Astronomy Principles and Practice Fourth Edition.pdf

Kirchhoff’s law 213
such a spectrum is provided in the visible region by a tungsten electric lamp filament. No clue is
given to the chemical composition of a body radiating a continuous spectrum. The energy–wavelength
distribution, however, gives a means of determining the body’s temperature.
Hot tenuous gases provide spectra with bright lines but with no continuous background. Emission
spectra, as they are called, may be used to identify the atoms and molecules in the source. Between
the emission lines, the spectrum generally displays gaps where there is no radiation present. Such
a gap acts as a transparent window. If a spectrometer were to be tuned to a zone of wavelengths
corresponding to a gap between emission lines, it would not receive radiation from the gas, which
would remain undetected and transparent. Any radiation emitted in that zone of wavelengths by an
object behind the gas would be detected by the spectrometer and the object would be ‘visible’ through
the gas.
In the laboratory, emission spectra are usually readily available in the form of discharge lamps
containing some selected gas at reduced pressure. Emission spectra of metallic elements can be
obtained by using electric arcs.
Absorption spectra may be produced as in Kirchhoff’s famous experiment. If light from a source
which would normally present a continuous spectrum is made to pass through a tenuous gas, then
energy may be removed from the wavelength positions which correspond to emission lines of the
particular gas. The amount of energy removed in the absorption lines—and hence the strength of these
features—increases if the gas is cooled.
The difference between a continuous spectrum and a line spectrum results from the circumstances
under which the atoms emit or absorb the radiation. It will be seen later in this chapter that spectral
lines are associated with the jumping of electrons within an atom from one discrete orbit to another.
In the case of gases at low pressure, the atoms are able to radiate and absorb without the influence of
neighbouring atoms and very sharp spectral lines are produced. An increase of the gas pressure causes
the spectral lines to broaden. For emission line spectra, the broadening increases with pressure until
lines overlap and merge to produce a continuous spectrum. In bodies which are liquid or solid, the
atoms are packed even closer together than for a gas at high pressure and they are unable to radiate
without being influenced by the neighbouring atoms. They, therefore, normally present a continuous
spectrum.
The application of Kirchhoff’s law and the principles relating to the understanding of laboratory
spectra immediately provides a simple explanation for the interpretation of stellar spectra. The most
common stellar spectra, as is the case of the solar spectrum, are absorption spectra. This suggests that
the stars have structure. The general wavelength–energy distribution curve must originate from some
body which must be a hot solid, liquid or dense gas and the absorption lines must be produced by
some surrounding cooler tenuous gas, being in effect the stellar atmosphere. This idea is summarized
pictorially in figure 15.2.
Thus, by correct interpretation of some of the possible basic measurements of solar light, we know
that its spectrum is produced by a hot dense gas which is surrounded by a cooler more tenuous layer.
From the general appearance of stellar spectra, it is not unreasonable to assume that the stars have this
same basic structure.
Immediately after Kirchhoff’s work, the problem of spectra identification was taken up by many
investigators and it was soon evident that, because of the many thousands of lines which were being
discovered, results by different workers could only be compared by using some definite standards. One
of the first sets of standards to be accepted was as a result of Ångström’s work on his Normal Solar
Spectrum; his wavelength values were given in terms of a known grating spacing to six significant
figures and expressed in units of 10 −8 cm. This unit has since been known as the ångström unit (Å).
In the remainder of this chapter, the basic physical concepts associated with the radiation
emanating from stars is presented with some indications as to how its collection and analysis leads
to basic information on these bodies.