By what standard technique did astronomers in the 1930s originally determine the luminosity class (I, II, III, IV, or V) of a star?
A) combining the apparent magnitude with the measured distance to the star
B) observing the diameter of the star on a photographic plate or CCD image
C) timing how long it takes for the star to be eclipsed by a companion in an eclipsing binary star system
D) studying the absorption lines in the star's spectrum

Question

Quizplus · Accepted Answer

The luminosity class of a star is determined by studying the absorption lines in the star's spectrum. These lines provide information about the star's atmosphere, including its temperature, composition, and density, which in turn allows astronomers to classify the star according to its luminosity.

By What Standard Technique Did Astronomers in the 1930s Originally