Astronomy clicker question: main sequence to giant/supergiant evolution

Astronomy 10, Spring Semester 2008
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)

Student responses
Section 4160
(A) : 10 students
(B) : 14 students
(C) : 6 students
(D) : 2 students

Section 5166
(A) : 26 students
(B) : 5 students
(C) : 20 students
(D) : 3 students

According to the Stefan-Boltzmann law, the luminosity of a star is proportional to its size (its surface area) and the fourth power of its temperature. As a medium-mass or massive main sequence star becomes a giant or supergiant, its outer layers expand and cool. Thus the size increases while the temperature decreases, resulting in approximately the same luminosity as it makes a horizontal track to the right across a Hertzsprung-Russell diagram.