Two stars have the same spectral classification,say,M2,but one of them is a red giant while the other is a main-sequence star.To determine which is which,we look at their spectra because (see Figure 10-9,Comins,Discovering the Essential Universe,6th ed.)the absorption lines in the giant will be:
A) narrower because of the pressure difference between the atmospheres of the two stars.
B) broader because of the pressure difference between the atmospheres of the two stars.
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

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The absorption lines in the spectrum of a red giant are narrower due to the lower pressure in its atmosphere compared to a main-sequence star.

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