Astronomy clicker question: main sequence to giant/supergiant evolution

Astronomy 10, Spring Semester 2007
Cuesta College, San Luis Obispo, CA

Astronomy 10 learning goal M3.1

Students were asked the following clicker question (Classroom Performance System, einstruction.com) in the middle of their learning cycle:

[0.3 points.] According to the Stefan-Boltzmann law, how does the luminosity of a medium mass or massive main sequence star change as its outer layers expand and cool off, as it becomes a giant or supergiant?
(A) It becomes dimmer.
(B) It remains the same.
(C) It becomes brighter.
(D) (Any of the above (A)-(C) choices, depending on how old the star is.)

Correct answer: (B).

The evolutionary track of a main sequence star as it becomes a giant or supergiant can be approximated as a horizontal path from left-to-right across an H-R diagram, which means that its luminosity remains constant while its surface temperature decreases. From the Stefan-Boltzmann law, constant luminosity means that size must increase while temperature decreases.

Student responses
Section 4136
(A) : 13 students
(B) : 3 students
(C) : 9 students
(D) : 5 students

Section 5076
(A) : 8 students
(B) : 6 students
(C) : 4 students
(D) : 2 students