Doppler redshift astronomy. Illustration showing how light from a distant galaxy can appear shifted towards the 'red' end of the spectrum (an increase in wavelength) if the observer and star are moving away from each other (left). The converse effect, when objects are moving towards each other, is called a blueshift (right). These Doppler shifts are observed and measured as the shift of known spectral lines (black bars), as shown here. Doppler shifts can be both due to the actual motion of objects, and also due to a cosmological redshift caused by the general expansion of the universe.
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