Around the tail and the teapot.
Globular clusters appear most abundant around the hub of our Milky Way, which is well placed for observing during July evenings. The absence of a familiar M or NGC prefix for the globulars I selected this month may be daunting to some observers. Conventional wisdom has held that objects without these tags were invisible in amateur telescopes. And this belief was further promoted in literature.
The first edition (1968) of E. J. Hartung's Astronomical Objects for Southern Telescopes refers to the Palomar list of globulars as "photographic" objects. Burnham's Celestial Handbook makes scant mention of globulars outside the NGC. And even the recently published Night Sky Observers Guide by George Kepple and Glen Sanner mentions only four of the 15 Palomar clusters, and one of them carries an NGC designation as well. While comprehensive, this work contains not a jot about globulars on the Terzan, Haute-Provence, Tonantzintla, Djorgovski, or Arp lists, yet many of these clusters are visible in fairly modest telescopes.
I suspect that one obstacle to the successful observation of these difficult clusters is an observer's expectations. A paradigm must be broken. Consider the appearance of M13 or 47 Tucanae--a bright haze with a myriad sprinkling of diamond dust and a dramatic central condensation. Take that mental picture and dispose of it! Most of these obscure objects, even in large scopes, don't look like a globular cluster at all. Most are either distant, small, heavily obscured by dust in the Milky Way, or are objects with very little central concentration. Sometimes they are all four. In most cases this adds up to a patch of faint haze, similar in many respects to the appearance of small, low-surface-brightness, face-on spiral galaxies. Hold that image in your mind's eye while searching for these globulars. You also need to guide your telescope to the exact location of the object. All the clusters described in the following text are plotted on charts in the Millennium Star Atlas and the new Uranometria 2000.0 Deep Sky Atlas. All are within reach of a 10-inch telescope under dark skies, and some can be seen with smaller scopes.
Starting with the easiest, Palomar 8 was unlucky to miss the NGC since I can think of several NGC globulars that are more difficult to observe, and I suspect a good 4-inch scope will show it. Although Palomar 8 is located a bit more than 2[degrees] east-southeast of the open cluster M25, I like to begin my search at the 3.5-magnitude star [Zeta.sup.2] ([[zeta].sup.2]) Sagittarii. An imaginary 4[degrees]-long line drawn from this star west-north-westward through 5th-magnitude 29 Sagittarii ends at the cluster, which is at the midpoint of a pair of 6th-magnitude stars separated by about 1[degrees]. The cluster is just east of a small triangle of 11th-magnitude stars.
Palomar 8 has a round disk about 2' in diameter, which grows gradually brighter toward the center without any pronounced central condensation. There are a couple of faint stars superimposed near the edge of the disk. My 12 1/2-inch reflector shows incipient resolution at 312x, which is consistent with the brightest members of the cluster being about magnitude 15 1/2.
Tonantzintla 2 and Tonantzintla 1 are in the tail of Scorpius amid the thronging Milky Way. Also known as NGC 6380, Ton 1 is 1 1/2 [degrees] due west of Kappa ([kappa]) Scorpii, and it enjoys the richly deserved reputation as being a horror of an NGC cluster. On many nights I searched vainly for it with my 10-inch reflector. Then, during the 1995 South Pacific Star Party in the Blue Mountains west of Sydney, I spotted it as an ill-defined blotch of haze about 1 1/2' in diameter with virtually no central brightening. There is a 10th-magnitude star lying right on the cluster's south edge. Larger scopes show barely more, and there is no resolution apparent in telescopes up to 20-inch aperture.
Ton 2 is located 40' northeast of Ton 1 and a scant 6' northwest of a 4.3-magnitude star. This star is a mixed blessing since it makes locating the field a snap, but seeing the cluster is very difficult due to the star's glare. To spot this globular it is essential to place the star outside the eyepiece field. An 11th-magnitude star marks the southern edge of the cluster, further pinpointing its location. Considerable concentration is needed with my 10-inch to detect it as an ephemeral 2'-diameter gossamer glow wafting in and out of view as the seeing allows. There is no hint of resolution or a central condensation. As with all these challenging clusters, moderate to high magnification tends to reduce sky glow and boost contrast.
