Table 1: Physical parameters for different types of HII regions.

Class Size Density EM
(pc) (cm−3) (cm−6 pc)

Hypercompact ≤0.03 ≥106 ≥1010
Ultracompact ≤0.1 ≥104 ≥107
Compact ≤0.5 ≥5 × 103 ≥107
Classical ~10 ~100 ~102