Astronomy final exam question: close-pair mass transfer

Astronomy 10 Final Exam, Fall Semester 2007
Cuesta College, San Luis Obispo, CA

Astronomy 10 learning goal Q10.3

[15 points.] The stars in a binary star system are a 4 M_Sun main sequence star and a 1 M_Sun giant, where no mass transfer is taking place right now. Discuss which star had originally first transferred mass to the other star, and explain your reasoning.

Solution and grading rubric:

• p = 15/15:
  Correct. Massive stars have shorter main sequence lifetimes than more massive stars. Star "B," the 1 M_Sun giant, had to have been the larger star in order to have reached the giant phase first. Since it is now less massive than star "A," the 4 M_Sun main sequence star, star "B" must have transferred mass to star "A."
  
• r = 12/15:
  Nearly correct (explanation weak, unclear or only nearly complete); includes extraneous/tangential information; or has minor errors.
  
• t = 9/15:
  Contains right ideas, but discussion is unclear/incomplete or contains major errors.
  
• v = 6/15:
  Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner.
  
• x = 3/15:
  Implementation/application of ideas, but credit given for effort rather than merit.
  
• y = 1.5/15:
  Irrelevant discussion/effectively blank.
  
• z = 0/15:
  Blank.
  

Grading distributions:
Section 1073
p: 10 students
r: 13 students
t: 4 students
v: 4 students
x: 0 students
y: 0 students
z: 0 students

A sample of a "p" response (from student 1000) is shown below:
Another "p" response (from student 1256):