Messier 24, NGC 6603, IC 4715 - Sagittarius Star Cloud, Delle Caustiche

Messier 24 is a star cloud in the constellation Sagittarius. It is a pseudo-cluster of stars spread thousands of light years along our line of sight, perceived through a tunnel in the Milky Way's interstellar dust. Some sources improperly identify M 24 as the faint open cluster NGC 6603.

Historical Identification

Though the Small Sagittarius Star Cloud is hardly the sort of thing that could be mistaken for a comet, Messier included it as object number 24 in his catalogue. Messier described M 24 as a "large nebulosity containing many stars" and gave its dimensions as being some 1.5° across, a description that fits the star cloud rather well.

Within M 24 is a dim open cluster, NGC 6603, of magnitude 11, visible in telescopes starting from 4-inches' aperture. Many catalogs identify Messier 24 as this cluster, despite Messier's description, which matches the cloud and not the cluster. Other sources identify M 24 as IC 4715, but their positions differ by 10 minutes of right ascension. At least the IC description of IC 4715, "extremely large cloud of stars with nebulosity", would match with the appearance of M 24.

M 24 is often referred to as the "Small Sagittarius Star Cloud", to distinguish it from the "Large Sagittarius Star Cloud", which lies to the south, and refers to the portion of our Galaxy's central bulge which is not obscured by foreground dust. M 24 was named "Delle Caustiche" by Father Secchi, from the "peculiar arrangement of its stars in rays, arches, caustic curves, and intertwined spirals."

Observing Messier 24

The Sagittarius Star Cloud is easily located, NE of μ Sgr, with the unaided eye under dark skies. This patch is 1.5° in size, three times the size of the full Moon. With a total magnitude of 4.6, it is the densest concentration of stars visible in binoculars; around 1,000 stars are visible in a single field of view. It is a stunning sight in rich-field telescopes, which reveal a vast number of stars, arranged in remarkable patterns.

Two prominent dark nebulae were cataloged by E. E. Barnard in the northern part of M 24 in 1919; these are B 92 and B 93. Also within the cloud are two less-conspicuous clusters: Collinder 469, near B 92; and Markarian 38 (also Biurakan 5), south of B 93. On the western edge of M 24 is the 12th-magnitude planetary nebula NGC 6567, about 8' in diameter; it is a foreground object at a distance of about 4,000 light-years. The Delta Cepheid variable WZ Sagittarii is found in the southern portion of M 24; this pulsating giant star varies between magnitude 7.45 and 8.53 over a period of 21.85 days.

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 are associated with the little open cluster NGC 6595.

Physical Properties

M 24 is a section of a far interior spiral arm of our galaxy, the Norma Spiral Arm, which we see framed by the dust clouds of the nearer Sagittarius-Carina Spiral Arm. These dust clouds, in which the Lagoon, Trifid, and Omega nebulae are embedded, lie about 5,000 to 7,000 light years from us. The stars which comprise the M 24 star cloud are probably 12,000 to 16,000 light years distant, and the "window" in the obscuring dust which lets us see them is approximately 600 light years wide.

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.