Dark Horse Observatory

Dark Horse Observatory
Kimberton, PA US


Image Details: NGC 6791

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NGC 6791

Object/Name: OCL 142 Image Details:
Type: Open Cluster - (Trumpler Class II 3 r) Luminance IRB:5.00 min. (9) – 45.00 min. 1x1
Constellation: Lyra Red:5.00 min. (9) – 45.00 min. 1x1
RA: 19 hours 20.7 minutes Green:5.00 min. (10) – 50.00 min. 1x1
Dec: +37 degreees 46 minutes Blue:5.00 min. (10) – 50.00 min. 1x1
Distance: 16,000.0 light years Total Time:190 minutes
Magnitude: 9.5 mag   
Size: 16.0 x 16.0 arc minutes   
Imaging Dates: 17 - 18 September 2007   
Location: Dark Horse Observatory, Kimberton, PA 19460 U.S.A.   
Telescope: Takahashi BRC-250 Mount: Mountain Instruments MI-250
CCD: SBIG ST-10XME Rotator: Optec Pxyis Rotator
Focuser: Finger Lakes Instruments DF2 Filters: Astrodon Series E
Software Used:
  CCDStack MiraPro Professional
Photoshop CS2 Russel Croman’s Gradient Xterminator
Noel Carboni’s Astronomy Tools  
Detailed Information:
NGC 6791

I became interested in the open star cluster NGC 6791 after reading about it in Mark Alison’s book, Star Clusters: and How to Observe Them, © Springer-Verlag London Limited 2006. In my readings I uncovered several things that make this open cluster interesting. From a visual perspective, this is an interesting object because it is classified as an open cluster, but it appears more like a loose globular cluster with 300 – 500 stars as you can see from the image that I took. This takes on an even more intriguing dimension when you consider its age.


According to Allison on page 15, most open clusters are between 1 million and 10 million years old with minor variation. The average age is typically 50 million years old. NGC 6791, on the other hand is currently accurately estimated to be about 9.5 – 10 billion years old. It places it among the oldest of the open clusters with Berkley 17, in Auriga. This age is well beyond half of the current estimates of the age of the universe. How has this open cluster escaped the ravages of time that tends to disperse most open clusters well before this? Or is it an open cluster at all?


Then there is the matter of stellar content. As you know, the oldest objects in the universe tend to be composed of primarily hydrogen and helium that they make during fusion. There is a smattering of some other heavier elements in these older stars, but the heavier elements are typically found in younger stars that are made up of the supernova remnants of stars that were formed much earlier in the history of the universe. Now since NGC 6791 is so old it should be relatively pure, but spectral examination of the cluster reveals that carbon, oxygen, iron, and other elements are present in the stars of NGC 6791 at levels similar to our own Sun, which was born only 4.5 billion years ago. How can the age of this open cluster be reconciled with the heavy element concentrations? This is yet a third mystery to be solved regarding this open cluster.


Recently, NGC 6791 became a focus of interest when it was targeted in the search for new exoplanets. Since higher metal containing stars are considered more like our sun, astronomers reasoned that they may be more likely to have planets orbiting them. Using photometry observers can see a characteristic fluctuation in the brightness of a star when a planet transits the disc of the star. Time will tell if this open cluster contains any exoplanets.


For all the above reasons this is an interesting open cluster. Additionally, it has been marked as a observable object for binocular observing by the IAAC.


I am including the NGC6791 link at the M45 website for the observers as well as 3 sky charts with Telrad markings included. Good luck with your observations!


The star charts were generated using the Project Pluto software Guide8

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