Nucleosynthesis

Nucleosynthesis is the process by which the elements are formed in the interiors of stars during the course of their normal evolution. Hydrogen and helium are thought for the most part to have been generated at the origin of the universe itself (nucleogenesis), while all other heavier elements are synthesized via nuclear reactions in stellar cores. The heaviest elements are created during the death throes of massive stars.

Elemental Synthesis

Two of the most fundamental questions of modern astrophysics have to do with the origin and composition of the universe's primordial matter: when it came into existence and how it relates to the Einsteinian space-time structure of the present universe. With developments in physics, the problems seem to be divided into two parts: the origin of the simplest elements, hydrogen and helium, during the initial formation of the present universe and the subsequent nucleosynthesis of the other elements in the pressure cookers known as stars.

To understand elemental synthesis, scientists must rely on experimental observations interpreted in the light of current theory. Such data principally have to do with abundances of nuclear species now and in the past. This data set is provided from composition studies of the earth, meteorites, and other planets, and from stellar spectra. The distribution of hydrogen and heavier elements in stars throughout the galaxy, particularly in what are referred to as population I and II stars (younger and older, respectively), indicates how the chemical composition of the Milky Way galaxy has changed over time. From these studies, most theorists conclude that the galaxy has synthesized 99 percent of its own heavy elements and thus that nucleosynthesis occurs during the natural evolution of stars. In the light of variations observed in stars of diverse ages, scientists have formulated theories regarding the formation of elements within stellar structures. A dramatic piece of evidence along that line, for example, was the discovery of technetium in the spectrum of a red giant, all of whose isotopes, being radioactive, are short-lived, indicating that the star in which it is found must be currently producing the element. The study of naturally occurring radioactive isotopes, long- and short-lived, not only allows for measuring the time of galactic and stellar nucleosynthesis but also provides evidence that synthesis of elements heavier than hydrogen must be occurring continuously throughout the universe.

Theory

Starting with the simplest element, hydrogen, which possesses one proton and one electron and is by far the predominant element in the universe, the study of nucleogenesis has progressed to a consideration of the origin of the universe. Beginning in 1946, Russian physicist George Gamow and others presented the theory that the entire structure started as a gigantic explosion of an extremely dense, hot “singularity,” or infinitesimally small object. The explosion would have been so intense as to provide the propellant for all subsequent motion of the outwardly expanding matter and for the creation of the elements. Such a “” concept has come to be accepted almost unanimously, with certain modifications. The discovery of an isotropic microwave background radiation, corresponding to a 3-kelvin temperature residual from the original fireball, lent support to the theory, along with the use of gigantic accelerators in the 1970s, which permits examination of the formation and interactions of the basic constituents and forces of nature.

Such physics has determined that elemental synthesis, via nuclear reactions, combining protons, electrons, and neutrons, could have occurred only when the temperature dropped to below 1 billion kelvins about three minutes after the explosion. Before that point, the energy of motion would have been too great either to form those particles or to let them cling together in electromagnetic interactions. That period of elemental synthesis probably lasted about one hour; eventually the temperature and pressure would have dropped too low to sustain any further reactions. Because of the instability of particles with atomic masses of 5 and 8, only traces of particle combinations beyond a mass of 4, including lithium and beryllium, would have been formed; thus the universe was probably composed of about 75 percent hydrogen and 25 percent helium. The formation of the helium nucleus, with a mass of 4, would have used up all the available neutrons. The reactions would have progressed in a certain order. First, neutrons and protons would combine to produce deuterium; deuterium and protons would then give helium 3; the collision of two helium 3 nuclei would produce a helium 4 nucleus and two protons, releasing energy in the process as gamma rays. This postulated process seems to be in excellent agreement with observational data and theoretical calculations.

For roughly one million years, radiation was so intense that electrons could not combine with nuclei to form neutral atoms. Only after the radiation pressure became low enough could neutral atoms begin to form galaxies and stars. At that state, the dominating force in the universe became gravity, with the galaxies and stars forming as a result of gravitational contraction. Scientists' understanding of galactic formation remains sketchy, but stellar evolution—from birth in dust-cloud nurseries to death—is well understood through a combination of observational data, laboratory measurements of nuclear reactions and their rates, and copious amounts of theoretical work. As stated best by Indian American astrophysicist Subrahmanyan Chandrasekhar, the working hypothesis generally accepted by astrophysicists is that the stars are the places where the transmutation of elements occurs, all the elements beyond hydrogen being synthesized there. All the energy available to a star throughout its life span, with minor exceptions, is derived from such transformations.

