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Other than a few light elements that formed in the Big Bang, the elements in the universe formed in stars in a process called stellar nucleosynthesis. Dust such as the silicates that constitute much of the earth's crust formed in supernovae explosions, and molecules, such as water and methane, formed in dark molecular clouds.
The extremely high temperatures and pressures in the cores of stars enable stellar (star) nucleosynthesis, which begins with protons forming helium nuclei. In contrast with rapid Big Bang nucleosynthesis in which there were available neutrons, stellar nucleosynthesis that takes billions of years. The proton-proton chain (Figure 2‑22) in stars requires beta plus decay of a proton to a neutron, which is rare. In this process, two protons fuse into a diproton (two protons) through quantum tunneling. Diproton is extremely unstable and quickly decays back to two protons; however, beta-plus decay infrequently transforms one of the protons in diproton into a neutron before the diproton breaks apart and forms deuteron with one proton and one neutron. Beta-plus decay takes place when the weak force transforms one of the up quarks in one of the protons into a down quark, which converts the proton to a neutron and also results in the emission of a positron and neutrino. The rarity of beta plus decay enables the sun and other stars to burn for billions of years. If the strong nuclear force were only 2% greater, then the force binding diproton would be more stable, and beta-plus decay would be more frequent. Stars would burn much faster and would not be a stable source of energy for billions of years. Deuteron combines with another proton and forms 3He, which then combines with another 3He, to form helium. The energy release comes from the decay of a proton to a neutron, which is lighter. Based on E = mc2, loss of mass releases energy.
Figure 2‑22. Hydrogen fusion in the sun by the proton-proton chain. Credit NASA.
The next step in the formation of heavy elements is the fusion of two helium nuclei (atomic mass 4) to form beryllium (atomic mass 8), which then joins with another helium (atomic mass 4) and forms carbon (atomic mass 12). Beryllium is extremely unstable and decays in a quadrillionth of a second. This is where the story becomes compelling. Fred Hoyle studied this process and realized that the time that a beryllium atom exists is so short that this would not give the third helium time to join with the first two to form a stable carbon atom. However, Hoyle figured out that if beryllium had exactly the right resonance frequency, then this would speed up the attachment time and allow the helium to attach to the beryllium m before the beryllium decayed. When he looked at atomic tables, he did not find this resonance frequency. Hoyle suspected that it should be there, and so he contacted nuclear physicist Willy Fowler at Cal Tech and asked him to look for this resonance frequency of carbon. Fowler at first declined, but Hoyle insisted, and Fowler found the resonance frequency when he experimented.
After carbon forms, protons are added in a series of steps, and the weak force causes a series of beta plus decays, forming nitrogen and oxygen in the CNO cycle. (Figure 2‑23). There is fine-tuning in the resonance frequencies in the oxygen atom, without which all of the carbon would convert to oxygen, and the abundance of carbon in the universe would be drastically reduced. In another case of fine-tuning of the CNO cycle, if the strong nuclear force increased or decreased by 0.4%, the CNO cycle would produce all carbon or all oxygen but not the balance of both as we have in our world. After hydrogen and helium, the three most abundant elements in the Solar System are oxygen, carbon, and nitrogen, in that order. Along with hydrogen and helium, these are the most important elements in life.
Figure 2‑23. The Carbon, Nitrogen, Oxygen (CNO) cycle. Credit NASA.By Borb, CC BY-SA 3.0
Figure 2-24. Carbon atom with six protons, six neutrons, six electrons, and four unfilled electron positions
Carbon nitrogen and oxygen have a second electron shell with eight electron positions. Electrons always fill the inner electron shell first, and the leftover electrons fill the next electron shell. These open electron positions are the key factor in the utility of these elements for life.
Carbon (Figure 2-24) has six protons and thus has six electrons. Thus, the carbon outer electron shell has four unfilled electron positions (black dots). Two characteristics of carbon make it an ideal molecule for the molecules of life. First, the electrons have a high energy level in the outer electron shell, which is useful for energy reactions such as respiration. Second, there are four open electron positions that will bond with other atoms, making carbon an ideal molecule for creating chemical bonds in organic molecules.
