Globular Clusters in Hercules
The constellation Hercules contains two of the finest globular star
clusters visible in the Northern Hemisphere: M13 (below) and M92 (not
shown -- yet!). Each contains several hundred thousand stars, and
they are about 25,000 light years from earth. They are directly
overhead during evenings in July, so I snapped a photo of M13 with
a 155 mm aperture f/7 refractor equipped with a 1.4X teleconverter,
giving an effective focal length of 1520 mm (f/10). Three 10-minute
exposures on Kodak Max 800 film were combined to give the images below.
Greater aperture and focal length would have given more impressive
results, but at least M13 is nicely resolved. Of course, only the
biggest and brightest stars in M13 are resolved by my small telescope!
The black and white images below were made with the green (above) and
blue (below) components of the negative. The green image is very
similar to the view of this object in a large telescope, where the
"streams" of bright stars extending from the core give M13 a crab-like
or tick-like aspect. These star streams are largely lost amid the more
numerous stars of the blue-derived image, and in professional photographs
they are not evident at all. Human eyes are most sensitive in the
green, and perhaps this is why the visual and photographic views are
so different for this object, but who knows?!
