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The insert highlights the unusual propeller shape structure inside this globular cluster. Click on the image to view a high resolution picture.
The insert highlights the unusual propeller shape structure inside this globular cluster. Click on the image to view a high resolution picture.
585 minutes UV/IR filter
ZWO FF107 107mm Astrograph (f/7)
ZWO ASI2600MC-Air (26 Megapixel astronomical color camera)
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The Propeller in M13
For casual observers, there’s a belief that most globular clusters have similar appearances. However, more seasoned enthusiasts know this is not completely true. Core concentration, brightness, size, shape, hues and the resolving power of optical instruments all help to create distinctions between globulars. There are other features, too.
For example, a good challenge is to glimpse or capture a unique structure in the Hercules Globular Cluster (M13) known as the propeller.
What is the M13 propeller?
The propeller is a pattern of three dark lanes converging to a common point that is offset to the southeast side of M13's core. The lanes are offset from one another by 120° and it’s this regularity that produces the pattern reminiscent of a propeller. The feature was originally reported by Bindon Stoney around 1850, an astronomer assisting William Parsons, the 3rd Earl of Rosse at Birr Castle in Parsonstown, Ireland. After Stoney's initial sighting became known, other observers confirmed the existence of these unique dark rifts through instruments as small as 6 inches. But as photography diminished the need for accurate visual observations, the M13 propeller became somewhat lost in the glare of the globular's intense core.
The propeller is not believed to be a physical structure. Instead, it's considered to be an optical effect caused by the stars' arrangement and how the human eye perceives them.
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