Question

How does a star rotation affect the appearance of its spectral lines?

Answer 1

Red or blueshifting of the spectra and broadening of lines.

Explanation:

The part of the star's light that is rotating towards Earth will show a blueshift while the part rotating away from us will be redshifted. The part in between at the middle of the star will show little to no Doppler shift.

The combined effect will result in broadening of spectral lines but also "smearing" or disappearance of thinner spectral lines close to the broad ones. The faster the star is rotating, the larger the broadening. See the image for more info:

Here's a link to a college class notes (not sure which college) that will show you how to work the math.