Roaming the Deep Sky #4
The Great Hercules Cluster, M13
By Don Clouse
In early June the summer constellations, Ophiuchus, Sagittarius, Aquila, and Cygnus, among others, begin their climb into prominence in our night skies. In early June, Hercules is high in the sky, straddling our southern meridian around midnight. The showpiece object of the constellation, and, arguably, of the entire northern sky, is the globular cluster M13. It must be my favorite object as I’ve logged eleven observations of it – five more than any other object I’ve observed. At 140 to 150 light years in diameter, M13 is one of the larger globulars. It is also one of the closer globular clusters at some 21,000 to 23,000 light years distant. This ancient, roughly spherical, gravitationally bound cluster contains several hundred thousand members at least. Being so large and so relatively close, it is both bright (magnitude 5.7) and large (12’ to 16’ in diameter) in the eyepieces of our telescopes.
M13 looks magnificent in any size telescope. However, the more aperture you view it with, the more stars you’ll see. One of my more memorable observations of M13 (but not the most recent time I’ve viewed it!), was on 5/9/99 at 2:34am at the "Doyle farm" in southern Indiana, very close to our Curby site. I used a 12.5mm Ultima (Celestron) eyepiece in my Celestar 8 (Celestron eight inch Schmidt-Cassegrain). This combo yields a magnification of 162x and a field of view of 18.6’. Here’s what I wrote: "Core area takes up 25% of field, more than half of the field is covered by the entire cluster. At least 100 stars visible with direct vision, many across the face. Detail and mottling apparent in central area even with direct vision. Averted vision – forget it!" M13 is a dazzling array of stars overlaying a wide milky glow. Several chains and arcs of stars trail from the central area. Two especially long ones curve away to the southeast and southwest. Truly, it is an awesome sight, which never fails to move me.
The chart (push the star!) shows the location of M13 along the western side of the Hercules Keystone between Eta and Zeta Herculis. The Keystone is located approximately 20 degrees due west of the constellation Lyra. You can find Hercules, the Keystone, and M13 near the zenith early on summer nights. Clear skies!
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