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Why are H II regions usually found near young massive stars?

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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 found near young, massive stars because these stars are the primary source of the intense energy required to create and maintain the ionized state of the gas.

Specifically, a massive star with a surface temperature exceeding 20,000 K produces copious amounts of ultraviolet radiation. This radiation is necessary to strip electrons from hydrogen atoms, a process known as ionization, which converts neutral hydrogen into the ionized plasma that defines an H II region.12

Because these massive stars have relatively short lifespans on an astronomical timescale, H II regions are typically associated with areas of active star formation, such as molecular clouds, where these hot, luminous stars have recently emerged.12