Open Clusters & Asterisms

Star clusters are groups of stars that have a common origin and are gravitationally bound.  There are two types of star clusters;  Globular Clusters and Open clusters. 

Globular Clusters are tightly bound groupings  of hundred of thousands of old stars located in the galactic halo and are remnants of the formation of our galaxy.   

Open Clusters are relatively young associations which can contain anywhere from a few to several hundred loosely bound  stars and are located in the spiral arms of our galaxy. 

Asterisms are random groupings of stars that to many observers take on a recognizable shape, for example “The Big Dipper”

Most, if not all, Astronomical objects have several names or designations depending on the type of object it is. For Star Clusters you will most often find them identified by a Messier, NGC, or IC number. The most popular objects often go by a popular descriptor, name, or moniker such as “The Butterfly Cluster” or “Beehive Cluster”

  • M – “Messier Number” – The Messier Catalog is a list of 110 astronomical objects which was first compiled in 1774 by French astronomer Charles Messier. The list contains most deep-sky objects that can be easily observed in the Norther-Hemisphere.
  • NGC – “New General Catalog” – A catalog of 7,840 astronomical objects (Galaxies, Star Clusters, and Nebulae) first compiled in 1888. It should be noted that all of the Messier objects have a NGC designation as well.
  • IC – “Index Catalog” – A supplement to the NGC catalog describing a further 5,386 astronomical objects published in 1895.
Located low to the horizon for northern observers, the open cluster M7 is the southernmost Messier object. The cluster has approximately 80 members with the brightest being of magnitude 5.6 which are set against the fainter and much more distant stars of the core of our galaxy – the Milky Way. The cluster is 980 light years distant with an estimated age of 200 million years. This photo was taken through my C9.25 Schmidt-Cassegrain telescope with a 16 minute integration time.
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The trapezium is a small cluster of stars within The Great Orion Nebula. It is the light from this group of stars that illuminates the nebula which is composed of interstellar dust and gas.
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A wide-field view of the Pleiades Cluster – also known as “The Seven Sisters” – peeking through layers of high thin clouds. It is the most easily visible star cluster to the naked eye. The stars range in brightness from magnitude 3.0 to 5.5. This cluster is also the inspiration for the multi-star logo found on Subaru cars.
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A closer view of the primary members of the Pleiades Cluster. The origin of the names of the stars are based on the the Greek God Atlas, his wife Pleione, and their seven daughters. Faint wisps of nebulosity, remnants of the dust cloud that was the source of their formation is visible around several of the stars.
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NGC 457 is variously called the “Owl Cluster or the “ET Cluster” (standing on his head).  It was discovered in 1787 by William Herschel and contains about 150 stars ranging from 12 to 15 magnitude.
Visible to the naked eye, the double cluster in the constellation of Persius is best viewed through binoculars or low power wide-field telescopes. The two clusters (NGC 869 & NGC 884) are relatively young at 12.5 Million years old and are both 7,500 LY distant. It is estimated that NGC 869 contains stars equivalent to 3,700 solar masses while NGC 884 contains stars equivalent to 2,800 solar masses.
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The star cluster NGC 2169 is also an example of an “Asterism” – a collection of stars which appears to take on a recognizable shape.  Look closely and you will see that the stars spell out the number “37”.
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With an apparent diameter nearly equal to that of the full moon, NGC 2477 is one of the most impressive open clusters in the night sky. It is located in the southerly constellation of Puppis and as such it barely rises above the horizon here in Colorado. I was able to get this photograph on a particularly clear and steady night with the telescope pointing nearly horizontal toward the southern horizon.
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“Kembles Cascade” is another example of an asterism. A stream of colorful stars seem to cascade downward from upper left to lower right anchored at the bottom by the cluster NGC 1502.
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A close-up view of the open cluster NGC 1502 which lies at the base of “Kimble’s Cascade” shown in the previous photo. This is a relatively young cluster with several “Hot” OB type stars.
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Kemble 2 in the constellation of Draco, perhaps more popularly known as “The Little Queen” or “The Mini Cassiopeia”, is an asterism 7th and 8th magnitude stars in the shape of a “W” making it look very much like the larger Constellation of Cassiopeia.
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This open cluster was discovered by Caroline Herschel in 1799 is 8,000 LY distant.  The loops of stars seem to take on the shape of rose petals if you stare at it for a time.
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A Small cluster in Scorpio that only rises 15 degrees above the horizon here in Castle Rock. The cluster is 6,800 light years distant with a physical diameter of 14 light years.
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NGC 6791 is one of the oldest open clusters known with an age of over 8 billion years.
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