How can we know the density of particles in interstellar or . . .
The density of material in the interstellar medium is inferred from (i) the electromagnetic radiation it emits; (ii) its effect on electromagnetic radiation passing through it. Often these approaches are combined to learn about different "phases" of the interstellar medium - e. g. hot, cold, high, or low density, ionised or not. For example, we can learn about how much atomic hydrogen there is . . .
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