A molecular cloud is a type of interstellar cloud whose density and size permits the formation of molecules, most commonly molecular hydrogen (H₂). Interstellar cloud is the generic name given to an accumulation of gas, plasma and dust in our and other galaxies. ... Molecular hydrogen, H2, is a molecule formed from two atoms of hydrogen. ...

This molecule is difficult to detect, and the molecule most used to trace the H₂ is CO (carbon monoxide). The ratio between CO luminosity and H₂ mass is roughly constant, although there are reasons to doubt this assumption in observations of some other galaxies.^[1] Carbon monoxide, with the chemical formula CO, is a colourless, odourless and tasteless gas. ...

Within a few million years the light from bright stars will have boiled away this molecular cloud of gas and dust. The cloud has broken off from the Carina Nebula. Newly formed stars are visible nearby, their images reddened by blue light being preferentially scattered by the pervasive dust. This image spans about two light-years and was taken by the orbiting Hubble Space Telescope in 1999.

This cloud of gas and dust is being deleted. ... This cloud of gas and dust is being deleted. ... NGC 3372, the Eta Carinae Nebula. ...

Occurrence

Within our own Galaxy molecular gas accounts 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 one-half of the total gas mass interior to the Sun's galactic orbit. The bulk of the molecular gas is contained in a molecular ring between 3.5 to 7.5 kiloparsecs from the centre of the galaxy (the Sun is about 8.5 kiloparces from the center).^[2] Large scale carbon monoxide maps of the galaxy show that the position of this gas correlates with the spiral arms of the galaxy.^[3] That molecular gas occurs predominantly in the spiral arms argues 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.^[4] The Milky Way (a translation of the Latin Via Lactea, in turn derived from the Greek Γαλαξίας (Galaxias), sometimes referred to simply as the Galaxy), is a barred spiral galaxy of the Local Group. ... The distribution of ionized hydrogen (known by astronomers as H II (aitch two) from old spectroscopic terminology) in the parts of the Galactic interstellar medium visible from the Earths northern hemisphere (from the Wisconsin H-Alpha Mapper Survey) In astronomy, the interstellar medium (or ISM) is the matter (interstellar... For other uses, see Sun (disambiguation). ... Stellar parallax motion The parsec (symbol pc) is a unit of length used in astronomy. ...

Vertically, the molecular gas inhabits the narrow midplane of the galactic disc with a characteristic scale height of approximately 50–75 parsec, much thinner than the warm atomic (Z=130-400pc) and hot ionized (Z=1000pc) gaseous components of the ISM. ^[5] The exception to the ionized gas distribution are HII 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. To meet Wikipedias quality standards, this article or section may require cleanup. ... Properties In chemistry and physics, an atom (Greek ἄτομος or átomos meaning indivisible) is the smallest particle of a chemical element that retains its chemical properties. ... An ion is an atom or group of atoms that normally are electrically neutral and achieve their status as an ion by loss or addition of one or more electrons. ... NGC 604, a giant H II region in the Triangulum Galaxy. ... OB stars are hot, massive stars stars which form in loosely organized groups called OB associations. ...

This smooth distrubtion 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.^[2]

Types of Molecular Cloud

Giant Molecular Clouds (GMCs)

Vast assemblages of molecular gas with masses of 10⁴–10⁶ times the mass of the sun are called Giant molecular clouds (GMC). The clouds can reach tens of parsec in diameter and have an average density of 10²–10³ particles per cubic centimetre (the average density in the solar vincinty is one particle per cubic centimetre). Substructure within these clouds is a complex pattern of filaments, sheets, bubbles, and irregular clumps.^[4]

The densest parts of the filaments and clumps are called "molecular cores", whilst the densest molecular cores are, unsurprisingly, called "dense molecular cores" and have densities in excess of 10⁴–10⁶ particles per cubic centimeter. Observationally molecular cores are traced with carbon monoxide and dense cores are traced with ammonia. The concentration of dust within molecular cores is normally sufficient to block light from background stars such that they appear in silhouette as dark nebulae.^[6] Wikipedia does not yet have an article with this exact name. ...

GMCs are so large that "local" ones can cover a significant fraction of a constellation such that 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 around the sun called the Gould Belt. ^[7]The most massive collection of molecular clouds in the galaxy, the Sagittarius B2 complex, forms a ring around the galactic centre at a radius of 120 parsec. The Sagittarius region is chemically rich and is often used as an exemplar by astronomers searching for new molecules in interstellar space.^[8] A picture of Barnards Loop, which is a primary component of the nebula complex. ... There are very few or no other articles that link to this one. ...

Small Molecular Clouds

Main article: Bok globule

Isolated gravitationally bound small molecular clouds with masses less than a few hundred times the mass of the sun are called Bok globule. The densest parts of small molecular clouds are equivalent to the molecular cores found in GMCs and often included in the same studies. An image of Bok globules in the H II region IC 2944, taken with the WFPC2 instrument on the Hubble Space Telescope A Bok globule is a dark cloud of dense dust and gas in which star formation is sometimes taking place. ... An image of Bok globules in the H II region IC 2944, taken with the WFPC2 instrument on the Hubble Space Telescope A Bok globule is a dark cloud of dense dust and gas in which star formation is sometimes taking place. ...

High Latitude Diffuse Molecular Clouds

Main article: Infrared Cirrus

In 1984 IRAS identified a new type of diffuse molecular cloud.^[9] These were diffuse filamentary clouds that are visible at high galactic latitudes (looking out of the plain of the galactic disc). These clouds would have a typical density of 30 particles per cubic centimeter.^[10] Infrared Cirrus are filamentary structures seen in infrared light. ... 1984 (MCMLXXXIV) was a leap year starting on Sunday of the Gregorian calendar. ... The Infrared Astronomical Satellite (IRAS) was a space-based observatory that performed a survey of the entire sky at infrared wavelengths. ... The anisotropy of the star density in the night sky makes the galactic coordinate system very useful for coordinating surveys, both those which require high densities of stars (at low galactic latitudes) and those which require a low density of stars (at high galactic latitudes) Many galaxies, including the Milky...

Processes

Star Formation

Main article: Star Formation Star formation is the process by which dense parts of molecular clouds collapse into a ball of plasma to form a star. ...

To our knowledge, the creation of newborn stars in the current Universe occurs exclusively within molecular clouds. This is a natural consequence of their low temperatures and high densities, and of the 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 external pressure (like clouds in the sky). 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 Pleiades, an open cluster of stars in the constellation of Taurus. ... In physics, the virial theorem states that the average kinetic energy of a system of particles whose motions are bounded is given by where ri and Fi are the position and force vectors on the i th particle respectively. ...

Physics

The physics of molecular clouds are 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. In fluid dynamics, turbulence or turbulent flow is a flow regime characterized by chaotic, stochastic property changes. ... Magnetic lines of force of a bar magnet shown by iron filings on paper In physics, magnetism is one of the phenomena by which materials exert an attractive or repulsive force on other materials. ... It has been suggested that hypersonic be merged into this article or section. ...

Molecular clouds, and especially "Giant" molecular clouds (GMCs), are often the home of astronomical masers. A dark nebula is a large cloud which appears as star-poor regions where the dust of interstellar medium seems to be concentrated. ... An astrophysical maser is a naturally occurring source of stimulated spectral line emission, typically in the microwave portion of the electromagnetic spectrum. ...

References

1. ^ Craig Kulesa. Overview: Molecular Astrophysics and Star Formation. Research Projects. Retrieved on September 7, 2005.
2. ^ ^{a b} Ferriere, D. (2001). "The Interstellar Environment of our Galaxy.". Reviews of Modern Physics 73 (4): 1031-1066.
3. ^ Dame et al (1987). "A composite CO survey of the entire Milky Way". Astrophysical Journal 322: 706-720.
4. ^ ^{a b} Williams, J. P.; Blitz, L.; McKee, C. F., (2000). "The Structure and Evolution of Molecular Clouds: from Clumps to Cores to the IMF". Protostars and Planets IV, 97, Tucson: University of Arizona Press.
5. ^ Cox, D. 2005, The Three-Phase Interstellar Medium Revisited, Annual Reviews of Astronomy and Astrophysics, 43, 337-85
6. ^ Di Francesco, J., et al (2006). "An Observational Perspective of Low-Mass Dense Cores I: Internal Physical and Chemical Properties". Protostars and Planets V.
7. ^ Grenier (2004). "The Gould Belt, star formation, and the local interstellar medium". The Young Universe. [http://uk.arxiv.org/abs/astro-ph/0409096 Electronic preprint
8. ^ Sagittarius B2 and its Line of Sight
9. ^ Low et al (1984). "Infrared cirrus - New components of the extended infrared emission". Astrophysical Journal 278: L19-L22.
10. ^ Gillmon, K., and Shull, J.M. (2006). "Molecular Hydrogen in Infrared Cirrus". Astrophysical Journal 636: 908-915.