M 24

Sagittarius Star Cloud, Delle Caustiche

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. 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. M 24 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)

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 objects, 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.

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."

The cluster NGC 6603 is classified by Shapley as type "g" and is of Trumpler class I,2,r (Trumpler and Götz) or I,2,r,n (Sky Catalog 2000). This 11.4 mag cluster consists of about 30 stars in a field of about 5 arc minutes in diameter, and is about 9400 light years remote (Kenneth Glyn Jones gives even 16,000 light years). Thus its linear diameter should be about 14 light years. The hottest stars are about B9 (pointing to an intermediate age of several 100 million years, an estimate of which is not known to the present author), and the brightest of photographic mag 14.

For observing NGC 6603, a medium-sized amatuer scope (starting perhaps at 4-inch refractors or equivalent) with medium to high magnification is required. This cluster is difficult to distinguish from the Milky Way background.

More images of M24

Region around M24 with M17 and M18

Right Ascension	18 : 16.9 (hours : minutes)
Declination	-18 : 29 (degrees : minutes)
Distance	10 (*1000 light-years)
Visual Magnitude	4.6
Apparent Dimension	90 (arc minutes)

Hartmut Frommert (spider@seds.org)