Doppler Broadening of Spectral Lines

This animation illustrates how random motion of sources for spectral lines leads to a broadening of the observed line. The illustration assumes a single absorption line from a set of stars, but the idea is the same irregardless of what the source of the spectral lines are, and whether they are absorption or emission lines. For the few stars shown here the total spectrum width would change with time. However, when averaged over many stars the broadened line would be rather constant in width, with the width determined by the average radial velocity of the sources of the spectral line.

Because the spectral broadening depends on the average velocities, the width can be used to infer the average velocity. This can, in turn, be used in conjunction with the gravitational law to infer the amount of mass present in the region observed, whether the mass is directly visible or not. Generally, the more mass present, the faster the sources must move. Usage: Click "Play" to begin. Continue to click "Play" when action stops. "Step" moves forward one step and "Back" moves backwards one step. "Home" returns to the beginning from any point.