20150525

Astronomy final exam question: too late to observe a type II supernova?

Astronomy 210 Final Exam, spring semester 2015
Cuesta College, San Luis Obispo, CA

"A supergiant 19 light years away exploded as a type II supernova 20 years ago. Is it too late to watch it happen?" This question was asked on an online discussion board, and two possible answers were given[*]:
qc: Yes. The "news" of the explosion washed through our solar system 19 years after the explosion--which, if it happened 20 years ago--would have been a year before today.
ogP: No, it will become visible to us next year. Not that we would know that it would happen ahead of time.
Discuss which answer is correct, and how you know this. Explain using the properties of mass and stellar lifetimes, and light.

[*] answers.yahoo.com/question/index?qid=20150506235144AAFW6fZ.

Solution and grading rubric:
  • p:
    Correct. Understands that:
    1. due to the finite speed of light (where it takes one year of time to travel a distance of one light-year), it will take 19 years for the light from this star to travel to Earth; and
    2. since the type II supernova occurred 20 years ago, and took 19 years for this light to travel to Earth, then it would have been observed one year ago.
    Thus "qc" has the more correct argument.
  • 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.
  • v:
    Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. Garbled discussion of mass, stellar lifetimes, and light.
  • x:
    Implementation/application of ideas, but credit given for effort rather than merit. Discussion not based on mass, stellar lifetimes, and light.
  • y:
    Irrelevant discussion/effectively blank.
  • z:
    Blank.
Grading distribution:
Section 30674
Exam code: finaln1N3
p: 18 students
r: 6 students
t: 1 student
v: 1 student
x: 1 student
y: 2 students
z: 0 students

Section 30676
Exam code: finals3nG
p: 12 students
r: 9 students
t: 8 students
v: 4 students
x: 9 students
y: 2 students
z: 1 student

A sample "p" response (from student 9419):

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