Dr. Aaron Titus | Department of Physics, High Point University
PHY1050      Astronomy of Stars, Galaxies, and the Cosmos
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doppler

By measuring the Doppler shift of the spectrum of a star, we can measure how fast a star moves and whether it is traveling toward us or away from us.

Suppose that a fire engine blowing its siren travels toward you. What will it sound like?

The simulation below (developed by Wolfgang Christian) shows a sound source (black dot). Click the Play button to view the animation. The circles represent the crests (wavefronts) of sound waves emitted from the source.

Animation 1 shows a sound source at rest. It emits sounds uniformly in all directions.

Animation 2 shows an "observer" (i.e. detector of some kind) traveling toward the sound source. Will the observer measure time between wavecrests to be greater or less than if she/he was at rest? Will the observer detect a higher frequency or lower frequency for the sound than if she/he was at rest? Will the observer calculate a longer wavelength or shorter wavelength for the sound?

How do your answers change after the observer passes the sound source?

Animation 3 shows the sound source moving toward the observer. Will the observer measure time between wavecrests to be greater or less than if she/he was at rest? Will the observer detect a higher frequency or lower frequency for the sound than if she/he was at rest? Will the observer calculate a longer wavelength or shorter wavelength for the sound?

How do your answers change after the source passes the observer?

Similar to a sound source, as a star travels toward you, it spectrum shifts toward higher frequency, shorter wavelength. We call this blue shift.

For example, here's a spectrum for hydrogen.

Here's the same spectrum, but for hydrogen that is moving toward us at four tenths of the speed of light.

Note that the previous two spectra are same except that all five lines have been shifted to lower wavelengths--the bluer region of the spectrum. We say that the spectrum is blue shifted.


If you are standing at a location behind the sound source as it moves away from you, then the wave crests are farther apart. That's a longer wavelength and a lower frequency. You would hear a lower frequency than if the source was sitting still.

Likewise, as a star moves away from you its spectrum shifts to lower frequency, longer wavelength. We call this red shift.

Again, here's a spectrum for hydrogen.

Here's the same spectrum, but for hydrogen that is moving away from us at four tenths of the speed of light.

Note that in this case all five lines have been shifted to longer wavelengths--the redder region of the spectrum. We say that the spectrum is red shifted.

By determining whether the spectrum of a star is red shifted or blue shited, we know whether a star moves away from us (red shifted) or toward us (blue shifted). By measuring how much the spectrum shifts, we can determine how fast the star is moving.

 

 

 

 

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