🚦 Doppler Redshift

📖 The Physics

Light from a star carries dark absorption lines at fixed wavelengths — a kind of barcode of the atoms in it. If the star moves away, its light waves are stretched and the lines slide toward red (redshift).

If it moves toward us the waves bunch up and the lines shift toward blue. The size of the shift tells us the star's speed along our line of sight.

Slide the velocity and watch the three hydrogen lines march across the spectrum. This same Doppler shift, seen in distant galaxies, is what revealed the expanding universe in Hubble's law.