Starlight reveals how fast galaxies are moving
The Doppler effect changes starlight's color, revealing galactic speeds and confirming the universe's expansion, even predicting future cosmic collisions.
The Doppler effect, which makes a siren's pitch change as it passes, also shifts the color of starlight. When a galaxy moves away, its light stretches red (redshift); if it approaches, light compresses blue (blueshift). Astronomers use these color shifts to measure how fast galaxies are moving, providing vital clues about the universe. Edwin Hubble discovered in 1929 that most distant galaxies show redshift, proving they are moving away from us. This led to Hubble's law, showing the universe is expanding, with galaxies moving faster the farther they are. For example, the Andromeda galaxy is blueshifted, heading for a collision with our Milky Way.