Origin of the chemical elements
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The relative abundances of the chemical elements provide significant clues regarding their origin. Earth’s crust has been affected severely by erosion, fractionation, and other geologic events, so that its present varied composition offers few clues as to its early stages. The composition of the matter from which the solar system formed is deduced from that of stony meteorites called chondrites and from the composition of the Sun’s atmosphere, supplemented by data acquired from spectral observations of hot stars and gaseous nebulas. The table lists the most abundant chemical elements; it represents an average pertaining to all cosmic objects in general.
| The most abundant chemical elements (by numbers of atoms per 109 atoms of hydrogen) | ||||||||||
| element | symbol | abundance | element | symbol | abundance | element | symbol | abundance | ||
| helium | He | 9.8 × 107 | magnesium | Mg | 38,000 | potassium | K | 133 | ||
| carbon | C | 501,000 | aluminum | Al | 3,000 | calcium | Ca | 2,200 | ||
| nitrogen | N | 100,000 | silicon | Si | 35,000 | titanium | Ti | 91 | ||
| oxygen | O | 794,000 | phosphorus | P | 320 | chromium | Cr | 473 | ||
| fluorine | F | 33 | sulfur | S | 17,400 | manganese | Mn | 288 | ||
| neon | Ne | 123,000 | chlorine | Cl | 250 | iron | Fe | 33,000 | ||
| sodium | Na | 2,100 | argon | Ar | 3,600 | nickel | Ni | 1,800 | ||
The most obvious feature is that the light elements tend to be more abundant than the heavier ones. That is to say, when abundance is plotted against atomic mass, the resulting graph shows a decline with increasing atomic mass up to an atomic mass value of about 100. Thereafter the abundance is more nearly constant. Furthermore, the decline is not smooth. Among the lighter elements, those of even atomic number tend to be more abundant, and those with an atomic number divisible by four are especially favoured. The abundances of lithium, beryllium, and boron are rare compared with those of carbon, nitrogen, and oxygen. There is a pronounced abundance peak for iron and a relatively high peak for lead, the most stable of the heavy elements.
The overwhelming preponderance of hydrogen suggests that all the nuclei were built from this simplest element, a hypothesis first proposed many years ago and widely accepted for a time. According to this now-defunct idea, all matter was initially compressed into one huge ball of neutrons. As the universe began to expand, its density decreased and the neutrons decayed into protons and electrons. The protons then captured neutrons (see neutron capture), one after another, underwent beta decay (ejection of electrons), and synthesized the heavy elements. A major difficulty with this hypothesis, among various other problems, is that atomic masses 5 and 8 are unstable, and there is no known way to build heavier nuclei by successive neutron capture.
A large body of evidence now supports the idea that only the nuclei of hydrogen and helium, with trace amounts of other light nuclei such as lithium, beryllium, and boron, were produced in the aftermath of the big bang, the hot explosion from which the universe is thought to have emerged, whereas the heavier nuclei were, and continue to be, produced in stars. The majority of them, however, are fashioned only in the most massive stars and some only for a short period of time after supernova explosions (see below Evolution of high-mass stars).
The splitting in the spectral sequence among the cooler stars can be understood in terms of composition differences. The M-type stars appear to have a normal (i.e., solar) makeup, with oxygen more abundant than carbon and the zirconium group of elements much less abundant than the titanium group. The R-type and N-type stars often contain more carbon than oxygen, whereas the S-type stars appear to have an enhanced content of zirconium as compared with titanium. Other abundance anomalies are found in a peculiar class of higher temperature stars, called Wolf-Rayet (or W) stars, in which objects containing predominantly helium, carbon, and oxygen are distinguished from those containing helium and nitrogen, some carbon, and little observed oxygen. Significantly, all these abundance anomalies are found in stars thought to be well advanced in their evolutionary development. No main-sequence dwarfs display such effects.
A most critical observation is the detection of the unstable element technetium in the S-type stars. This element has been produced synthetically in nuclear laboratories on Earth, and its longest-lived isotope, technetium-99, is known to have a half-life of 200,000 years. The implication is that this element must have been produced within the past few hundred thousand years in the stars where it has been observed, suggesting furthermore that this nucleosynthetic process is at work in at least some stars today. This heavy element upwells from a star’s core (where it is produced) to the surface (near where it is observed) in a phase called the third dredge-up, when material in deep helium-burning layers is brought to the surface through convection.
Researchers have been able to demonstrate how elements might be created in stars by nuclear processes occurring at very high temperatures and densities. No one mechanism can account for all the elements; rather, several distinct processes occurring at different epochs during the late evolution of a star have been proposed.
After hydrogen, helium is the most abundant element. Most of it was probably produced in the initial big bang. Furthermore, as described earlier, helium is the normal ash of hydrogen consumption, and in the dense cores of highly evolved stars, helium itself is consumed to form, successively, carbon-12, oxygen-16, neon-20, and magnesium-24. By this time in the core of a sufficiently massive star, the temperature has reached some 700 million K. Under these conditions, particles such as protons, neutrons, and helium-4 nuclei also can interact with the newly created nuclei to produce a variety of other elements such as fluorine and sodium. Because these “uneven” elements are produced in lesser quantities than those divisible by four, both the peaks and troughs in the curve of cosmic abundances can be explained.
As the stellar core continues to shrink and the central temperature and density are forced even higher, a fundamental difficulty is soon reached. A temperature of roughly one billion K is sufficient to create silicon (silicon-28) by the usual method of helium capture. This temperature, however, is also high enough to begin to break apart silicon as well as some of the other newly synthesized nuclei. A “semi-equilibrium” is set up in the star’s core—a balance of sorts between the production and destruction (photodisintegration) of silicon. Ironically, though destructive, this situation is suitable for the production of even heavier nuclei up to and including iron (iron-56), again through the successive capture of helium nuclei.
Evolution of high-mass stars
If the temperature and the density of the core continue to rise, the iron-group nuclei tend to break down into helium nuclei, but a large amount of energy is suddenly consumed in the process. The star then suffers a violent implosion, or collapse, after which it soon explodes as a supernova. In the catastrophic events leading to a supernova explosion and for roughly 1,000 seconds thereafter, a great variety of nuclear reactions can take place. These processes seem to be able to explain the trace abundances of all the known elements heavier than iron.
Two situations have been envisioned, and both involve the capture of neutrons. When a nucleus captures a neutron, its mass increases by one atomic unit and its charge remains the same. Such a nucleus is often too heavy for its charge and might emit an electron (beta particle) to attain a more stable state. It then becomes a nucleus of the next higher element in the periodic table of the elements. In the first such process, called the slow, or s-, process, the flux of neutrons is low. A nucleus captures a neutron and leisurely emits a beta particle; its nuclear charge then increases by one.
Beta decay is often very slow, and, if the flux of neutrons is high, the nucleus might capture another neutron before there is time for it to undergo decay. In this rapid, or r-, process, the evolution of a nucleus can be very different from that in a slow process. In supernova explosions, vast quantities of neutrons can be produced, and these could result in the rapid buildup of massive elements. One interesting feature of the synthesis of heavy elements by neutron capture at a high rate in a supernova explosion is that nuclei much heavier than lead or even uranium can be fashioned. These in turn can decay by fission, releasing additional amounts of energy.
The superabundant elements in the S-type stars come from the slow neutron process. Moreover, the observation of technetium-99 is ample evidence that these processes are at work in stars today. Even so, some low-abundance atomic nuclei are proton-rich (i.e., neutron-deficient) and cannot be produced by either the s- or the r-process. Presumably, they have been created in relatively rare events—e.g., one in which a quantum of hard radiation, a gamma-ray photon, causes a neutron to be ejected.
In addition, no known nuclear process is capable of producing lithium, beryllium, and boron in stellar interiors. These lightweight nuclei are probably produced by the breakdown, or spallation, of heavier elements, such as iron and magnesium, by high-energy particles in stellar atmospheres or in the early stages of star formation. Apparently, these high-energy particles, called cosmic rays, originate by means of electromagnetic disturbances in the neighbourhood of starspots and stellar flares, and they also arise from supernova explosions themselves. Some of these light-element nuclei also might be produced by cosmic rays shattering atoms of carbon, nitrogen, oxygen, and other elements in the interstellar medium.
Finally, the peculiar A-type stars comprise a class of cosmic objects with strange elemental abundance anomalies. These might arise from mechanical effects—for example, selective radiation pressure or photospheric diffusion and element separation—rather than from nuclear effects. Some stars show enhanced silicon, others enhanced lanthanides. The so-called manganese stars show great overabundances of manganese and gallium, usually accompanied by an excess of mercury. The latter stars exhibit weak helium lines, low rotational velocities, and excess amounts of gallium, strontium, yttrium, mercury, and platinum, as well as absences of such elements as aluminum and nickel. When these types of stars are found in binaries, the two members often display differing chemical compositions. It is most difficult to envision plausible nuclear events that can account for the peculiarities of these abundances, particularly the strange isotope ratios of mercury.
