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
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, well 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
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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