Astronomy clicker question: dark matter evidence

Astronomy 10, Spring Semester 2008
Cuesta College, San Luis Obispo, CA

Astronomy 10 learning goal Q11.3

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

[0.3 points.] How do we know that there is dark matter in the halo of the Milky Way (above/below its disk)?
(A) The temperature of empty space is not really absolute zero.
(B) Stars just outside of the Milky Way appear dimmer than they should be.
(C) Stars in the inner and outer parts of the disk orbit the center of the Milky Way at the same speed.
(D) The Milky Way has more black holes than expected.

Correct answer: (withheld until after whole-class discussion)

Student responses
Section 4160
(A) : 5 students
(B) : 19 students
(C) : 4 students
(D) : 3 students

Whole-class discussion
Instructor: "So why did you guys choose (B)?"

Student 1: "Because dark matter is dark?"

Instructor: "Fine. And what's the deal with (C)--does this look like Kepler's third law?"

Student 2: (Explains why it is not.)

Instructor: "Yes. That would certainly be strange, to see inner and outer orbits having the same speed. But it is exactly what is seen for the stars that orbit around the center of the Milky Way."

Class: "What?!?"

Instructor: "...And that is the evidence for dark matter."

Class: "What?!?"

Well, that's the way it might have been said.

Correct answer: (C)