Quick question. How many times (to the nearest thousand) have you viewed the great globular cluster M13 with your telescope? Next question. How many times (nearest thousandth) have you visited Hercules’ other great globular cluster M92? I doubt it would be an exaggeration to say that I’ve seen M92 once for every hundred times I’ve viewed M13.
The reason for M92’s relative anonymity can be summed up in three words – location, location, location! M13 is conveniently placed on the western edge of the “Keystone” of Hercules. M92, on the other hand, is positioned in a part of Hercules devoid of such landmarks. One way to find M92 is to trace line from delta (?) to pi (?) Herculis and extend it further by about half that length.
M92 may not be as impressive as M13, but it’s a marvelous sight, nonetheless. Were we to grade the two clusters on their visual impact, M13 would receive an “A+”, while M92 would garner an “A”. M92 shines at magnitude 6.5 – about a half magnitude fainter than M13. Its 14 arc-minute diameter is about half that of M13. Because the two lie at about the same distance from Earth (about 25,000 light years), M13 is intrinsically the larger.
Binocular users will have no trouble viewing M92. I captured it with a pair of 10X50 binoculars during a Messier Marathon last April. A 4-inch telescope at 120X will begin to resolve the cluster’s outer portion, while an 8 to 10-inch or larger Dob at 150-200X will capture M92’s true splendor. While comparing M92 with M13, I noticed that the latter had a brighter, more condensed core. I also discovered that M92 deserves far more attention than I’ve given it in the past. M92 was discovered by Johan Bode (of Bode’s Law fame) in 1777. Next clear night, take a few moments to discover (or rediscover) this dazzling globular.
Your comments on this column are welcome. E-mail me at firstname.lastname@example.org.