Evolution of Massive Stars

• Lifetimes ===> t ~ 10 [M/M(sun)]**(-3) billion years ===> for a 25 M(sun) star, t ~ 500,000 years (or so)

• Evolutionary track for a massive star.

  Illustrative numbers for the evolution of a 25 M(sun) star.

• Comments
  • While on the Main Sequence the star generates energy in its core through hydrogen burning using the CNO cycle. Again, while on the Main Sequence, since the chemical make-up of its core is changing, the star evolves slowly. The star's L increases while its surface T decreases (the star moves to the right and up in the HR diagram).

  • When the core is cleaned out of hydrogen, it starts to cool and thus starts to contract slowly. As the core contracts, it heats up and increases in density. The slow contraction of the core causes the overall star to become hotter and slightly more luminous (the star moves to the left and up in the HR diagram).

  • Next the region which is just outside of the core gets heated by the contracting core. Since the region just outside of the core has abundant hydrogen, when its temperature reaches several million Kelvins, hydrogen burning is ignited in the shell which surrounds the core. The formation of this so-called shell source causes the star to expand and cool (the star moves to the right in the HR diagram).

  • At this point, the energy transport in the star is by photons. However, as the star expands and cools, convection eventaully becomes more efficient than the photon transport of energy. At around a surface temperature of a few thousand Kelvins, convection takes over and the energy leaks out very quickly and the luminosity of the star shoots way up (the star rises almost vertically in the HR diagram). The star is now a Red Giant.

  • Note that at this time the core is still inert (a slowly contracting ball of helium). When the core finally reaches a temperature of ~ 100,000,000 Kelvins or so, helium burning is ignited. This occurs at the tip of the Red Giant branch. The helium burns in what is known as the Triple Alpha Process. An alpha particle is a helium nucleus. The origin of this name is historical in nature.

    An amusing point about helium burning:

    • helium + helium ===> berylium (which is unstable), so we can have that
    • berylium ===> helium + helium (with a half-life of 2.6x10**(-16) seconds!)
    • berylium + helium ===> carbon
    • (carbon + helium ===> oxygen as another way for the process to end)

    The addition of the third helium nucleus must occur within 2.6x10**(-16) seconds after the first reaction or the berylium will disappear. This requires high densities (to give many collisions) as well as high T (to overcome the large electrical barriers) in order for the Triple Alpha process to go. This is a very nasty business. By the way, this is why it is so difficult for heavy elements to be produced in the early Universe. It is very hard to get beyond helium, because the berylium (which is an important step in the building of heavier elements) is so unstable.

  • This causes the star to decrease in luminosity and to heat (causing the star to move down and to the left in the HR diagram). The star settles into this burning stage for a time which is roughly 10 - 20 % as long as its Main Sequence lifetime.

  • The star merrily cruises along in this state (helium burning in its core, surrounded by a hydrogen burning shell) until it scours out all of the helium from its core. The core of the star then cools, and starts to contract in an attempt to replace the heat lost by radiation from its surface. (This causes the star to start moving to the right in the HR diagram again).

  • Now, just as before when the contracting core heated the surrounding region and caused the ignition of hydrogen in a shell, the contracting core heats the surrounding helium and ignites helium burning in a shell. The star moves again rapidly to the right in the HR diagram.

  • When the conditions become right for convection to set in, the star rapidly increases in L (and moves almost vertically in the HR diagram). It ascends what is referred to as the Asymptotic Giant Branch (AGB).

  • It moves up the AGB until the core contracts enough to raise its temperature to the carbon ignition point at the tip of the AGB. When it ignites carbon, the star stops moving upward. Note--the lifetime of the carbon burning state is ~ 170 years. This is too fast for the star to make any appreciable changes in its overall structure and so the outward appearance of the star does not change during carbon burning.

  • When the carbon is scoured out of the core, the core contracts until it can start carbon burning in a shell around the core and neon burning in the core. The neon burning lasts ~ 1 year and so the outward appearance of the star does not change during this phase of evolution.

  • The nuclear game continues like this from neon burning to oxygen burning to silicon burning with the total time required for these phases being less than 1 year and so outward evolution of the star is not be visible for these stages.

  • After silicon burning, the star has an iron (Fe) core surrounded by many active shell sources.

  The nuclear game stops at this point, and the star is poised for a catastrophic event.

Mass Loss

A large uncertainty in the evolution of massive stars concerns mass loss during the course of their evolution. Hot, massive stars and cool, luminous stars are both sources of prodigous stellar winds; they can lose a solar mass of material on time scale ranging from hundreds of thousands of years to several hundred million years. These sound like long times, but given the time required for stellar evolution, these sorts of rates can affect the way in which stars evolve! The detailed physics by which such winds are generated are not well-understood and usually these effects are included in calculations in rather arbitrary manners. They are usually included by hand.