Astronomy final exam question: relative metallicities of red dwarfs

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

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

Pd: What differences are there between a star born a long time ago, from a star born a short time ago? Both stars have the same mass.
qcl: A red dwarf born long ago will consist of fewer metals than a red dwarf born a short time ago.

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

[*] answers.yahoo.com/question/index?qid=20141206170547AAKawke.

Solution and grading rubric:

• p:
  Correct. Understands that:
  1. metals are formed only by medium-mass and massive stars during their giant and supergiant phases, and these would be released out to universe during a type Ia or type II supernova explosion;
  2. a star that formed a long time ago in the early universe would start out metal-poor, while a star that formed more recently would be metal-rich, having formed from hydrogen enriched with metals produced by previous generation stars;
  3. and red dwarfs are extremely long-lived main-sequence stars that fuse hydrogen into helium, so they cannot make metals regardless of its age, which means that any metal content must come from the material that it formed from.
• r:
  Nearly correct (explanation weak, unclear or only nearly complete); includes extraneous/tangential information; or has minor errors. Two of the three points (1)-(3) correct, the third is problematic/incomplete.
• t:
  Contains right ideas, but discussion is unclear/incomplete or contains major errors. Only one of the three points (1)-(3) correct, other two are problematic/incomplete.
• 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.
• x:
  Implementation/application of ideas, but credit given for effort rather than merit. Discussion not based on metallicity and evolution rates of stars.
• y:
  Irrelevant discussion/effectively blank.
• z:
  Blank.

Grading distribution:
Section 70158
Exam code: finalst0P
p: 0 students
r: 17 students
t: 12 students
v: 4 students
x: 1 student
y: 0 students
z: 0 students

Section 70160
Exam code: finaln3rF
p: 1 student
r: 9 student
t: 12 students
v: 5 students
x: 2 students
y: 0 students
z: 0 students

A sample "p" response (from student 3314):
A sample "r" response (from student 9999):
A sample "v" response (from student 0620):
Another sample "v" response (from student 0902):
Another sample "v" response (from student 1717):
Another sample "v" response (from student 3980):