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51-467: 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. Near-infrared polarized light observations showed that the star BN is still surrounded by

102-608: A circumstellar disk . BN moves towards the northwest with respect to other stars in the Kleinmann-Low nebula . A proper motion of between 21 and 27 km/s in the northwest region and a redshift of about 11 km/s with respect to the OMC1 was measured for this star. BN is therefore considered a runaway star . It was proposed that Theta1Ori C ejected BN about 4000 years ago, but it is more likely that BN and two other runaway stars, called Source I (Src I) and Source n (Src n), were ejected from

153-408: A pre-main sequence or main-sequence star. Within its deep interior, the protostar has lower temperature than an ordinary star. At its center, hydrogen-1 is not yet fusing with itself. Theory predicts, however, that the hydrogen isotope deuterium (hydrogen-2) fuses with hydrogen-1, creating helium-3 . The heat from this fusion reaction tends to inflate the protostar, and thereby helps determine

204-576: A spectral line at a frequency of 1420.405 MHz . This frequency is generally known as the 21 cm line , referring to its wavelength in the radio band . The 21 cm line is the signature of HI and makes the gas detectable to astronomers back on earth. The discovery of the 21 cm line was the first step towards the technology that would allow astronomers to detect compounds and molecules in interstellar space. In 1951, two research groups nearly simultaneously discovered radio emission from interstellar neutral hydrogen. Ewen and Purcell reported

255-482: 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. Filaments are truly ubiquitous in the molecular cloud. Dense molecular filaments will fragment into gravitationally bound cores, most of which will evolve into stars. Continuous accretion of gas, geometrical bending, and magnetic fields may control

306-459: A crucial role in the initial conditions of star formation and the origin of the stellar IMF. 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 10 to 10 particles per cubic centimeter. Typical molecular cores are traced with CO and dense molecular cores are traced with ammonia . The concentration of dust within molecular cores

357-552: A detectable radio signal . This discovery was an important step towards the research that would eventually lead to the detection of molecular clouds. Once the war ended, and aware of the pioneering radio astronomical observations performed by Jansky and Reber in the US, the Dutch astronomers repurposed the dish-shaped antennas running along the Dutch coastline that were once used by the Germans as

408-424: A fast transition, forming "envelopes" of mass, giving the impression of an edge to the cloud structure. The structure itself is generally irregular and filamentary. Cosmic dust and ultraviolet radiation emitted by stars are key factors that determine not only gas and column density, but also the molecular composition of a cloud. The dust provides shielding to the molecular gas inside, preventing dissociation by

459-441: A molecular cloud assembles enough mass, the densest regions of the structure will start to collapse under gravity, creating star-forming clusters. This process is highly destructive to the cloud itself. Once stars are formed, they begin to ionize portions of the cloud around it due to their heat. The ionized gas then evaporates and is dispersed in formations called ‘ champagne flows ’. This process begins when approximately 2% of

510-505: A more complex interaction, such as the interaction of a tight binary with a compact star cluster . This dynamical interaction released a large amount of energy, causing an infrared-only flare on the scale of a nova or supernova with an energy of about 10 erg . Alternatively the explosion was not a multi-system interaction but a supernova. The remnant of the explosion is called Kleinmann-Low nebula . Multi-wavelength observations and carbon monoxide (CO) observations with ALMA reveal

561-423: A position about 500 years ago. Source I and Source n both move in opposite directions, away from BN. With more recent VLA proper motion measurements it was realised that at least six compact sources recede from a common point: BN, source I, Orion MR (formerly source n), X, IRc23 and Zapata 11. Almost all these sources were ejected about 500 years ago. The ejection of BN and source I was proposed to have occurred in

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612-461: A protostar is not detectable at optical wavelengths, and cannot be placed in the Hertzsprung–Russell diagram , unlike the more evolved pre-main-sequence stars. The actual radiation emanating from a protostar is predicted to be in the infrared and millimeter regimes. Point-like sources of such long-wavelength radiation are commonly seen in regions that are obscured by molecular clouds . It

663-441: A timescale shorter than 10 million years—the time it takes for material to pass through the arm region. Perpendicularly 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

714-427: A warning radar system and modified into radio telescopes , initiating the search for the hydrogen signature in the depths of space. The neutral hydrogen atom consists of a proton with an electron in its orbit. Both the proton and the electron have a spin property. When the spin state flips from a parallel condition to antiparallel, which contains less energy, the atom gets rid of the excess energy by radiating

765-460: A weak rotational and vibrational modes, making it virtually invisible to direct observation. The solution to this problem came when Arno Penzias , Keith Jefferts, and Robert Wilson identified CO in the star-forming region in the Omega Nebula . Carbon monoxide is a lot easier to detect than H 2 because of its rotational energy and asymmetrical structure. CO soon became the primary tracer of

816-471: Is a very young star that is still gathering mass from its parent molecular cloud . It is the earliest phase 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

867-475: Is commonly believed that those conventionally labeled as Class 0 or Class I sources are protostars. However, there is still no definitive evidence for this identification. Molecular cloud 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 absorption nebulae , the formation of molecules (most commonly molecular hydrogen , H 2 ), and

918-409: Is dispersed after this time. The lack of large amounts of frozen molecules inside the clouds also suggest a short-lived structure. Some astronomers propose the molecules never froze in very large quantities due to turbulence and the fast transition between atomic and molecular gas. Due to their short lifespan, it follows that molecular clouds are constantly being assembled and destroyed. By calculating

969-414: Is likely to be the main mechanism. Those regions with more gas will exert a greater gravitational force on their neighboring regions, and draw surrounding material. This extra material increases the density, increasing their gravitational attraction. Mathematical models of gravitational instability in the gas layer predict a formation time within the timescale for the estimated cloud formation time. Once

1020-505: 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

1071-502: The Big Bang . Due to their pivotal role, research about these structures have only increased over time. A paper published in 2022 reports over 10,000 molecular clouds detected since the discovery of Sagittarius B2. 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 it. The bulk of

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1122-469: The Milky Way per year. Two possible mechanisms for molecular cloud formation have been suggested by astronomers. Cloud growth by collision and gravitational instability in the gas layer spread throughout the galaxy. Models for the collision theory have shown it cannot be the main mechanism for cloud formation due to the very long timescale it would take to form a molecular cloud, beyond the average lifespan of such structures. Gravitational instability

1173-454: The ISM . The exceptions 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

1224-683: The Leiden-Sydney map of neutral hydrogen in the galactic disk in 1958 on the Monthly Notices of the Royal Astronomical Society . This was the first neutral hydrogen map of the galactic disc and also the first map showing the spiral arm structure within it. Following the work on atomic hydrogen detection by van de Hulst, Oort and others, astronomers began to regularly use radio telescopes, this time looking for interstellar molecules . In 1963 Alan Barrett and Sander Weinred at MIT found

1275-580: 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

1326-409: The beginning of star formation if gravitational forces are sufficient to cause the dust and gas to collapse. The history pertaining to the discovery of molecular clouds is closely related to the development of radio astronomy and astrochemistry . During World War II , at a small gathering of scientists, Henk van de Hulst first reported he had calculated the neutral hydrogen atom should transmit

1377-473: The clouds where star-formation occurs. In 1970, Penzias and his team quickly detected CO in other locations close to the galactic center , including the giant molecular cloud identified as Sagittarius B2 , 390 light years from the galactic center, making it the first detection of a molecular cloud in history. This team later would receive the Nobel prize of physics for their discovery of microwave emission from

1428-432: The dense core accrues mass from its larger, surrounding cloud, self-gravity begins to overwhelm pressure, and collapse begins. Theoretical modeling of an idealized spherical cloud initially supported only by gas pressure indicates that the collapse process spreads from the inside toward the outside. Spectroscopic observations of dense cores that do not yet contain stars indicate that contraction indeed occurs. So far, however,

1479-461: The detailed fragmentation manner of the filaments. In supercritical filaments, observations have revealed quasi-periodic chains of dense cores with spacing of 0.15 parsec comparable to the filament inner width. A substantial fraction of filaments contained prestellar and protostellar cores, supporting the important role of filaments in gravitationally bound core formation. Recent studies have suggested that filamentary structures in molecular clouds play

