The Small Sagittarius Star Cloud (also known as Messier 24 and IC 4715) is a star cloud in the

constellation A constellation is an area on the celestial sphere in which a group of visible stars forms Asterism (astronomy), a perceived pattern or outline, typically representing an animal, mythological subject, or inanimate object. The first constellati ...

of Sagittarius approximately 600

light years A light-year, alternatively spelled light year (ly or lyr), is a unit of length used to express astronomical distances and is equal to exactly , which is approximately 9.46 trillion km or 5.88 trillion mi. As defined by the International Astro ...

wide, which was catalogued by

Charles Messier Charles Messier (; 26 June 1730 – 12 April 1817) was a French astronomer. He published an astronomical catalogue consisting of 110 nebulae and star clusters, which came to be known as the ''Messier objects'', referred to with th ...

in 1764. It should not be confused with the nearby Large Sagittarius Star Cloud which lies about 10° to the south. Messier described the cloud as "a large nebulosity in which there are many stars of different magnitudes" and gave its dimensions as being some 1.5° across. Some sources, improperly, identify M24 as the small

open cluster An open cluster is a type of star cluster made of tens to a few thousand stars that were formed from the same giant molecular cloud and have roughly the same age. More than 1,100 open clusters have been discovered within the Milky Way galaxy, and ...

NGC 6603. The location of the Small Sagittarius Star Cloud is near the

Omega Nebula The Omega Nebula is an H II region in the constellation Sagittarius. It was discovered by Philippe Loys de Chéseaux in 1745. Charles Messier catalogued it in 1764. It is by some of the richest starfields of the Milky Way, figuring in the north ...

(also known as M17) and open cluster

Messier 18 Messier 18 or M18, also designated NGC 6613 and sometimes known as the Black Swan Cluster, is an open cluster of stars in the constellation Sagittarius. It was discovered by Charles Messier in 1764 and included in his list of comet-like objects. ...

, both north of M24. Messier 24 is not a distinct

deep-sky object A deep-sky object (DSO) is any astronomical object that is not an individual star or Solar System object (such as Sun, Moon, planet, comet, etc.). The classification is used for the most part by amateur astronomers to denote visually observed fa ...

, rather an open window through the Great Rift into deeper regions of the

Milky Way The Milky Way or Milky Way Galaxy is the galaxy that includes the Solar System, with the name describing the #Appearance, galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars in other arms of the galax ...

galaxy. It fills a space of significant volume to a depth of 10,000 to 15,000 light years away, including stars from the Scutum-Centaurus Arm, the major

spiral arm Spiral arms are a defining feature of spiral galaxies. They manifest as spiral-shaped regions of enhanced brightness within the galactic disc. Typically, spiral galaxies exhibit two or more spiral arms. The collective configuration of these arms i ...

between Earth and the

Galactic Center The Galactic Center is the barycenter of the Milky Way and a corresponding point on the rotational axis of the galaxy. Its central massive object is a supermassive black hole of about 4 million solar masses, which is called Sagittarius A*, a ...

. The star cloud is the most dense concentration of individual stars visible using

binoculars Binoculars or field glasses are two refracting telescopes mounted side-by-side and aligned to point in the same direction, allowing the viewer to use both eyes (binocular vision) when viewing distant objects. Most binoculars are sized to be held ...

, with around 1,000 stars visible within a single

field of view The field of view (FOV) is the angle, angular extent of the observable world that is visual perception, seen at any given moment. In the case of optical instruments or sensors, it is a solid angle through which a detector is sensitive to elec ...

. In

telescopes A telescope is a device used to observe distant objects by their emission, Absorption (electromagnetic radiation), absorption, or Reflection (physics), reflection of electromagnetic radiation. Originally, it was an optical instrument using len ...

it is best seen at low magnification, with a field of view of at least 2 degrees. Described as "a virtual carpet of stellar jewels", M24 is visible to the naked eye whenever the Milky Way itself is visible as well. The light of M24 is spread out over a large area, which makes estimating its brightness difficult. Older references give the star cloud's magnitude as 4.6, but more recent estimates place it a full two magnitudes brighter, at 2.5. HD 167356 is the brightest star within the Small Sagittarius Star Cloud, a white

supergiant Supergiants are among the most massive and most luminous stars. Supergiant stars occupy the top region of the Hertzsprung–Russell diagram, with absolute visual magnitudes between about −3 and −8. The temperatures of supergiant stars range ...

with an apparent magnitude of 6.05. This star is an Alpha-2 Canum Venaticorum variable, showing small changes in brightness as it rotates. There are three other stars in M24 with visual magnitudes between 6.5 and 7.0. The star cloud incorporates two prominent

dark nebula A dark nebula or absorption nebula is a type of interstellar cloud, particularly molecular clouds, that is so dense that it obscures the visible wavelengths of light from objects behind it, such as background stars and emission or reflection ...

e which are vast clouds of dense, obscuring interstellar dust. This dust blocks light from the more distant stars, which keeps them from being seen from Earth. Lying on the northwestern side is

Barnard 92 Barnard 92 (abbreviated to B92) is a dark nebula located in the Small Sagittarius Star Cloud. It was discovered by American astronomer Edward Emerson Barnard. B92 was initially referred to as "the black hole," given its appearance, after it was ...

, which is the darker of the two. Within the star field, the nebula appears as an immense round hole devoid of stars. American astronomer

Edward Emerson Barnard Edward Emerson Barnard (December 16, 1857 – February 6, 1923) was an American astronomer. He was commonly known as E. E. Barnard, and was recognized as a gifted observational astronomer. He is best known for his discovery of the high proper m ...

discovered this dark nebula in 1913. Along the northeast side lies Barnard 93, as large as Barnard 92 though less obvious. There are also other dark nebulae within M24, including Barnard 304 and Barnard 307. The Small Sagittarius Star Cloud also contains two planetary nebulae, M 1-43 and NGC 6567. Messier 24 holds some similarities with NGC 206, a bright, large star cloud within the

Andromeda Galaxy The Andromeda Galaxy is a barred spiral galaxy and is the nearest major galaxy to the Milky Way. It was originally named the Andromeda Nebula and is cataloged as Messier 31, M31, and NGC 224. Andromeda has a Galaxy#Isophotal diameter, D25 isop ...

References

External links

Finder Chart for Messier 24

* {{Messier objects Carina–Sagittarius Arm Star clouds Milky Way Sagittarius (constellation) Messier objects IC objects 17640620 Discoveries by Charles Messier

See also

References

External links