Astronomy final exam question: supergiant and white dwarf star cluster?

Astronomy 210 Final Exam, Fall Semester 2008
Cuesta College, San Luis Obispo, CA

[20 points.] Why is it not possible to have both white dwarfs and supergiants in the same star cluster? Explain using the properties of mass and stellar lifetimes.

Solution and grading rubric:

• p = 20/20:
  Correct. Discusses how all stars in a cluster are born at the same time and have the same age, but supergiants arise from massive stars, which have much faster evolution rates/shorter lifetimes than medium mass stars, which eventually become white dwarfs.
  
• r = 16/20:
  Nearly correct (explanation weak, unclear or only nearly complete); includes extraneous/tangential information; or has minor errors. At least understands how lifetimes depends on masses, but does not explicitly connect end stages to mass (i.e., may not distinguish convincingly between using "white dwarf" interchangeably with "red dwarf").
  
• t = 12/20:
  Contains right ideas, but discussion is unclear/incomplete or contains major errors. Some discussion of difference in lifetimes, but compounded/confounded with other factors. May argue that this is possible if the supergiants came from "elsewhere" to mingle with white dwarfs.
  
• v = 8/20:
  Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. May discuss how these stars feed on or disrupt each other, etc.
  
• 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 70158
p: 10 students
r: 25 students
t: 16 students
v: 16 students
x: 0 students
y: 1 student
z: 0 students

A sample "p" response (from student 4562):

Another sample "p" response (from student 9057):

And another sample "p" response (from student 3008):

A sample "r" response (from student 3578), conflating white dwarfs with red dwarfs:

Several other sample "r" responses (starting with student 1616), conflating white dwarfs with red dwarfs, but in the framework of the "house party" model (discussed in class) of stellar evolution rates in star clusters (massive stars = people who come early, stay briefly, leave early; low mass stars = people who show up very late, stay over and never leave):


(...and from student 1990):

(...and from student 2007):

A sample "t" response (from student 7787), positing that stars in the same cluster must all have the same mass:

Another sample "t" response (from student 4409), burying the correct answer with a list of ruminative speculation:

Two sample "v" responses (starting with student 1327), discussing how supergiants would somehow interfere with white dwarfs if they were in the same star cluster:

(...and from student 3825):

A sample "v" response (from student 4018) discussing how supergiants and white dwarfs in the same star cluster would incongruous: