Spectral lines in stars

In Chapter 2 we discussed the continuous spectra of stars and saw that they could be closely described by blackbody spectra. In this chapter, we will discuss the situations in which the spectrum shows an increase or decrease in intensity over a very narrow wavelength range.

3.1 I Spectral lines

We know that if we pass white light through a prism, light of different colors (wavelengths) will emerge at different angles with respect to the initial beam of light. If we pass white light through a slit before it strikes the prism (Fig. 3.1), and then let the spread-out light fall on the screen, at each position on the screen we get the image of the slit at a particular wavelength.

Both William Hyde Wollaston (1804) and Josef von Fraunhofer (1811) used this method to examine sunlight. They found that the normal spectrum was crossed by dark lines. These lines represent wavelengths where there is less radiation than at nearby wavelengths. (The lines are only dark in comparison with the nearby bright regions.) The linelike appearance comes from the fact that, at each wavelength, we are seeing the image of the slit. It is this linelike appearance that leads us to call these features spectral lines. If we were to make a graph of intensity vs. wavelength, we would find narrow dips superimposed on the continuum. The solar spectrum with dark lines is sometimes referred to as the Fraunhofer spectrum. Fraunhofer gave the strongest lines letter designations that we still use today.

The origin of these lines was a mystery for some time. In 1859, the German chemist Gustav Robert Kirchhoff noticed a similar phenomenon in the laboratory. He found that when a beam of white light was passed through a tube containing some gas, the spectrum showed dark lines. The gas was absorbing energy in a few specific narrow wavelength bands. In this situation, we refer to the lines as absorption lines. When the white light was removed, the spectrum showed bright lines, or emission lines, the wavelengths where absorption lines had previously appeared. The gas could emit or absorb energy only in certain wavelength bands.

Kirchhoff found that the wavelengths of the emission or absorption lines depend only on the type of gas that is used. Each element or compound has it own set of special wavelengths. If two elements which don't react chemically are mixed, the spectrum shows the lines of both elements. Thus, the emission or absorption spectrum of an element identifies that element as uniquely as fingerprints identify a person. This identification can be carried out without understanding why it works.

Whether we see absorption or emission depends in part on whether or not there is a strong enough background source providing energy to be absorbed (Fig. 3.2). The strength of the spectral lines also depends on how much gas is present and on the temperature of the gas. Sample emission and absorption spectra of stars are shown in Fig. 3.3.

If we allow white light to fall all over a prism, the red from one part will overlap the blue from another part, and we can't see a clear spectrum. Instead, we pass white light through a slit first.The beam of light is then spread out as it passes through the prism. On the screen, we are seeing a succession of images of the slit in different colors. If there is a color missing from the white light, this will show up as a gap on the screen in the shape of the slit.

If we allow white light to fall all over a prism, the red from one part will overlap the blue from another part, and we can't see a clear spectrum. Instead, we pass white light through a slit first.The beam of light is then spread out as it passes through the prism. On the screen, we are seeing a succession of images of the slit in different colors. If there is a color missing from the white light, this will show up as a gap on the screen in the shape of the slit.

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