Objective:

To learn how astronomers measure the moving speed of a galaxy.

New Concept:

Doppler Effect.

Although a galaxy may recede at a speed thousands of times faster than a jet plane, it does not appear to move even if you watch it for a lifetime. This is because the further away a moving object is, the more slowly it appears to move. For instance, the Moon revolves around us at the speed of our fastest jet fighter, yet it seems to be standing still if you watch it for only a few minutes. In astronomical terms, the distance between the Earth and the Moon is negligible -- merely hundreds of thousands of kilometers. The galaxies are millions of billions of times more distant. Therefore, it is impossible to measure the movement of the galaxies by direct observation. Fortunately there is a way around this problem, which is based on an effect, named after the Austrian physicist, Christian Johann Doppler (1803 -- 1853).

Fig. 1 When an ambulance is approaching, the wavelength of its sirens shortens and the pitch gets high. As the vehicle passes, the wavelength of the siren lengthens and the pitch drops. This is known as the Doppler Effect.

Doppler Effect

You may have noticed that when an ambulance approaches you with its sirens wailing, the sound of the siren is at very high pitch. When the ambulance speeds away, the pitch of the siren lowers. This is known as the Doppler effect. High pitched sounds have shorter wavelengths, and lower pitched sounds have longer wavelengths. Therefore, we can determine the speed of a moving sound source by measuring the wavelength change of the sound.

Figure Credits: Z. H. Zhou

The Doppler effect not only works for sound but also holds for light. If a light source is moving away from us, the wavelength of the light lengthens and the light shifts to the red side, which is called "redshift." Therefore, instead of measuring the receding velocity of a galaxy directly, astronomers measure the wavelength change of the light emitted from the galaxy. They then convert this change into the moving speed.

Surprisingly, lights emitted from distant galaxies are all shifted to the red side. The Doppler effect then tells us that all of these galaxies are receding from us.