Desk Top Stars: How Fast is it Moving?

In the last part of the laboratory, we mentioned that light comes in the form of waves. You may have heard that sound does the same thing! They are different kinds of waves, but sound and light both consist of waves.

One interesting property of waves is this: when the source of the waves is moving towards you, the wavelength of the waves you measure goes down. The faster the motion, the shorter the wavelength. If the source is moving away, the waves get longer. For sound, longer waves mean a lower pitch. If you've watched a train whiz by, you've probably noticed the pitch of the sound drop when the train goes by. This is called the Doppler effect.

For light, longer wavelengths mean a redder color. If a galaxy is moving away from us, the spectral lines from different elements (the lines you saw in the last experiment) will be shifted to a longer wavelength! But the pattern stays the same (though it is stretched out), so we can still use the lines to figure out what elements are in the galaxy.

Listen as the siren is moving towards and away from you. Can you tell the difference in pitch as the velocity changes?

Astronomers have a formula which tells them the speed at which an object is moving away or towards us once they have measured the shift of a line in its spectrum. This is exactly how a radar detector works, except that astronomers use it for the power of good, while police use it to raise revenue. The formula is:

The speed of light is called c, and its value is about 300,000,000 meters per second (m/s)--fast enough to go all the way around the world seven times in a second! The rest wavelength is the wavelength of a line from a source that is not moving (like the tubes in the lab), and the change in wavelength is the observed wavelength minus the rest wavelength.