Observing Globular Clusters
Follow the tips below to maximize your globular cluster viewing experience.
1. What are Globular Clusters
Omega Centauri - The Brightest and Biggest Globular Cluster in the Sky
Globular clusters are large spherical collections of stars that are gravitationally bound. In contrast to the random shapes of open clusters, globular clusters are round collections of stars with varying degrees of concentrations of stars. They are some of the oldest objects in the universe with the ages of their stars around 10 billion years old and older.
The origin and formation of globular clusters is a very active area of research, and their relationship with galactic evolution is still unclear. In contrast to open clusters, globular clusters are not found in the plane of galaxies but in their halos. Looking at the age distributions of stars in globular clusters, some appear to have formed almost all of their stars in one immense star formation event. A few globular clusters do not have stars of a single age but show an age distribution of stars similar to that of galaxies, with the remaining globular cluster believed to be the remnant core of a cannibalized ancient galaxy.
When viewing a globular cluster you are looking at the remnants of the early stages of galactic formation and evolution, laying and skimmering at immense distances away at the outer edges of our Milky Way galaxy.
2. Variations on a Theme
M 13 (left) a less dense globular cluster compared to M 92 (right), a highly dense globular cluster that has undergone core collapse.
In small telescopes and at low magnification, globular clusters can all look the same, fuzzy spherical orbs hanging in space, only varying in brightness. However, with larger telescopes and at high magnifications their stars and structure can be resolved.
Some globular clusters have incredibly dense central regions that seem to be endless regardless of magnification. Some are less dense and the nebulous appearance resolves into a dense cluster of resolved stars. Globular clusters with dense central regions are the result of cluster core collapse and account for 10-20% of globular clusters. This is believed to be caused by interactions between the stars in the cluster with the heavier stars losing energy falling to the center, and the lighter stars gaining energy going to the edges of the cluster or being ejected entirely.
Globular clusters can also be torn apart from tidal interactions with their host galaxy. Globular cluster orbits tend to have orbits that pass through the galactic disk rather than staying within and can have stars ripped from them until they fizzle away. Clusters with these orbits are likely to end with their stars scattered throughout the disc and halo of their host galaxy.
Understanding the properties of a deep sky object allows the observer to pick the best equipment and timing and have the best experience possible.
In light-polluted skies the name of the game for finding deep sky objects in managing surface brightness. An object may have a low magnitude giving the impression it is bright against the background sky. However, if an object is large the light from its stated magnitude is spread across the area it takes up and is dimmed to a much lower level. Surface brightness is the brightness of a given region of the object, not its stellar magnitude equivalent.
3. Observing Globular Clusters
M 5 at low magnification in a 10” telescope (left) and at high magnification in a 24” telescope (right)
A globular cluster with a dense central region will have a high surface brightness with its light concentrated to a few parsecs instead of tens of light years. A more diffuse globular cluster will have a lower surface brightness and be harder to see against the glow of the sky. For globular clusters with minimally resolved stars use low magnifications to maximize surface brightness. Highly concentrated globular clusters with high surface brightnesses at their cores will be easier to see and can more effectively punch through light pollution.
Large and closeby globular clusters that can be resolved into individual stars can avoid the surface brightness problem with sufficient magnification. When using higher magnification and smaller exit pupils the background sky darkens but resolved stars remain the same brightness. This allows for high magnifications on resolvable globular to increase contrast with the background sky and the resolved stars in the group. This means that even in light-polluted skies, a bigger telescope will produce better views. The glow from unresolved members in the clusters will still be dimmed at high magnifications.
4. Notable Globular Clusters
The following globular clusters are some of the biggest, brightest, and most unique. The biggest take up multiple degrees in the sky, the brightest can be seen naked-eye in a dark sky, and the most unique are the remnants of galactic collisions or on the far edges of our galaxy 50,000+ ly away.
All globular clusters below have an eyepiece recreation that best represents what they appear like from bright suburban skies. These recreations represent the view when the object is highest in the sky in ideal transparent moonless conditions, and views may be dimmer in reality. The views are also made assuming the viewer has had at least 3 minutes to dark adapt. If the listed objects are viewed with no dark adaptation or while looking at lights, the object will appear much dimmer than recreated.
To make the eyepiece recreation as realistic as possible set the screen brightness to the highest setting and lower it until the 8 rectangle is barely visible. A dark sky will provide much better views in comparison to the eyepiece recreations and is always recommended.
Eyepiece recreation screen calibration aid
Best and Brightest
These globular clusters are the best and brightest our night sky has to offer. These are famous well-known clusters and are some of the most observed objects in the sky. In a large telescope in a dark sky, they fill the eyepiece and appear as a spherical chandelier of countless diamonds hanging above. Brighter globular clusters will have eyepiece recreations at higher magnifications to reveal stellar structure.
Unique
These globular clusters, with the exception of Omega Centauri, are not the most visually impressive but are fascinating objects. Some of them are incredibly far away and others are the remnant cores of long-gone galaxies. These objects make me reflect on my place in the galaxy and how large our island in the universe is.
Diving Deep
These globular clusters are not the best and brightest nor are they particularly unique. Every globular cluster is slightly different and these are for those of you who want to see as much and as deep as you possibly can. Objects will be added to this list every few months as I observe more globular clusters and make eyepiece recreations.
Omega Centauri
NGC 5139 / Caldwell 80
Magnitude: +3.9
Observing Months: April - June
Aperture: Naked eye or Binoculars
Size: 55 Arcminutes
Distance: 15,800 ly
Omega Centauri is the brightest, biggest, and most visually impressive globular cluster in the night sky. It is one of the largest globular clusters in the Milky Way and houses an astounding 10 million stars. Omega Centauri is ancient and almost as old as the galaxy itself, forming between 10 and 12 billion years ago.
Due to how far south this globular cluster is, it is difficult or impossible to observe from mid-northern latitudes. If you are able to see Omega Centauri from mid-Northern latitudes, due to the high air from the object being low in the sky will make it appear a mere whimper of its true self.
47 Tucanae
NGC 104
Magnitude: +4.0
Observing Months: January - June
Aperture: Naked eye or Binoculars
Size: 50 Arcminutes
Distance: 16,700 ly
47 Tucanae is the second brightest globular cluster in the night sky following only after Omega Centauri. 47 Tucanae appears as a fuzzy object the size of the full moon to the naked eye in dark sky conditions.
Despite the object’s massive size, it was unknown to European astronomers until 1751 due to its place in the far southern reaches of the sky. It sits adjacent to the Small Magellanic Cloud and both objects can be seen within the same field of view with 10 x 50 binoculars.
47 Tucanae is one of the oldest objects in the sky with an estimated age of 12 billion years and possibly older.
Great Globular in Hercules
Messier 13 / NGC 6205
Magnitude: +5.8
Observing Months: April - October
Aperture: 50mm+
Size: 20 Arcminutes
Distance: 25,000 ly
The Great Globular Cluster in Hercules, M 13, is one of the most impressive globular clusters in the Northern sky. Lying on the line connecting the naked eye stars of Hercules, Eta and Zeta Herculis, M 13 is very easy to find.
In binoculars, M 13 appears as a small fuzzy orb in bright suburban skies and a glittering bright orb in dark skies. M 13 is easily resolved into hundreds of individual stars at magnifications of 100 and above. The lack of a strongly concentrated central region can make M 13 harder to observe than dimmer smaller globular clusters in city or hazy skies.
M 13 lies far out of the plane of the galaxy, sitting at a vantage point that allows for the entire disc of the Milky Way to be seen, taking up half of the sky of a planet in the cluster.
M 5
NGC 5904
Magnitude: +5.7
Observing Months: March - September
Aperture: 50mm+
Size: 23 Arcminutes
Distance: 23,000 ly
Messier 5 is one of my favorite globular clusters to observe and brilliantly fills the eyepiece at high magnifications, even in bright suburban skies. Not having any bright stars near it, it is likely that M 5 has failed to reach the same level of popularity as M 13 due to its difficulty in locating. With the adoption of tools such as Astrohopper that can turn any manual telescope into a push-to smart telescope, I am expecting M 5 to gain popularity.
M 5 appears as a fuzzy orb at low magnifications but develops into a collection of hundreds of resolved stars with high magnifications. Even with heavy light pollution, M 5 is a fantastic object to observe at can be viewed at high magnifications.
Large telescopes and small exit pupils are recommended to reveal many stars against a dark sky background.
M 22
NGC 6656
Magnitude: +5.2
Observing Months: May - September
Aperture: 50mm+
Size: 32 Arcminutes
Distance: 10,000 ly
Messier 22 is the brightest globular cluster visible from most mid-Northern latitudes and is one of the closest globular clusters to the Earth.
M 22 sits in the plane of the galaxy and not in the galactic halo. When observed in a dark sky, it can be seen against a rich starfield that makes up the glow of the Milky Way’s thick disc. Being in the plane of the galaxy M 22 suffers from heavy dust extinction and appears dimmer than it would be if it were placed in the galactic halo.
M 22 does not have a dense central region and is best observed at low magnifications and large exit pupils to retain surface brightness. In dark enough skies M 22 is an easy object to find with the naked eye.
M 92
NGC 6341
Magnitude: +6.5
Observing Months: October - April
Aperture: 50mm+
Size: 14 Arcminutes
Distance: 25,900 ly
Messier 92 is one of the most overlooked objects in the night sky due to its close proximity to its more impressive neighbor M 13.
Most objects in the night sky appear close to one another often due to an optical illusion caused by a lack of depth perception rather than actually being close to one another. M 92 and M 13 however are visually and actually (in galactic terms) close to one another only a few hundred light years apart.
M 92 has undergone core collapse and has a very densely populated center. Although M 92 is not as visually large as M 13, the densely populated core of the cluster allows it to be more easily seen than M 13 in light-polluted skies. M 92 however is harder to manually find than its neighbor M 13 with no close bright stars for reference.
For public telescope viewing events, M 92 is recommended instead of M 13 due to the higher central surface brightness allowing more people to notice the cluster. M 92 benefits from background sky darkening from high magnification and smaller exit pupils.
M 3
NGC 5272 / Melotte 3
Magnitude: +6.3
Observing Months: March - September
Aperture: 50mm+
Size: 18 Arcminutes
Distance: 31,000 ly
Messier 3 is one of the most studied globular clusters in the night sky. Visible from northen latitudes while nights are still long, far out of the plane of the galaxy, and bright with a magnitude making it barely visible to the naked eye in a dark sky, M 3 was an ideal object for the earliest detailed studies of globular clusters.
M 3 has a somewhat centrally dense center and it can be easily seen in very light polluted skies. The outer fainter extents of the cluster need darker skies to see. The core of the cluster does well at high magnifications at can benefit from smaller exit pupils. M 3 can appear unimpressive compared to its bright neighbor M 5 in bright skies but becomes stunning in dark skies.
M 12
NGC 6218
Magnitude: +6.1
Observing Months: May - October
Aperture: 50mm+
Size: 16 Arcminutes
Distance: 18,000 ly
Messier 12 is the brightest globular cluster in the forgotten zodiacal constellation of Ophiuchus. M 12 when originally discovered by Charles Messier was described as a nebula without stars. When visually observed this description can be seen as somewhat true, with very few resolved stars visible. The brightest stars in the cluster are only 12th magnitude.
M 12 is best seen as moderate magnifications between 75 and 150x to retain surface brightness. At high magnifications in large telescopes, the brighter members of the cluster begin to resolve. In small and medium telescopes M 12 remains a fuzzy orb hanging in the heavens.
M 2
NGC 7089
Magnitude: +6.6
Observing Months: July - December
Aperture: 50mm+
Size: 16 Arcminutes
Distance: 36,000
Messier 2 at 175 light-years in diameter is one of the largest globular clusters known. It is a rich and compact cluster containing over 150,000 stars. Large telescopes can begin to resolve the brightest stars in the cluster around 13th magnitude.
M 2 is one of the best globular clusters in the sky in autumn and early winter. Its bright compact core cuts through light pollution well and can be seen in binoculars in light-polluted skies.
In my experience, M 2 does not always have correct celestial coordinates listed in planetarium softwares, with some placing the cluster 3 degrees North of its true location in the sky. If you believe you are looking at the right place in the sky and do not see M 2 check another star chart to make sure you are looking in the correct location.
M 15
NGC 7078
Magnitude: +6.3
Observing Months: July - December
Aperture: Naked eye or Binoculars
Size: 18 Arcminutes
Distance: 33,000 ly
Messier 15 is the brighter neighbor of M 2 in Pegasus. M 15 is one of the most densely packed globulars in the galaxy and has undergone core collapse.
In most circumstances a more densely populated core makes a globular cluster appear more prominent in light polluted skies. M 15 however has a core so densely packed that it appears less impressive and smaller than its dimmer neighbor M 2. M 15 will begin to reveal its brighter members in large telescopes around magnitude 12.6.
M 15 does well at high magnifications and benefits greatly from smaller exit pupils darkening the sky background.
M 4
NGC 6121
Magnitude: +5.4
Observing Months: May - September
Aperture: 50mm+
Size: 36 Arcminutes
Distance: 7190 ly
Messier 4 is perhaps the globular cluster that frustrates the most number of those just getting started with a telescope. Messier 4 with its bright magnitude of 5.4 gives the impression that it should be quite obvious and easy to observe. This however in practice is not the case.
For mid-northern latitude observers, M 4 is low in the sky and dimmed by the larger air mass along the line of sight. M 4 is also a rather sparse globular cluster with no characteristics of core collapse. These two factors make M 4 challenging to visually observe from light-polluted or even moderately dark skies. In my experience, M 4 often appears as a fuzzy region slightly brighter than the background sky with a few resolved stars.
In stark contrast to visual observing, imaging M 4 is incredibly easy and the stars of the cluster pop up in modest exposures.
M 10
NGC 6254
Magnitude: +6.6
Observing Months: May - October
Aperture: 50mm+
Size: 20 Arcminutes
Distance: 13,000 ly
Messier 10 is the second brightest globular cluster in Ophiuchus. It is slightly less impressive than its brighter neighbor M 12 but it is still a great object to observe.
M 10 has not undergone core collapse and is not sparse, making it look like a featureless fuzzball at low magnifications. At high magnifications in larger telescopes, many stars are revealed in its core. M 10 is dim enough to begin to appear indifferent from the majority of globular clusters magnitude 7 and dimmer.
In dark skies, the cluster is easily visible in binoculars and can fill a medium-power eyepiece with many stars. The famous British astronomer William Herschel described M 10 as “a beautiful cluster with extremely compressed stars,” but in my experience I think a perfectly dark sky is needed to describe it as such.
NGC 6934
Caldwell 13
Magnitude: +8.9
Observing Months: June - November
Aperture: 100mm+
Size: 7.1 Arcminutes
Distance: 52,000 ly
NGC 6934 is a globular cluster that sits at the outer edge of the galaxy and has a highly elliptical and inclined orbit.
NGC 6934 is not an exciting globular object to observe but is a great test of your observing skills and sky conditions. At a magnitude of 8.9 this globular cluster is one of the faintest clusters visible from bright suburban skies. It is visible even with a nearly full moon if the transparency is high.
3 degress off of the tail of Delphinus the dolphin this globular cluster should be easy to find.
The Intergalactic Wanderer
NGC 2419 / Caldwell 25
Magnitude: +10.3
Observing Months: November - May
Aperture: 150mm+
Size: 4.6 Arcminutes
Distance: 275,000 ly
The Intergalactic Wanderer, NGC 2419, is one of the dimmest objects visible in brightly suburban skies, appearing as a faint smudge in large telescopes on the best of nights.
NGC 2419 as its name suggests, lies far at the edge of the galactic halo at a staggering 275,000 ly away. It is intrinsically one of the biggest and brightest globular clusters in the sky with a mass of nearly 1 million suns.
For observers in the Andromeda galaxy observing out Milky Way, NGC 2419 would be the brightest extragalactic globular cluster.