Messier 24, Small Sagittarius Star Cloud
On June 20, 1764, Charles Messier recorded this bright spot on the Milky Way as the 24th object in his list. He described the 1.5° large spot as follows: «Cluster on the parallel line of the preceding [M 23] and near the end of the Sagittarius arc in the Milky Way: A large nebula in which there are many stars of different magnitudes: The light that is in this pile spreads is divided into several areas.»
For a long time it was unclear which object Charles Messier meant by M 24. Some identified it with the large star cloud, others with the small star cluster NGC 6603, which is located in the northern part of it. However, it seems certain that the tiny star cluster is too faint to have been discovered with Messier's humble telescope. Also, his description doesn't fit at all. The star cluster NGC 6003 was discovered on July 15, 1830 by John Herschel
Edward Barnard encountered this cloud of stars again in 1905 while searching for dark nebulae, but made a mistake in the position (in epoch 1855: RA 18h 08m statt 18h 18m). Dreyer took over these incorrect coordinates and since he had not originally included Messier's number 24 in his first version of his «New General Catalogue», which appeared in 1888, there was no duplicate entry and so he later added it to the second version of his «Index Catalogue», which was later published added as IC 4715. The description «very large cloud of stars and nebulae» also fits M 24. [4, 196, 217]
M 24 is not a real deep-sky object, but a large star cloud in an arm of the Milky Way, which spreads over a distance of thousands of light years and is perceived through a random gap of interstellar dust. It is comparable to NGC 206 in our neighboring Andromeda Galaxy (M 31). Today M 24 is mostly called the «Small Sagittarius cloud». The "Big Sagittarius Cloud" is about ten degrees further south in the direction of the center of the Milky Way.
In the northern part of M 24 there are two prominent dark clouds: Barnard 92 and Barnard 93. Barnard 92 is the larger dark cloud with an extension of about 15 'in north / south direction and 10' in east / west direction. A single star with 11 mag (HD 312872) stands lonely and lost in the black void. E. E. Barnard described B 93 in his catalog published in 1919 as "meteoric". Originally it was assumed that these dark spots were empty holes or tunnels in the middle of star masses. The true nature of clouds of non-luminous dust, which blocks the light from stars behind, was not recognized until later. [4, 239] At the southern end of the B 93 is the small, open star cluster Collinder 469.
Name : M 24: Small Sagittarius Star Cloud R.A. : 18h 16m 30s Dec. : -18° 50' 00" Mag. : 4.6 mag Dim. : 95' x 35'
|NGC 6603||18 18 24.0||-18 24 24||OCL (I1r)||11.1||4.00||!, Cl, vRi, vmC, R, st 15 (M Way)||OCL 36, ESO 590-SC17, in M 24|
|IC 4715||18 18 48.0||-18 33 00||OCL (*Cloud)||eeL cloud of st and neb||M 24, ESO 591-**1, Sagittarius Star Cloud|
Messier 24 is located in the constellation Sagittarius. It can best be seen in the months of June to August. Then the constellation is highest above the southern horizon.