20150525

Astronomy final exam question: different mass stars, same star cluster, same metallicity?

Astronomy 210 Final Exam, spring semester 2015
Cuesta College, San Luis Obispo, CA

"Could supergiants and medium-mass main-sequence stars in the same cluster have the same amount of metals in their outer layers?" This question was asked on an online discussion board and two possible answers were given[*]:
Yng: Yes, because the metals in their outer layers came from the dust cloud from which the stars were formed. Although more metals would be forming deep within the supergiants.
ogP: No, because supergiants have shorter lifetimes and will be younger and 
metal-rich compared to longer lifetime medium-mass main-sequence stars.
Discuss which answer is correct, and how you know this. Explain using the properties and evolution of stars.

[*] answers.yahoo.com/question/index?qid=20150506235250AA57UAC.

Solution and grading rubric:
  • p:
    Correct. Understands that stars in the same cluster were born at the same time from the same dust and gas cloud, such that their outer layers will have the same metal-content (even though massive stars will evolve faster through their main-sequence lifetimes than medium-mas stars, reaching their supergiant stage while the medium-mass stars are still on the main-sequence, thus "Yng" has the more correct argument.
  • 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. Typically argues how younger, shorter-lived stars would have inherited their metals from earlier generations of stars undergoing type II or type Ia supernovae.
  • 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.
  • 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: finaln1N3
p: 6 students
r: 3 students
t: 10 students
v: 7 students
x: 2 students
y: 1 student
z: 0 students

Section 30676
Exam code: finals3nG
p: 11 students
r: 5 students
t: 19 students
v: 5 students
x: 1 student
y: 4 students
z: 0 students

A sample "p" response (from student 2000):

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