Astronomy quiz question: red dwarfs as giants?

Astronomy 210 Quiz 5, Fall Semester 2008
Cuesta College, San Luis Obispo, CA

A red dwarf will never become a giant because:
(A) it is made of degenerate matter.
(B) its core will never get hot enough to fuse helium.
(C) it will never run out of hydrogen to fuse.
(D) not a single red dwarf has died of old age anywhere in the universe.

Correct answer: (B)

A red dwarf has a main sequence lifetime much longer than the age of the universe, so none have actually "died" yet. However, when all of the hydrogen in a red dwarf has been depleted, it will gravitationally contract, but not enough to increase the temperature for helium to fuse, so it will never restart fusion to become a giant, as a medium mass main sequence star would at the end of its hydrogen fusion lifetime.

Section 70158
(A) : 2 students
(B) : 51 students
(C) : 6 students
(D) : 7 students

"Success level": 80%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.22

Section 70160
(A) : 1 student
(B) : 16 students
(C) : 8 students
(D) : 5 students

"Success level": 58%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.31