Astronomy final exam question: blue stars in old star cluster?

Astronomy 210 Final Exam 2, fall semester 2016
Cuesta College, San Luis Obispo, CA

An astronomy question on an online discussion board[*] was asked and answered:
Pd: Can there be old blue stars in an old star cluster?
rcp: Not really...
Discuss why this answer is correct, 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=20161125205646AADjj75.

Solution and grading rubric:
  • p:
    Correct. Understands that for stars in an old cluster the only blue stars must be massive main-sequence stars, and evolve very quickly, and will have already exploded as type II supernovae.
  • 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.
  • 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:
Grading distribution:
Section 70158
Exam code: finalS1tH
p: 18 students
r: 5 students
t: 6 students
v: 5 students
x: 0 students
y: 0 students
z: 2 students

A sample "p" response (from student 1209):

Another sample "p" response (from student 2013):

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