Fundamental Astronomy

◭ Fig. 15.18. The great nebula in Orion (M42, NGC 1976). The
nebula gets its energy from newly formed hot stars. The dark
regions are opaque dust clouds in front of the nebula. Radio
and infrared observations have revealed a rich molecular cloud
behind the nebula (Fig. 15.20). In the upper part of this picture
is the gas nebula NGC 1977, in the lower part the bright star
ι Orionis. (Lick Observatory)
In completely ionized hydrogen, ne = ni, and hence
nrec ∝ n 2 e . (15.22)
Most recombinations will include the transition n =
3 → 2, i. e. will lead to the emission of a Hα photon.
Fig. 15.19. The Lagoon nebula (M8, NGC 6523) in Sagittarius.
This H II region contains many stars of early spectral
types and stars that are still contracting towards the main
sequence. Small, round dark nebulae, globules, are also vis-
15.2 Interstellar Gas
Thus the surface brightness of a nebula in the Hα line
will be proportional to the emission measure,
EM =
n 2 e
dl , (15.23)
where the integral is along the line of sight through the
nebula.
The ionization of a helium atom requires more energy
than that of a hydrogen atom, and thus regions of ionized
helium are formed only around the hottest stars. In these
cases, a large H II region will surround a smaller central
ible against the bright background. These are presumably
gas clouds in the process of condensation into stars. (National
Optical Astronomy Observatories, Kitt Peak National
Observatory)
325