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How hot is the ionized gas in an H II region?

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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.

The ionized gas in a typical H II region has a temperature of approximately 8,000 K.1

This temperature is maintained by the ultraviolet radiation emitted by massive, hot central stars, which typically possess surface temperatures exceeding 20,000 K. When these stars ionize cold [molecular clouds]—where temperatures are roughly 25 K—the process raises the gas temperature to this 8,000 K level.1

It is important to note that H II regions are complex environments; while the "warm" ionized gas is at 8,000 K, these regions may also contain much hotter, tenuous material (reaching millions of degrees) produced by supernovae or stellar winds, as well as cold, neutral globules within the plasma that can be as cold as 100 K.1