Astronomy final exam question: metal rich vs. metal poor stars

Astronomy 210 Final Exam, Fall Semester 2008
Cuesta College, San Luis Obispo, CA

Older stars are metal poor, while newer stars are metal rich because:
(A) metals produced by older stars were released by supernova explosions, and became part of newer stars.
(B) dark matter gradually converts metal rich stars into metal poor stars.
(C) older stars concentrate more of their metals into their cores, leaving their outer layers metal poor.
(D) older stars have had more time to break down heavy elements into lighter elements.

Section 70158
(A) : 41 students
(B) : 1 student
(C) : 14 students
(D) : 12 students

Correct answer: (A)

Stars produce metals (elements heavier than hydrogen and helium) in their cores during their giant/supergiant phases, up through type Ia/II supernovae explosions. Along with their unused hydrogen, these metals are then scattered into the interstellar medium, which are then incorporated into later generations of stars. An old, early generation star will have metals only in its core, while a young, later generation star will have metals sprinkled in its outer layers.

This was asked previously this semester as a quiz question.

"Difficulty level": 64% (including partial credit for multiple-choice)
Discrimination index (Aubrecht & Aubrecht, 1983): 0.33