Stellar Spectra

All our information about the physical properties of stars comes more or less directly from studies of their spectra. In particular, by studying the strength of various absorption lines, stellar masses, temperatures and compositions can be deduced. The line shapes contain detailed information about atmospheric processes.

As we have seen in Chap. 3, the light of a star can be dispersed into a spectrum by means of a prism or a diffraction grating. The distribution of the energy flux density over frequency can then be derived. The spectra of stars consist of a continuous spectrum or continuum with narrow spectral lines superimposed (Fig.8.1). The lines in stellar spectra are mostly dark absorption lines, but in some objects bright emission lines also occur.

In a very simplified way the continuous spectrum can bethought of as coming from the hot surface of the star. Atoms in the atmosphere above the surface absorb certain characteristic wavelengths of this radiation, leaving dark "gaps" at the corresponding points in the spectrum. In reality there is no such sharp separation between surface and atmosphere. All layers emit and absorb radiation, but the net result of these processes is that less energy is radiated at the wavelengths of the absorption lines.

The spectra of stars are classified on the basis of the strengths of the spectral lines. Isaac Newton observed the solar spectrum in 1666, but, properly speaking, spectroscopy began in 1814 when Joseph Fraunhofer observed the dark lines in the spectrum of the Sun. He assigned capital letters, like D, G, H and K, to some of the stronger dark lines without knowing the elements responsible for the origin of the lines (Sect. 8.2). The absorption lines are also known as Fraunhofer lines. In 1860, Gustav Robert Kirchhoff and Robert Bunsen identified the lines as the characteristic lines produced by various elements in an incandescent gas.

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