Formation of the Elements
We will discuss in more detail later some of the amazing reasons that the relative amounts of the
elements are really important. We need lots of carbon and oxygen in order to have complex life. We
have lots of elements that we need for our health. Water, concrete, steel, air, trees, eyes, DNA, fingers,
glass, wine, light bulbs, electricity (flowing electrons in metals), all of it…protons, electrons, neutrons.
Hard solids exist because of the strength of the bonds between the atoms. Bond energies, bond angles.
Water ice is a bond angle thing. We have lots of raw materials to invent with; endless options to explore.
Where did all this come from?
The universe started basically as a bunch of hydrogen atoms and helium atoms—protons, neutrons,
electrons. Not much to build planets and life with. But the physics laws led to all the current complexity.
The densities and force laws (as we’ll see when we talk about the fine tuning) were just right so that
large stars formed. At the center of these large stars, huge pressures and temperatures cause nuclear
fusion that fuses hydrogen into helium. The radiation from the fusion pushes outward and keeps the
star from contracting further.
When most of the hydrogen is used up, the center of the star contracts because there is nothing to
balance the pressure. Then the center of the star gets even hotter, hot enough for helium to fuse. Two
heliums produces a nucleus that only lasts 10-16 sec. Helium fuses into carbon (3 heliums) and oxygen (4
heliums). The amazing process that leads to carbon fusion is described more in sections 4 and 6. Two
reactions, one that fuses helium into carbon, and one that creates oxygen, compete with each other
[Stellar Interiors by Hansen, Kawaler, and Trimble] until the helium is used up. The balance is very
sensitive to the excited level structures of the oxygen and carbon nuclei. It yields more carbon in stars of
smaller mass and more oxygen in more massive stars. The nucleus of neon has no appropriate excited
level into which the product of oxygen and helium fusion can land (oxygen and helium don’t fuse easily),
so helium fusion effectively stops with carbon and oxygen; that is, the reaction is very slow unless
temperatures are very high. This is fortunate for us!
When the helium is all used up, the star again contracts until it heats up even more, and fuses heavier
elements. (source: Astronomy magazine August 2012): Stars like our Sun, after they grow old, eventually
send about half their mass into space that is then used to form new stars. Scientists couldn’t explain
this, though, because the tiny dust grains would be burned up before travelling far. They recently
discovered that the dust and sand travels in a “super wind” millions of times stronger than ordinary
stellar winds. This dust and sand carries the heavier elements that become part of the clouds in space
and eventually new stars and planets, like Earth.
This continuing process of contracting and fusing heavier elements operates in more massive stars until
they get to iron. Moving to elements heavier than iron requires energy input rather than output, so that
the star, if it is big enough for this, implodes. It keeps contracting until really huge temperatures and
pressures are reached (the whole thing happens in seconds), and it explodes as a supernova. The
elements are dispersed into space, where they again become part of the clouds that produce new stars.
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The bigger stars live fast and die young. For example, whereas a star that is 1.25 times the Sun’s mass
spends 2.8 billion years burning hydrogen (the Sun has several billion more years left in addition to its
nearly 5 billion years so far), a star that is 9 times the Sun’s mass only spends 20 million years burning
hydrogen.
Sources of Element Production (Stellar Interiors)
H and He: Left from Big Bang (including 2H and 3H); also a bit of 7Li from the early universe
Li, Be, B: Made by cosmic ray CNO fragmentation in interstellar medium
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C, 16O: Helium burning
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C, 14N, 15N, 17C, 19C, F: CNO cycle burning (bigger stars) and its extension to higher temperature
Ne to iron peak: Carbon, neon, oxygen, and silicon burning
Z = 30 and beyond: s-, r-, and p-processes, the latter two primarily in supernovae
Advanced Nuclear Burning Phases of Massive Stars (Stellar Interiors)
Dominant fuel
Temperature
Duration
Important Products
Carbon
Neon
Oxygen
Silicon
500 million K
800 million K
1 billion K
3 billion K
1000-10000 yr
100-1000 yr
< 1 year
days
Ne, Na
Mg, some O
Si, some S, etc.
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Ni
The Sun is a star that has formed after other stars made all the elements that are in its constituents.
Complex life is not possible on planets until a certain level of the heavier elements exists. Our Sun has
spent its nearly 5 billion years fusing hydrogen into helium, and has been very stable over this time
period, getting slightly more luminous and expanding slightly over time.
I am struck by how dependent we are on the Sun, to continuously shine, and how much trouble we’d be
in if it didn’t give us its heat for just one day. And to think it’s been steadily doing its thing for 4 ½ billion
years so that intelligent life could evolve. That the physics provides for supernovas to rapidly create the
elements we need and for the amazingly steady Sun to work too; it’s astounding.
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