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M 24 - Sagittarius Star Cloud Delle Caustiche
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M 24 - Sagittarius Star Cloud, Delle Caustiche

February 18, 2005
Milky Way Patch M24 (IC 4715 ?; contains NGC 6603) in Sagittarius

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)

"Discovered" 1764 by Charles Messier.

Under entry No. 24 in his catalog, Charles Messier list a large object of 1 1/2 deg in extension, which he included on June 20, 1764, and describes it as "a large nebulosity in which there are many stars of different magnitudes."

Messier object number 24 is not a "true" deep sky object, but a huge star cloud in Milky Way, a pseudo-cluster of stars spread thousands of light years along the line of sight, perceived through a chance tunnel in the interstellar dust. They form a portion of a spiral arm of our galaxy. This cloud is the bright Milky Way patch slightly above the center of our image; among many other Deep Sky objects (clusters and nebulae) one can find 10 more Messier objects in this image.

The interstellar dust generally dims the light of stars behind it. But the dust is patchy. For some unknown reason it clumps in clouds typically 25 light years across: many such clouds can be clearly distinguished, projected against the star cloud. There are typically two such clouds in a line of sight 1,000 light years long in the Milky Way. But even over the 30,000 light-years to the central regions of the Galaxy there could be, and by chance are, clearer windows than normal in the interstellar medium. M24 is in effect one of these windows.

These clear windows through the Galaxy have great significance in the study of galactic structure, since they make it possible to study otherwise hidden, distant regions (after Murdin/Allen/Malin's Catalogue of the Universe, 1979).

A.M. Clerke, in 1905, remarked that this "dim cloudlet (for the naked eye) near Mu Sagittarii" was named "Delle Caustiche" by Fr. Secchi, "from the peculiar arrangement of its stars in rays, arches, caustic curves, and intertwined spirals." Alternatively, M24 is often referred to as "Sagittarius Star Cloud", or "Little" or "Small Sagittarius Star Cloud" (in contrast to the "Big" or "Large Sagittarius Star Cloud" which lies more to the south and consists of that portion of our Galaxy's central bulge which happens to be not obscured by foreground dust).

Although this is what Messier discovered, it is interesting that, within this stellar cloud which is easily visible to the naked eye, there's a dim open cluster, NGC 6603, of magnitude 11. Many catalogs give the Messier number to this object, despite Messier's magnitude (4.5 .. 4.6), diameter (1.5 degrees), and his description as a "large nebulosity in which there are many stars of different magnitudes," which matches well with the cloud and not the cluster.

The stars, clusters and other objects of M24 form a portion of a spiral arm (the Sagittarius or Sagittarius-Carina arm) which fills a space of significant depth, at a distance of 10,000 to 16,000 light-years. This object is probably similar to the starcloud NGC 206 in our galactic neighbor, the Andromeda Galaxy (M31).

NED identifies IC 4715 with M24, taking into account a possible position error of +10 minutes in Right Ascension. The IC description, "extremely large cloud of stars with nebulosity", would at least match with the appearance of M24.

E.E. Barnard has cataloged two prominent dark regions (dark nebulae) in the Northern portion of star cloud M24 as Nos. 92 and 93 in his catalog of dark nebulae (see Barnard 1913, Barnard 1919). Within the area of the sky covered by the starcloud, there are two further, less conspicuous clusters: Collinder 469 close to the lower right end of comet-like looking dark nebula Barnard 92 (sometimes nicknamed "Black Hole"), and Markarian 38 or Biur 5 (for Biurakan observatory) south of Barnard 93. Immediately south of the star cloud, separated by a dark band, is the emission nebula IC 1283-1284, with two adjacent reflection nebulae, NGC 6589 and NGC 6590, all these nebulae associated with the little conspicuous open cluster NGC 6595. On about the western edge of M24, 12th-magnitude planetary nebula NGC 6567, of about 8 arc minutes diameter, can be found, a foreground object at a distance of about 4,000 light-years. In the southern portion of M24, Delta Cephei variable WZ Sagittarii is found; this pulsating giant star varies in brightness between mag 7.45 and 8.53, and in spectral type between F8 and K1, in a period of 21.849708 days. For more objects in and near the M24 star cloud, and some more information on these objects, see our list.

Under considerably dark skies, M24 is easily located with the unaided eye as a Milky Way star cloud in Northern Sagittarius, i.e. well "above" the teapot asterism and just north of Mu Sagittarii; under less favorable conditions the slightest optical aid will confirm it. Telescopes will reveal a vast number of stars, arranged in remarkable patterns. NGC 6603 will be well visible starting from 4-inch, and larger scopes will show the other clusters mentioned above.


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