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Demystifying Messier 24

M24 is often misidentified as NGC 6603, but don’t let that stop you from seeking out this superb nebulosity.
ChapleGlenn
In the summer of 1764, the French comet hunter Charles Messier came upon a nebulous object “near the extremity of the bow of Sagittarius, in the Milky Way.” He went on to describe it as “a considerable nebulosity, of about one degree and a half extension: In that nebulosity there are several stars of different magnitudes; the light which is between these stars is divided in several parts.” It was recorded as the 24th entry in his catalog of nebulous objects.

Messier 24 (M24), visible to the unaided eye and three Full Moon diameters in length, was one of the last members of the Messier catalog that I officially notched. Why? It all has to do with resources that misidentified it as NGC 6603.

As I’ve mentioned in the past, my earliest cosmic excursions were made with a little 3-inch f/10 Edmund Scientific reflector. Throughout the 1970s, I used this scope exclusively to work my way through all the entries in the Messier catalog. Bright Messiers, like the Andromeda Galaxy (M31), the Orion Nebula (M42), and the Beehive Cluster (M44), fell quickly. Within a few years, I had moved on to less accommodating targets, like the Crab Nebula (M1), the Pinwheel Galaxy (M33), and the galaxies in the Coma/Virgo area.

As the decade drew to a close, only a handful of Messier objects had eluded my eye. One of those holdouts was M24. The problem was that several of my resources identified M24 as NGC 6603, when in fact the two are not the same object. M24 is a 1°-by-2° detached chunk of the Milky Way, also known as the Small Sagittarius Star Cloud. On the other hand, NGC 6603 is a small, faint open cluster, tucked away in the northern part of M24, that was discovered by John Herschel in 1830. John Louis Emil Dreyer’s 1888 edition of the New General Catalogue correctly described NGC 6603 as a “remarkable cluster, very rich, very much compressed, round,” but then incorrectly cross-identified it as M24. Thus began the confusion.

ASYGC0718
One of the sky’s richest star regions, M24 (central diagonal cloud) makes up part of the Milky Way’s Sagittarius spiral arm. On the other hand, NGC 6603 — a small, dim open cluster — is located in the northeastern section of M24.
Chris Schur
Messier’s description of M24 matches the appearance of the Small Sagittarius Star Cloud; Dreyer’s doesn’t. Moreover, NGC 6603 is likely too faint to have been visible through Messier’s telescopes. American astronomer Robert Burnham Jr. had said as much in the third volume of his Celestial Handbook. Nevertheless, I decided to withhold planting a flag atop M24 until I had notched NGC 6603. This was hardly an easy task, as this 5'-wide cluster shines at a feeble 11th magnitude and is more aptly viewed with a telescope twice the aperture of mine. Numerous were those summer nights when I swatted mosquitoes while struggling with this elusive target! Finally, near midnight on the evening of July 28, 1978, I glimpsed an “incredibly faint, but persistent averted vision haze.” To remove all doubt that I had indeed seen NGC 6603, I carefully sketched the field to include stars in the immediate vicinity of my suspect. Indoors, I compared my sketch with a photo of NGC 6603 in the Celestial Handbook. The match was perfect! Fist pump!

Many sources describe M24’s brightness as magnitude 4.5. I agree more with Stephen James O’Meara’s assessment that it’s about two magnitudes brighter. M24 is reasonably visible from my backyard, where the limiting magnitude is around 5. Under such skies, a 4.5-magnitude nebulous object would be extremely difficult to see.

Needless to say, the real Messier 24 (correctly identified as IC 4715) is best viewed through binoculars or a rich-field telescope. Through 10x50 binoculars, it appeared to me as a kidney bean-shaped nebulosity interspersed with numerous stars. The brightest four formed a kite-shaped asterism. As I suspected, the embedded NGC 6603 wasn’t visible. I’m sure a skilled observer working from a dark sky site can pick out NGC 6603 with a common 2.4-inch refractor and moderately high (75x to 100x) magnification. You’ll need a scope 6 inches or larger in slightly light-polluted suburban areas. Larger instruments will resolve individual stars.

If M24 is on your list of yet-to-see Messier objects, don’t torture yourself like I did. Look for a naked-eye patch of light hovering high above Sagittarius’ Teapot asterism, 2° north and slightly east of the 4th-magnitude star Mu (μ) Sagittarii. Get a close-up view with binoculars, take time to enjoy the sight, and then put a check mark next to M24 on your Messier catalog list. If you’re up for a small-telescope challenge, try your luck with NGC 6603 as well. Just remember to put on some bug spray!

Questions, comments, or suggestions? Email me at gchaple@hotmail.com. Next month: a backyard mission to Mars. Clear skies!

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