When the white light of a star passes through its atmosphere, the atoms present in that atmosphere selectively absorb the wavelengths corresponding to their energy transitions. The light that reaches Earth, then passed through a grating or a prism, shows a continuous spectrum interrupted by dark lines: these are the Fraunhofer lines. Each dark line is an unmistakable “signature” of a specific chemical element.
Absorption spectrum. On the continuous background of the colours of white light, dark lines appear at the wavelengths absorbed by the atoms of the stellar atmosphere: each line identifies an element (hydrogen, sodium, calcium, …).
Why are the lines dark rather than coloured? A hydrogen atom in the ground state can only absorb a photon of exactly eV to rise to , or of eV to rise to , and so on. The star’s white light contains all frequencies, but only these well-defined ones are “stolen” by the atoms of the atmosphere. The excited atoms then re-emit those photons — but in all directions, isotropically. In practice almost all the photons at those particular frequencies are deflected out of the line of sight, so that wavelength reaches the observer heavily attenuated: hence the dark line.
Principle — Spectra and levels
A dark line at frequency means that in the stellar atmosphere there exists an element with an exact energy transition . Measuring the lines is equivalent to measuring the differences between the energy levels of the atoms: if the levels were continuous, the spectrum would be uniformly attenuated, with no lines at all.
Example — How Fraunhofer started cosmic chemistry
In 1814 Joseph Fraunhofer observed hundreds of dark lines in the solar spectrum, numbering them with letters (A, B, C, …). Half a century later Kirchhoff and Bunsen showed in the laboratory that each of those lines coincided with the absorption spectrum of an element: the D line corresponds to sodium, the C to hydrogen, and so on. For the first time in history it was possible to know the chemical composition of an object without touching it — indeed, of an object km away from us. The discovery of helium (1868) even happened first on the Sun, in a line of the spectrum collected during an eclipse, and only years later on Earth too: hence the name, from the Greek hélios, “sun”.
Bright lines in nebulae
The same lines, but bright against a black background, are observed in ionised nebulae: the excited atoms, in the absence of a continuous background source, emit at exactly the same frequencies at which they absorb elsewhere. It is the physical complementarity between absorption and emission, always at the same quantised transitions.
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Topics: Quantum physics Concepts: Bohr model · Photon
Related exercises: Hydrogen levels, Balmer and Lyman · Worked exercise — a red photon from the hydrogen atom · True or false on quantum physics