| definition |
A type of emission nebulae composed of very hot gas (about 10^4 K), mainly ionized
hydrogen, created by the ultraviolet radiation of massive stars. H II regions originate
when O or early-type stars, born in giant molecular clouds, start heating up the cold
gas, causing it to become ionized and 'glow'. The effective temperatures of the exciting
stars are in the range 3 x 10^4 to 5 x 10^4 K, and throughout the nebula hydrogen
is ionized. Helium is singly ionized, and other elements are mostly singly or doubly
ionized. Typical densities in the H II region are of the order 10 to 10^2 cm^-3, ranging
as high as 10^4 cm^-3. Internal motions occur in the gas with velocities of order
10 km s^-1. The spectra of H II regions are mainly composed of strong H I recombination
lines and forbidden lines such as [O III], [O II], [N II].
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