Add a dash of random to your observing plans and you're guaranteed an adventure. We'll start ours with the famous globular M22 and see where it takes us.
One of the greatest things about observational astronomy is you can go anything you want. I can look up at the sky, pick a place to begin, and — using my atlas — star-hop from there to the nearest star cluster, dark nebula, double star, and on and on to my heart's content. When I've gone far enough, I can step away from the eyepiece and come back to Earth. The freedom to strike out on a path of one's choosing feels positively exhilarating.
I call these "star walks." Similar to a walkabout, a right of passage practiced by the Australian aboriginal peoples, we begin with the familiar but then leave it behind to explore new territory. We may encounter difficulties along the way, like poor seeing, uncertain identification, or out-and-out invisibility, but surprises and successes propel us onward. In the end, we pull away from the eyepiece with a kind of spiritual satisfaction.
This week, we'll begin our walk with one of my favorites, the globular star cluster M22 in Sagittarius. Located a little more than 2° northeast of Lambda (λ) Sagittarii at the top of the Teapot, M22 is one of the brightest globulars; it looks like a small, dense dot of fuzz when viewed with the naked eye from dark skies. Through a 4-inch or larger telescope it's a revelation of stars. In my 15-inch reflector at a magnification of 142× the central "ball" sharpens into countless grainy stars set within well-resolved halo. The core is broadly concentrated with an odd gash or vacuity cutting into its northeastern side, giving it a Pacman-like profile. A beautiful orange star catches the eye a short distance northeast of the cluster's core.
One of my favorite times to view M22 is in early twilight against a bluing sky, which gives the cluster a fragile delicacy without sacrificing resolution. From here, it's an easy hopscotch to three smaller globulars — M28, NGC 6638, and NGC 6642 — that crowd into a parallelogram just 2° long by 1° wide. M28 is bright enough to see in binoculars as a smaller, dimmer version of M22. Through a 6-inch and larger telescope it keeps its stars close in a dense core surrounded by a broad halo of faint stars that require an 8-inch or larger scope to see. In my 15-inch at 142× they're a hoard of tiny star-motes.
The other two globulars are fainter 10th-magnitude objects. NGC 6642 is small and very compact with a nearly stellar core at low magnification. Befitting its size (just 48″) and magnitude, it's much more remote than M22 at 26,400 light-years. NGC 6638 is a little brighter and twice as large with a less compressed core. Each cluster's misty halo resolved into a combination of granulation and very faint stars with a magnification of 245× in the 15-inch and averted vision. Smaller scopes won't hammer these globs into individual suns, instead showing them as faint comet look-alikes.
From globular richness we move on to singular planetary nebulae. Here in the direction of the galactic bulge, planetary nebulae gather like monarch butterflies on milkweed. Get your UHC or O III filter ready because some of these are starlike in appearance and tricky to identify unless they're "blinked" with the filter. Once you've found your suspect, slide the filter back and forth between your eye and the eyepiece, and you'll see it leap in brightness compared to the surrounding stars. Just 1.5° southwest of M22 look for NGC 6644, which resembles an 11th-magnitude "star" but looks duller, lacking the sharp sparkle of a true star. With a magnification of 200× and higher I've seen this 3″-wide planetary nebula as an slightly hazy object with a brighter center. Either way, it's a great blinker!
A little more than 1° north of M22 sit side-by-side planetaries, one not too difficult, one very difficult. The easy one is IC 4732, which I estimated at magnitude 13.5 and stellar in appearance. Although faint it blinks beautifully with a narrowband filter, so you'll have little problem identifying it. About 15′ to the southeast, challenge yourself with PK 10-6.2, listed at magnitude 15. Don't believe it! At first I couldn't see it without the filter, but once I blinked the wispy critter and saw it surge in brightness, I knew exactly where to look and discerned a very faint "spot" at the correct position without the filter's aid.
NGC 6629 is another easy 11th-magnitude planetary nebula that blinks wonderfully and shows a nice, fuzzy disk with a brighter center some 15″ across when examined at medium and high magnifications. Our final planetary is PK 9-4.1 also known as H1-67 and located 32′ north-northwest of NGC 6629. It's only 6″ across and listed at magnitude 14. My eyes saw a magnitude 13.5 star swaddled in a wad of haze at 142×. Once again, I confirmed my observation by blinking it with an O III filter.
Some dark nebulae are stark "holes" in the star field. B99 is more subtle. Observing at low magnification (64×) and tapping and moving the telescope to take best advantage of the eyes' motion-sensitive, night-vision rod cells, I could clearly make out a 10′ × 3′ elongated dark patch curving gently southward at its western end.
Before our star walk is complete, let's circle back to M22 and attempt one of the hobby's greatest eyebusters: spotting the faint planetary nebula GJJC1. Buried within the cluster, the tiny, ~14 magnitude nebula requires a night of excellent seeing. If the cluster stars appear steady and generally sharp at 200× or above and you have at least a 12-inch telescope, fire the engines. You'll also need a UHC or an O III filter to confirm that the tiny object you're seeking is the real thing. More observers report better visibility of this nebula with a UHC-style filter; an O III darkens the field of view too much.
Use the resources on this page and the photograph above to know exactly where to look. Once you spot the nebula's location, confirm by blinking with a UHC filter. I used the 15-inch working at 357×, starting at the little quadrilateral of stars just south of the dense core. The easiest star in the figure is the variable V8. A little less than an arc minute to the east-northeast of the quad are two 15th magnitude stars oriented north-south and separated by 10″. GJJC1 touches the south "side" of the southern star.
I found the two faint stars, but mushy seeing prevented me from separating the planetary from the star. So I upped the magnification to 490× and blinked back and forth with a UHC filter and finally saw the nebula brighten and fade compared to neighboring stars of similar magnitude. Whew!
You may have neither the skies nor the equipment to track down something so faint and obscure. If so, put it on a list of things you hope to see when you have access to a big scope. Then the next time you're at a star party, saunter over to that 24-inch Dob and ask the owner to point you to M22. Sometimes it takes years to complete a journey. Even random ones.