Astronomy quiz question: blue main-sequence star vs. blue supergiant

Astronomy 210 Quiz 4, spring semester 2012
Cuesta College, San Luis Obispo, CA

A blue main-sequence star will be ___________ compared to a blue supergiant of the same color.
(A) dimmer and smaller.
(B) dimmer and larger.
(C) brighter and smaller.
(D) brighter and larger.

Correct answer: (A).

On an H-R diagram, main-sequence stars are dimmer than supergiants of the same color (temperature); and from the Stefan-Boltzmann law:

Luminosity ∝ size x Temperature^4

the less luminous star will be smaller than a more luminous star of the same temperature.

Section 30676
Exam code: quiz04sLO8
(A) : 26 students
(B) : 2 students
(C) : 5 students
(D) : 2 students

Success level: 76% (including partial credit for multiple-choice)
Discrimination index (Aubrecht & Aubrecht, 1983): 0.50