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Interstellar Speed Of Sound

  1. Apr 5, 2013 #1


    Staff: Mentor

    I understand statistical mechanics. I understand that PV=nRT has no lower bound on density. Yet I'm bugged by the thought that the densities in interstellar (or even intergalactic) nebulas are so low that most particles will never experience any collisions at all, thus undermining the premise of statistical mechanics.

    I understand that some of the particles are charged, but dust clumps of molecules are neutral.

    I looked up the speed of sound in space (300 km/s), I see pictures on APOD clearly showing shock waves in planetary nebula or supernova remnants, or at the boundaries of the solar system where Voyager is heading. Some shock waves seem driven by radiation pressure, others not.

    On the other hand galaxies pass through each other all the time with seemingly no effect other than gravity. The other day on APOD they even showed the galaxy Messier 64 and said that it is two concentric counter rotating galaxies coexisting. How the heck can that happen without lots of violent collisions between nebula?

    The visible evidence seems contradictory. It bugs me; my left brain says the particles collide frequently, my right brain says most particles never collide with anything.

    Here's my question. Are ordinary statistical mechanics sufficient to describe interstellar and intergalactic clouds of gas and dust, or is there something big I'm missing?
  2. jcsd
  3. Apr 5, 2013 #2
    Your right that the probability of collisions or nebulae is extremely low but it sounds like you are having trouble understanding why.

    Lets take a hypothetical collision for simplicity we will ignore dust. Set the average distance between stars at 1 light year apart for both galaxies. (this is far denser than average).
    Knowing this calculate how much space is between two stars. Once you perform this calculation you will quickly realize the diameter of a star would be like a grain of sand on a football field. Two football regions cross each other what would you think the likely hood of two grains of sand colliding?
    then consider that nebulae form stars with those seperation distances. Lets just say even
    particles in a nebulae have a lot
    of room between particles.
    Here is a good experiment to see how dust interacts. Take a handful of talcum powder and have a friend do the same. Stand one meter apart. Throw the two handfuls towards each other. You may have to stand closer.
    How many particles do you see
    collide? Their might be a few but it would be so miniscule it would be unmeasurable.
    Last edited: Apr 5, 2013
  4. Apr 5, 2013 #3


    Staff: Mentor

    Thank you Modred. But is I start with the very sparse mental model I have a hard time visualizing how intersecting clouds could cause shock waves in each other.

    Clearly, the stream of solar particles causes shock waves at the edge of the solar system, and nova ejecta cause shock waves.

    The talcum powder analogy illustrates the non-interaction of sparse clouds, but in my mind shock waves illustrate the strongest imaginable kind of interaction.
  5. Apr 5, 2013 #4
    Interstellar shockwaves have a few different sources or reasons. Some from supernovae, or explosions, some from interactions with the solar winds there are othre causes as well.

    here is one article bom Berkely that discusses some of the principles and mathematics.


    if you look up interstellar shockwaves you can find numerous related articles
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