M71 Globular Cluster



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August 26, 2014

There are two main types of star clusters:  “open clusters” (OC) and “globular clusters” (GC).  OCs are also sometimes called “galactic clusters,” and usually contain a few dozen to a couple thousand stars.  OCs reside in the main spiral arms of our galaxy, the Milky Way.  GCs contain hundreds of thousands to millions of stars, are usually quite densely packed, and form a halo around the Milky Way galaxy (other galaxies also have GCs).

The object at the centre of this image is M71.  For a long time, M71 was classified as an OC.  However now it’s known to be a rather loosely packed GC, located about 12,000 light years from us.   To me it looks like a GC when I look through an eyepiece, but images make it look more like a rich open cluster (e.g. M11).  It is about 27 light years across, and relatively young at around 9 or 10 billion years old.  It has the mass of around 17,000 suns and the luminosity (basically, brightness) of around 13,000 suns.

I considered cropping the image more closely, but love the rich Milky Way star field surrounding the cluster.   You can, of course, zoom in on the image as it has relatively high resolution.


SBIG STL-11000M camera, Baader LRGB filters, 10″ f/6.8 ASA astrograph, Paramount MX. Guided with STL-11000′s remote guide head using 80 mm f/6 refractor. Acquisition and guidingwith Maxim DL. All pre-processing and processing in PixInsight. Shot from my SkyShed in Guelph, Ontario. No moon. Good transparency and seeing throughout.

10x10m R, 9x10m G, 11x10m B, 5x5m L , all unbinned frames (total=5hr25m).

The R, G, B files produced in Maxim were combined and then registered to the L file.  RGB and L were then each background-corrected with AutomaticBackgroundExtraction. R, G and B were combined into an RGB image, ColourCalibration was applied and the image was stretched using HistogramTransformation followed by ColorSaturation. The background-corrected L image was stretched with HistogramTransformation. ACDNR noise reduction was applied with a radius of 0.7 px to remove small scale noise in the background. Luminance was extracted from the RGB image, matched to the L image with LinearFit, and then added back into the RGB image using Channel Combine in the Lab space. LRGB Combine was used to add Luminance to the RGB image.  I completed the processing by adjusting contrast and colour saturation using Curves. Image scale is about 1.1 arc sec per pixel.


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