IT'S GLOBULAR TIME
by Don Clouse
As we move into Spring and on towards Summer, the Earth is swinging round in its four billion year old dance around the Sun. Our night skies now begin to face more and more toward the center of our home galaxy, the Milky Way, and with this change in perspective, more and more globular clusters come into view. Globular clusters are huge, spherical, gravitationally bound collections of stars. They range in size from a few hundred thousand to several million stars. They orbit the galaxy in a vast roughly spherical halo some 100,000 light years in diameter. Thus as we look toward the central regions of the galaxy in the summer many more globular clusters are apparent than in the winter when night skies direct our gaze away from the center toward the outer reaches of the Milky Way. About 150 globular clusters have been identified as belonging to the Milky Way. Many of these are distant and faint, but some are simply stunning when viewed through amateur telescopes. Which brings us to the main topic of this article: the 'best' spring and summer globular clusters to view from our area and what to look for when you find them. (Most of the following information concerning magnitudes, angular sizes, positions, and distributions is drawn from Deep Sky Planner 2.04, by Phyllis K. Lang, available from Sky Publishing Corporation.)
But first a slight digression. There are 151 globular clusters listed in the New General Catalogue (NGC). Many of these would appear to be extra-galactic. For example, 23 are listed as being in Dorado and 13 in Mensa. Dorado and Mensa are the constellations where the Large Magellanic Cloud (LMC) is located. The LMC is known to have (at least) 17 globular clusters. No doubt some of these 36 (23+13) NGC globular clusters are members of the LMC. This still leaves a difference of some 19 globular clusters. To this point, however, I haven't run across any information which sheds any light on this seeming discrepancy. In any case, the point is that even though many, if not most, NGC globular clusters are denizens of our galaxy, not all are. I have run across references to globular clusters with designations from other lesser known catalogues. The similarity of the two numbers (150 clusters known in our galaxy and 151 NGC clusters) is therefore somewhat of a coincidence. Enough. Let's begin veering back toward main point of the article. (We'll get there eventually. I promise.)
So, of these 151 globular clusters, how many can we reasonably expect to see with our backyard scopes? Well, naturally, it depends on lots of things like your observing site, sky conditions, your scope, your eyes, your observing technique, and your dedication to finding 'faint fuzzies'. But for the purposes of this article, we’ll try to narrow things down a bit. First, there's lots of deep sky objects we can't see simply due to our location at approximately 38 degrees north latitude. Technically, we should be able to see anything north of -52 degrees declination (90-38=52). However, in practice, things like hills, trees, houses, and a very thick atmosphere get in the way. Choosing a more realistic declination of -40 degrees (which may still be stretching things a bit) immediately pares the list to 85 NGC globular clusters. If we disregard all globular clusters of magnitude 10.0 or less, we're still left with a list of 67. Clearly, there's lots of interesting targets here for observing. All 29 of the Messier (M) globular clusters are in this list. Not surprisingly, the Messier globular clusters tend to be the brightest ones which can be seen from northern latitudes. However, there are a number of ‘non-Messier’ globular clusters which are brighter than M72 (magnitude 9.4) in Aquarius, the dimmest 'M' globular. "But what about the spring and summer globular clusters?", you are no doubt wondering. O.K, now we finally get around to the (so-called) main point of the article.
First up is M3 in Canes Venatici. At magnitude 6.4 and 16'.2 in angular size (hereafter I will use the format mag-sec, e.g., 6.4 - 16'.2, to denote these measures) this is one of the more prominent northern globular clusters. Recently (3/29/98), I observed M3 from my backyard in the Highlands neighborhood of Louisville. At 58x (through my 8" Schmidt-Cassegrain) it was a small 'fuzz' ball. At 85x with averted vision, I was able to detect some mottling but no stars were resolved. Needless to say, it would be much more impressive from a dark site. (Nearby in Coma Berenices is M53 at 7.7 - 12'.6.) About two hours later that same evening, I observed M13 (5.9 - 16'.6) in Hercules as it rose above a tree in my neighbor's yard. At 58x some mottling was discernible. At 85x about a dozen outlying stars were resolved with direct vision. Using averted vision, a few dozen tiny pinpoints sprang into view surrounding the softly glowing core. It was really quite beautiful. (I tried 162x on both objects but the image became very indistinct - probably due to the urban sky glow.) I had occasion to view M13 from the prospective LAS observatory site in southern Indiana this past September. It was breath taking indeed at high power under dark skies. Also in Hercules is M92 (6.5 - 11'.2), which should make a nice target, and NGC6229 (9.4 - 4'.5).
