Astronomy midterm question: small cool stars or large hot stars in young cluster?

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

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

dk: How can we tell that a star cluster is young (only 10 million years old or so)? Small, cool stars, [or] large, hot stars [on the main sequence]?
pl: Large, hot stars [on the main sequence] means the cluster is young.

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=20070625180152AADZE8x.

Solution and grading rubric:

• p:
  Correct. Understands that massive (large, hot) stars evolve faster than low-mass (small, cool) stars, such a younger star cluster will have massive stars that are on the main-sequence, while an older star cluster will have low-mass stars on the main-sequence.
• r:
  Nearly correct (explanation weak, unclear or only nearly complete); includes extraneous/tangential information; or has minor errors.
• t:
  Contains right ideas, but discussion is unclear/incomplete or contains major errors. 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 70160
Exam code: midterm02NuF7
p: 22 students
r: 2 students
t: 3 students
v: 0 students
x: 0 students
y: 0 students
z: 0 students

A sample "p" response (from student 0716):
Another sample "p" response (from student 7783), appealing to the "house party" model: