Chandeliers of the galaxy, these distant stellar swarms fire our sense of wonder. Hop on and we'll tour 16 of season's finest globular clusters!
When it comes to sidewalk astronomy, the Moon, planets, and bright double stars always rate two thumbs up. But never doubt the power of globular star clusters. These stellar treasure troves rate right up there with Saturn and Jupiter when it comes to making an impression on first-time stargazers.
While the visual and emotional impact of a planet is immediate, globular clusters have this delicious, built-in delay between recognizing what they're looking at and then truly seeing it. I love the moment of silence before the realization sets in, followed a second or two later by something along the lines of, "Oh my God, I can't believe all those STARS!"
Every season has its share of globulars, though pickings are slim in winter. Things heat up in the spring and grow white-hot in the summer as the Teapot of Sagittarius makes its appearance at nightfall. Globulars are densely packed, generally spherical star clusters with anywhere from 10,000 up to 1 million or more stars. They orbit the center of the galaxy in a halo, the reason we see far more of them in summer, when we face toward galactic center, than in winter, when we gaze outward toward the anti-center.
Globular clusters' approximately spherical distribution about the galaxy's central bulge is thought to trace the margins of our galaxy in its youth, when the Milky Way was in the process of collapsing from multiple smaller clouds of gas and dust. The originally modest rotation of the material increased during the collapse and spun itself into a flattened disk, leaving the globulars as lonely "sentries" guarding the fringes of a great stellar empire. It's no surprise then that the clusters are the Methuselahs of the night sky, ancient assemblages of stars 10 billion years and older. Many of their original Sun-like, main-sequence stars have evolved into bloated red giants.
Globular clusters come in a variety of sizes and are distinguished by how tightly their stars are packed. Some are standing room only with cores so compressed, they stubbornly resist resolution in even the largest amateur telescopes. Other clusters are loose enough to give away all their stars even at low magnification. Astronomers classify these "degrees of concentration" according to the Shapley–Sawyer Concentration Class on a scale of I–XII (1–12), where Class I is the most concentrated and XII the least.
Distance can also be an important factor in resolving a cluster. M4 in Scorpius, the closest globular to Earth, lies just 7,200 light years away, while NGC 2419 is more than 40 times that far, twice as distant as the Large Magellanic Cloud. I can easily resolve many stars of M4 in my 10-inch reflector but only a few faint twinkles in NGC 2419's outer halo.
For our 16 featured globular clusters, I chose those that are well-placed during early evening hours from mid-May through early June. The Sagittarius and Ophiuchus hoards follow closely on the heels of this spring sampler, so I encourage you to pursue these beautiful balls of stars right through the summer. Here's a list of them all to keep the fire burning.
The current selection varies across the spectrum in size and degree of concentration, offering a delightful cross-section of what to expect from this surprisingly diverse cluster type.
Globulars benefit from medium and high magnifications — I like between 125× and 225× as a good balance between resolution, seeing conditions, and aesthetic appeal. Too much magnification robs a glob's character and softens the stars, detracting from the starry pinpoint effect.
Let's check out some highlights! All observations were made from a somewhat light-polluted site with a 15-inch reflector and 10×50 binoculars. The clusters ranged in brightness from magnitude +5.7 to +10.8. All are visible in an 8-inch scope and most in a 6-inch with varying degrees of resolution.
- M68 — Near the meridian at nightfall, this rich, moderately compact cluster's halo is peppered with faint stars even at a magnification of 64×. Increasing the power to 142× revealed a good number of individual stars draped across the core.
- NGC 4147 — Dim, small and rather difficult to resolve. Brightest stars are only about magnitude +14.5, so use high magnification.
- M53 — Rich, medium-compressed with partial resolution of the halo using 64×, this cluster's brightest star (~13 magnitude) lies a short distance due north of the core. Some resolution of the central core occurs at 142×. Upping to 245×, two short tendrils of stars extended southeast and northwest through the halo.
- NGC 5053 — What strange ghost is this? Large and composed of very faint stars from about magnitude +14 on down, which give the cluster a misty appearance that nonetheless resolves in a swarm of the tiniest pinpoints even at 64×. At 142×, the cluster is well-resolved, and the stars seem spread about with no concentration. Astronomers have discovered a tidal bridge of stars that winds back toward neighboring M53, hinting that the two clusters may have interacted in the past.
