| 9 | | '''Warm nuclear medium''' - |
| | 9 | '''Molecular clouds''' - Traced by CO, held together by their own self-gravity rather than being in pressure equilibrium with ambient. Typically about 10 K, 40 pc across, 200 particles/cm^3^, 400,000 solar masses. Mostly made up of H2, but this is hard to see in transition lines and is instead inferred from CO detection. MCs make up ~ 1% of the filling fraction of the ISM, but contains about 30-60% of the mass. |
| | 10 | |
| | 11 | |
| | 12 | '''Warm nuclear medium''' - Traced by 21 cm line of hydrogen (emitted by neutral hydrogen atom), ~800 K, is intercloud gas. Has a very low density (~0.5 / cm^3^), but a substantial filling fraction (30-60%). |
| | 13 | |
| | 14 | |
| | 15 | '''Cold nuclear medium''' - Also traced by 21 cm line of hydrogen. Is about 100 K and organized in diffuse HI clouds. Occupies about 1-4% the total volume, and on average a cloud is about 10 pc across and has a density of 50 / cm^3^. |
