Astronomy quiz question: star cluster with white dwarfs

Astronomy 210 Quiz 6, fall semester 2017
Cuesta College, San Luis Obispo, CA

A star cluster with white dwarfs would also have __________ at the same time.
(A) massive protostars.
(B) medium-mass main sequence stars.
(C) red dwarfs.
(D) giants.

Correct answer (highlight to unhide): (C)

All stars in a star cluster are born at the same time, but undergo stellar evolution at different rates depending on their masses. Medium-mass stars start out as protostars, then reach their main sequence stage, and subsequently become giants, planetary nebulae, and then white dwarfs. So a star cluster with medium-mass stars in their terminal white dwarf stage would not have the earlier stages of medium-mass stars: main-sequence nor giants.

Since massive stars evolve from protostars much faster than medium-mass stars to reach their main-sequence stage, and then to become supergiants that explode as type II supernovae, none of them will be in their protostar stage in a star cluster where the medium-mass stars have already long ended their main-sequence lifetimes.

Low-mass stars evolve from protostars much slower than medium-mass stars to reach their main-sequence stage to become red dwarfs, and since these low-mass stars have such long main-sequence lifetimes (there have been no red dwarfs that have left the main-sequence since the universe began), and so low-mass red dwarfs can be found in an old star cluster where the medium-mass stars have reached their terminal white dwarf stage.

Section 70158
Exam code: quiz06sSu3
(A) : 5 students
(B) : 19 students
(C) : 9 students
(D) : 10 students

Success level: 25% (including partial credit for multiple-choice)
Discrimination index (Aubrecht & Aubrecht, 1983): 0.31