1530-642: The detection of the 21-cm line in March, 1951. Using the radio telescope at the Kootwijk Observatory, Muller and Oort reported the detection of the hydrogen emission line in May of that same year. Once the 21-cm emission line was detected, radio astronomers began mapping the neutral hydrogen distribution of the Milky Way Galaxy. Van de Hulst, Muller, and Oort, aided by a team of astronomers from Australia, published

1581-659: The emission line of OH in the supernova remnant Cassiopeia A . This was the first radio detection of an interstellar molecule at radio wavelengths. More interstellar OH detections quickly followed and in 1965, Harold Weaver and his team of radio astronomers at Berkeley , identified OH emissions lines coming from the direction of the Orion Nebula and in the constellation of Cassiopeia . In 1968, Cheung, Rank, Townes, Thornton and Welch detected NH₃ inversion line radiation in interstellar space. A year later, Lewis Snyder and his colleagues found interstellar formaldehyde . Also in

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1632-619: The formation of H II regions . 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 H 2 is carbon monoxide (CO). The ratio between CO luminosity and H 2 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 much dust and many gas cores reside, called clumps. These clumps are

1683-444: The gas is highly irregular, with most of it concentrated in discrete clouds and cloud complexes. Molecular clouds typically have interstellar medium densities of 10 to 30 cm , and constitute approximately 50% of the total interstellar gas in a galaxy . Most of the gas is found in a molecular state . The visual boundaries of a molecular cloud is not where the cloud effectively ends, but where molecular gas changes to atomic gas in

1734-488: 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 producing helium. The modern picture of protostars, summarized above, was first suggested by Chushiro Hayashi in 1966. In the first models, the size of protostars was greatly overestimated. Subsequent numerical calculations clarified the issue, and showed that protostars are only modestly larger than main-sequence stars of

1785-570: The larger substructure of the cloud, having the average size of 1 pc . Clumps are the precursors of star clusters , though not every clump will eventually form stars. Cores are much smaller (by a factor of 10) and have higher densities. Cores are gravitationally bound and go through a collapse during star formation . In astronomical terms, molecular clouds are short-lived structures that are either destroyed or go through major structural and chemical changes approximately 10 million years into their existence. Their short life span can be inferred from

1836-401: The mass of the Sun is called a giant molecular cloud ( GMC ). GMCs are around 15 to 600 light-years (5 to 200 parsecs) in diameter, with typical masses of 10 thousand to 10 million solar masses. Whereas the average density in the solar vicinity is one particle per cubic centimetre, the average volume density of a GMC is about ten to a thousand times higher. Although the Sun is much denser than

1887-544: The mass of the cloud has been converted into stars. Stellar winds are also known to contribute to cloud dispersal. The cycle of cloud formation and destruction is closed when the gas dispersed by stars cools again and is pulled into new clouds by gravitational instability. Star formation involves the collapse of the densest part of the molecular cloud, fragmenting the collapsed region in smaller clumps. These clumps aggregate more interstellar material, increasing in density by gravitational contraction. This process continues until

1938-481: 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

1989-549: The mostly spherical remnant of an explosion at the intersection point of the BN object and Source I. The ALMA observations revealed hundreds of CO streamers moving with up to 100 km/s. Some of these CO streamers nearly reach the shocked gas and dust observed in molecular hydrogen and iron [Fe II]. Becklin's star (IRC -10093) is located at 5h 35.3m / -5° 23', very near the Becklin-Neugebauer object. Protostar A protostar

2040-425: The predicted outward spread of the collapse region has not been observed. The gas that collapses toward the center of the dense core first builds up a low-mass protostar, and then a protoplanetary disk orbiting the object. As the collapse continues, an increasing amount of gas impacts the disk rather than the star, a consequence of angular momentum conservation. Exactly how material in the disk spirals inward onto

