Sky Object of the Month – August 2017

Sky Object of the Month – August 2017

(Courtesy LVAS Observer’s Challenge*)

M24 – Star Cloud in Sagittarius (Mag. 4.6 (2.5, according to Stephen O’Meara); Size 1 X 2 degrees)

NGC 6603 – Open Cluster in the Sagittarius Star Cloud (Mag. 11.1; Diam. 

             M24 is one of the more easily-seen Messier objects – at least from regions where skies are dark enough to afford a clear view of the Milky Way. Why, then, was it such a stern challenge when I first set out to observe all of the Messier Catalog objects back in the 1970s? The answer lay in identity confusion.

            The object Messier cataloged is a 1 by 2 degree patch of the Milky Way. In his description, he clearly refers to it as a “large nebula in which there are many stars of different magnitudes.” However, M24 is sometimes connected with the embedded open cluster NGC 6603, described in the New General Catalog as “very rich and very much compressed; diam 4’; about 50 stars mags 14…” Messier could not have seen this tiny 11th magnitude open cluster with the instruments he used in the 18th century. Nevertheless, I decided not to notch M24 until I had seen NGC 6603.

            On the evening of July 28, 1978, I made it official by observing M24 with 7X50 binoculars and then NGC 6603 with my 3-inch f/10 reflector and a magnifying power of 60X. Through the binoculars, M24 appeared as a “Large, oval patch of light, studded with a handful of bright stars.” Through the telescope, NGC 6603 was “Incredibly faint, but a persistent averted vision haze.”  

            On the LVAS website (www.lvastronomy.com) the August Observer’s Challenge is listed as “M24 – Star cloud in Sagittarius.” I’m going to take the liberty of re-defining the Challenge as “NGC 6603, Open Cluster in M24 (Sagittarius Star Cloud).”

            M24 was discovered by Charles Messier on June 20, 1764; NGC 6603 fell to William Herschel’s son, John, on July 15, 1830. If we accept a published distance of 9400 light years, NGC 6603 is 14 light years in diameter.

 Glenn Chaple for the LVAS

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 ESA/Hubble (ground-based image) M24 is at top left

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M24 and NGC 6603 (center) Image by Christ Steverson of the St. John’s Centre of the RASC

 

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M24 Drawing by Glenn Chaple (9/8/2013)

 *The purpose of the LVAS Observer’s Challenge is to encourage the pursuit of visual observing.  It is open to everyone that is interested, and if you are able to contribute notes, drawings, or photographs, the LVAS will be happy to include them in our monthly summary.  If you would like to contribute material, submit your observing notes, sketches, and/or images to either Roger Ivester (rogerivester@me.com) or Fred Rayworth (fred@fredrayworth.com). To find out more about the LVAS Observer’s Challenge or access past reports, log on to lvastronomy.com/observing-challenge.

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