Astronomy quiz question: dark matter evidence

Astronomy 210 Quiz 7, spring semester 2015
Cuesta College, San Luis Obispo, CA

Stars further out orbiting at the same speed as stars nearer the center of the Milky Way is evidence of:
(A) dark matter.
(B) the central supermassive black hole.
(C) the expansion of the universe.
(D) unequal amounts of matter and antimatter.

Correct answer (highlight to unhide): (A)

The orbital speeds of stars are nearly the same for all distances from the center of the Milky Way, instead of decreasing with increasing distance. This is due to the mass of the Milky Way not being concentrated at the very center (despite a supermassive black hole there), but being diffusely distributed above and below the disk of the Milky Way. Because this amount of mass is not visible in a manner such as luminous stars and gas, this unseen mass is termed "dark matter."

Section 30674
Exam code: quiz07Ctlu
(A) : 5 students
(B) : 11 students
(C) : 15 students
(D) : 5 students

"Success level": 18% (including partial credit for multiple-choice)
Discrimination index (Aubrecht & Aubrecht, 1983): 0.50
Section 30676
Exam code: quiz07Ctlu
(A) : 9 students
(B) : 16 students
(C) : 15 students
(D) : 4 students

"Success level": 26% (including partial credit for multiple-choice)
Discrimination index (Aubrecht & Aubrecht, 1983): 0.52