Astronomy final exam question: metals breaking down in stars?

Astronomy 210 Final Exam, spring semester 2018
Cuesta College, San Luis Obispo, CA

An astronomy question on an online discussion board[*] was asked and answered:
Alexis: Why are metals more abundant in newer stars than older stars?
tdloo: Because the metals haven't had time to break apart yet in newer stars.
Discuss why this answer is incorrect, and how you know this. Explain using the properties of mass and stellar lifetimes, and evolution of stars.

[*] answers.yahoo.com/question/index?qid=20130409190107AA1Ej3v.

Solution and grading rubric:
  • p:
    Correct. Discusses why the answer is incorrect by discussing at least one of the following arguments:
    1. metals are produced in the cores of massive stars (and medium-mass supergiants) by fusion, which entails the combination of atoms, rather than in breaking them apart; or that
    2. metals are typically produced in the cores of previous generation ("old") massive stars, which explode as type II supernovae to scatter these metals that get incorporated into the next generation of "new" stars, such that metals are more abundant in newer stars than older stars.
  • 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, 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:
Grading distribution:
Section 30674
Exam code: finaln3sT
p: 16 students
r: 0 students
t: 1 student
v: 1 student
x: 1 student
y: 0 students
z: 1 student

Section 30676
Exam code: finalSuB1
p: 19 students
r: 1 student
t: 2 students
v: 12 students
x: 2 students
y: 0 students
z: 0 students

A sample "p" response (from student 5069):

1 comment:

Patrick M. Len said...

Grading distribution:
Astronomy 210 Final Exam, fall semester 2017
Section 70158
Exam code: finalSe77
p: 23 students
r: 5 students
t: 4 students
v: 5 students
x: 0 students
y: 0 students
z: 0 students

Section 70160
Exam code: finalNo7D
p: 13 students
r: 0 students
t: 7 students
v: 3 students
x: 2 students
y: 2 students
z: 0 students