Globular clusters generally look great even in washed-out skies. Many globulars—especially the Messier globulars—are bright enough to be relatively easy to find even in heavy light pollution. And when you use high magnification to try to resolve the globular, you also improve the contrast by darkening the background. In washed-out skies it will certainly be harder to resolve some of the globulars, and you won’t see the fainter stars so you will loose a bit of star density. But any of the major globular clusters still look spectacular. When I have guests at the telescope, I first turn the telescope to any visible planets. But the very next thing I turn the telescope to is a globular cluster: I can count on it to generate plenty of “wows” from my audience.

Now there are a lot of globular clusters, so you might wonder which ones look best in an urban environment. Everyone has their favorites, but here are mine, all proven performers observing from washed-out skies…

The Best Washed-out Globular Clusters

M13

The Great Hercules Globular Cluster is on everyone’s short-list (at least in the northern hemisphere). It’s large and quite bright, and relatively easy to resolve, even under washed-out skies. My 4.25” (108 cm) Newtonian at around 150x consistently shows partial resolution, even under the Washington DC area light pollution. In my 10” (250 cm) Dob M13 is a spectacular sight. On the other hand, I prefer really compact globular clusters and at class V, M13 is not particularly compact. Still, its other great characteristics combined with the fact that rides high in the sky for northern observers make M13 hands-down the best globular to observe from urban locations. (Image: NASA/STScl/WikiSky)

M3

M3 is another large and bright globular that is also well positioned for northern observers. M3's brightest stars are nearly a magnitude fainter than M13's, making it a bit harder to resolve than M13.  Under washed-out skies you’ll need an aperture of at least 6” (150 cm) to get partial resolution. Still, even in light polluted skies M3 isn't too hard to resolve because M3's relatively high declination helps lift it out of the worst of the sky-glow. Because M3 is looser than M13, once resolved you can easily see stars clear to the center. M3's fainter stars actually contribute to its "wow" factor:  when you resolve it, M3 reveals itself to be a delicate cloud of fine stars—an absolutely beautiful view. (image: S. Kafka and K. Honeycutt, Indiana University/WIYN/NOAO/NSF)

M15

M15 is also a large and bright globular. It doesn’t ride as high in the sky as M13 or M3, but it is still high enough to get above of the worst sky-glow. Once it is out of the sky-glow, M15 is no harder to resolve than M3 . M15 is noticeably more compact than both M3 and M13, so M15 can be quite spectacular if you can get a clear view of it. The central region is particularly dense and somewhat similar in appearance to the star-like cores of some galaxies. In fact, M15 has undergone core-collapse and its central region is one of the densest known: half the mass of M15 resides within a circle roughly 1 minute of arc around the center. The superdense inner core is only 1.6 light years across (about 8 arc seconds). This gives M15 a very different look even in amateur scopes than the other globular clusters. (Image: NASA and The Hubble Heritage Team (STScI/AURA))

M2

M2 is also relatively large and bright, but what makes it stand out is that it is extremely compact (class II). M2 is the brightest class I /II cluster visible in the northern hemisphere. With member stars starting at magnitude 13.1, it’s definitely harder to resolve than the other globulars on this list, especially because it also sits lower in the sky. However, with an 8” (200 cm) telescope or better, you’ll see a truly spectacular view. It looks like a cone-shaped mound of very fine sugar grains—you see shimmering stars everywhere, but individual stars are only clearly visible at the very edge. The difference is quite dramatic when you look at M2 and M13 or M15 in quick succession. While M2 doesn't have M15's ultra-dense core, M2 is dense everywhere except for the outermost edges.  If you compare M22 with M2, M22 (class VII) almost looks like an open cluster. M2's down-side is that it generally doesn't get high enough above the horizon to clear the urban sky-glow. This really does degrade the view significantly. But even then M2 offers a unique and beautiful view. (Image: \NASA/STScI/WikiSky)

M22

M22 is the brightest, largest, and (in theory) easiest to resolve globular cluster visible from the northern hemisphere. The only reason it isn’t at the top of this list is that M22 rides so low in the sky. Northern hemisphere observers have limited opportunities to see it, and even when it is visible, M22 never gets out of the urban haze and sky-glow. This makes it very difficult to get a good view of M22 from most northern urban/suburban areas. However, M22's brightness and size mean that even low on the horizon, it is quite an impressive sight. With a little extra magnification to improve the contrast, M22 resolves quite well. M22 is quite big and relatively loose, so even under less than favorable conditions, the view is impressive. (Image: N.A.Sharp, REU program/AURA/NOAO/NSF)

M5

M5 is nearly a clone of M13, except that it sits much lower in the sky; so low that it may never get above the urban/suburban sky-glow. M5’s low position sitting in the sky-glow and slightly fainter stars make it harder to resolve than M13. Even then, M5 will resolve quite well with an 8” (200 cm) telescope. If you switch back and forth between M5 and M13, the slightly fainter stars in M5 are quite noticeable: it’s as if both globulars were piles of tiny diamonds, but M5 is made out of finer diamond dust and M13 is made of coarser diamond dust. (Image: NASA/STScI/WikiSky)