Two stars have the same spectral classification,say M2,but one is a red giant and the other is a main-sequence star.To determine which is which,we look at their spectra because the absorption lines in the giant will be
A) narrower because of the pressure-broadening effect.
B) broader because of the pressure-broadening effect.
C) narrower because of the temperature difference between the two luminosity classes.
D) broader because of the temperature difference between the two luminosity classes.

Question

Quizplus · Accepted Answer

The absorption lines in the spectrum of a giant star are narrower compared to those in a main-sequence star of the same spectral type due to the lower pressure in the giant's atmosphere, which reduces the pressure-broadening effect.

Two Stars Have the Same Spectral Classification,say M2,but One Is