Figure 2-25. Nitrogen atom
Nitrogen has seven protons and seven electrons. Thus, nitrogen (N) has 3 unfilled positions in its outer electron shell (Figure 2-25). In the atmosphere, which is ¾ nitrogen, nitrogen atoms combine in molecules of two nitrogen atoms where they share 3 electrons in 3 covalent bonds. Nitrogen, N2, is a stable molecule and thus does not react easily with other molecules and is somewhat inert in the atmosphere; however, fungi and bacteria in the soil transform atmospheric nitrogen into forms of nitrogen that are available to plants. Nitrogen is essential to cell function because it is forms amino acids in proteins and nucleic acids in DNA.
Figure 2-26. Oxygen atom with eight protons, eight neutrons, eight electrons, and two unfilled electron orbitals
Oxygen (Figure 2-26) has eight protons and eight electrons. It has two open electron positions in the outer shell. Oxygen’s formation of bonds with carbon enables biological respiration because it releases energy when it combines with carbon in carbon dioxide. After photosynthesis stores energy in sugars and other hydrocarbons, the release of energy during biological respiration and combustion is ultimately due to the formation of carbon dioxide when oxygen combines with sugar and other biological molecules. The bonding of carbon and oxygen in carbon dioxide releases four times more energy than bonds between other elements. If this type of bond and energy release were not possible, then sugars and other organic molecules would have a much lower energy density, and respiration would be an ineffective source of energy.
Figure 2‑27. Shells in massive star. Credit: R.J. Hall. Used here per CC BY-SA 3.0
In stars the size of the sun, the largest elements that form are carbon, nitrogen, and oxygen. Larger stars form heavier elements. After the CNO cycle in large stars, sequentially larger elements form in shells as the interior becomes hotter (Figure 2-27). Each of the fusion reactions up to iron fusion releases heat and expands the star.
The cycles become shorter as giant stars produce heavier and heavier elements. After iron is formed, fusion no longer releases energy and the pressure and temperature that kept the star from collapsing on itself is suddenly gone. A supernova is the result of the entire star collapsing on itself. Brian Green compared a giant dying star to an imploding building. Extremely high pressures and temperatures in supernovae were responsible for the formation of all elements heavier than iron. For example, gold and silver formed in supernovae explosions.
Figure 2‑28 shows which elements formed in supernovae (exploding massive stars), dying low mass stars such as the sun (yellow), and other stars.
Scientists think that the solar system formed in a dense cluster of stars. There is evidence that one or more supernovae exploded nearby as the sun and solar system formed. Thus, the elements from a supernovae were incorporated into the solar system and earth.
Figure 2‑28. Sources of elements in the solar system. Credit: ESA/NASA.
The lack of interstellar radiation in the interior of the dark molecular clouds allows for the formation of water, methane, and other organic chemicals. Water forms as oxygen atoms combine with hydrogen atoms on the surfaces of dust grains. If hydrogen had not been captured in water and methane ice (Figure 2-29) prior to the formation of the earth, all of the hydrogen on earth would have been lost to interstellar space because it would have escaped earth’s gravity.
Dark molecular clouds are extremely cold. The temperature drops to almost absolute zero, 10 K to 20 K, which is below the freezing point of water, so water forms as ice in dark molecular clouds. Eventually, water ice can become the most abundant solid in the cloud, even surpassing dust. Typical fractions of the non-gaseous substances in dark molecular clouds just prior to collapse are 50% water ice, 25% dust, and 25% methane ice. The total concentration of these solids is only about 2% of the total mass of the clouds. The primary substances in the clouds are hydrogen and helium gas.
After water, methane is the next most abundant non-gaseous molecule that forms in dark molecular clouds (Figure 2‑29). In addition to methane, molecular clouds have rich organic chemistry in which carbon, hydrogen, oxygen, and nitrogen bind together in organic molecules.
Figure 2-29. Methane (left) and water (right).
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The Sun. Credit: NASA. Public domain.
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