Astronomy quiz question: hottest star

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

Which star is the hottest?
(A) F5 supergiant.
(B) B0 main-sequence star.
(C) M5 main-sequence star (red dwarf).
(D) A0 white dwarf.

Section 30676
(A) : 12 students
(B) : 25 students
(C) : 2 students
(D) : 16 students

Correct answer: (B)

Using the OBAFGKM spectral type mnemonic, or reading off of an H-R diagram (provided on this quiz), the B0 main-sequence star is the hottest, followed by the A0 white dwarf, F5 supergiant, and M5 red dwarf. Apparently students chose the F5 supergiant for being the largest star, or the A0 white dwarf for being "white-hot" (when in fact the B0 main-sequence star would be a hotter white-, or even blue-hot in appearance).

"Difficulty level": 51% (including partial credit for multiple-choice)
Discrimination index (Aubrecht & Aubrecht, 1983): 0.81