Astronomy midterm question: supergiant seen as a main-sequence star?

Astronomy 210 Midterm 2, fall semester 2009
Cuesta College, San Luis Obispo, CA

Discuss how it would be possible to observe a massive star during its main-sequence lifetime, while this star is actually a supergiant. Explain using the properties of mass and stellar lifetimes, and light.

Solution and grading rubric:

• p:
  Correct. Explains how the finite speed of light causes distant objects to appear as they did in the past, such that a massive star will still appear to be in its main-sequence lifetime, despite having already become a supergiant.
  
• r:
  Nearly correct (explanation weak, unclear or only nearly complete); includes extraneous/tangential information; or has minor errors. At least recognizes that distant objects appear as they do in their past, or that the vast distances and/or finite speed of light is relevant.
  
• t:
  Contains right ideas, but discussion is unclear/incomplete or contains major errors.
  
• v:
  Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. Discussion based on short lifetimes, bright luminosities, and/or fast fusion rates of massive stars.
  
• x:
  Implementation/application of ideas, but credit given for effort rather than merit. Implausible evidence/methods/discussion.
  
• y:
  Irrelevant discussion/effectively blank.
  
• z:
  Blank.
  

Grading distribution:
Section 70160
p: 22 students
r: 5 students
t: 0 students
v: 15 students
x: 0 students
y: 0 students
z: 0 students

A sample "p" response (from student 7272):