Astronomy quiz question: comparing brightnesses of stars moved to 10 parsecs away

Astronomy 210 Quiz 5, spring semester 2018
Cuesta College, San Luis Obispo, CA

The distances and absolute magnitudes of two stars are listed below.

	d distance (parsecs)	M absolute magnitude
Gliese 556	44 pcs	+7.3
θ Ursae Majoris	44 pcs	+3.0

When placed at 10 parsecs away, the star that is brightest is:
(A) Gliese 556.
(B) θ Ursae Majoris.
(C) (There is a tie.)
(D) (Not enough information is given.)

Correct answer (highlight to unhide): (B)

The absolute magnitude of a star is the brightness it has when placed 10 parsecs away from Earth. Since θ Ursae Majoris has an absolute magnitude of +3.0 that is brighter than Gliese 556's absolute magnitude of +7.3 (as larger positive magnitude values correspond to dimmer brightnesses), if both stars were placed 10 parsecs away from Earth, θ Ursae Majoris will appear to be brighter in the night sky.

Section 70160
Exam code: quiz05Oor7
(A) : 1 student
(B) : 13 students
(C) : 2 students
(D) : 1 student

Success level: 78% (including partial credit for multiple-choice)
Discrimination index (Aubrecht & Aubrecht, 1983): 0.20