Astronomy quiz question: same temperature main sequence vs. giant star

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

A main sequence star will be ___________ compared to a giant star that has the same temperature.
(A) less luminous and smaller.
(B) less luminous and larger.
(C) more luminous and smaller.
(D) more luminous and larger.

Correct answer: (A)

From an H-R diagram, main sequence stars that are the same temperature as giants will be less luminous than the giants. From the Stefan-Boltzmann law (luminosity proportional to size and temperature^4), since the main sequence star has less luminosity, but is the same temperature as the giant, then the main sequence star must be smaller in size.

Section 70158
(A) : 35 students
(B) : 1 student
(C) : 7 students
(D) : 1 student

(Cf. blog post Astronomy clicker question: same luminosity main sequence vs. giant star.)

Success level: 81% (including partial credit for multiple-choice)
Discrimination index (Aubrecht & Aubrecht, 1983): 0.37