OBSERVING GUIDE: THE SPRING GLOBULAR CLUSTERS
by: Clay Sherrod
Spectacular Objects for any Telescope
There are two events that occur every spring and continue through the hot days of summer that have some very much in common. Both I am sure you can relate to.
The first is that first "tinkle" of the little bell or the music that plays through a very bad bull-horn atop an old beat up panel van painted white...as it slowly progresses down your neighborhood streets, swarms of kids encircle it, follow it and surround it in a huge circular mass until it comes to a stop. The all-American Sno-Cone truck, attracting kids like bees around honey for decades.
Springtime also brings another similar-looking phenomenon, although not quite as "Norman Rockwell-ish" as the humble Sno-Cone truck. Indeed, this sight is awe-inspiring and the facts behind it even more.
It is the springtime and summer "appearance" of the hundreds of fantastic deep sky objects that we fittingly call 'GLOBULAR CLUSTERS', strange globs of stars which encircle our galaxy (and others as well) just like kids encircling the Sno-Cone truck!
Believe it not the first globular cluster was "discovered" by Edmund Halley, of comet fame, in 1715 yet he was not able to distinguish the many thousands of stars which we know today comprise the cluster. It was not until about 50 years later that Messier added into his famous catalog, "M-13" denoting his thirteenth entry of about 100. Halley himself noted that Messier 13 was visible as a fuzzy star to the naked eye (it is 6th magnitude if that attests to early sky conditions!).
It was not resolved until the very late 18th century by Sir William Herschel with his great "40-foot reflector" into many scores of what appeared to be tiny stars....yet, after that, many people - T.W. Webb with a 4" refractor and Admiral Smyth with his famous 6" glass - were able to clearly see many of the brighter individual stars in this cluster.
Soon, it became recognized as one of many and closely associated with two other wonderful globulars, Messiers 3 and 5, both nearby.
Herschel suggested that Messier 13 perhaps had over 14,000 stars, and indeed this cluster contains well in excess of 35,000 stars to limiting magnitude 23, ALL of which are gravitationally bound together around a common orbital center.
ABOUT THE GLOBULAR CLUSTERS
There are two images that are permanently fixed in my mind from my over 30 years of observing: the first is my first look at Saturn through my 4" Unitron in 1966, which I had saved for for two years to get; the second and equally memorable is the sight of Messier 13 through a 40" Boller & Chivens Cassegrain from the dark cornfield in central Nebraska (darkest skies I have EVER seen) in the summer of 1973. With that telescope and those inky-black sky, I could actually make our the orange, yellow and red giant stars among the thousands that stood in what appeared to be a true three-dimensional presentation.
Your highly capable telescope is fully capable of searching out and giving you fantastic and memorable views of most of the brighter globular star clusters; in the medium aperture telescopes (say, 4-6 inch), many of the brighter stars become evident on very clear nights....the larger 8" and above can show hundreds of individual stars in the larger clusters and begin to break down the "fuzzy glow" into stellar components on many of the fainter and more distance clusters as well.
You observational appetite of these "gems" will most definitely be piqued by some knowledge of their locations, physical make-ups and distances.
We see MOST of the Milky Way galaxy's globular clusters (well over hundreds of them, only about a hundred clearly visible using common telescopes) during the spring and summer months, mostly in the constellations of Bootes, Hercules, Scorpious, Sagittarius, Ophiuchus and Ara. This is misleading, however, as the globulars ARE NOT concentrated in space ONLY in one location as they appear.
Our sun and its planets are about 30,000 light years from the center of the Milky Way, and thus our "view" toward the center is skewed somewhat; when we look at the sky in the summer/spring months we are looking INWARD toward the galactic center, into a nearby area where the clusters appear to be more concentrated; in winter months, our nighttime skies are peering AWAY from the center where the actual number of globulars decreases rapidly.
Knowing that, you will realize how "skewed" our vantage of the globular clusters really is when you note the apparent spherical and very UNIFORM distribution of globulars around the Milky Way. Just like those kids hovering around the Sno-Cone truck, the globulars - for whatever reason - hover like a swarm of bees around our galaxy in all directions! They are EVERYWHERE in our galaxy, but WE are located far to one side!