- Omega Centauri (NGC 5139)— Grand! With 10 times the mass of a typical globular and spanning an incredible 230 light-years, this is the Jabba the Hutt of clusters. Years ago, I easily saw its blur-ball shape with the naked eye from Tucson, Arizona. Even in a portable 3-inch scope, it was all glitter. Someday I'll get to see this from a proper latitude in a nice 12-inch scope. Definitely one for the bucket list. With a declination of –47½°, Omega Cen stands just 3° high at latitude 40° N — worth a try if you've got a good southern horizon. Look shortly before 11 p.m. local time in mid-May when the cluster crosses the meridian.
- M3 —Harbinger of spring and a perennial favorite. I must look at this cluster at least 10 times a year. Like a painting by Van Gogh, one never tires of revisiting a favorite deep sky object to appreciate the familiar and see something new. The bright halo stars are well-resolved in an 8-inch, while the stars on the core's near side hover over the unresolved background glow in a very 3-D way. The effect is very eye-catching. If you don't see it at first, use a little imagination.
- NGC 5466 — This is another faint, large and loose globular. It resembles NGC 5053, but it's a bit brighter, richer, and somewhat more compressed. The brightest stars sparkle around magnitude +13. It's well-resolved into a shimmery cloud of sparkles at 64×. Astronomers have recently uncovered a 45° long tidal stream of stars with an average width of 1.4° extending from this cluster to reach all the way to Ursa Major. Bread crumbs to find its way home? More likely stars stripped from the cluster during its passage through the flat disk of the Milky Way.
- NGC 5634 — What? A globular cluster in Virgo? OK, it looks like a galaxy, or even better, a comet, but it really is a pile of stars. At 64×, the cluster is small and compressed, with no stars resolved. Going to 142× puts a few faint stars in the outer halo, but things get much more exciting at 242×. At that magnification, the halo tentatively resolves into hundreds of tiny, faint sparks. Beautiful if fleeting! I couldn't crack the core, though. A bright, magnitude +8 orange star 1′ east of the globular adds pizzazz.
- M5 — I'm torn between this cluster, M13, and M22 as my all-time favorite. The core here is dense and somewhat small compared to that of some globs, but the stars seem to be drawn out in great whorls that make for an entrancing view. Look for a long, parabolic arc of stars extending from southwest of the core region, under (north of) of the core and back up to the southeast. The brightest member glimmers along core's southeast edge.
- NGC 5694 — The cluster is small and grows gradually brighter toward the center. I was unable to resolve this cluster but I'm not surprised. It's one of the most remote with a distance of 114,000 light-years.
- NGC 5824 — This one's tiny, with a bright, compact nearly stellar core. No stars resolved in my scope but that may have been because of the cluster's –33° declination in Lupus. This distant cluster is 104,000 light-years from Earth.
- NGC 5897 — It's little, concentrated, and faint, but still incredibly rich with dim stars. 142× gave a fantastic view. It resembles NGC 5466.
- NGC 5986 — At nearly –38°declination, NGC 5986 is a little too far south for a good view at my location. Other observers report it as bright and moderately compact, with a sprinkling of stars resolved across the face in an 8-inch scope. A bright magnitude +11.2 member lies northeast of the core.
- NGC 6229 — This glob is small and bright with a bright, concentrated core. It looks just like a comet. I wasn't able to resolve it at 64×, but 142× showed a grainy halo. In good seeing with averted vision and 245×, the halo crystallized into countless stars of magnitude +14 or fainter. The core resisted my efforts to resolve.
- M13 — This cluster is so jammed with starriness that even your dad would be impressed. 300,000 stars squeeze into the space of 20′. The halo is remarkable for the 4–5 stringy chains of stellar points that extend north and south of the core like the legs of a crab. Look for three narrow, relatively star-poor dark lanes east of the cluster's center nicknamed "the Propeller." A 6-inch scope will resolve many stars in M13's halo and even some across the core. In a 15-inch, it's downtown Las Vegas! Like M3, stars hover over the core, evoking a three-dimensional sense of depth.
- M92 — If you've come here by way of M13, the first thing you might notice is how much more tightly compressed this globular's core is. The halo is a diverse mix of both relatively bright and fainter stars and easy to resolve. Using 245× I could partially cleave the core into stars.
Ever wonder what it would be like to live on a planet in the heart of a globular cluster? The average density of stars in a typical cluster is about one per light-year, but in the core, it's more like one star per solar system diameter. Needless to say, the sky there would be crammed with thousands of stars so bright they'd cast more light than multiple full Moons! Something to think about the next time you look M13 in the eye.