Astronomy Principles and Practice Fourth Edition.pdf

212 The radiation laws
Table 15.1. The strong Fraunhofer lines.
Fraunhofer line Å Element
A 7594 Oxygen in Earth’s atmosphere
B 6867 Oxygen in Earth’s atmosphere
C 6563 Hydrogen (Hα)
D 1 5896 Sodium
D 2 5890 Sodium
E 5270 Iron
F 4861 Hydrogen (Hβ)
G 4340 Hydrogen (Hγ )
H 3968 Calcium
K 3933 Calcium
the constant being independent of the type of body under consideration. The actual value of the
constant evaded an evaluation by theory until the development of the quantum theory, which at the
same time gave an explanation of how individual atoms radiate.
It follows from Kirchhoff’s law that a body which is a good emitter of radiation must
correspondingly be a good absorber. Simple confirmation of this is given by the appearance of the
Sun. Being a good emitter of radiation, the Sun’s material also absorbs radiation efficiently. Radiation
which is generated at any depth inside the Sun is absorbed before travelling any appreciable distance.
Consequently, we are only able to see radiation which is emitted in the Sun’s outermost layers. Thus,
the Sun appears to have a sharp edge and we cannot see to any depth into its interior.
Kirchhoff’s law also opened the door to modern spectrometry whereby atoms and molecules are
identified by their spectra. By the law’s principle, a gas which produces bright spectral lines should
also, at the same temperature, be able to absorb energy at these wavelength positions from a beam of
light which would otherwise have produced a continuous spectrum. In his now famous experiment,
Kirchhoff passed a beam of sunlight through a flame containing common salt (sodium chloride) and
found that the yellow emission lines of the flame coincided exactly with the Fraunhofer D lines and that
the sodium flame enhanced the absorption feature. He put forward the explanation that the Fraunhofer
lines in the solar spectrum are caused by the absorption by the elements in the outer, cooler region of
the Sun of the continuous spectrum emitted by its hot interior.
The idea that each element gives rise to a characteristic spectrum rapidly gained momentum from
this point. In partnership with Bunsen, Kirchhoff continued his experiments by investigating the spectra
of other metals in their purest form then available and was able to identify most of them in the solar
spectrum. The elements corresponding to the strong Fraunhofer lines are presented in table 15.1.
Spectral analysis thus opened the way to performing analytical chemistry at a distance. In
particular, the methods could be applied to all astronomical bodies and the problem of determining
the composition of stars—a problem which had been said to be impossible by the philosopher, Comte,
only a few years previously—was now realistically solved.
The science of spectrometry has now, in fact, developed further than this in that, by detailed study
of the positions and shapes of spectral lines, it is possible to ascertain physical parameters of a source
such as its speed and the temperature and density of its materials.
Very broadly, a laboratory spectrum may be considered to be one of three types, although, on
some occasions, it cannot be categorized simply, i.e. there is no sharp division between the three types.
Many solids, liquids and very dense gases are opaque to all optical radiation and, therefore,
according to Kirchhoff’s law, provide spectra which are continuous. A continuous spectrum is one
which provides a continuous distribution of energy through the observed wavelengths and, for example,