Astronomy quiz question: same luminosity, but different size stars

Astronomy 210 Quiz 4, Spring Semester 2010
Cuesta College, San Luis Obispo, CA

A(n) __________ has the same luminosity as, but is larger than a G5 white dwarf.
(A) A0 main-sequence star.
(B) K0 supergiant.
(C) K5 giant.
(D) M5 main-sequence star (red dwarf).

Correct answer: (D)

Using an H-R diagram, the only star that has the same luminosity as a G5 white dwarf is the M5 red dwarf, which is also larger in size, from reading off the diagonal equiradial lines on the H-R diagram; and/or applying the Stefan-Boltzmann law, where given two stars of the same luminosity, the cooler star (G5) must be larger in size than a hotter star (M5).

Section 30676
(A) : 3 students
(B) : 6 students
(C) : 10 students
(D) : 45 students

"Success level": 72% (including partial credit for multiple-choice)
Discrimination index (Aubrecht & Aubrecht, 1983): 0.39