Red shift

This diagram shows the cyclical changes in wavelength of the light from a star circling a dark companion, as it first approaches then recedes from Planet Earth.

The term red shift refers to the phenomenon that the wavelength of light from an object moving away from an observer is stretched, or shifted, by that motion, to the longer, red side of the spectrum. In contrast, the wavelength of light from objects approaching an observer is compressed, or shifted to the blue side of the spectrum. The name for describing the change in wavelength of light from an object moving towards or away from an observer is the Doppler effect.

In 1929, after a study of the spectrum of distant galaxies, astronomer Edwin Hubble described finding how almost all distant galaxies were moving away from our own galaxy, because they showed a red shift. Further, he found that there was a linear relationship between the distance to those galaxies and how large a red shift they showed. Hubble's discovery was recognized by almost all astronomers as demonstrating the size of our Universe was expanding.