As the original gas and dust in a nebula collapse and contract, they heat up enormously, until the temperature in the core reaches some 10 million kelvins, at which point thermonuclear proton-proton reactions occur, to form deuterium and give off positrons and radiant energy. Further reactions occur, increasing the helium formed, decreasing hydrogen, and producing energy sufficient to halt the gravitational collapse of the star. For most stars, this stage probably occupies the greater part of their lifetime. The more massive the star is, however, the faster it will exhaust the hydrogen supply at the core and the shorter its time of stability will be. Stars such as the sun are in the range for forming helium. Some interesting side reactions occur also. Some 5 percent of the helium reacts to make beryllium, boron, and lithium. In an even rarer occurrence, proton capture produces the isotope boron 8. The latter is important because it is very sensitive to temperature and therefore acts as a good test of stellar theories; the reaction produces neutrinos, which earthbound astronomers can then study.

Carbon-Nitrogen-

In older stars, formed as second, third, or later generations, some heavier elements are present. In these, the so-called carbon-nitrogen-oxygen cycle proposed by Hans Albrecht Bethe works, again turning four protons to helium. Because a higher temperature is necessary to overcome the electrostatic (Coulomb) repulsion barrier, this cycle takes place only in larger stars. In either case, when a significant amount of core hydrogen is used up, with helium ash left, the star will contract. Meanwhile, the hydrogen-containing outer area expands, causing the star to become a red giant; its central temperature rises to 100 million kelvins. At this stage, helium burns to form beryllium, forming one beryllium atom per billion helium atoms. Also produced are carbon, oxygen, and neon, the principal source of energy being the conversion of three heliums to carbon-12 plus gamma radiation. This burn, however, is short-lived, lasting only 10 to 100 million years, as compared to more than 5 billion years for the present sun. Any further synthesis requires much higher thermal energy input than can ever occur.

Beyond this stage, in larger stars, the processes become more complex. When helium is exhausted, contraction starts again. For objects such as the sun, this shrinkage will continue until it is halted by electron degeneracy (a mutual repulsion of tightly squeezed electrons) to form white dwarfs, small and intensely radiative bodies losing their heat into space, with no further nuclear energy available. Many become surrounded by a halo of expanding gases, the so-called planetary nebula; material from the star flows into space as a last gasp of the red giant stage. In larger stars the temperature continues to climb, to 70 million kelvins, eventually causing new sets of elements to form, including magnesium-24, sodium-23, neon-20, silicon-28, and sulfur-32. With further contraction, until the temperature reaches 1 billion kelvins, elements up to and including iron are created. Synthesis stops here, however, because of the energy required to bind more stable nuclei together.

Neutron-Induced Reactions

Additional synthesis does not involve charged-particle reactions but rather neutron-induced reactions, which tie up neutrons and produce energy. Such reactions are called s-processes because they proceed very slowly, taking from 100 to 100,000 years per capture step. This process accounts for the heavier isotopes on even atomic number elements and the distribution of nuclides up to bismuth. This reaction, in conjunction with a p-process involving successful proton reactions, can account for all the stable isotopes up to bismuth. For higher elements, however, a more rapid neutron-capture chain called the r-process is required; it takes place when there is an enormous neutron flux so that many captures can take place in milliseconds. Conditions perfect for such acts occur in supernovas, stars that explode with some of the greatest violence seen in the universe. Type I supernovas are from old, small stars, with masses of 1.2 to 1.5 times that of the sun; in such an explosion the entire star is destroyed, pushing the temperature to 10 billion kelvins. Type II supernovas occur in stars with masses greater than ten times that of the sun. Under contraction, the temperature in the nucleus of the star rises to 5 billion kelvins, and iron and nickel nuclei rapidly absorb neutrons, producing many of the heavier neutron-rich elements. The collapse, which takes about one second, results in a core mass of neutrons, with explosive ejection of these heavy elements into the interstellar regions. Such explosions, which occur perhaps once in a hundred years in a galaxy, contribute all the material from which other stars, clouds, and planets such as Earth are formed.

Determination of Isotopic Abundances

Since the first theories of the processes of elemental origins were proposed, scientific understanding of nucleosynthesis has progressed greatly, thanks principally to an improved ability to determine abundances of elements particularly in stars and nebulas, and to better understanding of transformation conditions during synthesis. Nuclear physics data on reaction rates, particle formation, and interactions at diverse temperatures and energies, along with clearer notions of strong and weak force interactions, have contributed vital knowledge on both the universe's origin and the generation of elements in stellar bodies during and at the end of a star's life cycle.

Isotopic abundances can be determined from meteorites by the use of mass spectroscopy. In this experimental technique, particles are heated until they break apart into ionic forms; the bodies are then propelled, under the influence of electric and magnetic fields, through a vacuum chamber. The curved path followed depends on the mass and the charge on the elements. Collection at the end of the path allows detailed comparisons to be made, with particular attention to the anomalies that are critical to theories of nucleosynthesis.

