How hot must a star be to ionize hydrogen 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.

To ionize hydrogen in an H II region, a central star must typically have a surface temperature in excess of 20,000 K.1

This process occurs because neutral hydrogen atoms are highly efficient at absorbing ionizing radiation, specifically photons with an energy of at least 13.6 electron volts. Stars that are sufficiently hot emit the necessary ultraviolet radiation to convert surrounding molecular hydrogen into atomic hydrogen and subsequently ionize it into high-pressure H+ plasma.23