With the summer Milky Way climbing high, it only makes sense to turn attention to some of
its more interesting denizens. To me that’s always included the globular star clusters that hover around the galactic bulge. But there are plenty of interesting open galactic clusters too. So this month we take a look at both types. Hopefully these are some you don’t usually chase. In any case, get out there and enjoy!
NGC 6441 rating EASY
Globular cluster in Scorpius
RA 17h 50.2m Dec -37d 03.0’ (2000) Magnitude 7.2 NGC 6441 ranks among the easier globulars in the sky to find. It sits a mere 4’ east of 3.2-magnitude G Scorpii, which is itself just 4 degrees east of 1.6-magnitude Lambda Scorpii, Shaula, which marks the stinger in the tail of Scorpius. It was discovered by James Dunlop in Australia on May 13, 1826, and was originally cataloged as Dunlop 557.
Finding NGC 6441 and resolving it are two different things, of course. Located about 26,000 light-years away, the 3’ diameter cluster is dimmed by an estimated 3 magnitudes of absorption in the region, and so is difficult to resolve in small scopes. In larger scopes, you can go for a somewhat clearer view of the cluster by blocking the light of G Sco, for example, by putting an obscuring bar in the focal plane of the eyepiece, if that’s possible. The cluster is 260 times farther away than G, whose distance from us is a comparatively tiny 100 light-years. Interestingly, NGC 6441 contains a planetary nebula, JaFu2 (2000 coordinates RA 17h 50m 12.84s, -37d 03’ 03.9”). The planetary is about 5” across and is one of only 4 planetaries known to be associated with the 150 globular clusters in the Milky Way. It was discovered in 1997.
M26 rating MEDIUM
Open cluster in Scutum
RA 18h 45.2m Dec -9d 23.9’ (2000) Magnitude 8.0
The small constellation of Scutum, lodged between the southern border of Aquila and the northern border of Sagittarius, is bereft of bright stars but fairly crammed with star clusters. The best known of these is open cluster M11, the Wild Duck Cluster. But there are at least a dozen more within the constellation boundaries. One of these, M26, lies in central Scutum just 2 degrees ESE of 3rd Alpha Scuti. Even in the short distance between Alpha and M26 there are two more open clusters (NGC 6664 and Trumpler 34) plus the well-known variable star Delta Scuti, which lies less than a degree NW of M26.
First noted by French observer Le Gentil sometime before 1750, M26 was re-discovered by Messier in June of 1764. He considered it undistinguished in his small telescope and concluded it needed a better instrument. Later, in 1784, he reported seeing it very well in a Gregorian scope at 104x.
M26 is rather small (9’ in diameter), so in 7x35 and even 10x50 binoculars it appears as just a small hazy spot. In smaller scopes M26 is a small, tight group with the brightest star (magnitude 11) on its SW corner. Roughly 25 stars can be seen with a 6-inch to 8-inch scope in a larger dim, unresolved glow created by about 70 additional fainter members as faint as magnitude 13. Larger scopes may pull in as many as 40 members.
M26 is about 22 light-years in diameter and roughly 89 million years old. It’s probably a bit closer than M11, with a current distance estimate of about 5000 light years, and dimmed a rather significant 1.8 magnitudes by Great Rift dust. The entire cluster lies on a backdrop of patchy bright and dark nebulae, just what you might expect from the summer Milky Way!
NGC 6380 rating HARD
Globular cluster in Scorpius
RA 17h 34.5m Dec -39d 04.2’ (2000) Magnitude 11.5
Wrapped in the tail of Scorpius, globular cluster NGC 6380 is located less than 2 degrees west of 2nd- magnitude Kappa Scorpii, one of the stars in the tail.
The cluster has an interesting discovery history. It was first found by James Dunlop in 1826 in Australia and cataloged by him as Dunlop 538. But for unknown reasons, Dunlop’s discovery could not be verified by John Herschel, who independently rediscovered it from the Cape of Good Hope in 1834, and cataloged it as h 3688. Herschel described it as “a star of 9th magnitude, with an extremely faint nebulous wisp or tail, extending northwards about one arcminute”.
Dunlop’s discovery was eventually brought to light in the 1990s by Glen Cozens of Australia. Until the 1950s, NGC 6380 was thought to be an open cluster, until it was recognized as a true globular by A.D. Thackeray. It was also independently “discovered” one last time by Paris Pismis, who cataloged it s Tonantzintla 1, also known as Ton 1, and Pismis 25.
NGC 6380 is buried in a rich star field near the galactic plane, where intervening dust clouds dim its light by about 4 magnitudes. Until recently, it was was one of the few globulars whose distance was completely unknown. But in 1997 it was determined to be in the galactic bulge, roughly 35,000 light-years away.
A 12-inch scope shows NGC 6380 as a faint, broadly concentrated haze. In dark skies with a 13-inch Dobsonian, you’ll see only a low-surface-brightness haze just north of a 10th-magnitude star. Also be aware that a number of sources list the wrong position for the cluster.