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Object Name: Messier 24
Alternative Designations: M24, IC 4715, Sagittarius Star Cloud, Delle Caustiche
Object Type: Star Cloud – contains Open Cluster NGC 6603 and NGC 6595, Barnard 92, Barnard 93, Collinder 469, IC 1283-1284, NGC 6589/90 and planetary nebula NGC 6567
Right Ascension: 18 : 16.9 (h:m)
Declination: -18 : 29 (deg:m)
Distance: 10.0 (kly)
Visual Brightness: 4.6 (mag)
Apparent Dimension: 90 (arc min)
Locating Messier 24: From a dark sky location M24 is easily located with the unaided eye as a large hazy patch in northern portion of the constellation of Sagittarius about a handspan above the teapot asterism. For those observing under urban skies, even the slightest optical aid will easily reveal this massive cloud of stars! Spanning a degree and a half of sky means this huge object is going to cover anywhere from about 1/3 to 1/2 the field of view in most binoculars, easily be seen in all optical finderscopes and absolutely require minimum magnification in all telescopes. (Even then, you’ll only be able to study portions of the Sagittarius Star Cloud at a time.)
What You Are Looking At: Messier 24 is one of the most curious of the catalog entries because it really isn’t a star cluster – simply an oddity. What we are looking at is thousands of stars that belong to the Sagittarius arm of the Milky Way galaxy seen through a chance hole in the gas and dust… a clear “window” in space. And speaking of space… M24 fills a space of significant volume to a depth of 10,000 to 16,000 light-years. This is the most dense concentration of individual stars visible using binoculars, with around 1,000 stars visible within a single field of view!
When viewing this awesome area, take into account how many different objects you can spot just within this region – like dim open cluster, NGC 6603. E.E. Barnard has cataloged two dark nebulae in the northern region as objects 92 and 93. How about lesser known clusters like Collinder 469 and Markarian 38? Along the southern edge you’ll find emission nebula IC 1283-1284, with two adjacent reflection nebulae, NGC 6589 and NGC 6590. What’s their fueling source? Try the notable little open cluster NGC 6595! Take a tour on the western edge of M24 and see if you can spot 12th-magnitude planetary nebula NGC 6567. Need more? Then how about Delta Cephei variable WZ Sagittarii in the southern area. Its a pulsating giant star that varies in brightness between magnitude 7.5 and 8.5 in slightly less than 22 days!
History: As bright as the Sagittarius Star Cloud is, we know that Messier probably wasn’t the first to see it – but he was the first to catalog it. Let’s take a look at his notes: “In the same night, June 20 to 21, 1764, I have discovered on the same parallel as the star cluster I have just been talking about and near the extremity of the bow of Sagittarius, in the milky way, 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. I have determined approximately the position of the middle of this cloud of light; its right ascension is 270d 26′, and its declination 18d 26′, south.”
While other historic astronomers would also look at Messier’s “discovery”, they realized they were looking at a portion of the Milky Way and were somewhat less than enthusiastic. The Sagittarius Star Cloud was named “Delle Caustiche” by Fr. Secchi, “from the peculiar arrangement of its stars in rays, arches, caustic curves, and intertwined spirals.” Says Admiral Smyth: “A beautiful field of stars, below the sinister base of the Polish shield, and in a richly clustering portion of the Milky Way. This object was discovered by Messier in 1764, and described as a mass of stars — a great nebulosity of which the light is divided in several parts. This was probably owing to want of power in the instruments used, as the whole is fairly resolvable, though there is a gathering spot with much star dust [This is NGC 6603!].”
Gather some star dust of your own… There’s lots there to share!
Top image thanks to Tomas Mazon and bottom color image thanks to Vanessa Harvey, REU program/NOAO/AURA/NSF.