The distances of globular clusters from Earth, hence, vary greatly; we are afforded the BEST views only of those located very near the vantage point surrounding the center of our galaxy; since we are on "one side" of the galaxy, those opposite are VERY distant, very faint and very difficult to see in our telescopes. One of the most spectacular (and yes it CAN be seen by observers in the southern U.S., not to mention our observers in Australia, South America, Mexico and Africa) globulars is Omega Centauri, a clear 2nd magnitude naked eye "fuzz ball" about 17,000 light years away. It can be cleanly resolved into stars by a 90mm scope and even a 60mm during very good conditions and high magnification (see magnification guidelines below).
By contrast the closest is only 8,000 light years and the most distant (all the way to the "opposite" edge of the Milky Way from us) is an incredible 180,000 light years away.
A typical globular cluster contains some 120,000 stars, mostly olding or aging stars, and is a whopping 150 light years in diameter! Consider that size for ONE cluster: that is 75 times LARGER than the distance we are from our next closest star!
Without going into theoretical astrophysics here, you should know that little is known about the actual origin of the globulars and why they have this "hovering" relationship with our galaxy; it appears that perhaps all typical galaxies - even the elliptical ones - have globular clusters hanging around in similar fashion. Astronomers do no know how they got there, if they are from the common "stuff" of the galaxies, or even which got here first.....the galaxy or the clusters?!
OPENING THE DOOR: OBSERVING THE CLUSTERS
Following is a concise and quick guide to your spring and summer tour of globular clusters. Not all of these may be on your AutoStar computer control, but most are. All major designations and coordinates are provided so that you can quickly access these curious objects.
Scope Size and the Clusters -
as an "overall guide" the following pretty well sums up what you can expect to see with your particular telescope. Note that I have divided the globular clusters in to Class I and Class II, with "Class II" being the faintest and more difficult to see/resolve. Class I globulars are bright, large and fairly close typically; Class II are very tight and compact, usually very small and faint. This quick list summarizes for 1) dark sky conditions (no moon or lights), 2) the cluster being nearly overhead, and 3) a magnification of about 25x per inch of aperture which is PERFECT for maximum contrast, resolution and brightness (as well as image scale) for your telescope:
TELESCOPE CLASS I CLASS II DESCRIPTION (overall)
60 / 70mm Super Select Larger clusters appear mottled, very 3-D
90 mm Super Select Larger clusters resolved
125 mm Excellent Super More stars than -90; faint clusters mottled detail
200 mm+ Excellent Super Many stars in Class I, brighter stars in II
No matter what size scope you have, there is a true adventure with the globulars. They offer a rare opportunity to visualize a 3-dimensional aspect not seen in other deep sky objects and a chance to really put the optics of any size scope to the acid test, both in light gathering and resolution.
Selecting Your Magnification - I have always said that there are two ways of observing a globular: 1) the grandeur of the magnificent "fuzzy ball" suspended in a wonderful wide-field dark sky among splattered stars; and, 2) a remarkable close-up that begins to reveal stars like "star dust" as they seemingly flicker in and out of view. BOTH experiences are incredible and worth your distant trip to the globular clusters.
Obviously, for wide-field (1) observing any of our telescopes will do in a very dark sky location. You want a very low (but not overly so) power magnification. About 15 power (15x) per INCH of aperture is an IDEAL low power for globular observing. Hence, the 3-inch needs a wide field power of around 38x; 55x for a 4-inch will do nicely; 75x is ideal with a 6-inch scope; and 120x (or less) is suitable for an 8-inch.
These magnifications will show you a relatively wide field of view (to afford a panorama of sorts with other objects in the field) yet still provide enough image scale (size of the globular) to give satisfactory hints of the shape and size of the object.
Now, for (2) above....to really get "into" the cluster and attempt to make out individual stars....I have found that globulars are the exception to the "deep sky" rule of magnification. You use whatever it takes! typically, a magnification of around 50x per inch is the limit. What you are trying to do is: find an eyepiece for your scope that will provide: 1) maximum resolution (i.e. show or begin to show individual stars or mottling); 2) with still very dark background sky (contrast); and, 3) no loss of brightness (magnification fall-off). You likely have just such an eyepiece! Experimentation will tell you if you are using too much or too little power when observing a globular cluster.
Now, let's pick out the best of the best and I will tell you what to expect.
YOUR GO-TO TELESCOPE GLOBULAR CLUSTER BONANZA (by constellation)
The following alphabetical list includes the most "observable" globular star clusters as found on your AutoStar "Star Cluster" Index. When possible all designations (Messier/Caldwell/NGC, etc.) are given. The brightness of the object, its size, and the concentration of stars within the cluster have led to this scale for ETX and LX 90 users.
THE RATING ("A" though "D") is provided for your telescope aperture; a rating of "A" denotes an excellent and easy globular cluster for ALL telescopes (all sizes and types); "B" on the other hand will be more difficult and best seen in at least a 3"; "C" puts the limit no smaller than the 6-inch, and "D" denotes a globular that is best seen with the 8-inch or perhaps disappointing in some scopes. A very brief description of the globular seen in some of these telescopes is also listed.
The rating is a simple way to eliminate frustration if you think you should be seeing "more" than you do in your telescope....for example, if the rating is "C" and you are observing with a 60mm, then don't expect too much! On the other hand, if the rating is "A" you should be very pleased with the view no matter WHAT size scope you have.
Each globular has an "R =..." rated within its description.
This list is broken into NORTHERN HEMISPHERE globulars (first list) and SOUTHERN HEMISPHERE GLOBULARS. (NOTE: there is some carry-over between, as some can be ideally or at least partially seen in both southern and northern hemisphere, depending on proximity to the Earth's equator. For example, Omega Centauri, even though it is far south at -47 degrees, can be seen as high as latitude +35 because of its large size and brightness! Likewise, Messier 13 at + 36 degrees is a good target for those at -35 degrees southern latitude with an unrestricted northern horizon!
NORTHERN GLOBULARS IN AUTOSTAR and other GO TO programs (all in Spring/Summer skies)
R = C: ngc6981 / Messier 72 - RA 20 53; DEC -12 32 / Very faint and tight, no resolution.
R = A: ngc7089 / Messier 2 - RA 21 33; DEC +00 49 / Spectacular, bright, large, one of the best!
CANES VENATICI -
R = A: ngc5272 /Messier 3 - RA 13 42; DEC +28 23 / Fantastic, one of the best, very large.
R = B: ngc7099 / Messier 30 - RA 21 40; DEC -23 11 / Very tight but fairly bright, no resolution.
COMA BERENICES -
R = A: ngc5024 / Messier 53 - RA 13 13; DEC +18 10 / Small but bright; partial resolv. in 125, LX.
R = D: ngc6934 / Caldw. 47 - RA 20 32; DEC +07 24 / Very dim, compact, no resolution in any.
R = D: ngc7006 / Caldw. 42 - RA 21 01; DEC +16 11 / Even dimmer, very faint and difficult, tiny.
R = A: ngc6205 / Messier 13 - RA 16 42; DEC +36 28 / The "standard", resolves partially in all scopes.
R = C: ngc6341 / Messier 92 - RA 17 17; DEC +43 08 / Very small, medium bright; part. res. in LX.
R = D: ngc2419 / Caldw. 25 - RA 07 38; +38 53 / 10th mag. very difficult, even with LX; tiny.
R = B: ngc 6779 / Messier 56 - RA 19 17; DEC +30 11 / Nice even tho small; fairly faint,no resolution.
OPHIUCHUS (are you ready for all these?) -
R = D: ngc6171 / Messier 107 - RA 16 32; DEC -13 03 / VERY small, med. bright, tough one!
R = B: ngc6218 / Messier 12 - RA 16 47; DEC -01 57 / Small but bright, some resolution in LX.
R = B: ngc6254 / Messier 10 - RA 16 57; DEC -04 06 / Almost a carbon copy of M-12 above!
R = A: ngc6286 / Messier 62 - RA 17 01; DEC -30 07 / Tiny, but nice even in small scope; no resolution.
R = B: ngc6273 / Messier 19 - RA 17 03; DEC -26 16 / Very nice, bright, some resolu. in -125 & LX.
R = A: ngc6333 / Messier 9 - RA 17 19; DEC -18 31 / Excellent in all, but dim; some resol in -90 +.
R = C: ngc6402 / Messier 14 - RA 17 37; DEC -03 15 / VERY small and difficult; no resolution.
R = A: ngc7078 / Messier 15 - RA 21 30; DEC +12 10 / Beautiful, resolves in ETX 90+; a small one!
R = A: ngc6626 / Messier 28 - RA 18 24; DEC -24 52 / Bright, medium size; resolves some in -125.
R = C: ngc6637 / Messier 69 - RA 18 31; DEC -32 21 / Very small, med. bright, tough even in LX.
R = A: ngc6656 / Messier 22 - RA 18 36; DEC -23 54 / One of the best, large, resolves in -90 and up.
