Doppler Shift, Redshift & Blueshift: Physical Mechanisms

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.

For a binary star, periodic Doppler shifts occur because each component moves alternately toward and away from the observer around the barycenter. In a rotating galaxy, opposite sides show systematic red and blue patterns. Thermal motion broadens a line because individual atoms have a velocity distribution. Thus wavelength shifts and line widths carry different velocity information: bulk motion displaces the centroid, while velocity dispersion broadens the profile.