20170518

Astronomy final exam question: star cluster more likely to have red giants?

Astronomy 210 Final Exam, spring semester 2017
Cuesta College, San Luis Obispo, CA

An astronomy question on an online discussion board[*] was asked and answered:
Curi: Star cluster A is 12 billion years old, while star cluster B is one million years old. Which star cluster will have more red giants?
arsl: Star cluster A.
Discuss whether this answer is correct or incorrect, and how you know this. Explain using the properties of mass and stellar lifetimes, evolution of stars, and star cluster ages.

[*] answers.yahoo.com/question/index?qid=20110402021909AA2fbXz.

Solution and grading rubric:
  • p:
    Correct. Understands that star cluster A will have more red giants (while star cluster B would not have any) because:
    1. medium-mass stars become red giants at the end of their main-sequence lifetimes (approximately 10 billion years, much longer than one million years); and
    2. medium-mass stars in the younger star cluster B will not have had enough time to become red giants, while the medium-mass stars in the older star cluster A will have had enough time to become red giants.
  • r:
    Nearly correct (explanation weak, unclear or only nearly complete); includes extraneous/tangential information; or has minor errors.
  • t: Contains right ideas, but discussion is unclear/incomplete or contains major errors. May have conflated red giants with red supergiants, which are massive stars that have ended their very short main-sequence life, and concludes that star cluster B would have more red supergiants.
  • v:
    Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. Garbled discussion of properties and evolution of stars.
  • x:
    Implementation/application of ideas, but credit given for effort rather than merit. Discussion other than that of the properties and evolution of stars.
  • y:
    Irrelevant discussion/effectively blank.
  • z:
    Blank.
Grading distribution:
Section 30674
Exam code: finalnmSS
p: 1 student
r: 3 students
t: 7 students
v: 6 students
x: 1 student
y: 0 students
z: 0 students
A sample "p" response (from student 6877):

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