Final answer:
The Doppler Effect causes the observed wavelengths of spectral lines from stars and galaxies to shift due to relative motion, leading to broadened lines that provide information on rotational speeds or star movements within galaxies.
Step-by-step explanation:
The Doppler Effect is related to the Doppler broadening of spectral lines in stellar spectra because both phenomena involve changes in the observed wavelength or frequency due to relative motion between the source and the observer. In the case of stars, as they rotate, parts of their surface move toward us (causing a blue shift) while others move away from us (causing a red shift), resulting in broadened spectral lines. This broadening, also known as line broadening, provides information about the rotational speeds of stars or the movement of stars within galaxies. For galaxies, the composite spectrum reflects the individual motions of many stars, which when combined, can reveal the range of speeds and thus the gravitational forces within galaxies. This information is critical for determining the mass of galaxies like elliptical galaxies.
As each point on a star's surface has a unique Doppler shift, the width of the observed spectral line represents a blend of all these shifts. The broader the line, the higher the rotational speed or the range of speeds within the galaxy.