Messier 24

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Sagittarius Star Cloud, Delle Caustiche
Bode 51, IC 4715

Type  Star Cloud
Magnitude  4.6
Size  120' x 60' @ 90°
Right Ascension  18h 16'  (2000)
Declination  18° 33' S
Constellation  Sagittarius
Description  eeL cloud of st and neb
Observing Notes

Andrew Cooper
Jun 27, 2020    Waikoloa Quarry, HI (map)
8x42mm Nikon Prostaff 3S Binoculars @ 8x
Seeing: 7 Transparency: 7 Moon: 0%

Large, bright, a rich section of galactic starfield 4° north to south, and 2° east to west in extent, the brightest patch in the area rivalled only by Baade's Window 10° south, M16 and M17 visible to the north, M8 to the south, and M25 to the east

Andrew Cooper
Aug 12, 2017    Waikoloa, HI (map)
Hodgepodge w/TV-76mm f/6 APO @ 18x
Seeing: 6 Transparency: 6 Moon: 0%

A large patch of brighter Milky Way starfield visible to the unaided eye and easy to find, large at over 2° across, quite distinct, one can see whay it was noted by Messier for inclusion in his catalog, this particular patch stands out so well as it is demarcated by areas of dark nebulae, particularly by B92, B93, and B304 on the northwest margins, a larger dark region to the southeast, the region creating the south margin of the star cloud is quite dark and well defined in extent, the dark nebulae on the north margin are more complex with tendrils that reach into the mass of M24, two clusters can be noted in the northern region of M24, NGC6603 is embedded, M18 is off the north end towards M17 visible further to the north

Andrew Cooper
Aug 30, 2008    Hale Pohaku, HI (map)
76mm APO @ 14x
Seeing: 7 Transparency: 7.5 Moon: 0%

Beautiful rich field of innumerable stars, large 1°x3°, a distinct cloud offset by dark nebulae on several sides, including B92 and B93, the cluster NGC6603 in the north reach

Andrew Cooper
Aug 30, 2008    Hale Pohaku, HI (map)
76mm f/6 APO, TeleVue-76 @ 14x
Seeing: 7 Transparency: 7.5 Moon: 0%

Beautiful rich field of innumerable stars, large at 1° x 3°, a distinct cloud offset by dark nebulae on several sides including B92 and B93, the cluster NGC6603 in the north reach

Andrew Cooper
Jun 13, 1999    Pinal Peak, AZ (map)
20cm f/10 SCT

A patch of the Milky Way thick with thousands of stars, a binocular object, too large for the LX50, the field is just full of stars, side to side

Rev. T.W. Webb
May 19, 1885    Hardwick, Herefordshire, England (map)

Magnificent region, visible to the unaided eye as a kind of protuberance of the galaxy; and so considered by h., who gives stars 15 mg., It is accompanied by two little pairs. 2° n of μ Sagittarii.
― Celestial Objects for Common Telescopes, T. W. Webb, 1917

John Herschel
Jul 15, 1830    

A glorious concentrated part of Milky Way, almost amounting to a globular cluster. Stars 14m and 15m.

Charles Messier
Jun 20, 1764    

Cluster on the parallel of the preceding [M23] & near the end of the bow of Sarittarius, in the Milky Way: a large nebulosity in which there are many stars of different magnitudes: the light which is spread throughout this cluster is divided into several parts; it is the center of this cluster which has been determined.
― Connaissance des Temps, 1781

Harold Corwin

IC 4715 = M 24. Barnard's RA as published in AN 4239 is 10 minutes of time too large. This is either a typo, or a simple digit error. His description of the star cloud is accurate, though, as are his notes about the two dark nebulae on the northern edge. Also, his positions for the dark nebulae are correct. And the star cloud is too large to miss -- even Barnard's incorrect position is within its boundaries. The position I estimate is for the entire elongated cloud of stars, approximately 2 deg by 1 deg. M 24 may just be the northern part of this cloud, about a degree across with NGC 6603 near the center (NGC 6603 is often mistakenly equated with M 24, but the NGC object is too faint and too small to match Messier's description; this was first noted by John Herschel who has two observations of NGC 6603).

The position I've finally adopted from the DSS2R image is close to Barnard's, but just a few arcminutes further south. Either one will do nicely, of course; this thing is just too big to miss.

Dreyer copied Barnard's incorrect position into the second IC, so insured that a casual reader of the catalogues would not notice the identity with M 24. Brent Archinal was apparently the first to catch the error, and it was pointed out to me in March 2001 by Brian Skiff.

The star cloud is well-shown in Plates 54 and 55 of Barnard's collection in Lick Publications, Volume 11. I have not included the "wings" extending north and southeast from the southern end of the main cloud in my size estimates. The star cloud is certainly defined for our point of view by intervening dust. It has an extremely irregular outline, so determining a "size" is a matter of interpretation at a given wavelength.
IC Notes by Harold Corwin
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Messier 24