## 20140405

### Astronomy quiz question: comparing star distances given apparent, absolute visual magnitudes

Astronomy 210 Quiz 5, spring semester 2014
Cuesta College, San Luis Obispo, CA

 mapparentmagnitude MVabsolute visualmagnitude Star X +1 +3 Star Y –1 –3

Which star is closer to Earth?
(A) Star X.
(B) Star Y.
(C) (There is a tie.)
(D) (Not enough information is given.)

Correct answer (highlight to unhide): (A)

Star X appears to have a brightness of +1 as seen from its true distance from Earth, but when placed at the "fair" distance of 10 parsecs away, it becomes much dimmer (+3). This means that Star X was taken farther away from Earth when moved to 10 parsecs, and thus is located closer than 10 parsecs away from Earth.

Star Y appears to have a brightness of –1 as seen from its true distance from Earth, but when placed at the "fair" distance of 10 parsecs away, it becomes much brighter (–3). This means that Star X was brought closer to Earth when moved to 10 parsecs, and thus is located farther than 10 parsecs away from Earth.

Thus Star X is closer to Earth than Star Y.

Section 30674
Exam code: quiz05n3Ne
(A) : 11 students
(B) : 9 students
(C) : 2 students
(D) : 3 students

Success level: 47% (including partial credit for multiple-choice)
Discrimination index (Aubrecht & Aubrecht, 1983): 0.63

Section 30676
Exam code: quiz05suN7
(A) : 19 students
(B) : 16 students
(C) : 4 students
(D) : 0 students

Success level: 52% (including partial credit for multiple-choice)
Discrimination index (Aubrecht & Aubrecht, 1983): 0.82