Astronomy midterm question: massive star evolution

Astronomy 210 Midterm 2, fall semester 2010
Cuesta College, San Luis Obispo, CA

For the H-R diagram of a star cluster shown at right, decide whether the massive stars are in their protostar, main sequence, or supergiant phase, or if there are no massive stars in this star cluster, and how you know this. Explain your answer using the properties of stars and evolution times.

Solution and grading rubric:

• p = 20/20:
  Correct. Massive stars in this star cluster are supergiants (or there may be no massive stars left after exploding as type II supernovae). Understands that massive stars evolve faster than low-mass stars, such that by the time the low-mass stars to have nearly reached the lower right corner of the H-R diagram, the massive stars have already gone through their protostar and main sequence phases.
  
• r = 16/20:
  Nearly correct (explanation weak, unclear or only nearly complete); includes extraneous/tangential information; or has minor errors. As (p), but with minor omissions, or while correctly identifying massive stars as being in their supergiant phase, focus of discussion is a (complete, correct) description of life stages of massive stars rather than recognizing the relative evolution rates and placement of medium- and low-mass stars on the star cluster H-R diagram.
  
• t = 12/20:
  Contains right ideas, but discussion is unclear/incomplete or contains major errors. Identifies upper right corner group of stars off the main sequence as supergiants, but does not plausibly rule out the possibility that they might be protostars instead (by discussing the relative positions of the medium- and low-mass stars).
  
• v = 8/20:
  Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner.
  
• x = 4/20:
  Implementation/application of ideas, but credit given for effort rather than merit. Conclusion without substantive argument.
  y = 2/20: Irrelevant discussion/effectively blank.
  
• y = 2/20:
  Irrelevant discussion/effectively blank.
  
• z = 0/20:
  Blank.
  

Grading distribution:
Section 70158
p: 21 students
r: 10 students
t: 9 students
v: 4 students
x: 2 students
y: 0 students
z: 0 students

A sample "p" response (from student 1234), discussing a "house party" model:
Another sample "p" response (from student 1122), also discussing a "house party" model: