Observing Open Clusters
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1. Open Cluster Basics
Open clusters are groups of stars that are loosely gravitationally bound. They come in all kinds of shapes, sizes, and ages.
Open clusters are believed to form when a nebula collapses, forms stars, and has its remnants subsequently blown away bright the brightest of the newly born stars. Since open clusters are only loosely gravitationally bound, as they travel around the galaxy gravitational interactions with other objects will cause the stars in the cluster to be ejected one by one until the cluster dissolves away. The ejected stars then spend the rest of their lives orbiting the galaxy.
This process of open clusters dissolving is short compared to the lifetimes of most stars, making most open clusters appear blue with large hot stars. Larger stars are bluer and have short lives, while smaller stars appear redder and live tens to thousands of times longer than giant blue stars. These bright blue behemoths end their lives quickly in supernovae, and as the open cluster ages, it turns redder with only the smaller stars remaining.
The dissolving process of open clusters can happen fast in comparison to the age of most stars, resulting in most open clusters appearing blue.
M 44 - Beehive Cluster
2. Open Cluster Apparent Sizes
Aldeberaan (bright star on the left), the Hyades cluster, and surrounding galactic dust.
Open clusters are typically around 10 parsecs or about 30 lightyears in size. Most open clusters only vary by an order of 10 or so making most open clusters similar in size.
This makes the distance of an open cluster to the Earth a major determining factor in how large the cluster appears in the sky. The closest open clusters take up large sections of the sky 10 times the size of the moon or more. For example, the Hyades cluster is easily visible to the naked eye, with stars in its cluster making up the constellation Taurus. The Hyades cluster is about 150 light-years away and about 20 light-years across making it appear under 6 degrees wide in the night sky (14 times larger than the full moon).
Open clusters can dissolve into moving groups, no longer gravitationally bound collections of stars but retain their common motion from when they were a cluster. The big dipper in Ursa Major is a moving group with its members, all having once part of the same open cluster.
Open clusters farther away will appear smaller and more concentrated with many of the brighter open clusters about the size of the full moon. Open clusters that are several tens of thousands of light years away are often partially blocked by galactic dust along the line of sight. This dims and reddens the cluster and can give a young blue-rich open cluster the appearance of an older one.
3. Where to Find Open Clusters
Open Clusters, Dark Nebulae and Reddening from dust in Cassiopeia
Open clusters are the remnants of star-forming regions and retain the motion of the nebula that formed them. Stars form from gas clouds that undergo gravitational collapse. Gravitational collapse of a gas cloud can occur where the density is high enough for gravity to pull material in to make stars. The highest gas density is in the disc of a galaxy, where density waves and disturbances can trigger gravitational collapse.
Since open clusters retain the motion of the nebula that formed them, they are most commonly found in the plane of galaxies. The open clusters in our galaxy the Milky Way are concentrated in and adjacent to the galactic plane.
The highest apparent density of open clusters in the sky is in the direction of and past the galactic core in Sagittarius. Open clusters farther away in the direction of the galactic core are often subject due to reddening from dust present along the line of sight. This makes these open clusters redder and appear to lack the brilliant blues of other open clusters. Looking along the galactic plane but away from the galactic core open clusters have a bluer appearance from less dust extinction and reddening.
To see the reddening effects of dust, scan across the Milky Way near Deneb in Cygnus. Starting adjacent to the galactic plane will reveal a rich star field. Scanning toward the large collection of dark nebulae that makes up the Northern Coalsack, between Deneb and Alderamin, will make the rich starfield fade in a sparse collection of many dimmer redder background stars.
4. Notable Open Clusters
Eyepiece recreation screen calibration aid
The following open 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 have shapes that look like numbers, animals, and more! The list starts from the biggest and brightest and progresses to smaller and fainter open clusters.
All open 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 high in the sky, and an actual view may be dimmer when the object is low. 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.
Binocular / Naked Eye
Low Magnification Open Clusters (< 40x)
Medium Magnification Open Clusters (50x - 150x)
Hyades Cluster
Melotte 25 / Collinder 50
Magnitude: +1.5
Observing Months: October - April
Aperture: Naked eye or Binoculars
Size: 330 Arcminutes (5.5°)
Distance: 153 ly
The Hyades cluster makes up the face of the bull of Taurus. Aldebaran and the Hyades make a characteristic V-shape that is unmistakable once seen. In a dark sky the cluster is bright and easily visible to the naked eye. In bright suburban skies (Bortle 7+) the cluster begins to fade into the sky and binoculars are needed to see the dimmer components.
The Moon occasionally occults the stars in the Hyades and Aldebaran and the stars of the cluster can be seen disappearing and reappearing from behind the moon. A half-moon occultation allows for true night and a dim enough moon to see the cluster disappear behind the moon with the naked eye.
Alpha Persei Cluster
Melotte 20 / Collinder 13
Magnitude: +2.2
Observing Months: August - February
Aperture: Naked eye or Binoculars
Size: 366 Arcminutes (6.1°)
Distance: 570 ly
The Alpha Persei Cluster is centered around a bright foreground star Mirfak (centered in the binocular view). The cluster is in the center of the body of Perseus and has many naked-eye visible members from a dark sky. Using a binocular reveals many dimmer members of the open cluster.
The cluster is relatively young with an estimated age between 50 - 70 million years and contains many blue stars. The stars in the cluster have metallicities very similar to that of the Sun. The metallicity of a star cluster is not directly visible and is determined spectroscopically.
Coma Star Cluster
Melotte 111 / Collinder 256
Magnitude: +1.8
Observing Months: February - July
Aperture: Naked eye or Binoculars
Size: 450 Arcminutes (7.5°)
Distance: 280 ly
The Coma Star Cluster takes up a large portion of the Spring Sky in the constellation Coma Berenices. The sparseness of the open cluster makes it nearly impossible to see with the naked eye unless viewed in dark skies.
In contrast to the sparse nature of this open cluster, its neighbor the Coma Galaxy Cluster is a very dense region in the sky, housing hundreds of galaxies in an area the size of the full moon.
Pleiades
Messier 45 / Melotte 22
Magnitude: +1.6
Observing Months: October - April
Aperture: Naked eye or Binoculars
Size: 110 Arcminutes (1.8°)
Distance: 444 ly
The Pleiades is one of the most famous open clusters in the night sky. It is often referred to as the Seven Sisters for the seven stars that were visible to the naked eye in ancient times. One of those original seven has now dimmed in brightness, leaving six stars visible to the average observer under a dark sky. Eight or more stars can be seen using only the naked eye by experienced observers with excellent vision. From your location, see how many stars you can make out!
The Pleiades has a shroud of dust leftover from its formation and a striking reflection nebula can be revealed with imaging. With very deep imaging, a large collection of IFN (integrated flux nebula) can be revealed in the surrounding region. However, when observed visually no nebulosity can be seen.
Viewing the Pleiades at low magnification is always enjoyable, with tens of glittering blue stars filling the view. Using binoculars or a telescope reveals tens of stars across the field of view. Viewing the Pleiades with an ultra-wide angle eyepiece (82 degrees and larger) is an incredibly immersive and unforgettable sight.
Beehive Cluster
Messier 44 / Melotte 88
Magnitude: +3.1
Observing Months: February - June
Aperture: Naked eye or Binoculars
Size: 70 Arcminutes (1.2°)
Distance: 574 ly
The Beehive Cluster is an open cluster in Cancer that appears as a fuzzy spot in dark skies. With binoculars or a telescope, the field of view is filled with many stars.
The Beehive Cluster has a larger star population than most other nearby star clusters and this appears as a richer star field when viewed. This is one of the first open clusters I ever looked at in a telescope and was wowed by the number of bright stars I could see.
