Roaming the Deep Sky #6
IC4756
By Don Clouse
Tucked away "off the beaten path" in a rather remote area of the sky is the very nice, large, bright, open cluster, IC4756. The cluster is located in Serpens Cauda near its northern edge between Aquila and Ophiuchus, far from any bright stars. Perhaps that is why the cluster is not well known. It is, however, well worth the time it may take to track it down.
This cluster is much too large for a modest size telescope, but it is perfect for a small telescope. It has a listed diameter of 52’, but to me appeared to be at least 1.5 degrees in extent. (The observations described here were done on 6/14/99 at the Doyle property – located perhaps a mile southeast of LAS’s property.) Through 7x50 binoculars it was visible as a mottled haze just at the edge of resolution. Through the 9x60 finder scope that I use, as many as two dozen faint stars were visible against the soft glow of the unresolved members of the cluster. Viewed in this expansive, 6-degree field, the cluster could be seen in the midst of the rich surrounding Milky Way star fields. It’s a beautiful panorama.
The Trumpler classification of this cluster is "2 3 r" – meaning it is detached from the surrounding star field with no concentration toward the center, has a moderate range in star brightness, and is rich having more than 100 stars. Sources list this magnitude 4.6 cluster as having 80 stars. I’m guessing that it would take a bit more aperture and magnification to bring out the fainter members. Even though IC4756 has a lot of stars, they are spread over a large area. A large field of view is necessary to appreciate it. It will lose the appearance of an open cluster in a small field of view, its members becoming evenly spread over the entire field. I’m looking forward to viewing it again this summer – this time with my Celestron 80mm f/5 refractor. My 35mm eyepiece in this scope yields a magnification of 11x with a field of view of over 4 degrees. I can’t wait to try it. (I observed this object on 6/14/99. Now here it is 10/27/01, as I revise this article for the web, and I still haven’t viewed this OC again!) Note: visit the "Classroom" page on the LAS website, www.louisville-astro.org, for descriptions of the Trumpler classification of open clusters and the spectral classes of stars.
I would suggest beginning the hunt for this cluster by pointing your Telerad or red-dot finder at Beta Ophiuchi, Cebalrai. This magnitude 2.8, (spectral class) K2 star, 82 light years distant1, marks the eastern shoulder of Ophiuchus. IC4756 is at nearly the same declination as Cebalrai. Center Cebalrai3 in your finder or eyepiece and then move your scope about 1 degree north. Then, if your small scope is on an equatorial mount (and reasonably well polar aligned!), you have but to move the scope east through right ascension for about 14 degrees until you run into IC4756. Along the way you may notice a couple of other very nice clusters well suited to small scopes. One near Cebalrai, IC4665 (4.2, 40’, 30*, 3 2 m), and the other, NGC6633 (4.6, 27’, 30*, 3 2 m), near IC4756. Both are shown on the accompanying chart.2 Happy hunting and clear, dark skies!
Sources:
- Observer’s Handbook 2000, Edited by Roy Bishop, The Royal Astronomical Society of Canada, 1999
- MegaStar 4.0, E.L.B. Software, Willmann-Bell, Inc., 1997. The chart shows stars to eighth magnitude and is 23.2 by 14.6 degrees in size.
- http://www.astro.uiuc.edu/%7ekaler/sow/sow.html "STARS", created by Jim Kaler, Prof. of Astronomy, University of Illinois.
Visitors of This Page Since 2/27/02
