Messier 24, Small Sagittarius Star Cloud

Messier 24
Messier 24: Small Sagittarius Star Cloud M 24 and Open Cluster NGC 6603 (left from the middle). Crop from DSS2 Color [147]

History

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.»

NGC 6603
NGC 6603: Section of the DSS [160]

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]

Physical Properties

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.

Barnard 92/93
Barnard 92/93: Dark clouds within M 24. Section from DSS2 [147]

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'
«Revised New General Catalogue and Index Catalogue», «Historically Corrected New General Catalogue», Dr. Wolfgang Steinicke, 2021 [277]
NameRADecTypeBmagVmagDimDreyer DescriptionIdentification
NGC 6603 18 18 24.0-18 24 24OCL (I1r)11.14.00!, Cl, vRi, vmC, R, st 15 (M Way)OCL 36, ESO 590-SC17, in M 24
IC 4715 18 18 48.0-18 33 00OCL (*Cloud)eeL cloud of st and nebM 24, ESO 591-**1, Sagittarius Star Cloud

Further infos at CDS: Messier 24, IC 4715, NGC 6603, Barnard 92, Barnard 93, Collinder 469

Finder Chart

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.

Chart Messier 24
Chart created using SkySafari 6 Pro and STScI Digitized Sky Survey. [149, 160]

References

4«Burnham's Celestial Handbook: An Observer's Guide to the Universe Beyond the Solar System» by Robert Burnham; Dover Publications, Inc.; Voume I: ISBN 0-486-23567-X; Volume II: ISBN 0-486-23568-8; Volume III: ISBN 0-486-23673-0
147Aladin Lite; aladin.u-strasbg.fr/AladinLite (2020-12-23)
149SkySafari 6 Pro, Simulation Curriculum; skysafariastronomy.com
160The STScI Digitized Sky Survey; archive.stsci.edu/cgi-bin/dss_form
196Celestial Atlas by Curtney Seligman; cseligman.com/text/atlas.htm (2020-12-28)
217The Messier Catalog (SEDS); messier.seds.org (2021-01-01)
239«On the dark markings of the sky, with a catalogue of 182 such objects» Barnard, E. E.; Astrophysical Journal, 49, 1-24 (1919); DOI:10.1086/142439; Bibcode:1919ApJ....49....1B
277«Historische Deep-Sky Kataloge» von Dr. Wolfgang Steinicke; klima-luft.de/steinicke (2021-02-17)