Emission and Absorption Spectra
More
accurately, a gas cloud is only opaque within spectral lines, while a star is
opaque at all wavelengths. The brightness
of each depends on the usual T4 relation. If, as is usually the case, the cloud is colder than the star (or
the star’s atmosphere is colder than its surface),
then an absorption line spectrum is produced.
If one looks only at the cloud, the background (empty
space) is even colder, so you always get an emission line
spectrum. If you look at a cloud through a hotter cloud of gas, you will get an emission line spectrum which
includes a continuum.
