Long identified as a tightly grouped open star cluster, spectroscopic analysis in the 1970’s showed this group of stars is a loosely grouped globular cluster. It is located in the constellation Sagitta (The Arrow) and is about 13,000 light-years from the Sun. At that distance, the cluster is about 27 light-years in diameter.
NGC 6838 (M 71)[C:52x30s]
This image shows the cluster viewed through the disk of the Milky Way. Software analysis detects over 12,000 stars in the image.
The Great Hercules Cluster, viewed through clear dark skies, is just large enough and bright enough to be visible to the naked eye. First observed by Edmond Halley (of comet fame) in 1714, and later cataloged as number 13 in Charles Messier’s list of not-comets, the cluster, as the common name implies, is located in the constellation of Hercules.
By offsetting the cluster in the image I was able to include several other objects. Each has a vastly different distance providing a somewhat three dimensional view to this small patch of the night sky.
The bright star (HIP 81848) near the upper left is an orange (K2 class) star with a magnitude of 6.86, putting it just under naked eye visibility. While none of the objects we will discuss are close to us, this star is the closest. It lies well within the Milky Way at about 1,832 light years (LY) from the sun.
Next comes the globular cluster. It lies 27,400 LYs from the center of the Milky Way, also putting it within our galaxy. Globular cluster orbits are not confined to the disk of the galaxy and M 13’s current position is above the galactic plane. Its 300,000+ stars lie about 25,100 LYs from the sun.
![The Great Hercules Cluster (M 13) [C:40x30s]](https://ptobservatory.com/wp-content/uploads/2011/01/decon_3_M-13.png)
The Great Hercules Cluster (M 13) [C:40x30s]
Next on the hit parade is NGC 6207, the bright galaxy in the upper right corner of the image. It is classified as a SA(s)c spiral galaxy. At a magnitude of 12.2 this is a challenge to see with a small telescope but should be visible as a faint patch of light in an 8″ scope.
(NGC 6207) detail
What appears to be a very bright galactic core is, in fact, a fortuitously positioned star in our galaxy. Not visible here is NGC 6207’s own collection of globular clusters orbiting its center. The galaxy is located approximately 37 million LYs from us.
The most distant object in our discussion is IC 4617, the small faint galaxy slightly above and to the right of M 13. This Sbc class spiral galaxy comes in at a magnitude of 15.9 making it a much more challenging visual target.
(IC 4617) detail
The base image is a stack of 40 thirty second exposures taken on the evening of May 28th.
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Initially discovered in 1775, this globular cluster was independently recovered in 1777 by Charles Messier and added to his list. One of the most distant globular clusters, M 53 is currently 60,000 light years distant from the galactic center and almost as far from the solar system (58,000 LYs). The cluster is some 220 LYs in diameter, has a visual magnitude of 8.3 and is located in the northern constellation Coma Berenices (Berenices’ Hair).
This image was taken on the night of May 14. With a limiting magnitude of 19.1, it consists of 120 ten second exposures.
The great English astronomer William Herschel first used the term globular cluster (derived from the Latin globulus – little sphere) in 1789. Globular clusters are, usually, very distinct. Thousands to a million stars packed into a tight spherical area is the defining visual characteristic. Located in the constellation Coma Berenices ( Berenices Hair) NGC 5053 doesn’t look the part of globular cluster, but numerous spectroscopic analyses show that it is. NGC 5053’s height above the galactic plane and the lack of metals in its stars also indicate it is globular in nature. In 2006, a study found a faint 6 degree tidal tail indicating that its close path to the Milky Way may be responsible for the dispersed nature of the cluster.
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Nothing in this image is visible to the naked eye.
I took this image on the 17th of March. The image is a stack of 56 thirty second exposures and as usual north is to the right, east is up.