IC 4665

Right Ascension	17 : 46.3 (h:m)
Declination	+05 : 43 (deg:m)
Distance	1.4 (kly)
Visual Brightness	4.2 (mag)
Apparent Dimension	70 (arc min)

Discovered by De Chéseaux 1745-1746.

IC 4665 is one of the brighter Milky Way clusters missing not only in the Messier Catalog, but also in the NGC, because it is so loose and coarse. Nevertheless, this cluster in Ophiuchus is a good object for binoculars or wide-field telescopes.

IC 4665 has been discovered by the Swiss astronomer Philippe Loys de Chéseaux. As De Chéseaux's list didn't get really published, it was independently rediscovered by Johann Elert Bode in or before 1782, and contained in his 1782 list, and again by Caroline Herschel on July 31, 1783; on this day it was also observed by William Herschel but not included in any of his catalogs. In view of these facts, the present author decided to tentatively add this object to Caroline Herschel's small list as CH 21. Neither of these findings got known widespread enough, so that it didn't find its way into the NGC or first IC catalog, but was only added to the second IC of 1908 based on its discovery by Solon I. Bailey on photographic plates taken with Harvard Observatory's Bruce telescope (Bailey 1908).

IC 4665 is a very conspicuous, coarse star cluster in the opera glas, but difficult to detect in the telescope, probably a reason that Messier didn't find it. The Sky Catalogue 2000.0 gives 30 member stars, the brightest of which is of 6.86 magnitude, a Trumpler type of III 2 p, and an estimated age of 36 million years. At a distance of 1,400 light years, IC 4665 is approaching us at 12 km/sec.

The image in this page was obtained by Till Credner and Sven Kohle of the University of Bonn, Germany when evaluating their Constellation Ophiuchus photography. The photo was obtained on May 2, 2000 at 03:45 UT with f=50mm f/4.0 lens on Kodak Royal 400 Select color print film from Tenerife, Spain. It was exposed 35 min; the original image has a 27x39 deg field of view; our image is a magnification of a small region from this field, around IC 4665. The bright star in the lower right (SW) of IC 4665 is Beta Ophiuchi, a red giant of spectral type K2 III and apparent visual magnitude 2.77 (not associated with the cluster).

In John Caldwell's Best Sky Objects list. In the Astronomical League's Deep Sky Binocular Club List.

NGC Online data for IC 4665

References:

• Solon I. Bailey, 1908. A catalogue of bright clusters and nebulae. Annals of Harvard College Observatory, Vol. 60, pp. 199-230. [ADS: 1908AnHar..60..199B]

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Hartmut Frommert
Christine Kronberg
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Last Modification: March 12, 2009