## 20191024

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

Astronomy 210 Quiz 5, fall semester 2019
Cuesta College, San Luis Obispo, CA

The apparent magnitudes and absolute magnitudes of three stars are listed below.
 mapparentmagnitude Mabsolutemagnitude Aldebaran +0.9 –0.6 Capella Ab +0.9 +0.4

The star that is farthest away from Earth is:
(A) Aldebaran.
(B) Capella Ab.
(C) (There is a tie.)
(D) (Not enough information is given.)

Correct answer (highlight to unhide): (A)

Aldebaran appears to have a brightness of +0.9 as seen from its actual location from Earth, but when placed at the "fair" distance of 10 parsecs away, it becomes brighter (–0.6). This means that Aldebaran was brought closer to Earth when moved to 10 parsecs, and thus is located farther than 10 parsecs away from Earth.

Similarly, Capella Ab appears to have a brightness of +0.9 as seen from its actual location from Earth, but when placed at the "fair" distance of 10 parsecs away, it also becomes brighter (+0.4), and must also be located farther than 10 parsecs away from Earth.

However, because Aldebaran's increase in brightness when brought to 10 parsecs was greater than Capella Ab's increase in brightness, then Aldebaran's actual location is further out from 10 parsecs (and farther away from Earth) than Capella Ab.

Section 70158
Exam code: quiz05Sh0w
(A) : 21 students
(B) : 6 students
(C) : 1 student
(D) : 1 student

Success level: 73% (including partial credit for multiple-choice)
Discrimination index (Aubrecht & Aubrecht, 1983): 0.50

Section 70160
Exam code: quiz05NpRm
(A) : 9 students
(B) : 8 students
(C) : 3 students
(D) : 2 students

Success level: 44% (including partial credit for multiple-choice)
Discrimination index (Aubrecht & Aubrecht, 1983): 0.38