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Why is some hot gas inside H II regions detectable mainly in X-rays?

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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.

Within an H II region, the hot, tenuous gas that exists between the visible filaments is typically several hundred times less dense than the surrounding "warm" (8,000 K) gas. Because of this extremely low density, the material does not emit enough radiation to be observed through standard optical methods. Instead, this hot component is detectable primarily through X-rays, which are capable of revealing the presence of such high-temperature, low-density matter.1