It's not quite as pristine as this simple cartoon form
because there's some mixing between the layers.
But basically moving outwards we still have unfused hydrogen in the very center.
Followed by helium, followed by the heavier elements, carbon and so
on, up the periodic table,
until the very hottest center part of the sun contains an iron core.
This is an extraordinary form of iron in a high massive evolved star.
It's denser than water.
It has a temperature of billions of Kelvin and yet it's a gas, a plasma.
I said the heaviest stable elements because the fusion process only works to
get you up to iron.
In the binding energy per nucleon curve, there's a stability trough at iron.
And this means that in nature, you can get energy out of fusion and
going towards iron from lighter elements.
But you can get energy out of the fission of heavier nuclei moving towards iron.
Iron is the most stable element.
What this means in terms of a star's life is iron is the endpoint.
No more free energy is available by fusing beyond iron, so it creates a logjam.
Or a slag heap at the center of the star ending in iron.
Extra energy is required to go beyond iron.
Summarizing the stages of evolution of an extreme star like a 25 solar mass star,
we see that these different fusion stages involve a crescendo of time scales.
The carbon fusion of the massive star takes only 600 years.
The neon burning just a year.
Silicone forms in a day, and
the final collapsed state of the star takes less than a second.
In the blast wave that moves out at relativistic speeds from the collapsed
star, elements are synthesized all the way out to the transuranic
elements at the top of the periodic table.
In temperatures that instantaneously reach 10 billion Kelvin.
With this piece of the story added for
high mass stars producing heavy elements, we've explained most of
the characteristics of the periodic table chemical abundance.
The sawtooth pattern.
The fact that there's a third peak at iron, the most stable element.
And the subsequent falloff because of the difficulty of
creating even heavier elements requiring such high temperatures.
Summarizing cosmic chemistry, we have primordial hydrogen and
helium, the most abundant elements in the universe,
with everything else taken together just a trace element.
We have carbon, nitrogen, and
oxygen, the carbon produced in relatively no, low-mass stars.
And these are the life elements present at a few parts in a thousand
relative to hydrogen and helium.
We have the creation of the elements up to iron in massive stars,
in a flurry of activity at very high temperatures.
And then we have elements beyond iron produced in supernova explosions.
But also in a slow manner.
In practice, there are two methods for creating heavy elements called the R and
the S process.
The R process, or Rapid,
occurs in supernovae where the slow process is the gradual diffusion of
neutrons into an atomic nucleus to increase its atomic number by one.
And, this can occur in the envelopes and cores of massive stars.
The final features we can explain are the sawtooth pattern caused by the addition
of helium nuclei in the fusion as well as hydrogen nuclei, and
then the deep trough at the light elements caused by the unstable beryllium nucleus,
representing a bottleneck which leads to the low abundance of carbon
relative to hydrogen and helium.
Perhaps the final insight in cosmic chemistry
is that we have an erroneous way of thinking of stars.
It's inevitable to think of them as light bulbs in the sky.
But that's not actually what's going on.
Because we know that the efficiency of mass energy conversion is actually
quite low.
Less than a percent.
We should instead think of the stars as chemical factories.
It's as if you had a car factory somewhere in Michigan and you flew over it at night.