HI and HII Regions

Ionized hydrogen is produced when the ultraviolet radiation emitted copiously by hot newly-formed stars ionizes surrounding clouds of gas. The characteristic beautiful red colors of nebulae like the Orion Nebula (M42) shown in the adjacent figure are produced by visible light emitted when electrons recombine with the ionized hydrogen in these regions.

Ionized Hydrogen

Such regions of ionized hydrogen are called HII regions, while cold un-ionized hydrogen clouds are termed HI regions (with the I and II referring to the ionization state of the hydrogen). As we have already seen, the disk and its spiral arms are heavily populated by both HI and HII regions.

Technically Speaking: Conditions for Ionizing Hydrogen

How hot does a star have to be to ionize large volumes of surrounding hydrogen? It takes 13.6 electon-Volts of energy to ionize a hydrogen atom, which implies a wavelength of 91.2 nm. Consulting the Planck Law for distribution of intensity with wavelength, this requires a temperature of about 25,000 K, which implies spectral class B1 or hotter.

Thus, as a rule of thumb we expect significant H II regions near B1 or hotter stars. Of course, for cooler stars there is a little radiation of sufficiently short wavelength to ionize hydrogen, but not much so we make little error by assuming B1 as the coolest spectral class star that can produce H II regions.

The Orion Nebula

The Orion Nebula is relatively nearby, about 1500 light years away in the same spiral arm of the galaxy as our own Sun. The image shown above is a mosaic of Hubble Space Telescope images showing the inner 2.5 light years of this large nebula (which is visible as the middle "star" in Orion's sword). As we have already seen, M42 is the location of many stellar nurseries where stars are being born. It contains many H II regions because there are many hot young stars embedded in its gas and dust.