R = D: ngc6681 / Messier 70 - RA 18 43; DEC -32 18 / Very hard, small, faint, no resolution.
R = C: ngc6715 / Messier 54 - RA 18 55; DEC -30 29 / Very hard, small, interesting in ETX 125.
R = A: ngc6809 / Messier 55 - RA 19 40; DEC -30 58 / A very nice one! Large and easy; res in -125+.
R = D: ngc6864 / Messier 75 - RA 20 06; DEC -21 55 / Very faint and small, no resolution at all.
R = C: ngc6838 / Messier 71 - RA 19 54; DEC +18 47 / Small and very faint; some res. in the LX.
R = B: ngc6093 / Messier 80 - RA 16 17; DEC -22 59 / VERY small but interesting in all scopes.
R = A: ngc6121 / Messier 4 - RA 16 24; DEC -26 32 / One of the best, very large & bright; resolves 90+.
R = B: ngc6712 - RA 18 53; DEC -08 42 / Faint but very large; some mottling in LX at high power.
R = A: ngc5904 / Messier 5 - RA 15 19; DEC +02 05 / Maybe the best; res. in ETX 90, very large, bright!
SOUTHERN GLOBULARS IN AUTOSTAR (all in Spring/Summer skies)
R = C: ngc6352 / Caldw. 81 - RA 17 25; DEC -48 25 / A very faint and small one; resolves in LX.
R = A: ngc6397 / Caldw. 86 - RA 17 41; DEC -43 40 / Very large and bright; one of the best for all scopes
R = D: ngc6101 / Caldw.107 - RA 16 26; DEC -72 12 / Maybe the toughest on the list; "transparent."
R = B: ngc2808 - RA 09 12; DEC -64 52 / Tightly packed in center, bright but small.
R = A: ngc5139 / Caldw. 80 - RA 13 27; DEC -47 29 / Omega Centauri....the BEST of them!
R = C: ngc5286 / Caldw. 84 - RA 13 46; DEC -51 22 / Fairly bright but small, resolves in LX.
R = C: ngc1851 - RA 05 14; DEC -40 03 / ETX 125 resolves stars around edge, looks like a glow.
CORONA AUSTRALIS -
R = B: ngc6541 / Caldw. 78 - RA 18 09; DEC -43 42 / Bright, but fairly small, good in ETX 90 +.
R = D: ngc 4590 / Messier 68 - RA 12 39; DEC -26 45 / Faint and small, mottled in the 8" scopes.
R = D: ngc5694 / Caldw. 66 - RA 14 40; DEC -26 332 / VERY tiny and faint...a real test, even for the LX.
R = D: ngc1261 - RA 03 12; DEC -55 13 / A very tough, compact one. Extremely small, no resolution.
R = B: Messier 79 - RA 05 24; DEC -24 33 / Partially resolvable in -90 and above; very small.
R = D: ngc5897 - RA 15 17; DEC -21 01 / Faint and very small, rather un-interesting in all scopes.
R = B: ngc5986 - RA 15 46; DEC -37 47 / Fairly bright but small, partial resolve in ETX 90 +.
R = A: ngc4372 / Caldw.108 - RA 12 26; DEC -72 40 / Faint but very large, good in ETX 90 +.
R = A: ngc4833 / Caldw.105 - RA 12 59; DEC -70 53 / Good in all scopes, large & med. bright.
R = A: ngc6752 / Caldw. 93 - RA 19 11; DEC -59 59 / A Beaut! Very large & bright, easy in all scopes.
R = B: ngc0288 - RA 00 53; DEC -26 35 /. Very loose and nice sized, some stars in larger scopes.
R = A: ngc0104 / Caldw.106 - RA 00 24; DEC -72 05 / Magnificent! Very large and easy! ("47 Tucanae")
R = B: ngc0362 / Caldw.106 - RA 01 03; DEC -70 51 / Bright and large, resolves in 3" scope.
R = B: ngc3201 / Caldw. 79 - RA 10 18; DEC -46 25 / Small but bright, some res. in ETX 125 +.
Now, armed with that complete AutoStar summary of GLOBULAR STAR CLUSTERS, you can go out there and get all of them in one night! Remember...there are MANY more such clusters (IC and NGC objects) than are listed here, but these are the primary listed ones in all modern popular planetarium programs. With their seemingly concentration in the spring and summer skies, why not make a "party" out of it....
.....and have "GLOBS" of fun!
Arkansas Sky Observatory
Conway / Petit Jean Mountain