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Interstellar gas contains mainly Hydrogen and Helium, whose number ratio is approximately 10:1. Obtain the value of average molecular weight for the fully ionized interstellar gas with temperature $T=10^4$K (components are ionized H$^+$ (HII) and He$^{+2}$ (HeIII) and electron e$^{-1}$). How about the molecular gas ($T=10$K) containing molecular H$_2$, neutral He (HeI) and no electron?

Kohji Tomisaka 2007-07-08