpii (about 2° west) in the view, and look just south of the point midway between them. There glows the big, diffuse globular M4. It's a dim, gray, 6th-magnitude haze-glow in binoculars or a 50-millimeter finder. A telescope reveals it as a sugarpile of tiny stars. This is one of the closest globulars in the sky, being only about 7,000 light-years away (which, nevertheless, puts it far in the background of Antares and most other stars). Therefore it is more easily resolved in amateur telescopes than most globulars are. M4 displays a distinctive bar of brighter stars across its face, oriented nearly north-south.

M80, Small and Compact

Exactly midway between Antares and Beta (β) Scorpii is 7th magnitude M80, about as different from M4 as a globular can be. In binoculars or a finder it looks just about like a star — much smaller and fainter than the symbol on the map would suggest. Check against the surrounding pattern of faint stars to be sure you've got the right object. In a good telescope M80 is a wonderful little cotton ball, very condensed with a bright core.

M19: Stars Out of Round

Due east of Antares by 7° is M19, glowing at 7th magnitude. This is one of the Milky Way's most intrinsically luminous globulars. With an absolute magnitude of –9.2, it shines with 400,000 times the light of the Sun. In a telescope, M19's peculiarity is its squashed shape. My 10-inch scope shows a decidedly elliptical halo about 7 by 5 arcminutes in size, oriented nearly north–south. An inner zone about 3' by 2' is also elongated in the same fashion. At moderately high magnifications this zone resolves quite well, with many very faint suns superposed over a strong haze in the central area. Farther out, the cluster becomes suddenly much less dense. A pair of 12th-magnitude stars, one northwest and the other northeast, lie in the outer zone, which is well populated with faint specks of light.

M62, an Off-Center Cluster

About 4° south of M19 is M62, magnitude 6.4. Charles Messier first observed this cluster in April 1771; in his observing notes he remarked, "Very beautiful nebula, discovered in Scorpio, it resembles a little Comet, the center is brilliant & surrounded by a faint glow." From my backyard observatory in Sydney, Australia (latitude –34°), M62 transits near the zenith. Through my 12-inch (0.3-meter) Newtonian reflector at 150x it appears about 9 arcminutes in diameter. At higher magnifications the globular’s outer halo is well resolved into faint stars. There is a very marked increase in brightness toward the core, which is much brighter than the outer halo.

An interesting feature of this cluster is its off-center core, as first noted by William Herschel. The core's skewed position is particularly easy to see at moderate powers — the bright central region appears displaced toward the southeast. The reason for this is not conclusively known. In the past it was attributed to dust obscuring the outer halo on the northwestern side. More recently it has been shown that M62 is only about 6,000 light-years from the galactic center. The deformation may therefore be real and due to tidal interaction with the Milky Way’s massive center.

M9 and Company

Far off northeast, situated 3½° southeast of 2nd-magnitude Eta (η) Ophiuchi, is another of Messier's discoveries. M9 is 8th magnitude, a little dimmer than the globulars we've looked at so far. Messier first spied this cluster on May 28, 1764, and noted: "Nebula, without star, in the right leg of Ophiuchus; it is round and its light is faint. . . . Diam. 3 arcminutes."

It seems that William Herschel's son John was the first observer to resolve M9 into stars. In terms of intrinsic brightness M9 is somewhat above average, with an absolute magnitude of –8.0. My 10-inch resolves individual stars in M9's outer halo pretty easily. Overall, the halo is about 5' across, and the core is quite dense and unresolvable. Although I didn’t notice it, many other observers have commented that M9 is also elliptical in outline. How do you see it?

Just 1.2° to the northeast is another globular, NGC 6356 (not plotted on the map). It's quite similar to M9 in size but is a little fainter and harder to resolve.

Also, look for the prominent dark nebula B64 extending about ½° southwest of M9. At low powers it appears as an irregularly shaped void with a few faint stars.

M107, The "Youngest" Messier Object

Messier's contemporary and friend Pierre Méchain found M107 (NGC 6171) in April 1782. But unlike some other Méchain finds, it didn't get into Messier's catalog for nearly two centuries. While he was still alive, Messier added handwritten notes to a printed version of his catalog about the objects that, after his death, came to be known as M104 through M109. In 1947 Canadian astronomer Helen Sawyer Hogg "officially" added M107 to Messier's list, and the name has stuck.

William Herschel described this object as a "very beautiful, extremely compressed cluster of stars, extremely rich, 5' or 6' in diameter, gradually more compressed toward the center."
My 12-inch provides a very satisfying view; the cluster is about 6' across, showing broad to moderate compression toward the center. At high magnifications I can clearly discern about 50 to 100 stars mainly in the outer halo, with many other faint ones at the threshold of visibility. The cluster's central core is about 2½' in diameter and quite grainy in appearance.

Lots more globulars await your telescope in this general region of the sky. Use a detailed star atlas, such as Sky Atlas 2000.0, and you'll have plenty to keep busy with for just as long as you like.