Astronomy quiz question: distance from apparent and absolute magnitudes

Astronomy 210 Quiz 4, Spring Semester 2010
Cuesta College, San Luis Obispo, CA

Alpha Mensae is a star that has an apparent magnitude m = +5.08 and an absolute magnitude M_V = +5.05. How far away is Alpha Mensae from Earth?
(A) Much farther away than 10 parsecs.
(B) A little farther away than 10 parsecs.
(C) Exactly 10 parsecs away.
(D) Closer than 10 parsecs away.

Correct answer: (B)

Alpha Mensae appears to be +5.08 as seen from its true distance from Earth, but when placed at the "fair" distance of 10 parsecs away, its becomes slightly brighter (+5.05). Thus it was brought closer to get to 10 parsecs, and was located farther than 10 parsecs away from Earth.

Section 30676
(A) : 7 students
(B) : 48 students
(C) : 3 students
(D) : 7 students

"Success level": 76% (including partial credit for multiple-choice)
Discrimination index (Aubrecht & Aubrecht, 1983): 0.62

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