Astronomy final exam question: blue main sequence star vs. blue supergiant

Astronomy 210 Final Exam, Fall Semester 2008
Cuesta College, San Luis Obispo, CA

[4.0 points.] A blue main sequence star is known to be smaller than a blue supergiant because it is:
(A) same luminosity as, but cooler than a blue supergiant.
(B) same luminosity as, but hotter than a blue supergiant.
(C) same temperature as, but less luminous than a blue supergiant.
(D) same temperature as, but more luminous than a blue supergiant.

Section 70158
(A) : 11 students
(B) : 8 students
(C) : 41 students
(D) : 1 student

Correct answer: (C)

From Wien's law, the main sequence star and the supergiant 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.

This was asked previously this semester as a quiz question.

"Difficulty level": 67% (including partial credit for multiple-choice)
Discrimination index (Aubrecht & Aubrecht, 1983): 0.74