Messier 13 (M13, NGC6205) Hercules, and the "Elusive Propeller"
M13 is a large and bright globular cluster in Hercules, discovered by Halley in 1714, and documented by Messier in 1764. The cluster is composed of about 300,000 members with a total mass of 500,000 solar. The brightest star in the cluster is a Cepheid variable V11 with apparent magnitude of 11.9. The cluster is 22,200 LY distant, 145 LY in diameter, and 20 arcmin in angular size. It is estimated to be 11.65 billion years old. It is composed of ancient stars of low metallicity, created long before succeeding generations of supernovae enriched the galaxy with heavier elements. With apparent magnitude of 5.8, it is visible to the naked eye under good conditions, and easily identified in binoculars. A 4-5 inch telescope is needed to begin to resolve stars.
Three diverging dust lanes, known as the "propeller", cut into the SE edge of the cluster. It was first described in the 1850s as "three dark rifts" by Bindon Stoney, working for lord Rosse. An 8 inch telescope is required to detect the feature visually, but it can be photographed with much smaller apertures. The dust lanes are most likely not a part of the cluster, but lie in the foreground.
This reprocessed image was taken with a TSapo100q astrograph, Sigma apo 1.4x tele-extender, unmodified Canon 600D camera, Astronomik CLS-CCD filter, Celestron AVX mount, and Orion 60mm F4 SSAGpro autoguider.
This reprocessed image is a stack of 7 x 240 sec exposures at iso1600, 25% crop, 2x drizzle, stacked with 30 dark and 30 bias frames.
Software used was PHD2, DSS, XnView, and StarTools.
Since taking this image I have learned that for improved resolution globular clusters are best photographed with a large number of short exposures. A UV filter helps to reduce Rayleigh scatter.
Thank you for looking. C&C welcome.