Astronomy midterm question: giant end-stage

Astronomy 10 Midterm 3, Fall Semester 2007
Cuesta College, San Luis Obispo, CA

Astronomy 10 learning goal M3.1

[15 points.] Why can't a medium mass star live as a giant forever after its main sequence lifetime ends? Discuss why all giants must eventually become planetary nebulae and white dwarfs, by explaining the properties of giants and stellar evolution.

Solution and grading rubric:

• p = 15/15:
  Correct. In its giant phase, heavier and heavier elements are fused until the core becomes carbon, but temperatures and pressures are unable to continue heavier fusion. Without an energy source to balance gravity, the giant lifetime of the medium mass star ends as the core gravitationally contracts into a white dwarf, while giving off enough energy to disperse its outer layers as a planetary nebula.
  
• r = 12/15:
  Nearly correct (explanation weak, unclear or only nearly complete); includes extraneous/tangential information; or has minor errors. Understands how the giant lifetime of a medium mass star occurs, but does not completely explain why it must end.
  
• t = 9/15:
  Contains right ideas, but discussion is unclear/incomplete or contains major errors. May confuse discussion with that of supergiants and type II supernovae, but at least understands the importance of no more energy being released from fusion.
  
• 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: 8 students
t: 17 students
v: 4 students
x: 3 students
y: 0 students
z: 0 students

A sample of a "p" response (from student 0115) is shown below:

Another "p" response (from student 4040):