Pedagoguery

By mass, the average human body is composed of 61% oxygen, 23% carbon, 10% hydrogen, 3% nitrogen, and 3% other elements. With the exception of the hydrogen, none of the elements were present in the universe before the advent of stars. Stars are how the universe manufactures elements heavier than helium. The following is a description of how it is done.

For most of a star's life, it quietly converts hydrogen into helium in its core. During the current epoch, there are two ways of doing this: the proton-proton chain, and the CNO cycle. The CNO cycle takes place in hotter, more massive stars, and it requires the presence of carbon, so it would not have happened in the first stars. The proton-proton chain works like this: Two protons collide. If the circumstances are just right, they'll stick together long enough for one of them to transform into a neutron, emitting a positron and a neutrino, thus forming a deuterium nucleus. In short order, that deuterium nucleus will be hit by another proton, forming helium-3 and emitting a gamma ray photon. Finally, two helium-3 nuclei will eventually collide, resulting in helium-4 and two protons, which go on to continue the chain.

Now, you have helium, but nature has played a trick. There are no stable nuclei with atomic weights of 5 or 8. Thus, hitting the helium-4 nucleus with another proton, or colliding two helium-4 nuclei together will not produce any new elements. However, nature did leave us a way out. The carbon nucleus has an excited state that nearly matches the energy of three helium-4 nuclei. This means that if the concentration of helium is high enough, as it is in the cores of stars toward the end of their main sequence lives, two helium-4 nuclei can stick toether just long enough to be hit by a third one, and the resulting carbon-12 nucleus is stable. This is the triple-alpha reaction.

I mentioned the CNO cycle before, and it depends on the presence of carbon. Here is how it works. A carbon-12 nucleus absorbs a proton, becoming nitrogen-13. Nitrogen-13 is unstable, quickly decaying into carbon-13, emitting a positron and a neutrino. The resulting carbon-13 nucleus absorbs another proton, becoming nitrogen-14. The nitrogen-14 nucleus absorbs another proton, becoming oxygen-15. Oxygen-15 is unstable, decaying into nitrogen-15 and emitting a positron and an neutrino. Finally, the nitrogen-15 nucleus absorbs another proton, splitting into a carbon-12 nucleus and a helium-4 nucleus.

Knowledge of these three reactions is critical to knowing where most of the atoms that form us come from. First of all, let's take a look at what makes up most of us – oxygen. In a star that is producing energy in its core via the triple-alpha process, the core consists of primarily helium, with some carbon. However, if a carbon-12 nucleus and a helium-4 nucleus collide, the most common result is oxygen-16. This is how most oxygen is formed. Typically, stars like this will undergo a red giant phase. In such a phase, the outer envelope of the star undergoes convection, which brings up a great deal of core material to the surface. Strong solar winds will then blow the outer layers off the star, a stage we call a planetary nebula. This process liberates oxygen and carbon (our second largest constituent).

Nitrogen is produced primarily in stars that use the CNO cycle to produce energy. Of all the reactions of the CNO cycle, by far the slowest is the one where nitrogen-14 absorbs a proton to form oxygen-15. As a result, the abundance of nitrogen-14 builds up in the star over time. When such a star undergoes a red giant phase, nitrogen is brought up through convection and blown off into space.

This accounts for 97% of us. What about the rest? Some of them form in the envelopes of red giant stars. During this stage of a star's life, it can undergo thermal pulsations. These occur when one form of core burning falters and before another takes over. The star starts to collapse, the core heats up, and the new form of nuclear burning takes over. During such episodes for stars of between 1 and 8 solar masses, carbon-13 nuclei will fuse with helium-4 nuclei. This forms oxygen-16 plus a stray neutron. Another common reaction is neon-22 and helium-4, which yields magnesium-25 and a neutron. That neutron is critical, because it is the source of something called the s-process. Essentially, these neutrons will hit various atomic nuclei, where they will usually stick. Sometimes this will result in a stable isotope, other times it will not. And, while a lot of neutrons are buzzing around in these environments, it is not so much that a nucleus cannot stabilize itself between collisions. Thus, if an unstable nucleus results, it will have time to undergo beta decay (the neutron converts into a proton, releasing an electron and an anti-neutrino), thus becoming a nucleus of a different element. Most of the molybdenum, strontium, yttrium, zirconium, barium, lanthanum, cerium, and lead inside you is formed by this process. Like carbon, oxygen, and nitrogen, these elements are typically formed in the core of these stars, and during the red giant phase, are brought to the surface by convection and ejected by stellar winds.

The rest of the elements undergo a much more dramatic creation. For instance, most of the iron in your blood forms in type 1a supernovae. This is when a carbon-oxygen white dwarf gets enough mass added to it, usually from a companion star, to push it over the Chandrasekhar limit of 1.4 solar masses. This causes a runaway nuclear reaction in the core, which converts much of the mass of the white dwarf into nickel-56. Nickel-56 is unstable, decaying into cobalt-56 with a half-life of about 6 days. Cobalt-56 is also unstable, decaying into iron-56 with a half-life of about 77 ¼ days.

Type II supernovae form the remaining elements. This is when a massive star, which has been fusing hydrogen to helium to carbon to oxygen, sodium, neon, and magnesium to sulfur and silicon to iron. Iron is a dead end, since you cannot produce energy by fusing iron with anything. Thus, when the iron core gets large enough, the star undergoes a core-collapse supernova explosion. During the explosion, massive numbers of neutrons are produced and released. This does not allow unstable nuclei to decay before being hit by more neutrons. This is the r-process and it produces critical elements like calcium, magnesium, silicon, sulfur, and titanium, along with elements less necessary for life but still valued like gold.

Next time, will the science of cosmology cease to exist?