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Why are spectral lines shifted toward blue when a light source approaches Earth?

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When a light source approaches Earth, its spectral lines are shifted toward shorter, blue wavelengths—a phenomenon known as blueshift.1

This occurs due to the Doppler effect, which describes how the observed frequency of waves changes when a source and an observer move relative to one another. As a light source moves toward an observer, the waves are compressed, resulting in an increase in frequency. Because the speed of light remains constant, this increase in frequency corresponds to a decrease in wavelength, shifting the light toward the blue end of the spectrum.231

This principle is a vital tool in astronomy, allowing scientists to measure the relative motion of celestial objects, such as detecting the orbital motion of binary stars or identifying extrasolar planets by observing the periodic blueshifting and redshifting of a star's light.31