Astronomy clicker question: Milky Way mass

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

Students were asked the following clicker question (Classroom Performance System, einstruction.com) at the end of their learning cycle:

The total mass of the Milky Way can be estimated by:
(A) observing how quickly or slowly stars at different distances orbit its center.
(B) observing how quickly the supermassive black hole at its center is growing.
(C) carefully counting all the stars in the night sky.
(D) timing all of the Cepheid variable stars in the halo.
(E) (I'm lost, and don't know how to answer this.)

Section 70160
(A) : 14 students
(B) : 6 students
(C) : 0 students
(D) : 7 students
(E) : 0 students

Correct answer: (A)

Kepler's third law would be used to determine the total mass of the Milky Way. (This clicker question sets up the introduction of dark matter.) Response (D) would determine the luminosity of the Cepheid variable stars, from which the their distances could be found, resulting in the distance of the sun from the center of the Milky Way. Response (B) is a somewhat fanciful notion.