Astronomy midterm question: white dwarf and supergiant cluster

Astronomy 10 Midterm 3, Fall Semester 2007
Cuesta College, San Luis Obispo, CA

Astronomy 10 learning goal Q9.5

[15 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 = 15/15:
  Correct. White dwarfs are the end product of medium mass stars. Supergiants are near the end of massive stars. It will take longer for a medium mass protostar to reach the main sequence, end its main sequence lifetime, go through its giant phase to become a white dwarf than a massive protostar to go through similar stages. Thus stars in a star cluster, which started at the same time, cannot have white dwarfs and supergiants existing at a same later time.
  
• r = 12/15:
  Nearly correct (explanation weak, unclear or only nearly complete); includes extraneous/tangential information; or has minor errors. Understands that evolution times (protostar to main sequence, main sequence lifetime, main sequence to end stage) are inversely proportional to mass, but discussion of these stages has minor omissions or errors. Or understands evolutionary sequence that results in these end stages, but discussion of evolutionary times has minor omissions or errors.
  
• t = 9/15:
  Contains right ideas, but discussion is unclear/incomplete or contains major errors. As (r), but discussion of evolutionary stages, or times is problematic.
  
• 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 distributions:
Section 1073
p: 8 students
r: 10 students
t: 7 students
v: 6 students
x: 10 students
y: 1 student
z: 0 students

A sample of a "p" response (from student 4040) is shown below: