20170105

Astronomy final exam question: change of a star's absorption metal lines?

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

An astronomy question on an online discussion board[*] was asked and answered:
Pd: Would the spectrum of a massive star show more or fewer metal absorption lines as the cluster it is in gets older?
pub: More.
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=20161021230048AADAVpf.

Solution and grading rubric:
  • p:
    Understands that the spectrum of a massive star does not gain more metal absorption lines because:
    1. metal lines in a star's absorption spectrum are due to the presence of metals in its outer layers; and
    2. production of heavier elements from fusion only occur in the core of a massive star, and do not add to the presence of metals in the outer layers; such that
    3. the presence of metal absorption lines should remain constant throughout its main-sequence and supergiant stages.
  • r:
    Nearly correct (explanation weak, unclear or only nearly complete); includes extraneous/tangential information; or has minor errors. May interpret question to argue that an older massive star would show fewer absorption lines relative to newer stars.
  • 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:
    Blank.
Grading distribution:
Section 70158
Exam code: finalS1tH
p: 4 students
r: 12 students
t: 7 students
v: 9 students
x: 4 students
y: 0 students
z: 0 students

Section 70160
Exam code: finalnwhP
p: 2 students
r: 15 students
t: 1 student
v: 5 students
x: 1 student
y: 0 students
z: 0 students

A sample "p" response (from student 2016):

A sample "r" response (from student 4135):

Another sample "r" response (from student 0924), with a note for the semester's end:

A sample "v" response (from student 2326):

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