2091-437: The protostar is not yet understood, despite a great deal of theoretical effort. This problem is illustrative of the larger issue of accretion disk theory, which plays a role in much of astrophysics. Regardless of the details, the outer surface of a protostar consists at least partially of shocked gas that has fallen from the inner edge of the disk. The surface is thus very different from the relatively quiescent photosphere of

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2142-401: The range in age of young stars associated with them, of 10 to 20 million years, matching molecular clouds’ internal timescales. Direct observation of T Tauri stars inside dark clouds and OB stars in star-forming regions match this predicted age span. The fact OB stars older than 10 million years don’t have a significant amount of cloud material about them, seems to suggest most of the cloud

2193-409: The rate at which stars are forming in our galaxy, astronomers are able to suggest the amount of interstellar gas being collected into star-forming molecular clouds in our galaxy. The rate of mass being assembled into stars is approximately 3 M ☉ per year. Only 2% of the mass of a molecular cloud is assembled into stars, giving the number of 150 M ☉ of gas being assembled in molecular clouds in

2244-820: The relationship between molecular clouds and star formation. Embedded in the Taurus molecular cloud there are T Tauri stars . These are a class of variable stars in an early stage of stellar development and still gathering gas and dust from the cloud around them. Observation of star forming regions have helped astronomers develop theories about stellar evolution . Many O and B type stars have been observed in or very near molecular clouds. Since these star types belong to population I (some are less than 1 million years old), they cannot have moved far from their birth place. Many of these young stars are found embedded in cloud clusters, suggesting stars are formed inside it. A vast assemblage of molecular gas that has more than 10 thousand times

2295-417: The same mass. This basic theoretical result has been confirmed by observations, which find that the largest pre-main-sequence stars are also of modest size. Star formation begins in relatively small molecular clouds called dense cores. Each dense core is initially in balance between self-gravity, which tends to compress the object, and both gas pressure and magnetic pressure , which tend to inflate it. As

2346-460: The same year George Carruthers managed to identify molecular hydrogen . The numerous detections of molecules in interstellar space would help pave the way to the discovery of molecular clouds in 1970. Hydrogen is the most abundant species of atom in molecular clouds, and under the right conditions it will form the H 2 molecule. Despite its abundance, the detection of H 2 proved difficult. Due to its symmetrical molecule, H 2 molecules have

2397-461: The second most common compound. Molecular clouds also usually contain other elements and compounds. Astronomers have observed the presence of long chain compounds such as methanol , ethanol and benzene rings and their several hydrides . Large molecules known as polycyclic aromatic hydrocarbons have also been detected. The density across a molecular cloud is fragmented and its regions can be generally categorized in clumps and cores. Clumps form

2448-500: The size of the youngest observed pre-main-sequence stars. The energy generated from ordinary stars comes from the nuclear fusion occurring at their centers. Protostars also generate energy, but it comes from the radiation liberated at the shocks on its surface and on the surface of its surrounding disk. The radiation thus created must traverse the interstellar dust in the surrounding dense core. The dust absorbs all impinging photons and reradiates them at longer wavelengths. Consequently,

2499-494: The temperature reaches a point where the fusion of hydrogen can occur. The burning of hydrogen then generates enough heat to push against gravity, creating hydrostatic equilibrium . At this stage, a protostar is formed and it will continue to aggregate gas and dust from the cloud around it. One of the most studied star formation regions is the Taurus molecular cloud due to its close proximity to earth (140 pc or 430 ly away), making it an excellent object to collect data about

2550-419: The ultraviolet radiation. The dissociation caused by UV photons is the main mechanism for transforming molecular material back to the atomic state inside the cloud. Molecular content in a region of a molecular cloud can change rapidly due to variation in the radiation field and dust movement and disturbance. Most of the gas constituting a molecular cloud is molecular hydrogen , with carbon monoxide being

2601-405: The year 1475±6 (about 550 years ago). IRc23 was ejected only 300 years ago. At the time of the ejection four or more protostars dynamically interacted with each other, leading to the ejection of the stars in different directions. In the classical three-body scenario , the dynamical interaction either formed a compact binary or the merger of two stars. The large number of ejected stars suggest

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