Astronomy midterm question: globular cluster metallicity

Astronomy 210 Midterm 2, Fall Semester 2009
Cuesta College, San Luis Obispo, CA

[20 points.] Discuss whether globular cluster stars in the halo of the Milky Way are expected to be metal-rich or metal-poor, and why. Explain using the properties of mass and stellar lifetimes, and the Milky Way.

Solution and grading rubric:

• p = 20/20:
  Correct. Globular cluster stars in the halo are the old first-generation stars that formed when the Milky Way was spherical in shape and are composed primarily of hydrogen with little metal content, as metal content increases over time due to stars fusing and releasing fusion products to subsequent generations.
  
• r = 16/20:
  Nearly correct (explanation weak, unclear or only nearly complete); includes extraneous/tangential information; or has minor errors. Complete explanation of why metal-poor stars are older than metal-rich stars, but discussion of why the halo is the oldest part of the Milky Way is problematic, or vice versa.
  
• t = 12/20:
  Contains right ideas, but discussion is unclear/incomplete or contains major errors. As (r), but discussion of why the halo is the oldest part of the Milky Way is missing, or vice versa.
  
• v = 8/20:
  Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. Has only problematic discussion of how metal-poor stars are old stars (e.g., metals are used up by stars over time).
  
• x = 4/20:
  Implementation/application of ideas, but credit given for effort rather than merit. Implausible evidence/methods/discussion.
  
• y = 2/20:
  Irrelevant discussion/effectively blank.
  
• z = 0/20:
  Blank.
  

Grading distribution:
Section 70158
p: 14 students
r: 6 students
t: 9 students
v: 22 students
x: 0 students
y: 0 students
z: 0 students

A sample "p" response (from student 9221), with a fanciful depiction of increasing metallicity in subsequent generations of stars:

A more conventional sample "p" response (from student 1520):

Another sample "p" response (from student 1204), illustrating the monolithic collapse model:

A sample "v" response (from student 6636), typically discussing how stars use up metals over time to eventually become old and metal-poor: