Astronomy midterm question: star cluster with supergiants and white dwarfs?

Astronomy 210 Midterm 2, spring semester 2013
Cuesta College, San Luis Obispo, CA

An astronomy question on an online discussion board[*] was asked and answered:

Pd: Would a star cluster with both supergiants and white dwarfs be old, or young?
t153: It would certainly be very unlikely for any cluster to have both supergiants and white dwarfs.

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

[*] answers.yahoo.com/question/index?qid=20130419183134AAKacCx.

Solution and grading rubric:

• p:
  Correct. Understands that:
  1. stars in the same cluster are all born at the same time;
  2. massive stars evolve faster than medium-mass stars;
  3. 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:
  Nearly correct (explanation weak, unclear or only nearly complete); includes extraneous/tangential information; or has minor errors. Discusses only two out of the three concepts in (p) correctly. 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:
  Contains right ideas, but discussion is unclear/incomplete or contains major errors. Discusses only one out of three concepts in (p) correctly, typically at least understands correlation between mass and main-sequence lifetimes.
• v:
  Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner.
• x:
  Implementation/application of ideas, but credit given for effort rather than merit. Discussion other than that of the properties and evolution of stars.
• y:
  Irrelevant discussion/effectively blank.
• z:
  Blank.

Grading distribution:
Section 30674
Exam code: midterm02nNm3
p: 3 students
r: 14 students
t: 4 students
v: 3 students
x: 7 students
y: 0 students
z: 0 students

Section 30676
Exam code: midterm02s5Sz
p: 4 students
r: 29 students
t: 6 students
v: 1 student
x: 4 students
y: 0 students
z: 0 students

A sample "p" response (from student 1127):
Another sample "p" response (from student 7045):