Sunday, December 04, 2011

 

My Favorite Fuzzies: Unk’s Globular Star Cluster Top Ten


I am an admitted galaxy-a-holic, muchachos. Nothing fires my imagination like giant island universes shining dimly across the dark light years of intergalactic space. I’ve been one for a while, too, though doing The Herschel Project has further educated me about the beauty of the less visited of these massive night birds and their incredible diversity. Still, there was a time when galaxies were number two on my deep sky hit parade.

“Globs,” globular star clusters, are giant balls of suns orbiting the nucleus of the Milky Way, swinging out into deep space in the course of their enormous elliptical orbits. Some are older and some are younger and some are more massive and some less so, but all are old and huge as we reckon such things. They are typically composed of hundreds of thousands of metal-poor stars, very old suns, fading embers that may have witnessed the birth of the Milky Way itself.

There are exceptions to the above; there are always exceptions when you are talking about the strange and far away deep sky. Some of the largest globs are thought not to be globular clusters at all, but the remains of small galaxies the Milky Way has devoured. And not all globular stars are yellow or red. Most clusters are peppered with “blue stragglers,” old stars who’ve got a second lease on life, maybe thanks to collisions and mergers with other old stars, and blaze away like younguns.

That’s the science, but how do they look? In a telescope with less than 5-inches of aperture, globular clusters are pretty blah: round fuzzy blobs not unlike many of the other deep sky objects a small scope will reveal. Get to 8-inches, though, and their appearance changes dramatically. “Dramatic” is a good word to describe them. Globs are the most dramatic of deep sky objects. In a medium-size telescope, the brightest are shown as what they are, giant balls of stars. Go to 12-inches and the most prominent globs become mind-bending forests of suns, while the dimmer ones begin to give up their stars, too. Increasing aperture just brings ever more globulars into the dramatic camp.

Anyhoo, globs were Unk’s faves for a long time, since he got up the gumption to stop just pining for a 6-inch Newtonian like the expensive Edmund Super Space Conqueror and RV-6 Dynascope and do something about it, home brewing a pipe-mount six. I still like globs, but I don’t spend nearly as much time with them as I used to. If I give a globular more than a glance these days, it’s usually because I’m using it as a focus “tool” when I’m preparing to image galaxies. But even now I sometimes find myself staring in wonder at a far away globe of stars and pushing the shutter release on it in spite of myself.

All globulars star clusters are not created equal. I have to laugh when some (usually novice) amateur says they all look alike. Like people, some are kings and some are peasants. They range from Messiers possessed of too many suns to count, to dim little smudges like the Palomar globs that the biggest scope won’t resolve visually. When you look at them, really look, you’ll find all globular clusters have strange and wonderful features peculiar to them. Which globs show these things best for amateur visual observers? Like anything from butterflies to race horses, “the best” is subjective, but I’ve been looking at globs for a long time, and these are my loves…

Before we begin, we’d better edumacate the greenhorns about the Shapley - Sawyer Scale. What is it? Why should we need to know what it is? In the late nineteen-twenties, American astronomers Harlow Shapley and Helen Sawyer Hogg set out to classify globular star clusters according to their concentrations. The most highly concentrated, “tightest,” clusters are assigned a Roman numeral class of I. The least concentrated, “loosest,” are Class XII. So what? Its Shapley Sawyer class has everything to do with how a glob will look in your telescope. Not only its general appearance in the eyepiece, but how difficult it is to resolve. A Class VI, for example, is way easier to break into stars than a III. Okay? Let’s go glob busting on Unk’s top ten star balls.

1. Omega Centauri (NGC 5139). Magnitude 3.9, 36.3’x36.3’, Class VIII. M13 is the best globular cluster visible from the northern hemisphere? Uh-uh, nosir buddy. Even from higher northern latitudes mighty Omega steals the show, and once you get lower than 30 N, there is simply no comparison or contest.

How good is it? One Chiefland Star Party, I forgot to look at the Big O till it was nearly too late. It had got so low that it was partially obscured by my tent canopy. On a whim, almost, I sent the NexStar 11 over that-a-way anyhow. Despite me using maybe half my aperture, Omega was still more than spectacular, a great egg of countless teeny-tiny stars. Frankly, this thing is so large, about the size of the full Moon, that you don’t need a big scope to appreciate it. Hell, it looks as good in a 50mm finder as many globulars do in a C8.

My history with Omega Centauri, I’m embarrassed to say, doesn’t go back that far. Maybe because I’d spent a considerable portion of my formative amateur years at the high (relatively speaking) latitude of Little Rock, Arkansas. I just never thought to look at it. Till one evening about twenty years ago after I moved back to the Swamp and was out at our old club dark site just over the Mississippi state line. I was staring at the southern horizon. Hmm… Centaurus was culminating. That fuzzy star, wasn’t that Omega? I sent my 8-inch Coulter Odyssey to it (by muscle power, of course). The result? My fellow observers musta thought I’d gone nuts with all that hooting and hollering.

How far south do you have to be to get a good look at Omega? At its declination of -47, it is above the horizon all the way up to 43-degrees north latitude, but to get a decent view you gotta be farther south. At 31-degrees (Possum Swamp) it is getting there. At 29.5N (Chiefland) it is considerably better. From the keys (Winter Star Party), it is better still. My dream? To see it overhead from southern climes some day. I can scarcely imagine…

2. 47 Tuc(anae) (NGC 104). Magnitude 4, 30’x30’, Class III. “Is it M13 time now, Uncle Rod?” Not yet, Skeezix. Even our number two is much better than M13. Much. Or so I am told. I’ve not yet had the pleasure of feasting my eyes on the cluster that some Southern Hemisphere boys and girls will tell you tops Omega. 47 Tuc (“tuck”), they say, has all the majesty of Omega Centauri, but packed into an area that’s slightly smaller and with a considerably tighter concentration. These things tend to make it a better object for larger telescopes.

The pros are not as concerned about looks as we are, but find 47 Tuc nearly as interesting as we do—or maybe moreso. An amazing 22 pulsars (neutron stars resulting from supernovae explosions) have been found in 47 Tucanae. Why, you may ask, do Omega Centauri and 47 Tuc have primary designations more suited to stars? Because both are fairly easily visible with the naked eye, and early observers mistook them for stars.

One day I will get a look at this thing. But only when I finally get down south—way south, as in south of the equator. With a declination of -72, it’s never above the horizon till you get to latitude 19-degrees north. Add at least ten more degrees to that so it’s more than just over the horizon, and the result is that nobody in the continental U.S. of A. gets a look at 47 Tuc. Dammit. In fact, even at the southernmost U.S. possession, Palmyra Atoll (uninhabited Pacific island), it’s only 14 degrees up. Sigh.

3. M5 (NGC 5904). Magnitude 5.6, 17.4’x17.4’, Class V. And it is still not time for M13, campers. While M5 is a little smaller than M13, and it’s Shapley – Sawyer class is the same, and it’s only a little brighter, it seems better resolved in any scope I use on it. To me, it also looks noticeably brighter than M13, brighter than the small difference in magnitude would suggest. It just looks “flashier” to me, whatever the hell that means. Strangely, while M13 has a yellowish cast in my 12-inch, M5 always looks “blue” to me.

I’ve always liked M5, but like a lot of y’all, I didn’t look at it often. That changed one night in 1998. My friend, ATM Pat Rochford, had just finished converting my much-loved 12-inch Meade Dobbie, Old Betsy, from a plebian Sonotube scope to a more upscale (and easier to pack) truss tube job. I took first light at one of the Escambia Amateur Astronomers’ Association’s dark sites; one that was, if not perfect, at least very good. For whatever reason, I turned Betsy to M5 as her first light object, and was blown away. Hell, it looked way better than M13. Went to M13 and back to M5, and I didn’t change my mind. Ever since, M5 has displaced M13 in my affections.

4. M13 (NGC 6205). Magnitude 5.8, 23.2’x23.2’, Class V. Finally. My putting it at number four is just quibbles from of your curmudgeonly old uncle; it is a wonder even if you have looked at it thousands of times. My history with it goes back to my earliest days as an amateur, and if I had a love - hate relationship with it in the beginning, it’s pure love today. Rare is the evening when the Great Globular is over the horizon that I don’t at least take a quick peep at it.

Reservations? If you are like I was near 50 years ago, a wet-behind-the-ears newbie with big dreams but a small scope, don’t expect too much. At a Shapley-Sawyer rating of V, it is a tough nut for small telescopes to crack. I can’t do it with my 80mm Short Tube refractor. I tried as hard as I could one spring with my ST80 from the fairly good skies of Indian Springs State Park and the Peach State Star Gaze. Pumped the Short Tube up in magnification as high as I could, but nary a star did I see. I can resolve a few of its sparklers with a 4.5-inch StarBlast at 150 – 200x, and my 5-inch ETX MCT delivers the goods big time.

5. M22 (NGC 6656). Magnitude 5.2, 24’x24’, Class VII. In the number five slot we have a cluster that would put M13 and M5 both in the ground if it were a little farther north in declination. As it is, this Sagittarius glob with a dec of almost -24-degrees south is too low to look like much for most northern observers. Even down here in the Swamp, it’s a wee bit far into the horizon hash to strut its stuff like M13 and M5 do.

