Question

Why does a star rotating faster result in a broadening of its absorption lines shouldn't it just result in a shift of its absorption lines?

Answer 1

So many nuclear reactions occur in a star.When an atom goes from ground state to excited state, absorption line is produced.

As a star rotates faster then according to you this line should shift, yes that off course shifts but it doesn't mean that previous probabilities are lost ,there is also a finite probability of occurance of previous absorption hence we get a broadening which contains both previous absorptions and new absorptions.....

Statistics-

This can also be explained on the behalf of statistics as nuclear reactions are always statistical in nature and always there exists delta kind of variation in absorption and emission of radiation which results in broadening of curves...