Key Takeaways:
- Globular clusters are gravitationally bound, dense star collections containing tens of thousands to millions of stars, often located in galactic halos.
- Omega Centauri, the largest known globular cluster in the Milky Way, is visible to the naked eye and contains approximately 10 million stars, with a mass comparable to Sagittarius A*.
- M13, the Hercules Cluster, is a readily observable globular cluster located approximately 26,000 light-years away, exhibiting unique visual patterns not always captured in photographs.
- Globular clusters, readily observable through telescopes, particularly Dobsonians with large apertures, present visually striking details and patterns.
There is little I love more than gazing at a glittering globular cluster through a big, juicy Dob. The larger the mirror, the better! Each cluster looks a little different, and resolving individual stars is always a treat. Despite how easy they are to see, they are mysterious objects — and some of the oldest structures in the universe.
Globular clusters are dense collections of stars bound to each other by gravity. They can have anywhere from tens of thousands to millions of members.
Related: How do globular clusters remain intact for so long?
Globs, as many amateurs refer to them, tend to lurk in the halos (spherical areas surrounding the cores) of galaxies, and you can find a bright one to observe in the early evening sky just about any time of the year. The prolific German-British astronomer William Herschel coined the term globular cluster in 1782. At that time, there were 34 known globular clusters, and Herschel went on to discover 36 more.
Our Milky Way now has roughly 150 known clusters, and a few from the neighboring Magellanic clouds are visible through amateur telescopes as well. Here are some of my favorites.
The most impressive globular cluster to witness is Omega Centauri — so named because it was long thought to be a star. Visible to the naked eye even under moderate light pollution, it was known in antiquity. Its true nature was finally understood in the 19th century with John Herschel’s observations. I saw it for the first time from the Texas Star Party in Fort Davis. It is a Southern Hemisphere object with a declination of –47°, but from Fort Davis it just crests the hills to the south, rising to about 12° off the horizon, much to the delight of the northern astronomers who attend the event.
Slightly larger on the sky than the Full Moon, it fills the view of even small telescopes with hundreds or thousands of stars, many that are easy to resolve. It’s hard to take your eyes away! I could see it naked eye from La Serena, Chile, in the nights before the 2019 solar eclipse. My eye kept catching it whenever I was looking around the sky.
Omega Centauri lies a little less than 17,000 light-years away and is the largest known glob in the galaxy, home to an astonishing 10 million stars. It weighs in at 4 million solar masses, nearly the mass of our galaxy’s central black hole, Sagittarius A*. Researchers believe it originated from the core of a dwarf galaxy that was swallowed by the Milky Way.
A frequent stop of mine at public stargazes and evenings with my friends is M13, known as the Hercules Cluster for lying within the boundaries of the constellation of the same name. At a distance of 26,000 light-years and with a population of hundreds of thousands of stars, it’s a must-see on any summer night. Besides its beauty, my other favorite part is how easy it is to find. First look for the central part of Hercules, known as the Keystone. Spot the top two (eastern) or bottom two (western) stars of the square. M13 is almost in line between them, closer to Eta (η) than Zeta (ζ) Herculis. You can’t miss it!
While there are many deep-sky objects that look better in a photograph than to the eye, globular clusters aren’t one of them. Many globular clusters have apparent patterns in the stars that don’t show up in images. M13 is a good example. Careful observing reveals a propeller shape on the southeast side, the result of three dust lanes that appear to converge at the same point.
There are plenty more globs worth observing that I haven’t mentioned, but if you pan around the sky with binoculars or trawl through the Milky Way with a telescope, you’re bound to spot some of these fuzzy balls. If you get a chance to look through a Dobsonian with a big aperture, definitely aim it at one of these dazzling objects. You won’t be disappointed!
