Doppler Shift, Redshift & Blueshift: Foundations

Astronomers compare observed spectral-line wavelengths with laboratory rest wavelengths. The fractional shift z = (λobs − λrest)/λrest quantifies redshift; at low speeds, radial velocity is approximately c times this fractional shift.

For ordinary Doppler motion, successive wavefronts arrive more widely spaced when the source recedes and more closely spaced when it approaches. At cosmological distances, redshift also reflects the expansion of space and requires a relativistic cosmological interpretation rather than the simple low-speed formula.

Spectrographs measure line positions with high precision. Binary-star velocities, exoplanet reflex motion, galaxy rotation, and cosmic expansion are all inferred from systematic wavelength shifts matched to known spectral features.

Doppler shifts convert spectral-line displacement into information about motion along the line of sight. A source moving away is redshifted and one moving toward us is blueshifted in the ordinary kinematic case. For speeds small compared with light, fractional wavelength shift is approximately radial velocity divided by c. The method measures only the line-of-sight component, so a star can move rapidly across the sky while showing little Doppler shift.