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What kind of stars provide the ultraviolet radiation that ionizes hydrogen in H II regions?

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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 are ionized by massive, hot stars that possess surface temperatures exceeding 20,000 K. These regions are powered either by individual massive stars or by clusters of such stars. Specifically, O-type stars are frequently identified as the primary sources of the intense ultraviolet radiation required to convert molecular hydrogen into atomic hydrogen and subsequently ionize it into high-pressure H+ plasma.12