Stretching and shrinking of light

Light from a star is blue- and red-shifted as the star moves towards and away from us. The lines in the spectrum at the bottom of this image are called absorption lines. These correspond to a wavelength of light that has been absorbed by a certain type of matter.

Light has a particular wavelength. Blue is shorter, red is longer. When a star moves towards us, it "squashes" the light, decreasing the wavelength, and making it look more blue. When a star moves away from us, it "stretches" the light, increasing the wavelength, and making it look more red.

This animation shows how astronomers use very precise spectrographs such as the HARPS instrument on the 3.6-metre telescope at La Silla, Chile to find exoplanets. As the planet orbits its gravitational pull causes the parent star to move back and forth. This tiny radial motion shifts the observed spectrum of the star by a correspondingly small amount because of the Doppler shift. With super-sensitive spectrographs such as HARPS the shifts can be measured and used to infer details of a planet’s mass and orbit.