Cuesta College, San Luis Obispo, CA
The H-R diagrams of three star clusters are shown below.
[Version 1]Which star cluster is the youngest? (Do not circle the H-R diagrams; clearly circle a response below.)
(A) Star cluster X.
(B) Star cluster Y.
(C) Star cluster Z.
(D) (There is a tie.)
Section 30674
(A) : 7 students
(B) : 6 students
(C) : 5 students
(D) : 0 students
Correct answer: (A) (Star cluster X)
Massive stars will evolve the quickest, reaching the upper part of the main sequence line first, staying there the shortest amount of time, and then leaving first, compared to the low-mass stars, which evolve the slowest, reaching the lower part of the main sequence line last, and then staying there...and remain there, as the universe has not been around long enough for low-mass stars to have exhausted their main-sequence lifetimes. Thus star cluster X is younger than both star clusters Y and Z, which have massive stars that already left the main-sequence, and low-mass stars that already reached the main sequence.
[Version 2]
Which star cluster is the oldest? (Do not circle the H-R diagrams; clearly circle a response below.)
(A) Star cluster X.
(B) Star cluster Y.
(C) Star cluster Z.
(D) (There is a tie.)
Section 30674
(A) : 0 students
(B) : 12 students
(C) : 5 students
(D) : 0 students
Correct answer: (B) (Star cluster Y)
"Difficulty level": 56% (including partial credit for multiple-choice)
Discrimination index (Aubrecht & Aubrecht, 1983): 0.69
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