20170518

Astronomy final exam question: white dwarf vs. red dwarf metallicity

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:
Chr: Which dwarf star will have more metals? White dwarfs or red dwarfs?
t153: White dwarfs will have lots of metals compared to red dwarfs.
Discuss why this answer is correct, 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=20110318182328AA0dt07.

Solution and grading rubric:
  • p:
    Understands that white dwarfs are more metal-rich than red dwarfs because:
    1. white dwarfs are the end-stage of the core of a medium-mass star, and are composed solely of metals (carbon and oxygen); and
    2. red dwarfs are main-sequence stars that are still fusing hydrogen into helium (which is not considered a metal).
  • 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. May base argument on white dwarfs being metal-rich due to being born more recently and incorporating metals from a previous generation star, while red dwarfs would have been born long ago during a metal-poor universe.
  • 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 30674
Exam code: finalnmSS
p: 2 students
r: 4 students
t: 6 students
v: 2 students
x: 4 students
y: 0 students
z: 0 students

A sample "p" response (from student 6877):

A sample "t" response (from student 0907):

A sample "x" response (from student 9355):

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