It took me a while to turn on to this southern cluster’s charms, but when I did I was hooked. It’s sitting adjacent to the teapot’s lid, but it is so big and so bright that it burns through all that atmosphere nevertheless. Shortly after I tried the Short Tube 80 on M13 with disappointing results, I hauled it out to my buddy Pat’s observatory where, in less than perfect skies, the little refractor resolved stars in M22 like a champ. If you’ve got a small scope and yearn for globular stars, M22 is where you go.

6. M3 (NGC 5272). Magnitude 6.3, 18.6’x18.6’, Class VI. M3, yeah, M3. It’s a beauty, of course, but it suffers by being in the spring sky where it must compete for our attentions with the hordes of spring galaxies. It’s also got the misfortune to be just in advance of M13, and M5, and M22, and M94, and M10, and M12 and the rest of the summer gang. M3 is not the biggest or brightest or best resolved Messier glob, but, still, imagine how folks would rave about it if it were in the winter sky in place of M79.

My experience with M3? I’ve always liked it, but I’ve never longed for it. It’s a good meat and potatoes glob, but I don’t think I’d be wrong to say it’s a little on the blah side amongst the Messier spectaculars. Also, in the days before go-to, it suffered by being a bit of a pill to find. It’s in a relatively star poor area, and it’s hard to know how to approach it. From the east, from Boötes, or from the west, from Coma? I eventually learned the latter is easier. When I did find M3, I was always happy with it, if not crazy happy.

7. M53 (NGC 5024). Magnitude 7.7, 14.4’x14.4’, Class V. If M3 is sometimes ignored, M53 is the forgotten man of the Messier globs. It has three strikes against it. Like M3 it is in the spring sky when amateurs tend to be focused on intergalactic space, it’s a little lackluster at nearly magnitude 8, and it’s somewhat hard for small instruments to resolve at Class V. And yet, and yet… It’s still a Messier, and that means g-o-o-d. If nothing else, this one provides a welcome break when you tire of observing yet another faint fuzzie in Coma - Virgo. You do need 8-inches of aperture before M53 begins to look like much, but when you have at least that you may be surprised at how good it is.

One thing M53 has going for it, it is incredibly easy to find without go-to. It is located less than a degree northeast of the bright star Alpha Comae, Diadem. Not that I ever spent much time finding or looking at it. Till one year at the Texas Star Party when I was hunting NGC 5053 with my 12-inch Dob. NGC 5053 is a small, very loose (Class XI), very subdued glob that is Gilligan to M53’s Skipper. M53 entered the picture because I wasn’t using digital setting circles, and was using M53, which is only about a degree west of NGC 5053, as a signpost. After setting and resetting on M53 a few times on the way to its little pal, I was struck by what a beauty, a lustrous beauty, the Messier is when set in an exceptional sky.

8. NGC 6712. Magnitude 8.2, 4.3’x4.3’, Class IX. Sometimes a globular is great not because of imposing magnitude and size specs, but because of its field. That is true of M71, which we’ll get to shortly, and it is true of Scutum’s NGC 6712. On the face of it, this one shouldn’t be much. It is loose, approaching open cluster loose, it is small, and it is dim, past magnitude 8. One look, though, will knock your socks off; it is in the midst of one of Scutum’s amazingly rich star fields.

I didn’t discover NGC 6712 until 2002. If I’d glanced at it before, I didn’t remember it, and must have seen it from poor skies. It took one evening at the Chiefland Spring Picnic for me to realize this glob is, well, “WOWSERS!” Not that I’d have hunted it down (well, punched it into the go-to hand control) if I hadn’t been forced to work Scutum. The 2002 Spring Picnic’s weather was hot, and it was hazy, and it was sometimes stormy. We had some hours of absolutely incredible skies, but these were frequently punctuated by clouds. One evening when the largest sucker hole was centered on Scutum, I sent my new NexStar 11, Big Bertha, to the glob, took a good look with my 27 Panoptic, and the next thing I knew my buddies were picking me up off the ground. Yes, that good.

9. M71 (NGC 6838). Magnitude 8.3, 6.1’x6.1’, Class XI. In the number nine spot is M71, which is a lot like NGC 6712: a loose globular in a rich star field, this time in Sagitta. Despite its Messier number, it’s not quite as good as 6712, since it’s both dimmer and looser. Still, what a view! The cluster itself looks more like an open cluster than a globular, and is in a field literally packed with stars. For years, amateurs and professionals (some of ‘em, anyway) wondered whether this wasn’t really a rich M11-like open cluster. But one look at its color - magnitude diagram shows M71 to be old, very old, and it is indeed a member of the glob ranks.

