The origin of it is free free emission meaning, scattering of electrons on protons in fully ionized plasma, that is it has temperatures of tens of millions of kelvin.

But we can detect it in absorption, because of its physical state absorption lines of elements like oxygen or they're ionized multiple times, should be present and from the atomic physics, we can figure out if we observe some of these lines, and their strengths, and ratios, what is the temperature, and the density or Coulomb density of the gas.