## 20150412

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

Astronomy 210 Quiz 5, spring semester 2015
Cuesta College, San Luis Obispo, CA

The apparent magnitudes and distances of three stars are listed below.
 mapparentmagnitude ddistancefrom Earth Proxima Centauri +11 1.3 pcs Gliese 15 +11 3.5 pcs Struve 2398 +9 3.5 pcs

When placed at 10 parsecs away, the star that appears dimmest is:
(A) Proxima Centauri.
(B) Gliese 15.
(C) Struve 2398.
(D) (There is a tie. )

Correct answer (highlight to unhide): (A)

Proxima Centauri and Gliese 15 both appear to be equally dimmer (m = +11) as seen from their true distances from Earth than Struve 2398, which appears to be the brightest (m = +9). Gliese 15 is the same distance from Earth as Struve 2398, but when both are placed at 10 parsecs away from Earth, Gliese 15 would appear to be dimmer than Struve 2398, meaning that Gliese 15 would have a dimmer absolute magnitude M.

When Proxima Centauri and Gliese 15 are both placed 10 parsecs away from Earth, Proxima Centauri would appear dimmer than Gliese 15, as it would move a further distance back away from Earth.

Thus Proxima Centauri would be dimmest at 10 parsecs away from Earth (and thus have the dimmest absolute magnitude M) than Gliese 15, and Struve 2398 would be brightest at 10 parsecs away from Earth (and thus have the brightest absolute magnitude M).

Section 30674
Exam code: quiz05n36A
(A) : 17 students
(B) : 5 students
(C) : 3 students
(D) : 11 students

Success level: 49% (including partial credit for multiple-choice)
Discrimination index (Aubrecht & Aubrecht, 1983): 1.0