4
Expansion into a Red Giant
•
•For 5 MSun  “red giant”
–Outer part swells to 75 RSun
–Inner core still contains most of mass
–
•Why is it red?
Two equivalent ways to answer:
•
–Thick envelope insulates outside from hot core
–
–L µ R2 T4 and
•L  is not much greater (so far)
•R2 is »(75/3)2 =252 = 625 times larger
•T must decrease to compensate
From our text:  Horizons, by Seeds