Spectral Classes of Stars

Abstract

In the previous chapter we saw that a star’s color (its color index) reveals its surface temperature, but the color gives us only a rough picture of the nature of the light emitted by stars. Two stars may have the same color index and yet the detailed features of the radiation they emit may differ considerably. The reason for this is that the color of the star’s radiation is determined by the temperature of its photosphere, but before the light from the photosphere reaches us it passes through the star’s atmosphere, and each atom in this atmosphere leaves its imprint on this radiation. Two stars with the same surface temperatures may have atmospheres whose physical and chemical properties differ considerably. Thus, the pressures and the abundances of the various chemical elements in the two atmospheres may be quite different so that the effects of the two atmospheres on the radiation streaming through them will be different. For many years astronomers have investigated the interaction of a star’s atmosphere with the star’s radiation by passing the radiation through a spectroscope and studying the spectrum so obtained. We noted in the chapter on the sun that the spectrum of the light coming from a star consists of a continuous array of colors on which a series of dark lines (the absorption or Fraunhofer lines) is superimposed.

The spacious firmament on high,

And all the blue ethereal sky,

And spangled heavens, a shining frame,

Their great Original proclaim.

—JOSEPH ADDISON, The Spectator