Atlas of the Messier Objects

The Small Sagittarius

Star Cloud

Degree of difficulty 1 (of 5)

Minimum aperture Naked eye

Designation –

Type Star cloud

Class –

Distance 12,000 – 16,000 ly

Size –

Constellation Sagittarius

R.A. 18h 16.9min

Decl. −18° 29′

Magnitude 2.5

Surface brightness –

Apparent diameter 90′×30′

Discoverer Messier, 1764

History When, on the 20th of June 1764, Charles Messier noted “large nebulosity in which there are several stars of different magnitudes: the light which is diffused over this cluster is divided into several parts. 1°30′ diameter,” he did not mean NGC 6603, as has been believed by many later observers.

80 years later, John Herschel correctly wrote: “A glorious concentrated part of Milky Way, almost amounting to a globular cluster. Stars 14th and 15th magnitude.” The last remark clearly refers to NGC 6603, which appears to lie inside M 24. Smyth gave a similar description: “A beautiful field of stars, and in a richly clustering portion of the Milky Way, has a peaking spot with a lot of stardust.”

In 1905, Agnes Clerke commented: “Visible to the naked eye as a dim cloudlet near μ Sagittarii and named by Secchi as ‘Delle Caustiche’ from the peculiar arrangement of its stars in rays, arches, caustic curves and intertwined spirals.”

When he inspected photographic plates in 1918, Curtis described the neighboring dark nebulae, which were discovered by Barnard in 1913: “Two dark nebulae, the larger western object is about 14′×8′, and the contrast between the dense Milky Way region and the vacant spots is very striking.”

Astrophysics The designation M 24 has been given to the Small Sagittarius Star Cloud, which should be distinguished from the Large Sagittarius Cloud south of M 8. M 24 does not hold any other catalog names, as it does not have any physical identity. In reality, this is just a large number of stars at a considerable range of distances from 12,000 to 16,000 light-years, seen through a 1.5° by 0.5° window of reduced absorption in the curtain of interstellar dust, making them appear as a star cloud in projection along our line of sight.