## 20111016

### Astronomy quiz question: distance from apparent and absolute magnitudes

Astronomy 210 Quiz 4, fall semester 2011
Cuesta College, San Luis Obispo, CA

[Version 1]
Cygnus OB2-12 is a star that has an apparent magnitude m = +11.4 and an absolute magnitude MV = −10.6. How far away is Cygnus OB2-12 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.

Cygnus OB2-12 appears to be +11.4 as seen from its true distance from Earth, but when placed at the "fair" distance of 10 parsecs away, its becomes much brighter (−10.6). Thus it was brought much closer to Earth when moved to 10 parsecs, and was located much farther than 10 parsecs away from Earth.

Section 70158
Exam code: quiz04sIZ3
(A) : 17 students
(B) : 8 students
(C) : 0 students
(D) : 9 students

Success level: 48% (including partial credit for multiple-choice)
Discrimination index (Aubrecht & Aubrecht, 1983): 0.89

[Version 2]
σ Sagittarii is a star that has an apparent magnitude m = +2.05 and an absolute magnitude MV = -2.14. How far away is σ Sagittarii 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.

σ Sagittarii appears to be +2.05 as seen from its true distance from Earth, but when placed at the "fair" distance of 10 parsecs away, its becomes much brighter (-2.14). Thus it was brought much closer to Earth when moved to 10 parsecs, and was located much farther than 10 parsecs away from Earth.

Section 70160
Exam code: quiz04nuO6
(A) : 12 students
(B) : 10 students
(C) : 2 students
(D) : 0 students

Success level: 53% (including partial credit for multiple-choice)
Discrimination index (Aubrecht & Aubrecht, 1983): 0.83