The Sagittarius Star Cloud (also known as Messier 24 and IC 4715) is a star cloud in the constellation of Sagittarius, approximately 600 light years wide, which was discovered by Charles Messier in 1764. It is sometimes known as the Small Sagittarius Star Cloud to distinguish it from the Great Sagittarius Star Cloud located to the north of Gamma Sagittarii and Delta Sagittarii.
The stars, clusters and other objects comprising M24 are part of the Sagittarius or Sagittarius-Carina arms of the Milky Way galaxy. Messier described M24 as a "large nebulosity containing many stars" and gave its dimensions as being some 1.5° across. Some sources, improperly, identify M24 as the faint cluster NGC 6603. The location of the Sagittarius Star Cloud is near the Omega Nebula (also known as M17) and open cluster Messier 18, north of M24. M24 is one of only three Messier objects that are not deep sky objects.
M24 fills a space of significant volume to a depth of 10,000 to 16,000 light-years. This is the most dense concentration of individual stars visible using binoculars, with around 1,000 stars visible within a single field of view. The star cloud can be seen visible when the Milky Way itself is visible as well. Without the dust and gas that conceals the Milky Way, M24 holds a collection of numerous kinds of stars that are placed along the Milky Way and through the galaxy's obscuring band of interstellar dust.
HD 167356 is the brightest star that is located within the Sagittarius Star Cloud, a white supergiant 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 located in M24 with visual magnitudes between 6.5 and 7.0.
The star cloud incorporates two prominent dark nebulae 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, which is the darkest out of the two. Inside the field filled with stars, the nebula appears as an immense round hole that is devoid of any stars. American astronomer Edward Emerson Barnard discovered this dark nebula in 1913. Along the northeast side contains Barnard 93, which is less obvious and large as the other dark nebula. There are also other dark nebulae within M24, including Barnard 304 and Barnard 307. Both Barnard 92 and 93 have the most significant features shown in M24 due to them both blocking out several stars and being the most visually prominent.
The Sagittarius Star Cloud also holds two planetary nebulae, M 1-43 and NGC 6567. Located within a spiral arm of the Milky Way, Messier 24 holds some similarities with NGC 206, a bright, large star cloud.
|Observation data (J2000 epoch)|
|Right ascension||18h 17m|
|Distance||~10 kly (3070 Pc)|
|Apparent magnitude (V)||4.6|
|Apparent dimensions (V)||90′|
|Estimated age||220 million years|
|Other designations||Delle Caustiche, IC 4715|
Baade's Window is an area of the sky with relatively low amounts of interstellar "dust" along the line of sight from the Earth. This area is considered an observational "window" as the normally obscured Galactic Center of the Milky Way is visible in this direction. It is named for astronomer Walter Baade who first recognized its significance. This area corresponds to one of the brightest visible patches of the Milky Way.Carina–Sagittarius Arm
The Carina–Sagittarius Arm (also known as Sagittarius Arm or Sagittarius–Carina Arm, labeled -I) is generally thought to be a minor spiral arm of the Milky Way galaxy. Each spiral arm is a long, diffuse curving streamer of stars that radiates from the galactic center. These gigantic structures are often composed of billions of stars and thousands of gas clouds. The Carina–Sagittarius Arm is one of the most pronounced arms in our galaxy as a large number of HII regions, young stars and giant molecular clouds are concentrated in it.The Milky Way is a barred spiral galaxy, consisting of a central crossbar and bulge from which two major and several minor spiral arms radiate outwards. The Carina–Sagittarius Arm lies between two major spiral arms—the Scutum–Centaurus Arm the near part of which is visible looking inward i.e. toward the galactic centre with the rest beyond the galactic centre and the Perseus Arm, similar in size and shape but locally positioned outward. It is named for its proximity to the Sagittarius and Carina constellations as seen in the night sky from Earth, in the direction of the galactic center.
The arm dissipates near its middle, shortly after reaching its maximal angle, viewed from our solar system, from the galactic centre of about 80°. Extending from the galaxy's central bar is the Sagittarius Arm (Sagittarius bar). Beyond the dissipated zone it is the Carina Arm.Messier 18
Messier 18 or M18, also designated NGC 6613, 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. From the perspective of Earth, M18 is situated between the Omega Nebula (M17) and the Sagittarius Star Cloud (M24).This is a sparse cluster with a linear diameter of 8.04 pc, a tidal radius of 7.3 pc, and is centrally concentrated with core radius of 0.012 pc. It has a Trumpler class of II 3 p. The cluster is 33 million years old with an estimated mass of 188 M☉. It has one definite Be star and 29 B-type stars in total. There are three supergiant stars, all of class A or earlier. The brightest component (lucida), designated HD 168352, is a B-type giant star with a class of B2 III and a visual magnitude of 8.65.Messier 18 is 1,296 pc from the Earth and 6,830 pc from the Galactic Center. It is orbiting the Milky Way core with a period of 186.5 million years and an eccentricity of 0.02. This carries it to as close as 6.5 kpc to, and as far as 6.8 kpc from the galactic core. It passes vertically through the galactic plane once every 27.4 million years, ranging no more than 80 pc above or below.Messier object
The Messier objects are a set of 110 astronomical objects cataloged by the French astronomer Charles Messier in his Catalogue des Nébuleuses et des Amas d'Étoiles ("Catalogue of Nebulae and Star Clusters").
Because Messier was interested in finding only comets, he created a list of non-comet objects that frustrated his hunt for them. The compilation of this list, in collaboration with his assistant Pierre Méchain, is known as the Messier catalogue. This catalogue of objects is one of the most famous lists of astronomical objects, and many Messier objects are still referenced by their Messier number.
The catalogue includes some astronomical objects that can be observed from Earth's Northern Hemisphere such as deep-sky objects, a characteristic which makes the Messier objects extremely popular targets for amateur astronomers.A preliminary version first appeared in the Memoirs of the French Academy of Sciences in 1771,
and the last item was added in 1966 by Kenneth Glyn Jones, based on Messier's observations.
The first version of Messier's catalogue contained 45 objects and was published in 1774 in the journal of the French Academy of Sciences in Paris. In addition to his own discoveries, this version included objects previously observed by other astronomers, with only 17 of the 45 objects being Messier's.
By 1780 the catalogue had increased to 80 objects. The final version of the catalogue containing 103 objects was published in 1781 in the Connaissance des Temps for the year 1784.
However, due to what was thought for a long time to be the incorrect addition of Messier 102, the total number remained 102. Other astronomers, using side notes in Messier's texts, eventually filled out the list up to 110 objects.The catalogue consists of a diverse range of astronomical objects, ranging from star clusters and nebulae to galaxies. For example, Messier 1 is a supernova remnant, known as the Crab Nebula, and the great spiral Andromeda Galaxy is M31. Many further inclusions followed in the next century when the first addition came from Nicolas Camille Flammarion in 1921, who added Messier 104 after finding Messier's side note in his 1781 edition exemplar of the catalogue. M105 to M107 were added by Helen Sawyer Hogg in 1947, M108 and M109 by Owen Gingerich in 1960, and M110 by Kenneth Glyn Jones in 1967.Sagittarius (constellation)
Sagittarius is one of the constellations of the zodiac. It is one of the 48 constellations listed by the 2nd-century astronomer Ptolemy and remains one of the 88 modern constellations. Its name is Latin for the archer, and its symbol is (Unicode U+2650 ♐), a stylized arrow. Sagittarius is commonly represented as a centaur pulling back a bow. It lies between Scorpius and Ophiuchus to the west and Capricornus and Microscopium to the east.
The center of the Milky Way lies in the westernmost part of Sagittarius (see Sagittarius A).