Astronomy midterm question: star cluster age

Astronomy 210 Midterm 2, Fall Semester 2008
Cuesta College, San Luis Obispo, CA

[20 points.] Shown at right is an H-R diagram of a star cluster, where all of these stars have the same age. Discuss whether this star cluster is very young or is very old, and explain why.

Solution and grading rubric:

• p = 20/20:
  Correct. Massive stars evolve quickly, and have already gone through their main sequence lifetimes to the supergiant stage, so this cannot be a very young star cluster. Low mass stars evolve slowly, so for these stars to have reached the main sequence, this must be a very old star cluster.
  
• r = 16/20:
  Nearly correct (explanation weak, unclear or only nearly complete); includes extraneous/tangential information; or has minor errors.
  
• t = 12/20:
  Contains right ideas, but discussion is unclear/incomplete or contains major errors. May state that this is a young star cluster, but understands how massive and low mass stars evolve differently.
  
• v = 8/20:
  Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. Uses other criteria to determine age of star cluster.
  
• x = 4/20:
  Implementation/application of ideas, but credit given for effort rather than merit.
  
• y = 2/20:
  Irrelevant discussion/effectively blank.
  
• z = 0/20:
  Blank.
  

Grading distribution:
Section 70160
p: 14 students
r: 1 student
t: 11 students
v: 3 students
x: 0 students
y: 0 students
z: 0 students

A sample "p" response, discussing the "house party" analogy of how stellar evolution rates depend on mass (from student 3089):