Kirchhoff's Laws and Spectral Analysis

Gustav Kirchhoff (1824-1887) formulated empirical rules for spectroscopic analysis. We can place these in a physical context:

1. A hot dense gas or solid object produces a continuous spectrum with no dark spectral lines.

It radiates as a black body of temperature T, according to the Planck function.
2. A hot diffuse gas produces bright emission lines.
Emission lines occur when electrons move from a higher orbit to a lower orbit in the atoms, emitting a photon of exactly the right wavelength that corresponds to the energy difference in the orbits.

Example: A star-forming gas cloud in a distant galaxy (courtesy Stacy McGaugh)

3. A cool diffuse gas in front of a continuous source produces dark lines in the continuous spectrum.
Light of a wavelength that corresponds to energy differences in atomic orbits will be absorbed, boosting the electrons to a higher orbit.

The properties of spectral lines

True: I see lines of element X in a star. Therefore it must have X in it.
False: I don't see X lines in a star. Therefore it has no X in it.

Things which affect spectral lines:




Also, if an atom is ionized, the spectral lines change (or are non-existant). The fractional ionization is given by the Saha equation: