Lines of constant R in the H-R diagram
• Main sequence not quite
constant R
• B stars:   R ~10 RSun
• M stars:   R ~0.1 RSun
• Betelgeuse:  R~ 1,000 RSun
• Larger than 1 AU
• White dwarfs:  R~ 0.01 RSun
• A few Earth radii
• What causes the “main sequence”?
• Why “similar” size, with
precise R related to T?
• Why range of T?
• Why are a few stars (giants, white
dwarfs) not on main sequence?