The white ball represents the center of a galaxy. The colored balls represent stars revolving around the galactic center. The amount of red/blue seen represents the degree to which the spectrum is Doppler shifted. A star’s spectrum is shifted toward the blue as the star approaches the observer because its wavelength is compressed. As the star recedes from the observer, the spectrum shifts toward the red} since its wavelength is extended. A useful mnemonic device is “red for recede”.

The Doppler shifted frequency for light with observer and source moving toward and away from each other with relative speed v are given, respectively by

$$f_{observer}=\sqrt{\frac{1+\frac{v}{c}}{1-\frac{v}{c}}}f_{source} \ \text{ and } \ f_{observer}=\sqrt{\frac{1-\frac{v}{c}}{1+\frac{v}{c}}}f_{source}.$$

The effect clearly becomes more pronounced as the relative speed $v$ approaches the speed of light $c$.