One key measurement used to determine how fast a galaxy is moving away from Earth is its redshift, derived from the galaxy's light spectrum.
- Observe the galaxy’s spectrum and measure the wavelengths of identifiable spectral lines (for example, hydrogen lines).
- Compare the observed wavelengths to the known rest wavelengths of those lines to calculate the redshift z = (λ_observed − λ_rest) / λ_rest.
- For galaxies moving away, the redshift is positive. Convert redshift to speed using v ≈ c z when z is small (c is the speed of light, about 300,000 km/s).
- Example: if λ_rest = 500 nm shifts to λ_observed = 505 nm, z = (505−500)/500 = 0.01, so v ≈ 0.01 × 300,000 km/s ≈ 3,000 km/s.