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Open Cluster (NGC 6811) & Globular Cluster (NGC 6838)
Kelli Grimm & Kristina Wicks

Open Cluster Orion Nebula
NGC 6811 - Kelli Grimm NGC 6838- Kristina Wicks

On a particularly clear night, it is humbling to look up into the night sky and see the stars sparkling above. They seem countless and isolated but, in reality, many of these stars are grouped together in clusters. There are two types of star clusters: open clusters and globular clusters. These stars share an origin and are bound together by gravity. Two such clusters are NGC 6811 and NGC 6838. The former is an open cluster, and the latter is a globular cluster.

Star cluster NGC 6811 was discovered by John Herschel in 1829. It is an open cluster, which is a group of typically around a hundred stars that formed from a single molecular cloud. Open clusters are usually fairly young, less than a few hundred million years old and are found only in spiral and irregular galaxies where there is still active star formation occurring. This particular open cluster, NGC 6811, is in the constellation Cygus and contains approximately 70 stars of uniform brightness. The diameter of this cluster is 15 arc-minutes, which is around half the diameter of the moon when it is full. The age of NGC 6811 is about 700 million years old, which is confirmed by the colors of the stars in the cluster: some red giants and many white dwarfs. From our sun, this cluster is approximately 3000 light-years away. In this cluster, there have been two planets discovered, which surprised many astronomers as it was thought to be unlikely that planets could survive in such a crowded star cluster. These planets, labeled Kepler-66b and Kepler-67b, are about three-fourths the size of Neptune, and less than three times the size of Earth. The planets were discovered using the transit method and their discovery implies that more planets might be found in similar environments.

Globular star clusters are generally spherical because they are tightly bound by gravity. Globular clusters are found in the halo of a galaxy. Stars in globular clusters are composed of hundreds of thousands of low metalocity, older stars. NGC 6838, also known as Messier 71 (M 71), is a globular cluster in the constellation Sagitta. This globular cluster in particular is barely distinguishable from an open cluster. Some of NGC 6838’s distinguishing features are that it is red shifted, has high metalocity, is rich in faint stars, and has a short horizontal branch. Also, it looks like an arrowhead that is pointing westward. The stars in globular clusters are approximately the same stage in stellar evolution, meaning they formed at about the same time. Historically, Frenchman Philippe Loys de Cheseaux first discovered NGC 6838. Charles Messier logged correct information about NGC 6838 that Pierre Mechain contributed. NGC 6838 is 12,000-13,000 light years from Earth. It spans 27 light years across. NGC 6838 is between 9-10 billion years old. In addition, it contains 10,000 members. Some interesting facts about this globular cluster are that is contains millisecond pulsars and sometimes kicks out white dwarf stars.

There are many similarities between open and globular clusters. Open clusters and globular clusters are both groups of stars that had formed from the same molecular cloud and are stuck together by gravity. As seen in the pictures, the open cluster, NGC 6811, and the globular cluster, NGC 6838, are both more crowded towards the center of the clusters and are more spread out around the outside.

Open clusters are comprised of mostly young stars while globular clusters are comprised of older stars. Therefore globular clusters are generally older than open clusters. As shown in the pictures, the globular cluster is predomiantly comprised of yellow stars while the open cluster is mostly white stars. The open cluster, NGC 6811, is around 700 million years old and therefore quite younger than globular cluster, NGC 6838. Also, while globular clusters are very tightly bound together by gravity, open clusters are more loosely bound. This is also shown in the photographs.



Object NGC 6811 NGC 6838
Right Ascension (J2000) 19:37:18 19:53:46.1
Declination (J2000) +46:22:55 +18:46:44
Filters used B (Blue), R (Red), V (Green) B (Blue), C (Clear), R (Red), V (Green)
Exposure time per filter B (300s x 2), V (120s x 2), R (90s x 4) B (240s x 2), V (180s x 2), R (240s), and C (120s)
Image dimension 951 x 578 pixels; 20.6 x 12.5 arcminutes 485 x 324 pixels; 10.51 x 7.02 arcminutes
Date/time observed October 1, 2015, 1:08 UT October 1, 2015, 6:30 UT



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