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Thread: Molecular cloud collapse

  1. #1
    Join Date
    Dec 2011
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    Molecular cloud collapse

    I was just reading a page in Wikipedia and it said molecular clouds are in a state of hydrostatic equilibrium until something triggers a collapse. Is this correct?

    I always assumed these vase objects ‘slowly’ form from much larger more defuse clouds, but at these scales ‘slowly’ means material travelling many light-years at tens if not hundreds of km/s.
    As the speed of sound in molecular clouds is only about 0.2km/s these inflows are nearly always supersonic.

    How does a cloud get into a state of hydrostatic equilibrium when pressure waves can’t travel back up a supersonic stream to slow it down?
    Once a supersonic flow starts you need to put something very solid in its path to stop it, as pressure waves can’t travel back up the flow all the energy arrives at the point of impact.

    I just thought supersonic hot plasma & dust flowed in, got compressed as it punched its way through and supersonic Bok globules and protostars came out.

    I just can’t find anything on how a cloud of gas formed from supersonic flows stops the flows to enter an equilibrium state and then later produces supersonic protstars etc with cloud escape velocity later.
    See: 'The God Kit' -- 'The Brigadier And The Pit' -- Carl N Graham -- Sci-fi blog: The Alien Reporter

  2. #2
    Join Date
    Oct 2005
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    26,158
    Although this is a simplification, the basic idea is that molecular clouds build up in hydrostatic equilibrium until their gravity gets strong enough to cause a collapse that is more or less an internal free-fall. So the infall that creates stars becomes supersonic, but by then the molecular cloud is not in hydrostatic equilibrium. Then the more massive stars start to supernova, and the molecular cloud gets ripped to shreds, again supersonically.

  3. #3
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    I see what you mean about a simplification.
    There seems to be a large spread of views and interpretations of available evidence.

    Still unclear if whole molecular clouds truly reach hydrostatic equilibrium or stay in that state for any significant time?
    However, it looks common to assume hydrostatic equilibrium as starting point for modelling collapse.

    There seems to be evidence from back tracking stars to point of origin, galaxy interactions, etc, etc, that time from cloud formation to star formation with cloud dissipating can be relatively short.
    This appears to leave little or no time for the cloud to reach any form of equilibrium?

    Some clouds show evidence of cloud migration?
    Parts of the cloud are collapsing or have formed stars in the past, while other parts are bing inflated, probably by high velocity inflows.

    There looks to be both evidence for the collapse in star forming regions travelling faster than the speed of sound, but the speed of sound limiting the speed the collapse spreads outwards?

    Is it best to assumer molecular clouds show a wide range of behaviours or is some of the above just wrong?
    See: 'The God Kit' -- 'The Brigadier And The Pit' -- Carl N Graham -- Sci-fi blog: The Alien Reporter

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