molecular clouds

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molecular clouds

Cool dense regions of interstellar matter within which atoms tend to be combined into molecules. The clouds are composed principally of molecular hydrogen (H2 ), with between 300 and 2000 molecules/cm3. There is also a small admixture of cosmic dust comprising about 1% of the mass, with gas temperatures between 10 and 20 K. Hydrogen molecules do not usually emit at radio and infrared wavelengths, and molecular clouds were discovered only in the mid-1970s in surveys of radio emission from carbon monoxide, CO, which is 10 000 times less common than hydrogen molecules in the clouds. The 2.6-and 1.33-mm CO lines are still the prime means of mapping and investigating the clouds. A wide variety of molecules, in addition to H2 and CO, have been found in the clouds (see molecular-line radio astronomy); more than 80 types have been detected in the largest clouds, such as Sagittarius B2.

Several dense core regions, with about 105 hydrogen molecules/cm3 and mass about 102 to 103 solar masses, may be found within one molecular cloud. In giant molecular clouds, these dense cores contain infrared sources, H II regions, maser sources, and the peak CO temperatures, which suggest that these regions are sites of massive star formation. There are also smaller clouds, containing about 500 solar masses of molecular hydrogen, throughout which low-mass stars are forming and which may be relatively isolated, as in the Taurus–Auriga star-forming region.

Collins Dictionary of Astronomy © Market House Books Ltd, 2006
References in periodicals archive ?
The maps provide unprecedented detail of the structure of the Orion A molecular cloud, the closest star-forming region of high-mass stars.
Nevertheless, the AME emissivity is comparable to the more robust detections on molecular clouds and diffuse cirrus, although on average it is lower; in some HII regions there are only upper limits on AME.
In some regions of the Milky Way, hydrogen and other elements gravitate together to form giant molecular clouds. One of the nearest examples is the Orion Molecular Cloud, whose dark cloak of dust and gas spans 100 light-years in southern Orion.
General topics include turbulence and cosmic ray transport (including the implications of the Hall Effect for turbulent molecular clouds), astrophysical flows (including fragmentation and turbulence in the interstellar medium), space plasma flows (including numerical simulations of solar wind disturbances by coupled models), kinetic and hybrid simulations (including an implicit particle-in-cell model) and a range of papers on data handling, visualization and computation.
The diffuse gas eventually collapses into denser molecular clouds, and from these solar systems eventually form," Ziurys explains.
STARS form in molecular clouds - dense concentrations of interstellar gas and dust which orbit the galaxy.
For example, it now appears that supernovas (exploding stars) create vast "hot bubbles," along with cosmic rays that "raise the pressure of the interstellar medium; higher pressures, in turn, compress the dense molecular clouds and increase the chance they will collapse [and form] stars." Oversized bubbles may extend all the way to the halo of the galaxy, each forming a kind of cosmic chimney that transports hot gases from its supernova to the outer reaches of the Milky Way, where the gases cool and rain back on the galaxy.
Molecular clouds of massive interstellar clouds within which the dust protects the water and other compounds from damaging radiation, and the conditions inside also promote chemical reactions.
Frozen films containing methane and oxygen used in these experiments further mimic a spacelike environment, since various types of ice (not just frozen water) form around dust grains in the dense and cold molecular clouds that exist in the interstellar medium.
Because molecular clouds are lively star-forming regions, it's not unusual to find bright nebulosities--emission nebulae, reflection nebulae, HerbigHaro objects--interrupting their deep reaches.
Chapters show how energetic particles first organized into atoms, then molecular clouds (the star factories), then protoplanetary disks and eventually the diverse residents of our planetary neighborhood including asteroids, Pluto and its plutino neighbors.

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