Evolution of High-Mass 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:

     Mass	Main-Sequence Lifetime
     1MSun 3MSun 15MSun 25MSun	9 x 109 years 500 x 106 years 15 x 106 years 3 x 106 years

     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 M > 2M_Sun the cores are hot enough that the e^- 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 M > 4M_Sun the carbon cores are hot enough to start carbon fusion.

  3. For M > 8M_Sun (maybe this should be 6M_Sun) 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.

Nuclear Fusion in Massive Stars

• The production of new elements is called Nucleosynthesis

  Temperature	Fusion Reaction
  10 x 106	H -> He
  100 x 106	4He2 -> 12C6
  600 x 106	12C6 -> 16O8 (24M12)
  1500 x 106	16O8 -> 20Ne10 (32S16)
  etc...	etc...

• In massive stars there is an "onion skin" of fusion shells with the outer layers dropping fuel to lower layers and heavier and heavier nuclei being cooked up as you move towards the center of the star.
  

• A very common path to building elements is via the Alpha Process where He nuclei (for historical reasons these are known as "alpha particles") are added to exisiting nuclei.

• Now, all the statistically likely reactions are such that the PRODUCT nuclei have less mass than the input nuclei with the "mass defect" being converted into energy.

  This is the way Nature likes to work - these reactions release energy.

• This proceeds all the way to Iron (actually to Nickel that decays back down to Cobalt and Iron).

  For EQUILIBRIUM fusion reactions, Iron is the end of the line

  

• If a reaction lead to MORE binding energy per nucleon (or less nuclear potential energy per nucleon) then this is a reaction that occurs in equilibrium.

  Therefore light elements fuse to push elements up the curve. Similarly for the heavy elements, fission reactions tend to push elements up the curve from the right and release energy. Fusing He to make Be goes the wrong way and this reaction requires energy INPUT to make it happen. Nature doesn't care for these sorts of reactions.

• I like to plot the Potential Energy tied up in the nuclei verse atomic number to make the analogy with Gravitational potential energy clearer:
  

• So Iron (Fe) is the most stable of all the nuclei and if the temperature of the Universe were high enough, this is where everything would be headed.

• By the usual cosmic standards, this all happens very fast. For a 25M_Sun stars:

  Stage	Central Temperature	Duration
  H fusion	4 x 107K	7 x 106 yr
  He fusion	20 x 107	5 x 105 yr
  Carbon fusion	60 x 107K	600 yr
  Neon fusion	120 x 107	1 yr
  Oxygen fusion	150 x 107K	6 mo
  Silicon fusion	270 x 107K	1 day