Small Sagittarius Star Cloud

The Small Sagittarius Star Cloud, also cataloged as Messier 24 and IC 4715, spans roughly 600 light-years across in the constellation Sagittarius. Charles Messier first recorded it in 1764. It is often confused with the Large Sagittarius Star Cloud, which sits about 10° to the south. Messier himself described it as "a large nebulosity in which there are many stars of different magnitudes," noting its apparent size of about 1.5°. Some sources mistakenly identify M24 as the open cluster NGC 6603. The cloud lies near the Omega Nebula (M17) and the open cluster Messier 18, both to its north.

Rather than being a true deep-sky object, M24 is an opening through the Great Rift, offering a view into deeper parts of the Milky Way. It occupies a volume extending 10,000 to 15,000 light-years away, containing stars from the Scutum-Centaurus Arm. This star cloud is the densest grouping of individual stars visible through binoculars, with roughly 1,000 stars appearing in a single field. Through a telescope, low magnification (a field of at least 2°) works best. Often called "a virtual carpet of stellar jewels," M24 is visible to the naked eye whenever the Milky Way itself is up.

Because its light spreads over a large area, gauging its brightness is tricky. Older references list its magnitude as 4.6, but newer estimates put it at 2.5—a full two magnitudes brighter. The brightest star within the cloud is HD 167356, a white supergiant of magnitude 6.05. It is an Alpha-2 Canum Venaticorum variable, whose brightness shifts slightly as it rotates. Three other stars in M24 have visual magnitudes between 6.5 and 7.0.

The cloud contains two prominent dark nebulae—dense, obscuring interstellar dust that blocks light from more distant stars. On the northwestern side lies Barnard 92, the darker of the pair, appearing as a large, round hole devoid of stars. American astronomer Edward Emerson Barnard discovered it in 1913. Along the northeast side is Barnard 93, similar in size but less obvious. Other dark nebulae within M24 include Barnard 304 and Barnard 307. The Small Sagittarius Star Cloud also hosts two planetary nebulae: M 1-43 and NGC 6567. It shares some similarities with NGC 206, a bright star cloud in the Andromeda Galaxy.