I can’t remember a time when I didn’t love M71. This was one that, unlike M13, my humble Palomar Junior 4.25-inch Newtonian could resolve from the OK but less than perfect skies of Mama and Daddy’s backyard in the 1960s.

10. M15 (NGC 7078). Magnitude 6.4, 12.3’x12.3’, Class IV. And so we come, inevitably, to number ten. Fall’s M15 rings down the curtain on the globular show for another year. Yeah, we’ve still got M79 in Lepus to look forward to, but it ain’t much, muchachos, it ain’t much. The claim to fame of the Horse’s Nose Cluster (M15 is in Pegasus not far from Enif)? Not that it’s easy to resolve, it is not at Class IV. The draw is the opposite: its incredibly compact, bright core shining bravely in the lonely skies of autumn.

The attraction for me as a sprout was not seeing M15’s stars—I couldn’t—but that it was so bright and easy to find. As the years went on and we learned about black holes and the pros began to wonder whether there might be one at this glob’s heart (they’ve gone back and forth on that a few times), this mysterious globular just became more intriguing. My best view? There’ve been many, but maybe the most memorable one was the night I set up my new 12.5-inch Dobbie in the uber light polluted backyard of Chaos Manor South and turned her to M15. Old Betsy blew this one apart and into stars without even breathing hard.

11. NGC 2419. Magnitude 10.4, 6.2’x6.2’, Class II. Yeah, I know this is eleven, but I figured I am due one more since I ain’t actually seen 47 Tuc. The problem? Which one? Wasn’t M2 entitled? Howsabout M92, my fave underappreciated globular? Or the weird looking M30? When the rubber hit the road, I decided on the offbeat, which is thisun in spades.

The Intergalactic Wanderer, as it is sometimes known, is definitely out in left field. Waaay out. Not only is it in a seldom visited constellation, Lynx (Unk has spent considerable time there tracking down Herschel galaxies), it is far, far away. NGC 2419 is one of the most distant globulars known at 91.5 kiloparsecs out in space—beyond the Magellanic Clouds. In fact, at one time it was thought not to be bound to the Milky Way at all—hence the “Wanderer” bidness. That wasn’t true, it turned out. The globular is indeed orbiting the galaxy, but what an orbit. It takes three billion years to complete one trip around the Milky Way.

I’ve looked at this one a bunch of times, starting when I read Scotty’s column about it in a long ago “Deep Sky Wonders” and was intrigued. And I’ve always been fairly satisfied with it. Yeah, it’s dim and it’s tight, but my C8, Celeste, easily teased a few stars out of it at the Deep South Regional Star Gaze a couple of years ago.

It’s the fate of top-ten list makers that nobody ever agrees with them. And I don’t expect you to agree with all my choices, much less my rankings. What I want to know is what you think. You can add comments to this here blog, you know, and I’d be very pleased to see your Best of the Best.

Next time: Come with Unk to the hottest star party ever.

Comments:
Uni Rod, number uno in my hit parade is M22.....then Omega Centauri, then M13, then everything else. Not a summer evening goes by that I don't start off gazing at M22. I like to start before complete darkness and watch all those stars gradually come into view. Can't comment on those south of the equator. Maybe someday.
 
In our northern (Iowa) Skies, m13 is KING. But I have always enjoyed m22. Being almost 42 degrees north I have never seen the other Southern Beauties.

TH
 
I agree with your rankings 1-4. Although I think Omega Cen hardly even looks like a glob. I remember when I first raised my binoculars to it in Australia, I got a fright. You need to go and see the southern skies, Uncle Rod.
 
My first time to see Omega Centauri from Jamaica's southwest coast nearly made my humble 6x travel binocs FLY out of my hand and into the surf. WOWOWOW! is all I can save. In a proper scope, I cannot imagine. I hope someday to have another crack at it in my 10" travel scope. Worth a trip all by it's lonesome. (Don't get me started on Jewel Box, Coal Sack, Running Chicken, Southern Pleiades, & their nebulous, dark sky bino friendly neighbors.)
 
My first time to see Omega Centauri from Jamaica's southwest coast nearly made my humble 6x travel binocs FLY out of my hand and into the surf. WOWOWOW! is all I can save. In a proper scope, I cannot imagine. I hope someday to have another crack at it in my 10" travel scope. Worth a trip all by it's lonesome. (Don't get me started on Jewel Box, Coal Sack, Running Chicken, Southern Pleiades, & their nebulous, dark sky bino friendly neighbors.)
 
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