Monthly
Messier Star-Hop; September #2
By
Art Russell
Overhead this month we find the constellation of Cygnus, "The Swan," also known as the "Northern Cross." When I see Cygnus high over head I know that the cooler days of Autumn will soon follow and with it a general clearing of the haze and humidity so prevalent during our southern summers. However, even the presence of the summers' impediments to viewing have not stopped most of us from cruising through some of the season's brighter objects. This month's star-hop takes us to two of the northern skies' more prominent members, the globular clusters M13 and M92. Along the way, we'll also take in two much lesser known galaxies in close proximity to M13, NGC 6207 and the seldom observed IC 4617.
Star-Hop
#1. M13, NGC 6205
Star-Hop #2: NGC 6207
Star-Hop #3: IC 4617
Star-Hop #4: M92, NGC 6341
Star-Hop #1. M13, NGC 6205. Also known as the "Great Cluster in Hercules," M13 is undoubtedly the most well known globular cluster in the northern skies. In fact many beginning astronomers cut their teeth in learning how to locate M13. Big and bright, it is easily seen in the moderately sized binoculars and small telescopes which are often used by the beginner. As can be imagined, it is even more impressive in larger telescopes and so offers something for beginning and advanced observers as well. Take your time to find M13. Over the years you'll find yourself returning here time and time again.
Locating M13 at this time of year is not difficult. Beginning near the zenith, directly overhead, we find ourselves in the constellation Cygnus. Scan the sky about 15 degrees (the distance spanned between your index and little fingers spread against the night sky at arms' length) west of Cygnus is the bright star Vega which prominently marks the constellation Lyra and its nearby "Ring Nebula", M57. From Vega, scan the sky to find the "Keystone" consisting of the four stars Zeta, Eta, Pi and Epsilon Herculis in the heart of the constellation Hercules. The Keystone consists of 4 moderately bright stars and is about 20 degrees (the distance spanned by your thumb and little finger spread against the night sky at arms length)to the west-southwest of Vega. Once you've located the Keystone, you've solved the problem of locating M13. M13 is located on the west side of the Keystone and is 1/3rds of the way along an imaginary line extended south from Eta Herculis to Zeta Herculis. M13 appears very prominently in binoculars and smaller telescopes, often presenting a bright core which fades uniformly towards its edges. In larger telescopes at higher magnifications one can resolve many of the globular cluster's constituent stars and discern random patterns within the cluster yielding a truly spectacular view!
Image produced by Megastar 4 (http://www.willbell.com/)
Star-Hop #2: NGC 6207. This small 11.9 magnitude galaxy is located less than a half degree away from M13. In fact many observers never notice this small galaxy and are surprised to find it located so close to such a prominent galaxy.
Locate NGC 6207 1/2 degree from the center of M13 to the northeast. This galaxy is best observed at higher powers so locate M13 first and then star-hop from there to M13. This object is definitely an object for beginner's binoculars. In larger telescopes at higher magnifications you may observe that NGC 6207 is generally oriented north to south, is very faint and may suggest some structure within the halo of the galaxy.
Star-Hop #3: IC 4617. By the way; on your star-hop to NGC 6207, did you happen to notice a VERY (!) faint (magnitude 15.5) galaxy about half way between NGC 6207 and M13? You might have noticed it next to a parallelogram of 4 stars which remind me of the parallelogram in Lyra which locates the "Ring Nebula," M57. If you noticed this small galaxy (definitely use a large telescope at high power!) count yourself among the fortunate few. IC 4617 isn't even charted on Uranometria! In a large telescope at high magnification this galaxy will appear very small, dim, and best seen with averted vision. If you try to track this galaxy down, welcome to a very small and exclusive group. Very few have seen IC 4617, but not because it is so difficult to view. Rather, because it is not catalogued in many of the standard references used by amateurs.
Star-Hop #4: M92, NGC 6341. M92 is the other Messier object located in the constellation Hercules. In fact, if it weren't for the fact that its overshadowed by its larger cousin, M13, M92 would be recognized as a very attractive globular cluster in its own right. As it turns out, everybody looks at M13, but nobody looks at M92Locating M92 from the "Keystone" is as easy as finding M13. Start at the eastern side of the Keystone and find the star Pi Herculis. Extend a line north of Pi Herculis for a distance of about 6 degrees, or a little more than the distance spanned by 3 fingers held together at arm's length against the night sky. Search this area with a moderate set of binoculars and you'll rapidly come upon the pale glow of the thousands of stars comprising this splendid globular cluster. Easily seen even through my finder scope, the core of M92 is bright with many resolved stars.