Below Hercules in the sky is Serpens Caput and Ophiuchus. Serpens Caput contains a single globular cluster, M5 (5.8 - 17'.4). According to the figures in Deep Sky 2.0, this is the sixth brightest globular cluster in the sky. Brighter in fact than M13 which ranks eighth. M5 is almost 35 degrees lower in the sky however. Therefore, due to atmospheric extinction may appear dimmer. The only opportunity I've had (thus far) to observe M5 was at low power through someone else's scope at the Patoka Lake star party last summer and so can't really report on how it looked. I'm looking forward to observing it soon - it's big and bright and so should make a really nice observing target.
To the east of Serpens Caput lies Ophiuchus which contains twenty one NGC globular clusters! The seven largest and brightest of these are Messier objects. They are M9 (7.9 - 9'.3), M10 ( 6.6 - 15'.1), M12 (6.6 - 14'.5), M14 (7.6 - 11'.7), M19 (7.2 - 13'.5), M62 (6.6 - 14'.1), and M107 (8.1 - 10'0). This past fall I observed M10 and M12 through tripod mounted 7x30 binoculars from the parking lot adjacent to the Otter Creek observatory. They were visible as small, softly glowing, fuzzy disks. I would expect them both to be very nice sights through a larger scope. One source ("Universe Guide to Stars and Planets", Ian Ridpath and Wil Tirion, Universe Books, 1984) indicates that individual stars can be resolved through apertures of 150mm (6 inches) or more.
Below Ophiuchus, Scorpius encloses ten (NGC listed) globular clusters within it's boundaries. Two of these are "M" objects, M4 (5.9 - 26'.3) and M80 (7.2 - 8'.0). M4 will definitely be a primary summer target for me. It is the third largest globular cluster (and 7th brightest, tied with M13) in the sky coming behind only Omega Centauri and 47 Tucanae. Ridpath (see previous citation) indicates that a 100mm (4 inch) scope can resolve individual stars. NGC6388 (6.9 - 8'.7) and NGC6441 (7.4 - 7'.8) may also be worth trying for. Both however are quite low in the sky at about -45 degrees and -37 degrees declination respectively. East (of the sun and west of the moon, ... oh, sorry) of Ophiuchus and Scorpius lies Sagittarius, the Archer (not ‘the Teapot’ - I'm sorry but it's just too darn prosaic). A full twenty NGC globular clusters grace the confines of Sagittarius. Seven of them are Messier objects. Here's the list. M22 (5.1 - 24'.0, 3rd brightest - 5th largest), M28 (6.9 - 11'.2), M54 (7.7 - 9'.1), M55 (7.0 - 19'.0), M69 (7.7 - 7'.1), M70 (8.1 - 7.'8),and M75 (8.6 - 6'.0). Also, you may want to look for NGC6723 (7.3 -11'.0). M22 is one of more spectacular objects visible from our latitude. Ridpath (same source as previously sited) indicates that apertures as small 75mm (3 inches) "begin to resolve it's outer regions ...''. This is what I wrote about M22 upon seeing it for the first time, "(at 116x it) resolved into hundreds of tiny stars overlaying a softly glowing background - stunning". This was the same September evening, alluded to earlier, on which I first observed M13. Keep in mind that this was the first time I had observed these two bright globular clusters under high power, large aperture (well, I suppose it's relative - eight inch aperture is large to me), and dark skies, so it's possible this description (actually written a couple of days after the fact) may be a bit over stated. In any case, I'm looking very much forward to observing both M22 and M13 again this year. Much higher in the sky, there are two other summer Messier globular clusters, M71 (8.3 - 7'.2) in Sagitta and M56 (8.3 - 7'.1) in Lyra.
When observing globular clusters, what should you be looking for? The following list of
suggestions is from "Observe the Herschel II Objects", by Carol Cole and Candace
Pratt, the Astronomical League, 1997. Should you choose to record your observations, this
list of things to check for will provide an excellent basis for your logs.
- Can this cluster be seen with direct vision or averted vision required?
- Is the core unusually bright, compact or not distinguishable.
- Is this globular cluster highly or loosely concentrated?
- Is any part of the cluster resolved into stars?
- Is any mottling visible?
- Can the edges be resolved?
- Are there any other deep-sky objects in the same field of view? If so, what?
To this list I would add only "observe at different powers" and note how the appearance changes. And lastly, clear skies and happy hunting!
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