Astronomy midterm question: star cluster with white dwarfs?

Astronomy 210 Midterm 2, spring semester 2016
Cuesta College, San Luis Obispo, CA

Discuss which H-R diagram is more likely to be a star cluster with type II supernovae (which are deliberately not shown). Explain using the properties of mass and stellar lifetimes, evolution of stars, and star cluster ages.

Solution and grading rubric:

• p:
  Correct. Understands that:
  1. stars in the same cluster are all born at the same time, but massive stars evolve faster than medium-mass stars, which evolve faster than low-mass stars (the "house party" analogy);
  2. medium-mass stars end their main-sequence lifetimes by becoming giants, planetary nebulae, and then white dwarfs;
  3. such that for medium-mass stars to have reached their end stage, massive stars have already gone through all their stages and have exploded as type II supernovae (and no longer appear on an H-R diagram), while low-mass stars have just begun their main-sequence life as red dwarfs.
• 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. 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. Garbled discussion of properties and evolution of stars.
• 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 30676
Exam code: midterm02n4s5
p: 3 students
r: 6 students
t: 10 students
v: 3 students
x: 0 students
y: 0 students
z: 0 students

A sample "p" response (from student 1503) discussing the "house party model":
Another sample "p" response (from student 1758):