| Type | density (no. of atoms/cubic centimeter) |
Temperature (K) | Comments |
| ISM | 1 | larger than 3 | "empty space" |
| Filamentary Clouds | 10 | up to a few 1000 | - sufficient collisions for molecules to form |
| Diffuse IS Clouds | few 100 | 40 to few 100 | - have dust
- more atomic hydrogen (HI) than molecular hydrogen (H2) |
| Molecular Clouds (MC) | a few thousand to a million | about 10 | - many molecules form since high density 10-20 ly in diameter |