Redshift and Blueshift: What Light Tells Us About Motion

0:00How can astronomers tell if a galaxy is moving toward us or away?

0:05Start with sound. A moving siren's waves are squeezed in front of it and stretched behind it.

0:12It sounds higher coming toward you and lower moving away. This is the Doppler effect.

0:19Light is a wave too. Its wavelength, the distance from one crest to the next, sets its colour.

0:27A spectrum spreads light out by wavelength: blue light has shorter waves, red light longer ones.

0:34Light from a source moving away is stretched toward red. This is redshift.

0:40Light from a source moving toward us is squeezed toward blue. This is blueshift.

0:47Gases in stars absorb certain wavelengths, leaving dark absorption lines. A laboratory shows their normal positions.

0:56From a galaxy moving away, the same lines are shifted toward red.

1:01From a galaxy moving toward us, they are shifted toward blue.

1:05A bigger shift means faster motion toward or away from us.

1:11Nearly all galaxies are redshifted, and generally, the farther away they are, the bigger the redshift.

1:18So the universe is expanding: galaxies are getting farther apart.

1:24A few nearby galaxies are blueshifted, like Andromeda, which is moving toward us.

1:30Careful: redshift does not mean a galaxy looks red. It means its lines moved to longer wavelengths.

1:39Redshift means moving away, and blueshift means moving toward us.