Open your bag wide as we go trick-or-treating in the "W" of Cassiopeia, home to more than 100 star clusters.

Open cluster watering hole
The Milky Way runs through the "W" of Cassiopeia. This is one of the reasons the region is so rich in open clusters, several of which are shown here.
Bob King

More than 1,100 open clusters are known in the Milky Way galaxy with undoubtedly many more hidden beyond the hub in the galaxy's far-away arms. Of that number about 106, or nearly 10%, reside in Cassiopeia, a modest constellation ranking 25th in area. Pretty much anywhere you point a telescope in the "W", you're not far from a sweet swarm of stars. Today we'll explore a dozen of these beauties.

Open or galactic star clusters are comprised of relatively young stars numbering from handful to several thousand, all born from the same cloud of gas and dust. Most reside in the galaxy's spiral arms in the plane of the Milky Way, unlike the gargantuan globular clusters which hover about the central bulge like moths around a flame.

Downtown vs. the suburbs
Globular clusters are typically far richer, older, and more compact than open clusters. The globular M2 (left) in Aquarius contains around 150,000 stars; NGC 457 (right), also known as the ET Cluster, in Cassiopeia numbers 200 members.
Hunter Wilson Astrophotography

A typical open cluster lives a relatively short life compared to a globular — a few hundred million years vs. more than 10 billion. Their lives are brief because they're more easily tugged apart by gravitational encounters with large gas clouds and other star clusters as they cycle about the galactic center. In contrast, globular clusters bulge with hundreds of thousands up to a million stars, the combined gravity of which acts to restrain each individual star's tendency to wander.

Man with a plan
Robert Trumpler, Swiss-American astronomer.

Many open clusters possess a denser core of stars surrounded by a looser nimbus of cluster members. Some can be surprisingly compact and nearly as difficult to resolve into stars as the far more compressed globulars. Others lack a central concentration of any kind, making it a challenge to tease them apart from random groups of unrelated field stars.

Sizes vary but most open clusters span about 13-16 light years across with cores one-half to one-third as wide. They range in distance from the nearest, the Hyades, at 153 light-years, to Berkeley 29, located halfway across the galaxy 51,000 light-years away. As with stars, so with clusters — distance can turn a grand cluster into a grainy smudge.

Open clusters are classified according to three criteria developed by early 20th century Swiss-American astronomer Robert Trumpler: the number of stars observed in a cluster; how concentrated they are in its center; and the range of their apparent brightness. Along the way, Trumpler cataloged 37 star clusters, most of them new discoveries and one of which, Trumpler 1, made our list of 12.

Here's the breakdown on Trumpler's classes. You'll find his tidy categories helpful if you take notes on what you see through the telescopel:

Degree of concentration 
I. Detached cluster with a strong central concentration ("detached" meaning "how easily the cluster stands out in the field of view")
II. Detached cluster with little central concentration
III. Detached cluster with no noticeable concentration
IV. Not well detached, resembling field stars

Range of brightness
1. Most of the cluster's stars are of similar brightness
2. Moderate range of brightness
3. A mix of everything from bright to faint

Number of stars
p = poor. Star clusters with fewer than 50 stars
m= medium-rich with 50-100 members
r = rich with more than 100 stars

The letter 'N' is added if the cluster is involved in nebulosity. For example, the Pleiades is classified as "II3rN" while M103 is "II2m".

Open cluster treasure hunt
Use this map, which shows stars down to magnitude +9.5, to cluster-hop around Cassiopeia. We'll start at Delta (δ) and proceed east, but feel free to explore other regions. Numerous clusters dot the other half of the "W". Click for a large map you can print out and use at the telescope.
Chris Marriott's SkyMap

M103 is exactly where we start our Cassiopeia cluster hop. I could have easily chosen 15 clusters or mined another part of the constellation, but picked M103 because it's a personal favorite. Other groups we'll visit along the way illustrate the full diversity of open clusters. All my observations were made with a 15-inch (37-cm) reflector or 10×50 binoculars.

Multi-faceted beauty
M103 is one of the most distant Messier open clusters at 8,500 light-years. The cluster measures about 14 light-years across.

M103 has so many attractive attributes — the compact triangular shape, a rich smattering of stars within the figure, a striking ruby-red member, and a triple star (Struve 131) at the triangle's north apex. The cluster's also easy to find, located ½° north and 1° east of bright Delta Cassiopeiae.

Cute queue
Trumpler 1, located a similar distance from Earth as M103, displays an unusual "belt" of four similarly bright stars in a row.
Hunter Wilson

From there we shift our scope less than a degree to the north-northeast and alight on the side-by-side clusters Trumpler 1 and Czernik 4. The former pops nicely with about 30 stars (by my count), but its most interesting feature is the almost perfectly straight line of four stars along the cluster's western border like a miniature Orion's Belt with one extra punch. Look closely because one of the stars splits into an attractive, close double. Czernik 4 appears small and sparse with one bright central star surrounded by 10 dimmer hangers-on.

Cousin to ET?
Bright NGC 663 packs in some 400 stars and makes a pretty target in any telescope. It's also easily visible in binoculars.
Hunter Wilson

Moving along 1.2° to the southeast, we next arrive at NGC 659, a starry pentagon set against a slightly hazy background of fainter stars that responds well to averted vision. The first person on Earth to see the cluster was Caroline Herschel back in 1783. NGC 659 serves as the warm-up for splashy, bright NGC 663, located in the same low power field of view about ½° to the north. I swear it's the cousin of the popular "ET or Extraterrestrial Cluster (NGC 457)," with two bright starry eyes and chubby arms flung to either side. Even if you don't see an alien, you'll enjoy all the glitter here.

Continuing ½° to the east, we meet the curiously compact Berkeley 6. Based on its Trumpler classification II2p, you might think there's not much to see here, but the cluster's 20 stars are crammed into a blob only a few minutes across that mimics a small comet with a bright nucleus. Definitely a treat, not a trick.

Rich but hidden
Rich but faint, IC 166 may require averted vision to see. The cluster is reddened by intervening interstellar dust.
Markus Blauensteiner

NGC 654, another rich, moderately compact cluster similar to but half the size of NGC 663, lies 1° back to the northwest. A 7th-magnitude star pins its southern border. From here we shoot 1° due east and encounter IC 166, a faint, nebulous patch 8′ across. A 9th-magnitude star on its western edge helps to locate the cluster. Try as I might, I couldn't resolve any stars in this ghostly nebula-imposter until I upped the magnification to 286× and used averted vision. Finally, the eastern half of the group resolved into a tremulous grainy haze.

According to Archinal and Hynes' Star ClustersIC 166's brightest stars shine at only 17th magnitude. That seemed a little too faint; I'd put them at closer to 15. What do you see?

Berkeley 7, a very small cluster of 14th-magnitude and fainter stars ½° northeast of IC 166, may prove challenging. About 15 faint 14th-magnitude stars were visible at 142×, but it took effort to see them as a distinct cluster. Czernik 5, 1° farther south, won't knock your socks off either, but in spite of its tiny size and faint magnitude, it stands out well even at 64× as a hazy group comprised of some 10 stars.

From here, we hop a bit more than 1° southeast to the lovely harp-shaped cluster NGC 743. Though lacking a distinct core, this pretty group catches the eye even at low magnification.

The road never ends
Our 12th star cluster, Stock 2, marks the end of the trail, unless, of course, you wanted to take one last hop down to the magnificent pair of open clusters, NGC 884 and NGC 869.
Bob King

After digging deep, there's nothing more refreshing than finishing with something big and bright. Stock 2 fits the bill. You can imagine all kinds of patterns in this gangly assemblage of old stars: a man bent over a cane, streams of raindrops, even someone lifting weights. The cluster's arguably more beautiful in binoculars and small telescopes where a large field of view makes it pop.

Go 2° south of Stock 2, and you'll fall headlong into the riches of the Perseus Double Cluster. We could on from there, but it's probably time to pack up the telescope and get some sleep.

Like trick-or-treating at Halloween, you never know for sure what's going to drop into your bag when you explore a chunk of sky. That's why it's so much fun. Just pick a patch rich with deep sky objects, make a chart and amble from one object to the next. You'll lose yourself for hours, enhance your observing skills, and bump into some of the night sky's most fascinating creatures.

Name Magnitude Diameter Class RA Dec Comments
M103  7.4  6.0′  II2m  01 33.3  +60 39.5  Stunning gem
Trumpler 1
 8.1  3.0′  II2p  01 35.6  +61 17.2  Peculiar aspect
Czernik 4  ~ 9.0  4.0′  IV2p  01 35.6  +61 28.5  Sparse
NGC 659  7.9  6.0′  I2m  01 44.3  +60 40.2  Small, pretty
NGC 663  7.1  15′  II3r  01 46.2  +61 13.1  Rich, bright
NGC 654  6.5  6.0′  II2r  01 44.0  +61 53.0  Small, rich
Berkeley 6  ~ 9.5  5.0′  II2p  01 51.2  +61 03.6  Comet-like
IC 166  11.7  8.0′  II1r  01 52.3  +61 51.3  Faint, nebular
Berkeley 7  ~ 11  4.0′  II2p  01 54.2  +62 22.2  Difficult, vague
NGC 743  ~ 8.0  7.0′  IV1p  01 58.5  +60 10.0  Harp-shaped
Czernik 5  ~ 12.0  2.0′  III1p  01 55.6  +61 21.4  V. small, dim
Stock 2  4.4  60′  I2m  02 14.7  +59 29.1 Huge, nice in binoculars!

* Data from Star Clusters by Brent A. Archinal and Steven J. Hynes. I estimated magnitudes when none were listed.

Looking for star clusters? You can find them in the Sky & Telescope Pocket Sky Atlas!


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October 21, 2015 at 12:37 pm

Awesome! I've gone open cluster-hunting in Cassiopeia many times, but now I'm inspired to chase down a few more.

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Bob King

October 21, 2015 at 2:07 pm

Great to hear, Frank. Enjoy the adventure!

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Anthony Barreiro

October 23, 2015 at 7:29 pm

Thanks Bob, this is wonderful.

This past weekend I offered an introductory skywatching program at a hot springs resort with a very dark sky. Among other topics, I spoke about the history of our understanding of the milky way. When I got to Galileo I pointed a pair of mounted 11x56 binoculars at exactly this patch of sky to suggest what Galileo saw when he first looked at the milky way through a telescope.

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December 2, 2017 at 9:35 am

This is terrific. I’d just finished imaging the 3 Auriga clusters and M35 in Gemini and was looking for more nice open clusters. Thank you!!!

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Bob King

December 2, 2017 at 11:36 pm

Hi Jim,

Enjoy! Cassiopeia is so rich and it won't be long before the moon departs.

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