How do H II regions form around hot stars?

H II regions form when a massive, hot star—typically with a surface temperature exceeding 20,000 K—begins its life within a molecular cloud. This star emits copious amounts of ultraviolet radiation, which interacts with the surrounding cold, neutral gas.12

The process occurs in two primary stages:
* Dissociation and Ionization: The ultraviolet radiation first converts molecular hydrogen ($H_2$) into atomic hydrogen and subsequently ionizes it into high-pressure, ionized hydrogen ($H^{+}$).2
* Pressure Expansion: This ionization significantly increases the temperature of the gas—from roughly 25 K to 8,000 K—and increases the number of particles per cubic centimeter. Because gas pressure is proportional to the product of temperature and particle density, the pressure within the newly formed H II region becomes hundreds of times greater than that of the surrounding cold gas. This pressure imbalance causes the ionized gas to expand violently into the dense molecular cloud.1

In their earliest, densest stages, these are known as "ultracompact" H II regions, which are often observed via radio and infrared radiation due to their immersion in thick dust.2