Astronomy final exam question: are globular cluster stars metal-poor or metal-rich?

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:

anon: Are stars in globular clusters considered metal-rich or metal-poor?
Cirr: Since globular clusters formed a long time ago, their stars have been fusing for a long time, and so will be metal-rich.

Discuss why this answer is 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=20100307080114AA5ua6g.

Solution and grading rubric:

• p:
  Explains why the answer is incorrect, because:
  1. stars in globular clusters ("formed a long time ago") will be metal-poor, having formed from essentially just hydrogen; as
  2. newer stars are metal-rich, having formed from hydrogen enriched with metals produced by and released from previous generation stars that have undergone supernovae explosions.
• 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. Typically has only one of (1)-(2) complete and correct.
• 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, such as breaking down of metals; masses and evolution rates.
• 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 30676
Exam code: finalSBr6
p: 12 students
r: 5 students
t: 12 students
v: 2 students
x: 4 students
y: 0 students
z: 2 students

A sample "p" response (from student 1231):
Another sample "p" response (from student 4321):
A sample "t" response (from student 1214):
A sample "v" response (from student 1236):
Another sample "x" response (from student 8008):