ASTRO-110 Lecture Notes - Lecture 29: Triple-Alpha Process, Helium Flash, Degenerate Matter
Astro 110
The Visible Universe
Summer 2017
Notes
The degenerate helium core:
● as hydrogen continues to fuse in the shell above the core, the helium ash from this
fusion settles onto the core, increasing its mass
● degenerate matter is odd – the more massive the core gets, the smaller it gets
● so, even though the core has become degenerate, the increasing helium causes the
core to continue to shrink and heat
● this increased heat causes even more fusion in the shell situation continues to get
more extreme with…
Helium Burning:
● when temperatures in the core finally reach about 100 million K, the helium there
finally can begin to fuse, creating carbon 12C and some oxygen 16O
● helium fusion doesn’t begin gradually. in the degenerate matter, fusion spreads rapidly
through the core, causing it to heat even further and fuse even faster – helium flash
(sudden event, but not dramatic)
● fortunately, the onset of helium fusion is not very violent, destroying only the
degeneracy of the core and causing the core to expand. this cools the layer of gas
above the core and hydrogen shell fusion slows the star shrinks
● it was hydrogen fusion in the shell above the core that was the stars main source
of energy. when the core expands, fusion slows in the shell, the luminosity
drops, and the star shrinks and heats up
● with degeneracy in the core destroyed, the stars thermostat works again and
the helium-burning star is stable as long as helium lasts. the star is now on the
“horizontal branch”
● it seems that all helium burning low mass stars have nearly the same luminosity. when
we look at HR diagrams of clusters old enough to have helium burning stars, we find
them on a “horizontal branch”
● the cores of all helium burning low-mass stars have the same power output
(luminosity). the variations we see in the star’s temperatures are caused by differences in
the masses of the stars after the red giant phase
From main sequence to helium burning:
● as the star enters the red giant phase, it grows in size by a factor of about 100 and
increases its luminosity by a factor of about 2000. Even so, the temperature drops and
the star appears red
● as a helium-burning star, its size and temperature depend on the mass of the star
following the red giant phase. But, the luminosity is still around 100 times the star’s
original main sequence value
“Intermediate” and High-Mass Stars
● for stars with M>2M nuclear burning proceeds much differently than for stars with
