Astronomy quiz question: red dwarf star vs. red giant

Astronomy 210 Quiz 4, Spring Semester 2009
Cuesta College, San Luis Obispo, CA

A red dwarf star is known to be smaller than a giant star that has the same red-hot color because it is:
(A) less luminous than the giant star.
(B) cooler than giant star.
(C) more luminous than the giant star.
(D) hotter than the giant star.

Section 30674
(A) : 28 students
(B) : 2 students
(C) : 5 students
(D) : 0 students

Correct answer: (A)

From Wien's law, the red dwarf star and the giant must have the same temperature, because they have the same color. From the Stefan-Boltzmann law, for two stars of the same temperature, the less luminous star must be smaller in size.

"Difficulty level": 78% (including partial credit for multiple-choice)
Discrimination index (Aubrecht & Aubrecht, 1983): 0.40