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Spectral lines are gaps in the ordinarily continuous distribution of frequency in light. They result when emitted light is partially absorbed by matter, usually a gas. One type of spectral line is an absorption line, which is caused when some frequencies of light are absorbed from emitted radiation. An emission line, on the other hand, can be seen in light that was absorbed and then re-emitted. Spectral lines can give astronomers clues about the composition and velocity of astronomical objects.
The first of the two types of spectral lines are absorption lines. Absorption lines occur when light is emitted from an object, such as a star, and then passes through a gas before reaching an observer. In this case, light from a hot star will be emitted strongly over a broad range of frequencies; in other words, it will have a continuous spectrum of radiation. If it passes through a cool gas, however, light with certain frequencies can be absorbed by the gas. When an observer analyzes the light, which originally came from the star, it will show absorption lines—noticeable gaps in certain frequencies of radiation.
The second kind of spectral line is an emission line. Starlight cannot be continually absorbed by a gas because the light contains energy. Rather, it is absorbed by the gas and then re-emitted at a later time. When it is re-emitted, the light only contains the frequencies that were absorbed by the gas. Therefore, the frequencies of emission lines should match up with the frequencies of corresponding absorption lines. This in fact has been verified by astronomers observing re-emitted light; the two types of spectral lines are essentially inverses of one another.
Spectral lines exist because the light emitted by elements and chemicals has a continuous spectrum of frequencies. Rather than being a homogenous entity, visible light is actually electromagnetic radiation within a certain frequency range. Radiation that is invisible to the naked eye also has a continuous frequency distribution, though it is concentrated outside of the visible range. A scientific instrument known as a spectrometer can be used to analyze the frequencies of electromagnetic radiation.
Studying these lines can give astronomers two important pieces of information. First of all, with knowledge of which gases produce which absorption lines, spectral lines can help identify the composition of planetary atmospheres. Astronomers have used this technique to characterize the atmospheres of planets outside the solar system. Secondly, recognizable patterns of spectral lines can indicate the velocity of astronomical objects—the velocity of an object shifts the frequency of its radiation, including spectral lines.