Sunday, August 28, 2016
Issue 507: Messier XI
|M71: it's a glob!|
We are now well past the halfway point and on the downhill slope of the Messier list. Finally. I know it’s been slow going lately, but here’s another batch. Unfortunately, these will have to last you for a while, since after a quick breather I’m back on The Road again. Anyhow, here’s seven more treats beginning with one of my absolute favorites.
What does everybody want to know about Messier 71? Is it a globular star cluster or is it a galactic (open) star cluster? What’s all the hubbub about, bub? One look will show you. Get your scope on its position near the center of the little constellation Sagitta’s arrow asterism, throw in a medium power wide-field eyepiece, and you will soon be scratching your head. At first, it seems you are looking at a rich galactic cluster. Like M11, maybe. But keep staring and it becomes obvious it has a suspiciously strong central condensation.
So what’s the big problem? Let’s just take a look at M71's color magnitude diagram. Unfortunately, that, too, is ambiguous. It could be an older galactic or it could be a younger globular. The professionals wondered about this for many years, going back and forth on M71’s classification. It seems pretty evident today, though, and has since the 1970s, that it is a glob, since the cluster’s HR diagram does show a horizontal branch, which is a feature of globulars. The conclusion, which has gained increasing credence over the last 40 years, is that it’s a young glob of relatively high metallicity.
You don’t have to know pea-turkey about horizontal branches and metallicity and color magnitude diagrams to appreciate M71, however; you just have to like pretty things. M71 is a beaut when it’s riding high in its little constellation, which lies just off the rich Cygnus Milky Way. I know it looked good in my old (and sold) C8, Celeste, one long ago night, even from Chaos Manor South’s bright backyard:
This curious cluster looks very much like an open cluster rather than a glob in the light pollution. I can see quite a few cluster stars, but get only fleeting glimpses of its core. The group seems shapeless. One of the big attractions of this object, the beautiful rich field around it, is missing in the city. Still a lovely sight, though. Best seen at 127x on this humid July evening.
Messier 72 is not a bad little cluster. If it were “only” an NGC object it would actually be considered pretty good. But it is an M, and we tend to thing that should mean something special. This one is not special, but it is OK.
While it is fairly loose with a Shapley – Sawyer concentration class of IX, and is dim for a Messier glob at magnitude 9.2, M72's reasonably small size, 6.6’, means it stands out well when it is well up and as far away from the horizon as it gets—which is fairly high for most northern observers given the object’s -12 degrees declination. The problem is locating the little booger if you don’t have goto or digital setting circles.
Probably the best way to run down this Aquarius globular is to move 3-degrees 22’ southeast of Abali, Epsilon Aquarii. This magnitude 3.75 star should be easy even in a smallish finder even in a suburban sky. When you are on the spot (a magnitude 6.0 SAO star lies about 40.0’ to the northeast and will be in the same field as the cluster in a wide-field ocular) scan around carefully at medium power. Depending on your skies and scope, the globular may be nothing more than a subdued round brightening of the sky background.
“Subdued round brightening?!” Yep, sorry; that’s about all you will see from the typical backyard with a 4-inch or even 6-inch telescope. An 8-inch will make it look “grainy” under those conditions, and may even reveal a few stars around the periphery at high power, but to gain much resolution, you’ll have to move that 8-incher to a dark site. How do you really make the cluster look like much? Use a 10 – 16-inch under a dark sky. Still ain’t gonna be M13, though.
If you thought M72 wasn’t much, you really aren’t going to be impressed by M73. What it is is a group of four stars that may not even be a “real” deep sky object. This may just be an asterism, a pattern of stars created by our line of sight. The collective brightness is not bad, 8.9. What is bad is finding this little 3.0’ across patch of stars in the sun-poor wastes of Aquarius.
The easy way to locate M73 is to go to M72 first. There, move 1-degree 18.0’ almost due east. How hard is this thing to see? Even in a 4-inch, not that hard. What you have is a triangular pattern of four stars with the brightest being just a bit dimmer than magnitude 10 and the dimmest being almost at magnitude 12.
And that is kinda it. Use a medium-high power to get a nice view of the group and move on. If it makes you feel better about spending your time on this second-most-blah Messier of them all, perhaps this will make you feel better: the group is now suspected to be a (very old) open cluster and not just a “meaningless” asterism. Still feeling put out about being here? The beautiful Saturn Nebula is 1-degree 45.0’ to the northeast, so after you’ve seen all there is to see of puny M73, give yourself a treat.
M74 “The Phantom”
M74, the Phantom Galaxy, a beautiful near face-on Sc spiral galaxy in Pisces, is one of the best Messier galaxies and also one of the true wonders of the northern sky. Assuming you can see it at all.
At least getting on the proper position of this object is not difficult without electronics. While Pisces is not the most striking constellation in the sky, you should have no trouble spotting its magnitude 3.8 Eta star when the constellation is well away from the horizon. From there, move 1-degree 18.0’ northeast. Use a medium power ocular and search carefully for a subtle glow in the field. And good luck.
M74’s size is a manageable 10’ 30”, and it’s “bright” for a galaxy, magnitude 9.39. BUT. It’s a face-on and that almost always spells trouble. Its light is badly spread out, making it quite difficult to see under less than perfect skies. There’s a reason it is called the “Phantom,” alas. Many observers consider it more difficult even than M101, and some folks claim it is invisible from suburban skies.
Well, not quite. When I was writing my book, The Urban Astronomer’s Guide, I hunted up M74 multiple times from a very compromised site. It was often detectable in my 8-inch f/5 Newtonian at higher powers, and was always visible with my C11. It wasn’t something that would put your eye out, and there was no detail, but I could see it as a vague round brightening.
How do you get a good look at it, though? How do you see spiral structure? It depends more on your conditions than your scope. The sky needs to be dark, sure, but also dry. Any humidity just kills this one. The seeing, the atmospheric steadiness, needs to be good as well. When these prerequisites have been met, however, M74 has shown off its spiral arms in stark relief to my rather humble 12-inch Dobsonian, Old Betsy, as you can read here.
This Sagittarius globular star cluster is a fairly bright looking little guy despite shining at only magnitude 8.6. That’s because that magnitude is coupled with a smallish size, 6.8’. While it’s somewhat low in declination for some Northern Hemisphere observers, it’s not bad for most of us at -21-degrees.
Wanna look at it? Use a goto scope. You don’t own a goto? Well, I’ll tell you how to find it, but you aren’t gonna like it. M75 lies in the relatively unvisited part of Sagittarius to the northeast of the Teapot’s “handle.” While it is technically in Sagittarius, it is right on the border of Capricornus, and is easier to locate using the stars of the Seagoat.
The glob is 8-degrees southwest of one of dimmish Capricornus’ more prominent stars, Magnitude 3.0 Dabih, Beta Capricornii. 5-degrees 37’ farther to the northwest from the area of the cluster you’ll find a distinctive pattern of 5th magnitude stars, a triangle of suns that’s easy in a finder. While looking for M75, use a medium magnification eyepiece, and be on the lookout for something that resembles a fuzzy star.
And a fuzzy star, or at least a bloated fuzzy star, is about all you can expect in 8-inch and smaller scopes, even from fairly good locations. To see a few stars you’ll usually need those good conditions and a 10-inch telescope and high power. 12-inches is decidedly better. In addition to its small size, M75 has a couple of other strikes against it. It is a highly compressed group—it is a Class I—and it is distant for a glob, lying some 67,500 light years from our cozy little rock.
M76 “The Little Dumbell”
The skies have rolled on now, and the stars of winter are on the rise, including the suns of Perseus. That constellation’s M76, a planetary nebula, is one of the true beauties of the list. It’s a little small, about 3.0’ x 2.0’, but that makes it look bright even at its magnitude of 10.1.
Finding is not a hassle for the computer deprived. The Little Dumbell lies about 7-degrees south-southwest of the magnificent Double Cluster and 1-degree northwest of a magnitude 4.0 star, Phi Persei.
And when you get there? M76 is easy to see in small telescopes, being obvious with my 60mm ETX, Snoopy, from suburban light pollution. Doing more than just making out the nebula requires more aperture, however. In 6 – 8-inchers, the nebula looks like a, yes, small dumbbell (it looks more like a dumbbell than its big brother M27) or maybe a peanut. With 10-inch and larger instruments you’ll begin to pick up dark patches and streamers of gas. Whatever the size of your telescope, use higher powers on this small object and employ an OIII or UHC filter if you have one, as I did on one pleasant Chiefland Astronomy Village night with my C11, Big Bertha:
M76 is very good this evening. In addition to the two lobes and brightness variations across these lobes, the streamers of nebulosity wrapping around the main body of the nebula are fairly easy to make out.
M77 “Cetus A”
|M77 in 1990 with the Palomar Junior|
What’s troubling you, Bunky? You are observing from your light polluted backyard in the fall and want to see galaxies? M74 ain’t making it with you? Well, there’s one island universe you can see under surprisingly poor skies, M77, a face-on Sb spiral in Cetus. Now, I’ve already told you face-ons are difficult. What makes M77 different? An intensely bright center. This is an AGN (“active galactic nucleus”) object, a “Seyfert” galaxy, which pumps its integrated magnitude up to 8.7. With a size of just 7’6”, M77 is hard to miss even for small backyard scopes.
Finding is a trivial affair, since the galaxy lies less than a degree southeast of a fairly prominent star (as the stars of Cetus go), magnitude 4.0 Delta Ceti. Scan from Delta with a medium power wide field eyepiece and you’ll soon run across a suspiciously fat star.
What’s the reward? You get to see a galaxy, if not one that shows much in the way of details. As I discovered 26-years ago when I used my 4-inch Palomar Junior to inspect M77 from my urban backyard:
Nice, bright galaxy. Easily seen with direct vision but handicapped by its southwestern position, which puts it right into the worst of the light pollution. Round with a bright central region. Diffuse, round outer envelope.
And, frankly, that’s all you’ll see even with considerably larger apertures from dark sites. 10-inch and bigger scopes will make the galaxy’s actual (tiny, star-like) nucleus visible, however.
That is it, y’all. I’d like to keep going, but I’ve got to turn to other tasks. I’ve got a Sky & Telescope Test Report to get underway, and I need to at least think about packing for my next gig, the Almost Heaven Star Party. In my absence, why not get out and see some Ms for yourself? Especially if you, unlike me, are lucky enough to live somewhere where there’s a hint of fall in clear skies.
FTA on M77: "Finding is a trivial affair, since the galaxy lies less than a degree southeast of a fairly prominent star (as the stars of Cetus go), magnitude 4.0 Delta Cephei".Post a Comment
Shouldn't this be Delta Ceti?
Shouldn't this be Delta Ceti?