Cuesta College, San Luis Obispo, CA
[Version 1]
[20 points.] Shown at right could be an H-R diagram of a star cluster, where all of these stars would have the same age. Discuss whether or not this H-R diagram is possible for a star cluster, and explain why, using the properties of stars and evolution times.Solution and grading rubric:
- p = 20/20:
Correct. Understands that massive stars evolve faster than low-mass stars, such that massive stars reach the upper left of the main sequence first, spend the least amount of time there, and leave well before low-mass stars become main-sequence stars. Therefore the H-R diagram is not possible for a star cluster where the stars would have the same age. - r = 16/20:
Nearly correct (explanation weak, unclear or only nearly complete); includes extraneous/tangential information; or has minor errors. Argument based on interpreting low-mass stars lying just off of the main sequence line as leaving the main sequence, which is yet to be observed given the age of the universe (while ignoring the presence of massive stars on the main sequence). - t = 12/20:
Contains right ideas, but discussion is unclear/incomplete or contains major errors. At least understands how massive and low-mass stars evolve at different rates to get to (and leave) the main sequence. - 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. - y = 2/20:
Irrelevant discussion/effectively blank. - z = 0/20:
Blank.
Section 30674
p: 34 students
r: 1 student
t: 5 students
v: 3 students
x: 1 student
y: 0 students
z: 0 students
A sample "p" response (from student 1802):
Another sample "p" response (from student 0310):
Yet another sample "p" response (from student 0299), mentioning the "house party" analogy discussed in class:
A sample "v" response (from student 2569), noticing an apparent lack of medium-mass stars on the main sequence:
[Version 2]
[20 points.] Shown at right could be an H-R diagram of a star cluster, where all of these stars would have the same age. Discuss whether or not this H-R diagram is possible for a star cluster, and explain why, using the properties of stars and evolution times.Solution and grading rubric:
- p = 20/20:
Correct. Understands that massive stars evolve faster than low-mass stars, such that massive stars reach the upper left of the main sequence first, spend the least amount of time there, and leave well before low-mass stars become main-sequence stars. Therefore the H-R diagram is possible for a star cluster where the stars would have the same (old) age. - r = 16/20:
Nearly correct (explanation weak, unclear or only nearly complete); includes extraneous/tangential information; or has minor errors. - t = 12/20:
Contains right ideas, but discussion is unclear/incomplete or contains major errors. At least understands how massive and low-mass stars evolve at different rates to get to (and leave) the main sequence. - 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. Conclusion without substantive argument. - y = 2/20:
Irrelevant discussion/effectively blank. - z = 0/20:
Blank.
Section 30676
p: 35 students
r: 5 students
t: 4 students
v: 25 students
x: 7 students
y: 0 students
z: 0 students
A sample "p" response (from student 0264), also describing the cause of different evolution rates for massive and low-mass stars:
A sample "p" response (from student 8282), appealing to the "house party" analogy:
A sample "v" response (from student 1001), with a whimsical threat:
A brutally self-honest "x" response (from student ), in manga form:
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