Analysis of Extraterrestrial Objects

Spectral analysis has been the principal tool for studying extraterrestrial abundances. In such analysis, light from the observed object is passed through a prism or diffraction grating so that it is broken into all of its component colors; the resulting spectrum ranges from the blue to the red region of the visible section of the spectrum. Invariably, the background will be crossed by dark or bright lines, depending on whether it is an emission or absorption spectrum. These lines, identifiable in the physics laboratory, act as fingerprints, quickly showing such information as which elements are present and their abundances. Observation of material emitted by supernovas, for example, not only shows how heavy elements are enriched in space but also contributes greatly to theories of explosive charged-particle nucleosynthesis.

Similar analyses, using spectroscopes, telescopes, and various light-intensity-enhancing instruments such as charge-coupled devices (CCDs), have been done of other objects, including medium-mass stars with s-process element formation, nova explosions, and mass flows from solar-type stars. The latter can be studied best in the sun, by analyzing the composition of the solar wind with data returned by meteorological and scientific satellites. Detectors placed above the atmosphere can be equipped to detect charged particles such as protons or electrons. Experiments to view the universe in some region of the electromagnetic spectrum besides visible light, such as radio, gamma, infrared, or ultraviolet, also must be placed beyond the disturbing influence of the atmosphere.

Advances in Theoretical Study

Much of the information usable for the theoretical study of nucleosynthesis comes from two terrestrial sources. First, experimental studies using nuclear reactors and particle accelerator machines have provided comprehensive measurements of reaction rates and of the actions of the weak force in nature. Increasingly reliable determinations of critical cross-sections, representing the space in which reaction occurs between two particles, and of the neutron-capture process, which is responsible for the bulk of nuclei more massive than iron, have become possible with highly refined accelerators and electrical detectors. Theoretical predictions and experimental results are thus more in harmony than ever before.

The second important advance has been in computer technology, which has made possible greatly increased numerical calculations of structures and the evolution of astrophysical objects. The advent of high-speed computers has allowed much greater predictive ability for the standard model regarding the formation of elements at various stages in the stars. Such detailed models, particularly of massive stars, in terms of hydrodynamic phases, have shown, for example, that supernovas are immensely important in the synthesis of heavy elements. Models for actions at extreme temperature and density conditions are very close to what is observed during the expansion, cooling, and mass ejection processes of the dying stars. Computer technology has helped identify further problems through capture modeling, such as the sites necessary for r-process neutron-capture nucleosynthesis.

Importance to Astronomy

Nucleogenesis and nucleosynthesis are two of the most important topics in astronomy and hence the earth sciences, promising to cast light on not only the evolution of stars but the ultimate origin of the universe as well. The understanding of universal origins has been advanced greatly by the advent of particle accelerators of remarkably high energies. These instruments provide physicists with clearer pictures of the elementary particle structures of the universe and of their interaction under the four forces controlling them. During the creative process, these four forces—strong and weak nuclear, electromagnetic, and gravitational—were unified as one, separating only as initial conditions of temperature, pressure, and density changed. Under their actions, radiation and particles ultimately formed, with radiation finally dispersing enough that protons and helium nuclei could combine with electrons to form neutral atoms.

Although the modern understanding of nucleosynthesis is thought to be quite satisfactory, there are still problems unsolved. Certain elemental anomalies have not been explained by either experiments or theory. Predictions of energy fluxes and solar winds from other stars, particularly red giants, represent other unsolved problems. The answer to what causes a dust cloud to begin to contract to form a star is unknown; a widely accepted notion is that the contraction is prompted by the shock wave of a supernova. Problems remain with the theory itself, so that modified theories, such as the “inflationary universe,” have been proposed. The investigation of such problems of modern physics and astronomy has led to numerous insights, including the possibility that planets may be by-products of stellar formation; in such a scenario, the galaxy may be filled with planets and, possibly, life-forms. Further fine-tuning of reaction rates, mechanisms, and such experimental topics as element reactions may solve some of the deepest philosophical and scientific mysteries of modern science.

Principal Terms

big bang theory: the theory that the universe was created via an initial explosion that resulted in the formation of hydrogen and helium

charged-particle reaction: a nuclear reaction involving the addition of a charged particle—proton or electron—to a nucleus

deuterium: an atom built of one proton and one neutron; an essential stepping-stone in the proton-proton cycle in solar-type stars

isotope: an atom with the same number of protons as another but differing in the number of neutrons and the total weight

neutron reaction: a nuclear reaction in which a neutron is added to increase the atomic mass of the nucleus, forming an isotope

nucleons: positively charged protons and neutral neutrons; large particles that occupy the atomic nucleus

supernova: a massive star that explodes after available energy in the interior is used up and the star collapses

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