20111126

Astronomy midterm question: red dwarf vs. massive main-sequence star metallicity

Astronomy 210 Midterm 2, fall semester 2011
Cuesta College, San Luis Obispo, CA

An astronomy question on an online discussion board[*] was asked and answered:
Pd: Which star today would have more metals: a red dwarf or a massive main-sequence star?
qp: I'm guessing a massive main-sequence star; because a red dwarf is likely very old...
Discuss whether or not if this answer is correct, and how you know this. Explain using the properties and evolution of stars.

[*] answers.yahoo.com/question/index?qid=20110601124741AA76NMC.

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;
    2. red dwarfs take a long time to become main sequence stars, such that they had to have started forming a long time ago, are thus are metal-poor, while massive stars take a short time to form, such that they formed recently, and thus are 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) is correct, other is problematic.
  • t:
    Contains right ideas, but discussion is unclear/incomplete or contains major errors. Only one of the two points (1)-(2) correct, other is missing, or both are problematic.
  • 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.
  • y:
    Irrelevant discussion/effectively blank.
  • z:
    Blank.
Grading distribution:
Section 70158
Exam code: midterm02s3Ar
p: 9 students
r: 3 students
t: 14 students
v: 8 students
x: 0 students
y: 0 students
z: 0 students

Section 70160
Exam code: midterm02n1cT
p: 9 students
r: 3 students
t: 9 students
v: 6 students
x: 1 student
y: 0 students
z: 0 students

A sample "p" response (from student 2541):
Another sample "p" response (from student 1863), using a "House Party" analogy:
Yet another sample "p" response (from student 1985), lavishly illustrated:
A sample "v" response (from student 0628), where older stars would have had more time to absorb metals than recent stars:
Another sample "v" response (from student 1226), where older stars have more time to break down metals:
Yet another sample "v" response (from student 1559), where older stars have more time to fuse metals by breaking them down:

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