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Moving Targets
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Absorption lines and the Doppler Effect
To see the various colors that make up starlight, astronomers can use a prism to separate the colors. Common elements like helium and calcium in the atmospheres of stars block certain colors of light, creating "absorption lines." These gaps in the otherwise smooth spectra from stars serve as familiar tunes. Seeing how far they are shifted up or down the spectrum is the same as listening to how much higher or lower a note from our trumpeter sounds. If the star is moving, the lines will be shifted. How much they're shifted tells us how fast the star's moving.

Illustration: stationary light source, hydrogen-gas-enclosed light source, moving light source

Illustration: stationary light source, hydrogen-gas-enclosed light source, moving light source

The light from a stationary source is composed of many different colors. A prism can separate the colors out into a spectrum. Notice that the spectrum from this source doesn't have any breaks or gaps. It is called a continuous spectrum.
This source is covered by a glass jar filled with hydrogen gas. Notice that the spectrum is no longer continuous, as it is above. The hydrogen "blocks out" some of the colors from the source, creating gaps in the spectrum call absorption lines. Each element has its own set of absorption lines. The spectrum on the right shows that there is hydrogen between the prism and the light source.
In this scenario, the gas is part of the star's atmosphere, and the star is in motion. Recognizing the pattern of absorption lines in the star's spectrum, astronomers can determine how fast the star is moving by noting how much the lines are shifted by the Doppler effect up or down the spectrum.

When the star moves away from us, the absorption lines shift towards the red end of the spectrum; when the star moves toward us, the lines shift toward the blue. How much they shift depends on how fast the star is moving.

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