Astronomy final exam question: giant versus supergiant metallicity

Astronomy 210 Final Exam, spring semester 2014
Cuesta College, San Luis Obispo, CA

An astronomy question on an online discussion board[*] was asked and answered:

Pd: Right now, which would be more metal-rich in its outermost layers today: a giant or a supergiant?
th: The correct answer is a giant.

Discuss why this answer is incorrect, and how you know this. Explain using the properties and evolution of stars.

[*] answers.yahoo.com/question/index?qid=20140405002159AARyDpo.

Solution and grading rubric:

• p:
  Correct. Understands that (1) older stars are metal-poor having formed from essentially just hydrogen, while newer stars are metal-rich, having formed from hydrogen enriched with metals produced by previous generation stars; and (2) supergiants are the end-stage of massive stars, which have evolved rapidly (having short main-sequence lifetimes), such that they had formed very recently than giants (end-stage of medium-mass stars, which have longer main-sequence lifetimes), and are thus metal-rich.
• r:
  Nearly correct (explanation weak, unclear or only nearly complete); includes extraneous/tangential information; or has minor errors. One of the two points (1)-(2) correct, other is problematic/incomplete.
• t:
  Contains right ideas, but discussion is unclear/incomplete or contains major errors. Both points (1)-(2) problematic/incomplete, or one point correct while other is missing.
• 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. Discusses factors other than relevant to Hubble's law, redshifts/recession velocities, expansion of space, big bang, etc. Discussion not based on metallicity and evolution rates of stars.
• y:
  Irrelevant discussion/effectively blank.
• z:
  Blank.

Grading distribution:
Section 30674
Exam code: finaln0oN
p: 5 students
r: 1 student
t: 9 students
v: 8 students
x: 1 student
y: 0 students
z: 0 students

Section 30676
Exam code: finals0N6
p: 5 students
r: 2 students
t: 16 students
v: 7 students
x: 1 student
y: 1 student
z: 2 students

A sample "p" response (from student 0923):
Another sample "p" response (from student 6288), using the "house party" analogy of stellar evolution rates:
A sample "t" response (from student 1920), only explaining how giants are younger than supergiants:
Another sample "t" response (from student 6124), only explaining how younger stars inherit the metals from previous-generation stars that have gone supernovae: