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Giant Molecular Cloud

Page history last edited by PBworks 13 years, 11 months ago

Giant Molecular Clouds (GMCs)

A dark nebula is a large molecular cloud, the largest of which are termed GMCs. They appear as star-poor regions where the dust of the interstellar medium seems to be concentrated.


Dark nebulae can be seen if they obscure part of an emission or reflection nebula (eg. the Horsehead Nebula) or if they block out background stars (eg. the Coalsack Nebula).


The form of such dark clouds is very irregular: they have no clearly defined outer boundaries and sometimes take on convoluted serpentine shapes. The largest dark nebulae are visible to the naked eye, appearing as dark patches against the brighter background of the Milky Way.



  • Primarily molecular dihydrogen
  • can be over 1 million solar mases
  • up to 150 light years across
  • avg density: 100-300 molecules/cm^3
  • internal temperature: 7-15K
  • clouds have an internal magnetic field that provides support against their own gravity


Star Formation

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 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.


Molecular clouds, and especially "Giant" molecular clouds (GMCs), are often the home of astronomical masers.

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