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Thread: Differentiation of star formation

  1. #1 Differentiation of star formation 
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    Why is it that some nebulae collapsed to form stars and others did not? As we know, the distribution of mass in the universe is anisotropic, for reasons that I can imagine, but why would some nebulae completely collapse to form stars while others still largely remain "uncollapsed"? I would think they would all roughly at the same time convert all of their mass into stars, but that is clearly not the case. Does it have anything to do with variation of temperature in space? Perhaps variation in elemental content? Apparently Helium and Lithium were formed in space during the very early universe....maybe uneven distribution of those elements cause variations in how readily nebulae form stars?


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    Moderator Moderator Janus's Avatar
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    It mostly comes down to two factors:
    Outside influence and density

    In order for star formation to occur, the nebula has to be "stirred" up from outside. Otherwise, the molecules are perfectly happy orbting each other. A nearby supernove can disturb the cloud enough to form dense pockets from which stars can form.

    But even with outside influence, some clouds are just too thin to form pockets dense enough to intiate star formation even when they have been disturbed.


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    External triggers are reasonable, and there are many examples that suggest such a scenario, but the final confirmation is still to be found.

    Mostly, the triggers for collapse are found within the clouds in the shape of turbulence. The drivers of these turbulences are still under investigation, but it seems clear that internal sources like outflows from already forming stars are too weak. So, scientists are looking into collisions of clouds or the interaction with the friction and tidal forces of the galaxy in spiral arms.

    The most simple law that governs the onset of collapse is the Jeans law, where the equilibrium of internal pressure (thermal, i.e. temperature distribution is crucial) and self gravity plays the main role. Nowadays additional forces like magnetic fields, external pressure, heating and cooling are also considered. But the central point is that one needs a dense enough core that produces enough self gravity to begin the collapse. The general idea is that random fluctuations in clouds can produce denser areas that fulfil this criterion.

    The numerous starless cores like B 68 are just not dense enough to produce such a gravitational force.
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    Universal Mind John Galt's Avatar
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    In short, chaos and catastrophe theory reign supreme?
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    Hmmm....interesting. I thought about the orbits of gas within nebulae preventing a collapse, but I didn't think those orbits even formed until a collapse had begun.

    In time, won't all of the thinner regions of gas agglutinate together via gravity to form dense enough pockets of gas for star formation to occur? I find it interesting that 13.5 billion years later we still have things like the interstellar medium just sitting there without coming together to form more significant clusters.

    Is there any way we currently possess to quantify the "star making power" of a nebula? The more mass one has, the greater the pressure that can be exerted and therefore the greater the density....that would increase the rate of star production, yet at the same time, the larger amount of gas would take longer to be converted into stars; it seems like at a certain point more mass no longer expedites the process but rather just retains an equilibrium of production. As for turbulence, isn't this factor essentially probabilistic? Won't most gas clouds demonstrate the same average rate of turbulence over billions of years?
    Of all the wonders in the universe, none is likely more fascinating and complicated than human nature.

    "Two things are infinite: the universe and human stupidity; and I'm not sure about the universe."

    "Great spirits have always found violent opposition from mediocrities. The latter cannot understand it when a man does not thoughtlessly submit to hereditary prejudices but honestly and courageously uses his intelligence"

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    Quote Originally Posted by Cold Fusion
    Hmmm....interesting. I thought about the orbits of gas within nebulae preventing a collapse, but I didn't think those orbits even formed until a collapse had begun.
    Could you please explain, what you mean with "orbits of gas"? The motion in pre-stellar clouds and clumps is very much random.
    Quote Originally Posted by Cold Fusion
    In time, won't all of the thinner regions of gas agglutinate together via gravity to form dense enough pockets of gas for star formation to occur? I find it interesting that 13.5 billion years later we still have things like the interstellar medium just sitting there without coming together to form more significant clusters.
    Not all matter in clouds is transformed into stars. In fact, it is usually only a small fraction. The rest is dispersed as soon as the first stars form. Their intense wind and radiation pressure can prevent the formation of additional stars and stop or reduce the accretion process by which the protostar constantly gains mass. In addition, interstellar matter is replenished all the time. Although the key ingredient hydrogen is primordial, a lot of other stuff that is needed to form stars was produced by previous stellar generations. Motions on galactic scales sweep up diluted material that form new clouds.
    Quote Originally Posted by Cold Fusion
    Is there any way we currently possess to quantify the "star making power" of a nebula? The more mass one has, the greater the pressure that can be exerted and therefore the greater the density....that would increase the rate of star production, yet at the same time, the larger amount of gas would take longer to be converted into stars; it seems like at a certain point more mass no longer expedites the process but rather just retains an equilibrium of production. As for turbulence, isn't this factor essentially probabilistic? Won't most gas clouds demonstrate the same average rate of turbulence over billions of years?
    These are all good questions, and precise answers cannot yet be given. One still does not know exactly, how stellar clusters form. There are a few scenarios that are being investigated. Yes, turbulence is probabilistic. And maybe this is exactly the reason for the uniformity of the Initial Mass Function. This is the mass distribution of forming stars. It seems that the mass distribution is independent of the parental cloud, as long as the volume and total mass of the entire cloud is big enough.
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