Cuesta College, San Luis Obispo, CA
[20 points.] An astronomy question on an online discussion board(*) was asked and answered:
Jayden: While looking at the H-R diagram I noticed the brighter, hotter stars were bigger, and the dimmer, cooler stars were smaller. Is this always the case [for] stars on the main sequence?Discuss why this statement would be incorrect for stars not on the main sequence, and how you know this. Explain using Wien's law, the Stefan-Boltzmann law and/or an H-R diagram.
DLM: For main sequence stars, that's essentially an accurate statement...
*Adapted from: http://answers.yahoo.com/question/index?qid=20090920095747AAfgLH2.
Solution and grading rubric:
- p = 20/20:
Correct. Uses Wien's law, the Stefan-Boltzmann law and/or interprets H-R diagram to give a concrete example of two stars off the main sequence (or one star off the main sequence, compared to a main sequence star) that are not consistent with the brighter/hotter/bigger and dimmer/cooler/smaller trend on the main sequence.
- r = 16/20:
Nearly correct (explanation weak, unclear or only nearly complete); includes extraneous/tangential information; or has minor errors. As (p), but typically discusses the Stefan-Boltzmann law for two stars with arbitrary parameters inconsistent with the brighter/hotter/bigger and dimmer/cooler/smaller trend on the main sequence, but not a concrete example of stars found on an H-R diagram.
- t = 12/20:
Contains right ideas, but discussion is unclear/incomplete or contains major errors.
- v = 8/20:
Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. At least attempts to use Wien's law, the Stefan-Boltzman law, and/or the H-R diagram.
- x = 4/20:
Implementation/application of ideas, but credit given for effort rather than merit. Discussion not based on Wien's law, the Stefan-Boltzman law, and/or the H-R diagram.
- y = 2/20:
Irrelevant discussion/effectively blank.
- z = 0/20:
Exam code: finalSor3
p: 9 students
r: 9 students
t: 1 student
v: 5 students
x: 4 students
y: 0 students
z: 1 student
A sample "p" response (from student 0640):