H II region
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H II regionA region of predominantly ionized hydrogen in interstellar space, existing mainly in discrete clouds. The ionization is usually caused by photoionization by ultraviolet photons in regions of recent star formation, but cosmic rays, X-rays, or shock waves in the medium may sometimes be responsible. In comparison with the 21-cm radio emission of neutral hydrogen in H I regions, the ionized hydrogen of H II regions emits radio waves by bremsstrahlung (thermal emission) and recombination line emission; the ionized hydrogen also emits recombination lines in the infrared, ultraviolet, and optical, the latter making an H II region appear as an emission nebula. Younger H II regions are often roughly spherical with a sharply delineated boundary (see Strömgren sphere). Their size is usually less than 200 parsecs, the largest being relatively constant in diameter from galaxy to galaxy. By studying the apparent diameters of the H II regions in a distant galaxy, the distance to the galaxy can be estimated. See also extragalactic H II region; interstellar medium.
H II region[¦āch ′tü ‚rē·jən]
A region of interstellar space occupied by gas that is largely atomic hydrogen and mostly ionized.