Astronomy final exam question: white dwarf and red dwarf binary star system?

Astronomy 210 Final Exam, fall semester 2014
Cuesta College, San Luis Obispo, CA

An astronomy question on an online discussion board[*] was asked:

Pdg: Can there be a white dwarf and a red dwarf in a binary star system? Even though they were born at the same time?
t153: Certainly. Binary star system GJ 169 is 17.6 light years away in the constellation of Camelopardalis, and consists of a red dwarf and a white dwarf. Because the red dwarf is less massive than the sun, and the white dwarf is more massive than the sun.

Discuss why this answer is correct, and how you know this. Explain using the properties and evolution of stars.

[*] answers.yahoo.com/question/index?qid=20141121105328AAGj3la.

Solution and grading rubric:

• p:
  Correct. Understands that for these stars in the same cluster that were born at the same time:
  1. the red dwarf is a low-mass star that evolves slower than a medium-mass star, and thus stays in its main-sequence stage for an extremely long time;
  2. the white dwarf is the remnant of a medium-mass star (after going through its giant and planetary nebula phases), that evolved faster than the red dwarf.
• r:
  Nearly correct (explanation weak, unclear or only nearly complete); includes extraneous/tangential information; or has minor errors. One of the two points (1)-(2) correct, other is problematic/incomplete.
• t:
  Contains right ideas, but discussion is unclear/incomplete or contains major errors. Only one of the two points (1)-(2) correct, other is missing, or both are problematic.
• v:
  Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. Garbled discussion of properties and evolution of stars.
• x:
  Implementation/application of ideas, but credit given for effort rather than merit. Discussion other than that of the properties and evolution of stars.
• y:
  Irrelevant discussion/effectively blank.
• z:
  Blank.

Grading distribution:
Section 70158
Exam code: finalst0P
p: 3 students
r: 9 students
t: 9 students
v: 5 students
x: 7 students
y: 1 student
z: 0 students

Section 70160
Exam code: finaln3rF
p: 7 students
r: 8 student
t: 4 students
v: 5 students
x: 3 students
y: 2 students
z: 0 students

A sample "p" response (from student 1109):
A sample "y" response (from student 8898):