Astronomy midterm question: H-R turnoff points

Astronomy 10 Midterm 3, Spring Semester 2008
Cuesta College, San Luis Obispo, CA

Astronomy 10 learning goal Q9.5
[15 points.] Show what the turnoff point of a star cluster is on an H-R diagram, and explain how this can be used to estimate the age of the stars in the cluster.

Solution and grading rubric:

• p = 15/15:
  Correct. "Turnoff point" is where stars from a cluster deviate from the main-sequence line. A turnoff point near the top left indicates a young cluster, as the massive stars have not yet reached their supergiant phases, turnoff point near the bottom right indicates an older cluster, as only the low-mass stars are still on the main sequence. Clearly indicates/draw/describes what a typical turnoff point looks like, and how its position is correlated with age. May instead describe how the different evolution tracks/rates of different mass stars that are observed on the main sequence line can be used to determine the age of a star cluster, without explicit mention of the "turnoff point."
  
• r = 12/15:
  Nearly correct (explanation weak, unclear or only nearly complete); includes extraneous/tangential information; or has minor errors.
  
• t = 9/15:
  Contains right ideas, but discussion is unclear/incomplete or contains major errors. At least understands mass determines evolution tracks/rates.
  
• v = 6/15:
  Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner.
  
• x = 3/15:
  Implementation/application of ideas, but credit given for effort rather than merit.
  
• y = 1.5/15:
  Irrelevant discussion/effectively blank.
  
• z = 0/15:
  Blank.
  

Grading distribution:
Section 5166
p: 9 students
r: 4 students
t: 15 students
v: 10 students
x: 7 students
y: 7 students
z: 3 students

A sample "p" response (from student 9518):
A sample "y" response (froms student ), who like many others felt compelled to write something down (for minimal partial credit) rather than leave the answer blank: