The Doppler effect is a phenomenon that affects the wavelength of waves. As applied to light, when an object that radiates light is traveling towards you, it tends to compress the wavelengths, making them appear to shift in the color they appear to be. The order of visible light wavelengths, from longest to shortest, are red, orange, yellow, green, blue, indigo, and violet. So a star that is traveling towards us here on Earth, if it were a yellow star, like our sun, might experience a shift and appear to be green in color. Consequently, a star that is moving away from us will stretch out the wavelengths and appear to shift to a lower wavelength of light. A yellow star moving away from us on Earth might experience a shift from the yellow color to an orange or red color. This apparent shift in wavelength is known as the Doppler effect.