Astronomy quiz question: same luminosity white dwarf vs. red dwarf

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

A white dwarf will be ___________ compared to a red dwarf that has the same luminosity.
(A) cooler and smaller.
(B) hotter and smaller.
(C) cooler and larger.
(D) hotter and larger

Correct answer: (B)

From Wien's law, white dwarfs must be hotter than red dwarfs. From the Stefan-Boltzmann law (luminosity proportional to size and temperature^4), since the white dwarf has the same luminosity, but is at a higher temperature than the red dwarf, then the white dwarf must be smaller in size.

Section 70160
(A) : 1 student
(B) : 32 students
(C) : 2 students
(D) : 8 students

(Cf. blog posts Astronomy clicker question: same luminosity main sequence vs. giant star, Astronomy quiz question: same temperature main sequence vs. giant star.)

Success level: 76% (including partial credit for multiple-choice)
Discrimination index (Aubrecht & Aubrecht, 1983): 0.38