Doppler shift of light
/ DOP-ler /
You have heard the Doppler effect on the street: an ambulance siren rises in pitch as it races toward you and drops as it speeds away. The pitch change happens because motion squeezes or stretches the sound waves. Light does exactly the same thing. When a source of light moves toward us its waves are squeezed to shorter wavelengths (shifted toward the blue), and when it moves away they are stretched to longer wavelengths (shifted toward the red).
Astronomers detect this not by overall color but by watching sharp spectral lines — features at known, fixed wavelengths produced by atoms. If those lines all appear shifted toward longer wavelengths, the object is moving away; shorter, it is approaching. The size of the shift gives the speed along our line of sight, the radial velocity. For everyday astronomical speeds, the fractional shift in wavelength simply equals the velocity divided by the speed of light, so a line shifted by one part in a thousand means a speed of about 300 km per second.
The Doppler shift is one of astronomy's most productive tools. It reveals how fast stars and galaxies move, exposes unseen companions by the wobble they impose on a star (the heart of one exoplanet-finding method), measures the spin of galaxies, and weighs star clusters from the spread of their velocities. One crucial caution: the Doppler shift only measures motion through space. The redshift of very distant galaxies is something subtly different — the stretching of space itself as the universe expands — even though it looks like a Doppler shift at first glance.
A star's hydrogen line, normally at 656.3 nanometers, is measured at 656.5 nanometers — shifted toward the red. The 0.2-nanometer stretch reveals the star is receding from us at about 90 km per second, even though it appears perfectly still in the sky.
Squeezed waves (blueshift) mean approach; stretched waves (redshift) mean recession.
The Doppler shift measures only motion toward or away (radial velocity), not sideways motion. And it is not the same as cosmological redshift: distant galaxies are not flying through space away from us so much as the space between us is stretching.