Howdie. This question started from what was to be a "simple" analysis of heating the air in a room. So much for simplicity when you start to question what exactly is heat and what is happening to atoms and molecules. SOOOO...the question I have (which will have bearing on my INITIAL quest), is this. Does anyone have data for range of density vs. range of temperature for the following Interstellar Medium objects? 1. Cold, dark nebulae 2. Diffuse nebulae 3. Ionized gas 4. Plasma In my web searches I get vague answers for densities and temperatures but never any rough correlation between the two. Thanks for any responses!