Cuesta College, San Luis Obispo, CA
An astronomy question on an online discussion board(*) was asked and answered:
Pd...: Is it possible that a blue supergiant can be as bright as a red supergiant, (assuming they are the same distance from Earth)?Discuss whether or not if this answer is correct, and how you know this. Explain using Wien's law, the Stefan-Boltzmann law and/or an H-R diagram.
per..gogy: Technically the blue supergiant would be the brighter one. Blue stars are more luminous than red stars...
Solution and grading rubric:
- p = 20/20:
Correct. Wien's law: blue stars are hotter, and are located on the left side of an H-R diagram, while red stars are cooler, and are located on the right side. Uses Stefan-Boltzmann law and/or interprets H-R diagram to show that either (a) blue supergiants and red supergiants can have the same size, such that the blue supergiants (being hotter) are more luminous; or (b) blue supergiants and red supergiants can have the same luminosity, but the blue supergiants (being hotter) are smaller in size.
- 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.
- 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, H-R diagram and/or the Stefan-Boltzman law.
- x = 4/20:
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-Boltzman law.
- y = 2/20:
Irrelevant discussion/effectively blank.
- z = 0/20:
Exam code: midterm02n1cT
p: 13 students
r: 6 students
t: 6 students
v: 3 students
x: 0 students
y: 0 students
z: 0 students
A sample "p" response (from student 8386), discussing how the blue supergiant while hotter, would be smaller than the red supergiant in order to have the same luminosity:
Another sample "p" response (from student 1027) instead arguing that a blue supergiant will be brighter than a red supergiant, assuming that they are the same size: