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Physics of the ISM

  1. Jan 18, 2015 #1
    1. The problem statement, all variables and given/known data
    Answer the following questions. Pleas show all of your work and your line of thinking and state your assumptions.

    There are several phases of the ISM. Consider the following three phases with different typical number densities ## n ## and temperatures ## T ##.
    • The cold neutral medium (CNM) with ## n \simeq 10 cm^{-3} ## and ##T \simeq 100 K##;
    • The warm ionized medium (WIM) with ## n \simeq 0.1 cm^{-3} ## and ##T \simeq 10^4 K##;
    • The hot ionized medium (HIM) with ## n \simeq 10^{-3} cm^{-3} ## and ##T \simeq 10^6 K##;
    (a) Compute the pressure of the three phases assuming the ideal gas law, and show that they are all equal. Describe the significance of this in a few sentences.

    (b) If the gas in a galaxy were equally distributed between the phases so that 1/3 of the mass were in each of the three phases above, what would be the relative volumes filled by the three phases?

    (c) For the Milky Way, half of the mass is thought to be in the warm phase, and it also resides in half of the volume of the galaxy. What are the relative volumes and masses of the other two phases?

    2. Relevant equations
    Ideal gas law
    ##PV = NRT##
    ##n = \frac{N}{V}##
    ##V_{total} = V_{CNM} + V_{WIM} + V_{HIM} ##

    3. The attempt at a solution
    Part (a) was very simple to do. I found the pressure in each to be ## 8314 \frac{J}{cm^3 \cdot mol} ##. For whatever reason, I cannot figure out part b, and I feel that c will easily follow. I've tried assuming the mass of the Milky Way, dividing it into thirds, and then using the molar mass of each phase of the ISM to find the relative volume of each phase. The problem is, the entire mass of the Milky Way is not in gaseous form for one, and another thing is that the CNM is mostly neutral hydrogen and the WIM is mostly singly ionized hydrogen. I can easily calculate the number of atoms of each necessary to make up their respective 1/3 of the mass of the galaxy, but the rest of the mass coming from the HIM is made up of everything else, so I can't calculate the number of particles in the HIM to find the amount of volume it occupies.
  2. jcsd
  3. Jan 19, 2015 #2


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    Staff: Mentor

    (a) Your pressure has a strange unit (especially with the mol). Why don't you calculate it in Pa?

    (b) If you have 1kg of cold, 1kg of warm and 1kg of hot medium, what is the total volume they will occupy? Can you scale this, if necessary?
    (c) can be solved similar to (b), right.
  4. Jan 19, 2015 #3
    I converted my answer for pressure. I apologize, I simply forgot to do that before. I got that the pressure for each, assuming each follows the ideal gas law, would be about ## 1.38 \times 10^{-14} Pa ##, which is right around the vacuum pressure out in space. For part (b), I made the assumption that number of molecules in each phase was the same, and I used that to calculate their relative volumes. I got a total volume that added up to 0.9999, so, about 99.99% of the total volume of a galaxy with such a distribution was represented (assuming that the volume of all three added to 1, where 1 is the normalized volume of whatever galaxy you're working with). I'm trying to solve these using only the information provided in the assignment proper.

    I must be missing something important, or I'm an idiot, because I'm still having trouble with (c).
  5. Jan 20, 2015 #4


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    Staff: Mentor

    I don't understand your description of (b). What does the 0.9999 mean and how did you get it?

    What did you try for (c)?
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