Astronomy midterm question: massive main-sequence stars and white dwarfs in same star cluster?

Astronomy 210 Midterm 2, fall semester 2012
Cuesta College, San Luis Obispo, CA

[20 points.] An astronomy question on an online discussion board(*) was asked and answered:

Bob: What's wrong with a [star cluster that has massive] main-sequence stars, and white dwarfs?
Alexis: [These stars] could not have formed from the same hydrogen cloud.

Discuss why this answer is correct, and how you know this. Explain using the properties and evolution of stars.

*Adapted from: http://answers.yahoo.com/question/index?qid=20111029161602AATbPgo.

Solution and grading rubric:

• p = 20/20:
  Correct. Understands that (a) stars in the same cluster are all born at the same time, (b) massive stars evolve faster than medium-mass stars, and (c) white dwarfs are the remnant of medium-mass stars (after going through its giant and planetary nebula phases), such that it is not possible for a massive star on the main-sequence to be the same age as a medium-mass star that has already ended its main-sequence lifetime to become a white dwarf.
• r = 16/20:
  Nearly correct (explanation weak, unclear or only nearly complete); includes extraneous/tangential information; or has minor errors. May confuse white dwarfs (medium-mass stars that have long ago ended their main-sequence lifetime) with red dwarfs (low-mass stars on the main-sequence).
• t = 12/20:
  Contains right ideas, but discussion is unclear/incomplete or contains major errors. At least understands correlation between mass and main sequence lifetimes.
• 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. Discussion other than that of the properties and evolution of stars.
• y = 2/20:
  Irrelevant discussion/effectively blank.
• z = 0/20:
  Blank.

Grading distribution:
Section 70158
Exam code: midterm02s0Ur
p: 5 students
r: 13 students
t: 8 students
v: 5 students
x: 2 students
y: 0 students
z: 0 students

A sample "p" response (from student 0916):
Another sample "p" response (from student 0402), appealing to the "house party" model: