A molecular cloud, sometimes called a stellar nursery (if star formation is occurring within), is a type of interstellar cloud, the density and size of which permit the formation of molecules, most commonly molecular hydrogen (H2). This is in contrast to other areas of the interstellar medium that contain predominantly ionized gas.
Molecular hydrogen is difficult to detect by infrared and radio observations, so the molecule most often used to determine the presence of H2 is carbon monoxide (CO). The ratio between CO luminosity and H2 mass is thought to be constant, although there are reasons to doubt this assumption in observations of some other galaxies.
Within molecular clouds are regions with higher density, where lots of dust and gas cores reside, called clumps. These clumps are the beginning of star formation, if gravity can overcome the high density and force the dust and gas to collapse.
Within the Milky Way, molecular gas clouds account for less than one percent of the volume of the interstellar medium (ISM), yet it is also the densest part of the medium, comprising roughly half of the total gas mass interior to the Sun's galactic orbit. The bulk of the molecular gas is contained in a ring between 3.5 and 7.5 kiloparsecs (11,000 and 24,000 light-years) from the center of the Milky Way (the Sun is about 8.5 kiloparsecs from the center). Large scale CO maps of the galaxy show that the position of this gas correlates with the spiral arms of the galaxy. That molecular gas occurs predominantly in the spiral arms suggests that molecular clouds must form and dissociate on a timescale shorter than 10 million years—the time it takes for material to pass through the arm region.
Vertically to the plane of the galaxy, the molecular gas inhabits the narrow midplane of the galactic disc with a characteristic scale height, Z, of approximately 50 to 75 parsecs, much thinner than the warm atomic (Z from 130 to 400 parsecs) and warm ionized (Z around 1000 parsecs) gaseous components of the ISM. The exception to the ionized-gas distribution are H II regions, which are bubbles of hot ionized gas created in molecular clouds by the intense radiation given off by young massive stars and as such they have approximately the same vertical distribution as the molecular gas.
This distribution of molecular gas is averaged out over large distances; however, the small scale distribution of the gas is highly irregular with most of it concentrated in discrete clouds and cloud complexes.
A vast assemblage of molecular gas with a mass of approximately 103 to 107 times the mass of the Sun is called a giant molecular cloud (GMC). GMCs are around 15 to 600 light-years in diameter (5 to 200 parsecs). Whereas the average density in the solar vicinity is one particle per cubic centimetre, the average density of a GMC is a hundred to a thousand times as great. Although the Sun is much more dense than a GMC, the volume of a GMC is so great that it contains much more mass than the Sun. The substructure of a GMC is a complex pattern of filaments, sheets, bubbles, and irregular clumps.
The densest parts of the filaments and clumps are called "molecular cores", while the densest molecular cores are called "dense molecular cores" and have densities in excess of 104 to 106 particles per cubic centimeter. Observationally, typical molecular cores are traced with CO and dense molecular cores are traced with ammonia. The concentration of dust within molecular cores is normally sufficient to block light from background stars so that they appear in silhouette as dark nebulae.
GMCs are so large that "local" ones can cover a significant fraction of a constellation; thus they are often referred to by the name of that constellation, e.g. the Orion Molecular Cloud (OMC) or the Taurus Molecular Cloud (TMC). These local GMCs are arrayed in a ring in the neighborhood of the Sun coinciding with the Gould Belt. The most massive collection of molecular clouds in the galaxy forms an asymmetrical ring about the galactic center at a radius of 120 parsecs; the largest component of this ring is the Sagittarius B2 complex. The Sagittarius region is chemically rich and is often used as an exemplar by astronomers searching for new molecules in interstellar space.
Isolated gravitationally-bound small molecular clouds with masses less than a few hundred times that of the Sun are called Bok globules. The densest parts of small molecular clouds are equivalent to the molecular cores found in GMCs and are often included in the same studies.
In 1984 IRAS identified a new type of diffuse molecular cloud. These were diffuse filamentary clouds that are visible at high galactic latitudes. These clouds have a typical density of 30 particles per cubic centimeter.
The formation of stars occurs exclusively within molecular clouds. This is a natural consequence of their low temperatures and high densities, because the gravitational force acting to collapse the cloud must exceed the internal pressures that are acting "outward" to prevent a collapse. There is observed evidence that the large, star-forming clouds are confined to a large degree by their own gravity (like stars, planets, and galaxies) rather than by external pressure. The evidence comes from the fact that the "turbulent" velocities inferred from CO linewidth scale in the same manner as the orbital velocity (a virial relation).
The physics of molecular clouds is poorly understood and much debated. Their internal motions are governed by turbulence in a cold, magnetized gas, for which the turbulent motions are highly supersonic but comparable to the speeds of magnetic disturbances. This state is thought to lose energy rapidly, requiring either an overall collapse or a steady reinjection of energy. At the same time, the clouds are known to be disrupted by some process—most likely the effects of massive stars—before a significant fraction of their mass has become stars.
Molecular clouds, and especially GMCs, are often the home of astronomical masers.
AFGL 2591 is a star forming region in the constellation Cygnus. Its dense cloud of gas and dust make its interior invisible to optical telescopes. Images in the infrared show a bright young stellar object, with an associated reflection nebula seen as a glowing cone projecting from the young star. A cluster of stars is forming within the molecular cloud, but most of the infrared radiation is coming from this star, AFGL 2591-VLA3.Initially AFGL 2591 was thought to be a single young, massive star expelling clouds of gas and dust in multiple events. It was estimated to be about 10 times the mass of the sun and at a distance of only 1,000 parsecs (3,300 light-years).Barnard's Loop
Barnard's Loop (catalogue designation Sh 2-276) is an emission nebula in the constellation of Orion. It is part of the Orion Molecular Cloud Complex which also contains the dark Horsehead and bright Orion nebulae. The loop takes the form of a large arc centered approximately on the Orion Nebula. The stars within the Orion Nebula are believed to be responsible for ionizing the loop.
The loop extends over about 600 arcminutes as seen from Earth, covering much of Orion. It is well seen in long-exposure photographs, although observers under very dark skies may be able to see it with the naked eye.
Recent estimates place it at a distance of either 159 pc (518 light years) or 440 pc (1434 ly) giving it dimensions of either about 100 or 300 ly across respectively. It is thought to have originated in a supernova explosion about 2 million years ago, which may have also created several known runaway stars, including AE Aurigae, Mu Columbae and 53 Arietis, which are believed to have been part of a multiple star system in which one component exploded as a supernova.Although this faint nebula was certainly observed by earlier astronomers, it is named after the pioneering astrophotographer E. E. Barnard who photographed it and published a description in 1894.Becklin–Neugebauer Object
The Becklin–Neugebauer Object (BN) is an object visible only in the infrared in the Orion Molecular Cloud. It was discovered in 1967 by Eric Becklin and Gerry Neugebauer during their near-infrared survey of the Orion Nebula.
The BN Object is thought to be an intermediate-mass protostar. It was the first star detected using infrared methods and is deeply embedded within the Orion star-forming nebula, where it is invisible at optical wavelengths because the light is completely scattered or absorbed due to the high density of dusty material.Cygnus OB7
Cygnus OB7 is an OB association in the Milky Way Galaxy in the Cygnus molecular cloud complex, which also contains the star-forming regions Cygnus X, the North America Nebula, and the Pelican Nebula. The Northern Coal Sack lies in the foreground of this region.DR 21
DR 21 is a large molecular cloud located in the constellation Cygnus, discovered in 1966 as a radio continuum source by Downes and Rinehart. DR 21 is located about 6,000 light-years (1,800 pc) from Earth and extends for 80 light-years (25 pc). The region contains a high rate of star formation and is associated with the Cygnus X star forming region. It has an estimated mass of 1,000,000 M☉.A number of different molecules have been detected in the region by their radio emission, including formaldehyde, ammonia, water and carbon monoxide.In this region, some of the most massive stars in the Milky Way have been observed. DR 21 contains complex patterns of dust and gas, which glow in the infrared due to the presence of organic compounds known as polycyclic aromatic hydrocarbons. Jagged patterns within DR 21 result from interactions with interstellar wind, radiation pressure, magnetic fields, and gravity.An estimated population of 2,900 stars have been formed in this molecular cloud, similar to the population of the Orion Nebula cluster, which are distributed in groups associated with cloud clumps. Feedback from the massive stars may ultimately disrupt the cloud; however, this has not happened yet due to the region's extreme youth. Study of these stars by the Spitzer Space Telescope has shown signs of protoplanetary disks.Horsehead Nebula
The Horsehead Nebula (also known as Barnard 33) is a small dark nebula in the constellation Orion. The nebula is located just to the south of Alnitak, the easternmost star of Orion's Belt, and is part of the much larger Orion Molecular Cloud Complex. It appears within the southern region of the dense dust cloud known as Lynds 1630, along the edge of the much larger HII nebula region called IC 434.The Horsehead Nebula is approximately 460 parsecs or 1400 light years from Earth. It is one of the most identifiable nebulae because of its resemblance to a horse's head.Messier 43
Messier 43 or M43, also known as De Mairan's Nebula and NGC 1982, is a star-forming nebula with a prominent H II region in the equatorial constellation of Orion. It was discovered by the French scientist Jean-Jacques Dortous de Mairan some time before 1731, then catalogued by French astronomer Charles Messier on March 4, 1769. The De Mairan's Nebula is part of the Orion Nebula (Messier 42), being separated from the main nebula by a dense lane of dust known as the northeast dark lane. It is part of the much larger Orion Molecular Cloud Complex.
The main ionizing star in this nebula is HD 37061 (variable star designation NU Ori), which is positioned near the center of the H II region and located 1,300 ± 160 ly (400 ± 50 pc) from the Sun. This is a triple star system with the brighter component being a single-lined spectroscopic binary. The main component is a blue-white hued B-type main-sequence star with a stellar classification of B0.5V or B1V. It has 19±7 times the mass of the Sun and 5.7±0.8 times the Sun's radius. The star is radiating over 26,000 times the Sun's luminosity from its photosphere at an effective temperature of 31,000 K. It is spinning rapidly with a projected rotational velocity of around 200 km/s.The H II region is a roundish volume of ionized hydrogen centered on HD 37061. It has a diameter of about 4.5′, corresponding to a linear size of 2.1 ly (0.65 pc). The net hydrogen alpha luminosity of this region is (3.0±1.1)×1035 erg s−1; equivalent to 78 L☉. There is a dark lane crossing in front of the region from north to south, known as the M43 dark lane.Messier 78
Messier 78 or M 78, also known as NGC 2068, is a reflection nebula in the constellation Orion. It was discovered by Pierre Méchain in 1780 and included by Charles Messier in his catalog of comet-like objects that same year.M78 is the brightest diffuse reflection nebula of a group of nebulae that includes NGC 2064, NGC 2067 and NGC 2071. This group belongs to the Orion B molecular cloud complex and is about 1,350 light-years distant from Earth. M78 is easily found in small telescopes as a hazy patch and involves two stars of 10th and 11th magnitude. These two B-type stars, HD 38563 A and HD 38563 B, are responsible for making the cloud of dust in M78 visible by reflecting their light.The M78 cloud contains a cluster of stars that is visible in the infrared. Due to gravity, the molecular gas in the nebula has fragmented into a hierarchy of clumps, the denser cores of which about to form stars with masses of up to 5 M☉. About 45 variable stars of the T Tauri type, young stars still in the process of formation as well as some 17 Herbig–Haro objects are known in M78.NGC 1579
NGC 1579 (also known as the Northern Trifid) is a diffuse nebula located in the constellation of Perseus. It is referred to as the Northern Trifid because of its similar appearance to the Trifid Nebula, which is located in the southern celestial hemisphere of the sky. It is a H II region, a region of star formation.The star cluster contains the emission-line star LkHα 101, which provides much of the ionizing radiation in the nebula.NGC 1579 lies within a giant molecular cloud known as the California Molecular Cloud.NGC 7129
NGC 7129 is a reflection nebula located 3,300 light years away in the constellation Cepheus. A young open cluster is responsible for illuminating the surrounding nebula. A recent survey indicates the cluster contains more than 130 stars less than 1 million years old. NGC 7129 is located just half a degree from nearby cluster NGC 7142.
The nebula is rosebud-shaped; the young stars have blown a large, oddly shaped bubble in the molecular cloud that once surrounded them at their birth. The rosy pink color comes from glowing dust grains on the surface of the bubble being heated by the intense light from the young stars within. The ultra-violet and visible light produced by the young stars is absorbed by the surrounding dust grains. They are heated by this process and release the energy at longer infrared wavelengths as photographed by the Spitzer Space Telescope. The reddish colors in the false-colour infrared image suggest the distribution of hydrocarbon rich molecular material.The much cooler molecular cloud outside the bubble is mostly invisible to Spitzer. However, three very young stars near the center of the nebula are sending jets of supersonic gas into the cloud. The collision of these jets heats carbon monoxide molecules in the nebula. This produces the complex nebulosity that appears like a stem of a rosebud.OB star
OB stars are hot, massive stars of spectral types O or early-type B that form in loosely organized groups called OB associations. They are short lived, and thus do not move very far from where they formed within their life. During their lifetime, they will emit much ultraviolet radiation. This radiation rapidly ionizes the surrounding interstellar gas of the giant molecular cloud, forming an H II region or Strömgren sphere.
In lists of spectra the "spectrum of OB" refers to "unknown, but belonging to an OB association so thus of early type".Open cluster
An open cluster is a group of up 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 many more are thought to exist. They are loosely bound by mutual gravitational attraction and become disrupted by close encounters with other clusters and clouds of gas as they orbit the galactic center. This can result in a migration to the main body of the galaxy and a loss of cluster members through internal close encounters. Open clusters generally survive for a few hundred million years, with the most massive ones surviving for a few billion years. In contrast, the more massive globular clusters of stars exert a stronger gravitational attraction on their members, and can survive for longer. Open clusters have been found only in spiral and irregular galaxies, in which active star formation is occurring.Young open clusters may be contained within the molecular cloud from which they formed, illuminating it to create an H II region. Over time, radiation pressure from the cluster will disperse the molecular cloud. Typically, about 10% of the mass of a gas cloud will coalesce into stars before radiation pressure drives the rest of the gas away.
Open clusters are key objects in the study of stellar evolution. Because the cluster members are of similar age and chemical composition, their properties (such as distance, age, metallicity, extinction, and velocity) are more easily determined than they are for isolated stars. A number of open clusters, such as the Pleiades, Hyades or the Alpha Persei Cluster are visible with the naked eye. Some others, such as the Double Cluster, are barely perceptible without instruments, while many more can be seen using binoculars or telescopes. The Wild Duck Cluster, M11, is an example.Orion Molecular Cloud Complex
The Orion Molecular Cloud Complex (or, simply, the Orion Complex) is a star forming region with stellar ages ranging up to 12 Myr. Two giant molecular clouds are a part of it, Orion A and Orion B. The stars currently forming within the Complex are located within these clouds. A number of other somewhat older stars no longer associated with the molecular gas are also part of the Complex, most notably the Orion's Belt (Orion OB1b), as well as the dispersed population north of it (Orion OB1a). Near the head of Orion there is also a population of young stars that is centered on Meissa. The Complex is between 1 000 and 1 400 light-years away, and hundreds of light-years across.
The Orion Complex is one of the most active regions of nearby stellar formation visible in the night sky, and is home to both protoplanetary discs and very young stars. Much of it is bright in infrared wavelengths due to the heat-intensive processes involved in stellar formation, though the complex contains dark nebulae, emission nebulae, reflection nebulae, and H II regions. The presence of ripples on the surface of Orion's Molecular Clouds was discovered in 2010. The ripples are a result of the expansion of the nebulae gas over pre-existing molecular gas.The Orion Complex includes a large group of bright nebulae, dark clouds in the Orion constellation. Several nebulae can be observed through binoculars and small telescopes, and some parts (such as the Orion Nebula) are visible to the naked eye.Orion–Eridanus Superbubble
The Orion–Eridanus Superbubble, or Eridanus Soft X-ray Enhancement is a superbubble located west of the Orion Nebula. The region is formed from overlapping supernova remnants that may be associated with the Orion OB1 stellar association; the bubble is approximately 1200 ly across. It is the nearest superbubble to the Local Bubble containing the Sun, with the respective shock fronts being about 500 ly apart.The structure was discovered from 21 cm radio observations by Carl Heiles and interstellar optical emission line observations by Reynolds and Ogden in the 1970s.Protostar
A protostar is a very young star that is still gathering mass from its parent molecular cloud. The protostellar phase is the earliest one in the process of stellar evolution. For a low mass star (i.e. that of the Sun or lower), it lasts about 500,000 years The phase begins when a molecular cloud fragment first collapses under the force of self-gravity and an opaque, pressure supported core forms inside the collapsing fragment. It ends when the infalling gas is depleted, leaving a pre-main-sequence star, which contracts to later become a main-sequence star at the onset of Hydrogen fusion.Rosette Nebula
The Rosette Nebula (also known as Caldwell 49) is a large spherical H II region (circular in appearance) located near one end of a giant molecular cloud in the Monoceros region of the Milky Way Galaxy. The open cluster NGC 2244 (Caldwell 50) is closely associated with the nebulosity, the stars of the cluster having been formed from the nebula's matter.
The complex has the following NGC designations:
NGC 2237 – Part of the nebulous region (Also used to denote whole nebula)
NGC 2238 – Part of the nebulous region
NGC 2239 – Part of the nebulous region (Discovered by John Herschel)
NGC 2244 – The open cluster within the nebula (Discovered by John Flamsteed in 1690)
NGC 2246 – Part of the nebulous regionThe cluster and nebula lie at a distance of some 5,000 light-years from Earth) and measure roughly 130 light years in diameter. The radiation from the young stars excites the atoms in the nebula, causing them to emit radiation themselves producing the emission nebula we see. The mass of the nebula is estimated to be around 10,000 solar masses.
A survey of the nebula with the Chandra X-ray Observatory has revealed the presence of numerous new-born stars inside optical Rosette Nebula and studded within a dense molecular cloud. Altogether, approximately 2500 young stars lie in this star-forming complex, including the massive O-type stars HD 46223 and HD 46150, which are primarily responsible for blowing the ionized bubble.
Most of the ongoing star-formation activity is occurring in the dense molecular cloud to the south east of the bubble.A diffuse X-ray glow is also seen between the stars in the bubble, which has been attributed to a super-hot plasma with temperatures ranging from 1 to 10 million K.
This is significantly hotter than the 10,000 K plasmas seen in HII regions, and is likely attributed to the shock-heated winds from the massive O-type stars.
On April 16, 2019 the Oklahoma Legislature passed HB1292 making the Rosette Nebula as the official state astronomical object. Oklahoma Governor Kevin Stitt signed it into law April 22, 2019.Star formation
Star formation is the process by which dense regions within molecular clouds in interstellar space, sometimes referred to as "stellar nurseries" or "star-forming regions", collapse and form stars. As a branch of astronomy, star formation includes the study of the interstellar medium (ISM) and giant molecular clouds (GMC) as precursors to the star formation process, and the study of protostars and young stellar objects as its immediate products. It is closely related to planet formation, another branch of astronomy. Star formation theory, as well as accounting for the formation of a single star, must also account for the statistics of binary stars and the initial mass function. Most stars do not form in isolation but as part of a group of stars referred as star clusters or stellar associations.Taurus Molecular Cloud
The Taurus Molecular Cloud is a molecular cloud in the constellations Taurus and Auriga. This cloud hosts a stellar nursery containing hundreds of newly formed stars. The Taurus Molecular Cloud is only 140 pc (430 ly) away from earth, making it the nearest large star formation region. It also reveals characteristics that make it ideal for detailed physical studies. It has been important in star formation studies at all wavelengths.The cloud is notable for containing many complex molecules, including cyanopolyynes HCnN for n=3,5,7,9.Westerhout 40
Westerhout 40 or W40 (also designated Sharpless 64, Sh2-64, or RCW 174) is a star-forming region in our galaxy located in the constellation Serpens Cauda. In this region, interstellar gas forming a diffuse nebula surrounds a cluster of several hundred new-born stars. The distance to W40 is 436±9 pc (1420±30 light-years), making it one of the closest sites of formation of high-mass O- and B-type stars. The Ionizing radiation from the massive OB stars has created an H II region, which has an hour-glass morphology.Dust from the molecular cloud in which W40 formed obscures the nebula, rendering W40 difficult to observe at visible wavelengths of light. Thus, X-ray, infrared, and radio observations have been used to see through the molecular cloud to study the star-formation processes going on within.W40 appears near to several other star-forming regions in the sky, including an infrared dark cloud designated Serpens South and a young stellar cluster designated the Serpens Main Cluster. Similar distances measured for these three star-forming regions suggests that they are near to each other and part of the same larger-scale collection of clouds known as the Serpens Molecular Cloud.
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