Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Intergalactic gas temperature

  1. Feb 26, 2012 #1
    I understand that intergalactic gas is a strong source of X-rays due to high temperature of upto 10^8K.
    If this gas is also known to be of very low density, will it 'feel' hot?
    I am a little confused, because I understand that densities can be so low as to produce forbidden spectral lines.. So how could such low densities impart heat to a more solid object .. Thanks
  2. jcsd
  3. Feb 27, 2012 #2
    it really wouldn't feel hot at all. According to wikipedia, there's about 10 to 100 hydrogen atoms per cubic meter.

    Compare that to a cubic meter of water with 3.345x10^28 molecules.
  4. Feb 27, 2012 #3


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    The temperature just indicates the mean of the statistical distribution of particle kinetic energies in the gas. Since the temperature is so high, it means that, on average, the particles in the gas are moving around pretty fast.

    You are right that, due to the very low density (and hence the very low frequency of collisions with your body), this gas would be very ineffective at heating you up by means of conduction. I think that how hot something "feels" depends on how well it can transfer heat to you by conduction.

    You still probably wouldn't want to go there. Those X-rays are ionizing radiation, which is very dangerous. (Of course, I'm sure you are already aware that X-rays are dangerous). I wonder, also, if the individual gas particles themselves might not have enough energy to cause damage upon collision with your molecules. The average particle energy seems to be just under 10 keV, which sounds like it could be well into the ionizing regime.
  5. Feb 27, 2012 #4
    That is an interesting question. How warm you feel is based upon the body temperature of the individual and how much heat it absorbs or confers to the surrounding environment.

    The interstellar medium will "warm you up" (it will transfer more energy to you than vice-versa), however, it will not warm you up quickly enough to make up for the huge amount of energy you are radiating into space.

    Think about it. The human body probably generates about 100 Watts of power in base metabolism. At maybe 2 m^2 of surface area, from the Stefan–Boltzmann law we know that you would be losing about 1000 W of power due to black body radiation, so effectively you would be losing about 54000 J of energy per minute, or about 130 C. That means that for every minute you spent in the interstellar medium, your total body temperature would decrease by maybe half a degree (assuming that your body doubled or tippled its metabolism to attempt to compensate and assuming your body is 100% H2O).

    I am not sure about the internal thermodynamics of the human body in a vacuum, but your skin would probably get a lot colder a lot faster than the rest of your body. Since your cold receptors are in your skin, I would imagine this would feel extremely cold, and evaporation of the moisture on the skin would make your skin cool down even more quickly than radiation alone.

    So yes, the interstellar medium may be "hot" but you would feel very cold.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook