Astronomy final exam question: T Pyxidis

Astronomy 210 Final Exam, Fall Semester 2010
Cuesta College, San Luis Obispo, CA

[20 points.] Earlier this year, an online news journal published an article discussing T Pyxidis, located approximately 1,000 parsecs away:
A star primed to explode in a blast that could wipe out...Earth was revealed by astronomers... [T Pyxidis] will self-destruct in an explosion called a [type Ia] supernova with the force of 20 billion billion billion megatons of TNT...

The blast from the thermonuclear explosion could strip away our ozone layer that keeps out deadly space radiation. Life on Earth would then be frazzled...

Experts said the Hubble space telescope has photographed the star gearing up for its big bang with a series of smaller blasts or "burps", called novas...
--Paul Sutherland, "The Death Star," The Sun, January 6, 2010, http://www.thesun.co.uk/sol/homepage/news/2795981/Supernova-may-wipe-out-the-Earth.html.

T Pyxidis is actually known to be a binary star system. Discuss specifically the type of star in the T Pyxidis system that "burps" and may eventually "self-destruct," and why this star is doing this. Explain using the properties of binary star systems, compact objects, novae and type Ia supenovae.

Solution and grading rubric:
  • p = 20/20:
    Correct. Discusses a companion star is feeding hydrogen to a white dwarf, which fuses its outer hydrogen shell and explodes periodically as a nova, and may eventually undergo complete core fusion and totally explode as a type Ia supernova.
  • 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. At least identifies "burps" as novae that may lead up to a type Ia supernova "self-destruct," and that a companion star is feeding hydrogen to some type of compact object.
  • v = 8/20:
    Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. Discussion may involve processes other than a white dwarf with a companion star.
  • x = 4/20:
    Implementation/application of ideas, but credit given for effort rather than merit.
    y = 2/20: Irrelevant discussion/effectively blank.
  • y = 2/20:
    Irrelevant discussion/effectively blank.
  • z = 0/20:
Grading distribution:
Section 70160
p: 7 students
r: 11 students
t: 7 students
v: 7 students
x: 5 students
y: 0 students
z: 1 student

A sample "p" response (from student 7735):

A sample illustrated "p" response (from student 7133):

Another sample "p" response (from student 3363):

Yet another sample "p" response (from student 1104):

A sample "t" response (from student 4305), while mistaken about the nature of T Pyxidis, is still nonetheless scared of it:

A sample "x" response (from student 1111), essentially regurgitating the given facts:

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