Astronomy 10, Summer Session 2007
Cuesta College, San Luis Obispo, CA
Astronomy 10 learning goal Q8.5
Students were asked the following clicker question (Classroom Performance System, einstruction.com) near the beginning of their learning cycle:
[0.3 points.] Why is a white dwarf star smaller than a main-sequence star that has the same white-hot color?
(A) It is less luminous than the main-sequence star.
(B) It is more luminous than the main-sequence star.
(C) It is cooler than the main-sequence star.
(D) It is hotter than the main-sequence star.
Correct answer: not revealed yet (see discussion).
This is the follow-up question after a short lecture (20 minutes) and an in-class activity (20 minutes) on how Wien's law and the Stefan-Boltzmann law describe blackbody radiation. Initial responses below:
Student responses
Section 8027
(A) : 3 students
(B) : 0 students
(C) : 3 students
(D) : 7 students
A leading question for the students: "Which star is hotter, and why?" Some students will have already realized that because these two stars have the same color, then they must be at the same temperature (application of Wien's law), and the class discusses why this must be the case. The same question is asked again, after the students collectively come to realization that both responses (C) and (D) cannot be true.
[0.3 points.] Why is a white dwarf star smaller than a main-sequence star that has the same white-hot color?
(A) It is less luminous than the main-sequence star.
(B) It is more luminous than the main-sequence star.
(C) It is cooler than the main-sequence star.
(D) It is hotter than the main-sequence star.
Correct answer: (A)
Student responses
Section 8027
(A) : 12 students
(B) : 1 student
(C) : 0 students
(D) : 0 students
The Stefan-Boltzmann law states that luminosity is proportional to size (that is, surface area) and temperature (T^4). Since they are the same temperature, then the less luminous star must be smaller in size than the more luminous star, which is larger in size.
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