I have to calculate the number of degrees of freedom within an HII region. I am asked to find the number of degrees of freedom before and after the hydrogen gas gets ionized by the star and forms an HII region. Does each particle have 3 degrees of freedom? I am given the number of ionizing photons per second, the temperature of the HII region, and the density of molecular hydrogen. I have calculated the stromgren radius. To find the number of degrees of freedom in the resulting HII region, do I just multiply the volume of the region times the molecular hydrogen density and then multiply by 6 to take into account the fact that the hydrogen atom gets decomposed into protons and electrons?? I have no idea how to find the number of degrees of freedom in the molecular hydrogen gas before it gets ionized. What volume would I use?