Terzan 3 rides on the Scorpion's back and is one of the easiest Terzan globulars. It is located 50' southwest of a 4th-magnitude naked-eye star that is 4[degrees] due west of Epsilon ([epsilon]) Scorpii. The cluster is quite large in my 12 1/2-inch scope, spanning 3' by 2' and appearing clearly elongated with a weakly hazy appearance that can be held easily using averted vision. There is a 14th-magnitude double star marking the cluster's eastern boundary and a single star of similar brightness at the northern edge.
Terzan 8 and Arp 2 are near an old friend, the globular M55. The surrounding field is littered with faint galaxies, so extinction due to the Milky Way must be relatively light in this part of the sky. Terzan 8 is 3.1[degrees] south of M55, conveniently located just 20' east of a distinctive V of 10th- and 11th-magnitude stars shaped like the Hyades. Although not as big or easy as Terzan 3, Terzan 8 can be held steadily with averted vision in my 12 1/2-inch scope. The 2 1/4' round glow rises slightly in brightness at the center. The brightest stars in this cluster are reputed to be around magnitude 15, which means they should be within range of a 12-inch scope, but I see no sign of resolution.
Arp 2 lies 3.9[degrees] west-northwest of M55, 45' southeast of a 5th-magnitude star. It is a marginal object in my 10-inch scope but slightly easier in the 12 1/2-inch, appearing nearly 2' across with no central brightening. A trio of 10th-magnitude stars about 20' from the cluster form a wobbly diamond with the cluster at the eastern tip.
Terzan 7 is located 3.3[degrees] northeast of Gamma ([gamma]) Coronae Australis. The surrounding field contains many faint galaxies as well, indicating only slight extinction due to the Milky Way. There is a close pair of 8th-magnitude stars 8' north of Terzan 7. Appearing as a 2'-diameter patch of haze with a slight central brightening, this is in my opinion the best of the Terzan globulars. There's a 13th-magnitude star marking the eastern edge, and the cluster can be held easily in the 12 1/2-inch scope with averted vision. During moments of good seeing Terzan 7 seems slightly mottled with a ragged edge. The faint galaxy ESO 397-15 is only 5' southeast of the cluster.
Terzan 7 and 8 and Arp 2 appear to have a common bloodline, being formerly attached to the Sagittarius Dwarf Galaxy before being forcibly adopted by the Milky Way along with Ruprect 106 in Centaurus and Palomar 12 in Capricornus. Nearby M54 is also a likely member of the family.
Leaving the best till last, Djorgovski 2 in Sagittarius was discovered only in 1987. Also known as E456-SC38, it is located 20' west-northwest of the sparkling open cluster NGC 6520, with the dramatic dark nebula Barnard 86 sitting between. I consider this one of the best fields in the entire sky, every view brimming with innumerable faint stars. The 2'-diameter cluster sits in a small box of one 8th- and three 10th-magnitude stars. It exhibits a slight brightening toward the center, and at 362x my 12 1/2-inch shows weak granulation and an odd faint sparkle now and again, particularly near the edges.
While observing globular clusters "beyond" the NGC is a path less trodden, it is well worth the effort. As with other challenging deep-sky projects, the important thing to remember is that the mind perceives and appreciates the true nature of what the eye can barely see.
Some Globular Clusters "Beyond" the NGC Name R.A. (2000.0) Terzan 3 [16.sup.h][28.7.sup.m] Tonantzintla 1 [17.sup.h][34.5.sup.m] Tonantzintla 2 [17.sup.h][36.2.sup.m] Djorgovski 2 [18.sup.h][01.8.sup.m] Palomar 8 [18.sup.h][41.5.sup.m] Terzan 7 [19.sup.h][17.7.sup.m] Arp 2 [19.sup.h][28.7.sup.m] Terzan 8 [19.sup.h][41.7.sup.m] Name Dec. Constellation Terzan 3 -35[degrees] 21' Scorpius Tonantzintla 1 -39[degrees] 04' Scorpius Tonantzintla 2 -38[degrees] 33' Scorpius Djorgovski 2 -27[degrees] 50' Sagittarius Palomar 8 -19[degrees] 50' Sagittarius Terzan 7 -34[degrees] 40' Sagittarius Arp 2 -30[degrees] 21' Sagittarius Terzan 8 -34[degrees] 00' Sagittarius
Contributing editor LES DALRYMPLE finds great pleasure in hunting down some of the heavens' most challenging deep-sky objects. He can be reached by e-mail at firstname.lastname@example.org.
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|Title Annotation:||rich hunting ground for globular star clusters|
|Publication:||Sky & Telescope|
|Date:||Jul 1, 2002|
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