STELLAR EVOLUTION - Massive stars

• The crucial difference between more and less massive stars is the core temperature.

  Remember that as the mass goes up, so does the central temperature as required to maintain hydrostatic equilibrium. This leads to two major differences between solar mass and more massive stars.

  1. Everything happens faster. For example, the main-sequence lifetime:

     

     Why is this? Because Luminosity increases much faster than Mass.

  2. The central temperature is high enough to fuse elements beyond Carbon and Oxygen.

• A few points:
  1. For the cores are hot enough that the have lots of momentum and the phase-space cells don't get filled up. (or, the cores never become degenerate.)

     So, no helium flash, no horizontal branch phase.

  2. For the carbon cores are hot enough to start carbon fusion.

  3. For (maybe this should be ) the cores are hot enough to have fusion reactions all the way to Fe (Iron) production.

  This last point is very important and leads into two fascinating topics - Supernovae II and Galactic Chemical Evolution.