The above diagram shows the specturm (component colors) of a galactic star. The different wavelengths(the distance from one wavecrest to the next) of light are what human eyes see as colors. The shortest wavelengths appear at the blue end of the spectrum. The longest wavelengths appear at the red end of the spectrum. The numbers on the top measure wavelength in nanometers. The deep black lines in the color spectrum are absorption lines. Each chemical element in a star's atmosphere absorbs a certain color. By looking at the absorption lines in a spectrum, we are able to determine what elements are present in a stars atmosphere (the missing colors are the colors that the elements in the stars atmosphere absorb).
Notice how the pattern of absorption lines shifts from the blue end to the red end as the galatic star becomes fainter. This is known as the Red Shift.
The Doppler Effect is used to explain the Red Shift. The Doppler Effect states that if a source that is emitting waves moves away from us, the wavelength of the waves we recieve from it will be longer. This implies that stars moving away from us will be red shifted(light with longer wavelength appears at the red end of the spectrum). In the 1920's, Edwin Hubble discovered that many galaxies appeared red shifted. This means that these galaxies are moving away from us! The universe is expanding! The idea that the universe is expanding supports the big bang theory.
 Gravitational redshifts. Light rays from the stars must travel vast distances to reach us. It has been proven that the pull of gravity, from the stars the light rays pass, could indeed cause a loss in light-wave energy葉hus moving that light toward the red on the spectrum. Einstein was the first to predict that gravity would affect starlight, and this was shown to be true in the 1960s.用. 35.
Albert Einstein was the first to predict that gravity would be able to affect the transmission of light. This fact could easily explain the redshifts which have been found.用. 42.
 Second-order Doppler shift. It is known that a light source moving at right angles to an observer will be redshifted. Compare this fact with the known fact that all stars are definitely circling galaxies. In addition, many scientists suspect that, just as all planets and stars are kept in position by orbiting, so, for purposes of stability, the entire universe is probably circling a common center!用p. 35-36.
 Energy-loss shift. Light waves could themselves lose energy as they travel across the long distances of space. This is called "tired light." The energy-loss shift is probably the primary cause of the redshift.用. 36.