Emission Nebulae: H II Regions
In many areas of space hydrogen does not occur as neutral atoms but is ionized. This is particularly true if the cloud surrounds a hot star which radiates largely in the ultraviolet region of the spectrum. If there is enough hydrogen surrounding the star then this region will be visible as an emission nebula. An example of an emission nebula is shown below:
Unlike a star, the spectrum of a cloud of ionized gas is dominated by a few narrow emission lines. The continuous spectrum of H II regions is relatively weak whilst the Balmer emission lines in the visible region are particularly clear. These are formed when a hydrogen atom recombines into an excited state and then returns to the ground state via a sequence of radiative transitions. A typical hydrogen atom in an H II region can remain ionized for several hundreds of years.
Whilst the majority of interstellar gas is hydrogen there are other molecules in these clouds. For example there is also a large amount of helium. More energy is required to ionize helium than hydrogen but around the hottest stars there will be regions of ionized helium as well as ionized hydrogen. Although the most common species in these clouds is hydrogen it does not mean that their emission lines will be the strongest. At the beginning of this century it was proposed that some of he strong unidentified emission lines observed were due to forbidden lines of O+, O2+ and N+.
We can use some of the information obtained from these spectra to determine the physical properties of these gas clouds. For example the ionization of hydrogen steadily propagates away from the star, however as neutral hydrogen absorbs so efficiently in the ultraviolet, there is a sharp boundary between the H II region an the neutral gas. We also know that the temperature of an H II region is greater than that of he surrounding gas so they tend to expand, until they eventually become so diffuse they merge with the general interstellar medium.