20110115

Astronomy final exam question: supergiants cooler and dimmer than giants?

Astronomy 210 Final Exam, Fall Semester 2010
Cuesta College, San Luis Obispo, CA

[20 points.] An astronomy question on an online discussion board (http://answers.yahoo.com/question/index?qid=20090417123734AAHUYUu) was asked and answered:
XxalicexX: What is the difference between a giant star and supergiant star?

Delta V: Supergiant stars are...cooler, and dimmer than giant stars.
Discuss whether or not if this answer is correct, and how you know this. Explain using the Stefan-Boltzmann law and/or an H-R diagram.

Solution and grading rubric:
  • p = 20/20:
    Correct. Discusses why statement is incorrect as while supergiants can be cooler than giants, supergiants must always be brighter (more luminous) than giants, based on the Stefan-Boltzmann law and/or interpreting an H-R diagram.
  • 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. Some merit in applying the Stefan-Boltzmann law and/or interpreting an H-R diagram.
  • v = 8/20:
    Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. At least some attempt in applying the Stefan-Boltzmann law and/or interpreting an H-R diagram.
  • x = 4/20:
    Implementation/application of ideas, but credit given for effort rather than merit. Arguments used other than the Stefan-Boltzmann law and/or H-R diagram, despite being instructed to.
    y = 2/20: Irrelevant discussion/effectively blank.
  • y = 2/20:
    Irrelevant discussion/effectively blank.
  • z = 0/20:
    Blank.
Grading distribution:
Section 70158
p: 31 students
r: 2 students
t: 5 students
v: 1 student
x: 3 students
y: 0 students
z: 0 students

A sample "p" response (from student 1992), using an H-R diagram:

Another sample "p" response (from student 0404), using a graphical chart form of the Stefan-Boltzmann law:

A sample "p" response (from student 0302), with various graphical Stefan-Boltzmann law charts, and a kudos for the instructor:

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