Ptolemy Cluster
Messier 7 / NGC 6475
Magnitude: +3.3
Observing Months: May - September
Aperture: Naked eye or Binoculars
Size: 75 Arcminutes (1.3°)
Distance: 1000 ly
The Ptolemy Cluster has been known since antiquity. It is named after the Greek-Roman astronomer Ptolemy who first recorded the object as a nebula in 130 AD. The cluster is in between the tail of Scorpius and the bow of Sagittarius the archer.
This open cluster is visible as a nebulous fuzz similar in brightness to the glow of the surrounding milky way in a dark sky. In bright suburban skies, the cluster needs binoculars or a telescope to be seen. The large angular size of the cluster makes it resolve into individual stars in small finder scopes.
In a dark sky, in the same view at low magnification, the open cluster Trumpler 30 and the globular cluster NGC 6543 can also be seen.
Brocchi’s Cluster
Cr 399
Magnitude: +3.6
Observing Months: May - October
Aperture: Naked eye or Binoculars
Size: 60 Arcminutes (1°)
Distance: N/A - Asterism
Brocchi’s Cluster, also called the Coathanger or Al Sufi’s Cluster, is an asterism in the summer triangle between Vega and Altair in the constellation Vulpecula the Fox. An asterism is a collection of stars that create a pattern but are smaller than constellations. Most asterisms are not gravitationally bound and Brocchi’s cluster is no exception. The closest star in the group is only 235 ly away while the farthest is 1735 ly away.
The cluster resembles the shape of a coathanger and has a striking line of 5 blue stars almost evenly spaced apart. To the naked eye the cluster appears as a faint smudge but with binoculars or a telescope resolves into its iconic shape.
Christmas Tree Cluster
Cr 399
Magnitude: +4.1
Observing Months: October - April
Aperture: Naked eye or Binoculars
Size: 60 Arcminutes (1°)
Distance: 2300 ly
The Christmas tree cluster lives up to its name and traces the outline of a Christmas tree with a bright blue star at its base. The Christmas tree cluster also just happens to be highest in the late evening around Christmas, making it a great festive target for holiday season star parties.
The cluster is in a hydrogen-rich region of the sky and lies directly centered between the Fox Fur and Cone Nebula. These nebulas are both not visible visually, however, their dust extinguishes the light from background stars and helps make the Christmas tree shape pop out against a velvet-black background sky.
Butterfly Cluster
M 6 / NGC 6405
Magnitude: +4.2
Observing Months: April - October
Aperture: Naked eye or Binoculars
Size: 33 Arcminutes
Distance: 2000 ly
The Butterfly Cluster is one of my favorite summer objects. This bright open cluster comfortably fits in the fields of small and large telescopes. The stars of the Butterfly Cluster outline and fill out the shape of a butterfly with its wings spread in flight.
The cluster is around 95 million years older and contains a single orange supergiant that is in the final stages of its life. This cluster is a great example of the evolution of stars in a cluster with the most massive evolving into red giants and dying before the less massive stars.
Double Cluster
Caldwell 14 / NGC 869 and NGC 884
Magnitude: +4.3
Observing Months: September - March
Aperture: Naked eye or Binoculars
Size: 60 Arcminutes (1°)
Distance: 7640 ly
The Double Cluster in Perseus is one of the best objects in the Northern sky. The double cluster consists of two individual clusters that appear side by side. Both clusters are physically close to each other and are about 7500 ly away.
In a dark sky, the double cluster appears as two very close fuzzy spheres or a fuzzy peanut-shaped luminous region. Binoculars will resolve the two clusters apart from one another and reveal their brighter stars.
The richer of the two clusters is NGC 869 and shows many small resolved stars at its center under high magnification.
Near the Double Cluster is a dimmer but similarly sized open cluster named Stock 2 can be seen under dark skies with binoculars accompanying these two bright clusters.
37 Cluster
NGC 2169
Magnitude: +5.9
Observing Months: November - April
Aperture: 100mm+
Size: 6 Arcminutes
Distance: 3400 ly
The 37 Cluster is a celestial oddity, resembling the number 37 written out in the sky with stars. The small size of the cluster requires a telescope or large pair of binoculars to properly resolve the 37. The 37 is made of 8th to 10th magnitude stars which will not be visible in regular 10 x 50 binoculars.
The 37 Cluster is near the Christmas Tree Cluster and is best seen in the winter sky. This cluster does well at high magnifications.
Wild Duck Cluster
M 11 / NGC 6705 / Mel 213
Magnitude: +5.8
Observing Months: May - October
Aperture: 50mm+
Size: 11 Arcminutes
Distance: 6000 ly
The Wild Duck Cluster is one of the richest and most compact known open clusters containing about 2900 stars. This open cluster has an estimated age of about 250 million years and as a result, is no longer blue in appearance.
This open cluster lies within a dense star field near the galactic core, and under dark skies, the eyepiece is filled with countless stars. The compactness and massive size of the cluster make its appearance lie between typical open clusters and globular clusters. This is one of my favorite open clusters to observe.
M 35
NGC 2168
Magnitude: +5.1
Observing Months: November - April
Aperture: Naked Eye, Binoculars
Size: 25 Arcminutes
Distance: 2800 ly
Messier 35 is one of the brightest open clusters and can be seen as a fuzzy nebulous object naked eye under dark skies. M 35 is easily spotted in dark skies near the feet of Gemini but has personally been difficult for me to find in very light-polluted skies due to a lack of bright adjacent stars. This cluster has an estimated age of around 175 million years.
The compact open cluster NGC 2158 can be seen in the same field of view as M 35 at low magnifications under dark skies. NGC 2158 may look unimpressive in comparison to M 35 but is nearly six times farther away at 16,000 ly away.
Auriga Salt and Pepper
M 37 / NGC 2099
Magnitude: +5.6
Observing Months: May - October
Aperture: Possibly Naked Eye, 50mm+
Size: 15 Arcminutes
Distance: 4590 ly
Auriga Salt and Pepper is one of the lovely open clusters visible in the winter sky. Accompanied by M 36 and M 38 in Auriga, M 37 shines brightest. This rich open cluster resolves into hundreds of stars under medium to high magnifications.
Auriga Salt and Pepper is in the antipodal direction, opposite of the galactic core as seen from Earth. The cluster is estimated to be about 450 million years old and has a yellow appearance with a spattering of remaining blue stars.
Owl Cluster
NGC 457 / Caldwell 13
Magnitude: +6.4
Observing Months: July - March
Aperture: 100mm+
Size: 20 Arcminutes
Distance: 7890 ly
The Owl Cluster is a wonderful open cluster in the constellation Cassiopeia. The Owl cluster as its name suggests has the appearance of the large beady eyes of an owl with an accompanying arc of stars that appears to follow the body profile of an angled owl.
This cluster is found in binoculars or a finderscope by its characteristic bright two stars that make up the eyes of the Owl. Surprisingly the two bright stars that give this cluster its recognizable appearance are actually not in the foreground but 70 percent farther away. These two distance luminous stars outshine the entire closer open cluster.
Northern Jewel Box
Mel 153 / NGC 6231 / Collinder 315
Magnitude: +2.6
Observing Months: May - August
Aperture: Naked eye, Binoculars
Size: 14 Arcminutes
Distance: 5600 ly
The Northern Jewel Box is a stunning open cluster that is elusive to observe from mid-Northern latitudes. This open cluster lies among the southernmost stars of Scorpius and never rises high in the sky.
The low altitude of the open cluster makes it difficult to observe, demanding a low unobstructed horizon. This open cluster is incredibly young, estimated to be between 2-7 million years old. The cluster is filled with incredibly luminous massive young stars that shine tens to hundreds of thousands of times brighter than our Sun.
The Northern Jewel Box shines like a collection of sparkling diamonds but from mid-Northern latitudes appears only a shell of its true self, attenuated by the large air mass along the line of sight. Despite having Northern in its name, this cluster is best viewed from the Southern hemisphere.