What fundamental property of a star
varies along the main sequence?
• T and R vary smoothly (and together) along the main sequence
• B stars are ~4 times hotter and ~10 times bigger than sun
• M stars are ~2 times cooler and ~10 times smaller than sun
• Presence of a line implies that a single fundamental property
 is varying to make some stars B stars and some stars M stars
• That fundamental property then controls T, R
• A second property controls whether we get a giant or dwarfs
• Fundamental properties we could measure
• Location:         Doesn’t seem to be of major importance
• Composition:  Outside composition of most stars similar (H, He, ...)
• Age:         Will be important – but put off till Chapter 9
• Mass:         Turns out to be the most important parameter