M 24
M 24
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.
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), 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."
| Right ascension | 18 : 15.5 (hours : minutes) |
|---|---|
| Declination | -18 : 27 (degrees : minutes) |
| Distance | 10.0 (light-years*10^3) |
| Visual magnitude | 4.6 |
