Astronomy final exam question: luminosity/temperature of sun as a red giant?

Astronomy 210 Final Exam, fall semester 2016
Cuesta College, San Luis Obispo, CA

An astronomy question on an online discussion board[*] was answered:

nscr: When the sun becomes a red giant, it will be more luminous and cooler than it was as a main sequence star.

Discuss why this answer is correct, and how you know this. Explain using Wien's law, the Stefan-Boltzmann law and/or an H-R diagram.

[*] answers.yahoo.com/question/index?qid=20090624203955AA2Wzer.

Solution and grading rubric:

• p:
  Correct. Discusses how the Stefan-Boltzmann law and/or H-R diagram shows that as the sun (a medium-mass star) becomes a red giant, it becomes more luminous and cooler (moving higher up, and to the right on the H-R diagram).
• 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. At least discussion demonstrates understanding of Wien's law, H-R diagram and/or the Stefan-Boltzmann law.
• v:
  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, H-R diagram and/or the Stefan-Boltzmann law.
• x:
  Implementation/application of ideas, but credit given for effort rather than merit. Discussion not based on Wien's law, H-R diagram and/or the Stefan-Boltzmann law.
• y:
  Irrelevant discussion/effectively blank.
• z:
  Blank.

Grading distribution:
Section 70158
Exam code: finalS1tH
p: 33 students
r: 0 students
t: 2 students
v: 1 student
x: 0 students
y: 0 students
z: 0 students

Section 70160
Exam code: finalnwhP
p: 16 students
r: 1 student
t: 5 students
v: 1 student
x: 0 students
y: 1 student
z: 0 students

A sample "p" response (from student 0924):
Another sample "p" response (from student 3168), using both the "box method" to fill in relative Stefan-Boltzmann law parameters: