The observer sees two lines (except when the stars are moving perpendicular to her line of sight) because one star is moving away and one star is moving toward the observer. These give red-shifted and blue-shifted spectral lines, respectively. The arrow below the spectrum indicates the location of the unshifted line. Note that the colors in the above animation are used only to distinguish one star from another; if the stars were actually different colors, they would not have the same spectrum. Usage: Click "Play" to begin. You may stop the motion at any time with "Stop". "Step" moves forward one step and "Back" moves backwards one step. "Home" returns to the beginning from any point.