The spectral luminosity class of the star α Arae is B2 V,and that of π Herculi is K3 II.Using only this information,without referring to tables or diagrams,we can tell that α Arae is:
A) hotter but intrinsically fainter than π Herculi.
B) hotter than π Herculi.
C) cooler and intrinsically fainter than π Herculi.
D) cooler than π Herculi.

Question

Quizplus · Accepted Answer

The spectral luminosity class of a star provides information about its temperature and luminosity. 
The number in the spectral luminosity class indicates the temperature, with higher numbers representing cooler temperatures. 
The letter in the spectral luminosity class indicates the luminosity, with I representing supergiants, II representing bright giants, III representing giants, IV representing subgiants, V representing main sequence stars, and VI representing subdwarfs.

α Arae has a spectral luminosity class of B2 V, indicating that it is a main sequence star with a temperature higher than a B3 star, but cooler than a B1 star. 
π Herculi has a spectral luminosity class of K3 II, indicating that it is a bright giant with a cooler temperature than a K2 star, but warmer than a K4 star.

Based on this information, we can tell that α Arae is hotter than π Herculi. We cannot determine their absolute luminosities from this information alone, but we can say that α Arae is likely less luminous than π Herculi since it is a main sequence star while π Herculi is a bright giant. Therefore, choice B is the best option.

The Spectral Luminosity Class of the Star α Arae Is