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

Answer 1

Red or blueshifting of the spectra and broadening of lines.

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.

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Answer 2

Henry Asbill

The Doppler effect causes a star's spectral lines to broaden when it rotates because different parts of the star move in different directions from us, shifting the spectral lines' wavelengths. This rotational broadening can reveal details about the star's orientation and rotation speed.

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Answer from HIX Tutor

When evaluating a one-sided limit, you need to be careful when a quantity is approaching zero since its sign is different depending on which way it is approaching zero from. Let us look at some examples.