As a star moves toward or away from Earth,
its light -- and absorption lines -- are shifted
by the Doppler effect.

• Stars and galaxies moving toward us are catching up with their own light waves, so the wavelengths of their absorption lines are shorter
  (a BLUESHIFT)

• Stars and galaxies moving away from us are pulling away from their own light waves, so the wavelengths of their absorption lines are longer (a REDSHIFT).

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