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?Discuss why this answer is correct, and how you know this. Explain using the properties and evolution of stars.
Alexis: [These stars] could not have formed from the same hydrogen cloud.
*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:
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: