Star clusters are groups of gravitationally bound stars that formed millions of years ago from the same vast cloud of dust and gas.
When you gaze up at the night sky, it looks like a lot of stars are on their own.
But a solitary-looking star may be a member of a group; part of a star cluster.
Star clusters can contain anything from as few as ten stars to as thousands or even millions.
They're formed in clouds of cosmic gas and dust called 'interstellar clouds'.
More about star clusters
There are objects known as super star clusters, like Westerlund 1, which may have be responsible for vigorous star formation in our early Galaxy.
They travel together through space, but gentle tidal forces eventually cause the stars to move apart until they begin to merge into the general starry background.
There are many fine examples of newer and older star clusters out there, perfect for looking at with binoculars (read our top tips for binocular astronomy).
As a rule of thumb, you can pretty much assume that the younger the cluster, the more compact it appears, since the stars haven’t had much time to drift apart.
Globular clusters
Globular clusters are much bigger than regular star clusters, consisting of hundreds of thousands or millions of generally reddish, older stars.
Whereas star clusters are found and made within the plane of our Galaxy, globular clusters form a halo around it and their creation is much less well understood.
The majority of star clusters are found in or close to that misty river of stars stretching across the sky, the Milky Way, while globular clusters are seen all over the sky.
When looking at them with the naked eye you’ll see only fuzzy patches, but a pair of binoculars will reveal some truly spectacular gems.
Have a go at observing some star clusters for yourself. Below we've listed 5 star clusters and 5 globular star clusters.
If you're observing during the brighter months, read our pick of the best summer star clusters.
5 star clusters
The Pleiades
- Constellation Taurus
- RA 03h 45m 48s
- Dec. +24° 22’ 00”
The Pleiades, or Seven Sisters, is one of the most splendid clusters in the night sky. With the naked eye, six stars of the cluster are easy to see, but counting up to 10 is possible.
The cluster actually contains hundreds of stars, and a decent pair of binoculars will be able to reveal many of them.
NGC 869 and NGC 884
- Constellation Perseus
- RA 02h 19m 00s
- Dec. +57° 09’ 00”
This is the ‘Sword Handle’, a wondrous double cluster with two star clusters sitting side by side. They are both 0.5º in diameter and are easily visible to the unaided eye.
Try sweeping the area with binoculars – the hundreds of stars, set against the backdrop of the Milky Way, make for a fine sight.
Messier 7
- Constellation Scorpius
- RA 17h 53m 54s
- Dec. –34° 49’ 00”
Also known as the Ptolemy Cluster, this appears to be twice the size of the full Moon.
To the eye, the 80 stars of the cluster appear as a bright clump in the Milky Way, but through binoculars the stars are resolved.
Messier 35
- Constellation Gemini
- RA 06h 08m 54s
- Dec. +24° 20’ 00”
Messier 35 contains upwards of 200 stars and can just be seen with the unaided eye on good clear nights.
Binoculars bring out the brightest 20 or so stars, while the rest form a diffuse oval wash behind.
Beehive Cluster
Constellation Cancer
RA 08h 40m 06s dec. +19° 59’ 00”
Known as the Beehive Cluster, M44 contains hundreds of stars and can be seen as a misty patch with the naked eye. Binoculars are the best way to see M44: through them you’ll see a dozen or so of its brightest stars.
An added bonus is the Beehive Cluster's proximity to the ecliptic, which means it's often visited by planets like Mars or Venus.
5 globular clusters
Hercules Globular Cluster
- Constellation Hercules
- RA 16h 41m 42s
- Dec. +36° 28’ 00”
Known as the Great Globular Cluster, this is the best of its kind in the northern hemisphere.
From a dark site, the Hercules Globular Cluster can just be seen with the unaided eye, but its bright, round form is a stunning sight through a pair of binoculars.
Messier 5
- Constellation Serpens
- RA 15h 18m 36s
- Dec. +02° 05’ 00”
This is thought to be one of the oldest of all globular clusters. It is easily found in binoculars and has a slightly oval-shaped appearance.
What you’ll see is a fuzzy blob, hinting at the vast number of stars it contains.
Messier 3
- Constellation Canes Venatici
- RA 13h 42m 12s
- Dec. +28° 23’ 00”
This is another stunning globular cluster. It can just be seen with the unaided eye, but binoculars will reveal its bright, round shape that holds around 500,000 stars.
274 of these are known to be variable, the largest number in any known globular cluster.
Messier 22
- Constellation Sagittarius
- RA 18h 36m 24s
- Dec. –23° 54’ 00”
One of the brightest globular clusters, M22 is easily visible with the unaided eye, and a great sight through binoculars.
It’s larger than M13, which is impressive, but its place in the Milky Way’s river of stars makes this areal jewel in the crown.
Messier 15
- Constellation Pegasus
- RA 21h 30m 00s
- Dec. +12° 10’ 00”
Looking like a slightly more compact M13, this densely packed object is an ideal target for binoculars. It appears as a round smudge with quite a compact central region, giving this distant star cluster a real sense of depth.]
What are your favourite star clusters? Have you managed to capture an image of any? Let us know by emailing contactus@skyatnightmagazine.com