202 13. EQUILIBRIUM
the process of photoionization also heats the gas, typically to a temperature of about 10;000 K.
Left behind is a hydrogen nucleus without its electron, an H II ion.
While the high-energy photons from the hot Trapezium stars are knocking electrons off
neutral hydrogen atoms, previously freed electrons are finding random H II ions and rejoining
them to make neutral atoms again; this is the process of recombination. For the gas as a whole,
photoionization and recombination are in an equilbrium, and both processes occur at the same
rate.
When an electron recombines with an H II ion, it usually joins up with the atom in an
excited state, rather than joining directly to the ground state. is means that the process of
photoionization followed by recombination produces, in a roundabout way, atoms in an excited
state. e electrons in these excited states then spontaneously drop to lower energy levels, emit-
ting photons as they go. ey eventually arrive at the ground state, unless a high energy photon
from the Trapezium comes along first, and reionizes the atom.
And so a photoionized gas emits light—and it emits wavelengths related to all of the
possible differences in energy levels of the atom. For hydrogen gas, all of the possible changes
in energy result in either ultraviolet or infrared lines visible only from space, with the exception
of the Balmer lines.
But in a nebula, these wavelengths are emitted, rather than absorbed. And so the gas glows
with an emission or bright-line spectrum. Since the brightest of the hydrogen lines is usually
red H˛ at 656 nm, such an emission nebula often appears red in a long-exposure photograph.
13.3 REFERENCES
Bradley W. Carroll and Dale A. Ostlie. An Introduction to Modern Astrophysics, 2nd ed., Cam-
bridge University Press, 2017. DOI: 10.1017/9781108380980 194
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