## 20110610

### Astronomy final exam question: main-sequence star colors vs. sizes

Astronomy 210 Final Exam, Spring Semester 2011
Cuesta College, San Luis Obispo, CA

Jayen: While looking at the H-R diagram I noticed the [bluer] stars were bigger, and the [redder] stars were smaller. Is this always the case [for] stars in the main sequence?
lithiumdeuteride: Not necessarily...
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.

• 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. Interprets H-R diagram to show that for main sequence stars, blue stars are larger than red stars.
• 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. Applies Wien's law, but does not limit scope of discussion to main sequence stars.
• 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:
Blank.
Section 30674
Exam code: finaln4St
p: 12 students
r: 7 students
t: 8 students
v: 4 students
x: 4 students
y: 1 student
z: 0 students

A sample "p" response (from student 7924):