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How do H II regions differ from molecular clouds?

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H II region Centre of the Orion Nebula (M42). Astronomers have identified some 700 young stars in this 2.5-light-year-wide area. They have also detected over 150 protoplanetary disks, or proplyds, which are believed to be embryonic solar systems that will eventually form planets. These stars and proplyds generate most of the nebula's light. This picture is a mosaic combining 45 images taken by the Hubble Space Telescope.

H II regions and molecular clouds differ primarily in their physical state, temperature, and composition, though they are closely linked in the lifecycle of stars.

Physical State and Composition: An H II region consists of interstellar matter where hydrogen is ionized by the intense ultraviolet radiation of hot, massive stars. In contrast, molecular clouds are opaque, cold regions where hydrogen exists primarily in the form of H₂ molecules.123
Temperature: H II regions are relatively hot, with temperatures typically around 8,000 K. Molecular clouds are significantly colder, with internal temperatures ranging from 7 to 15 K.13
Relationship and Evolution: Molecular clouds serve as the birthplaces of stars. When a massive star forms within a molecular cloud, its radiation ionizes the surrounding gas, transforming it into an H II region. This process increases the temperature and particle density, creating high pressure that causes the H II region to expand into the surrounding cold molecular gas.312

While H II regions are often found on the borders of molecular clouds, the latter are characterized by their dust content and complex chemistry, which remain hidden from visible light but detectable via infrared and radio observations.132