Joel Selbin Lousiona State University Baton Rouge, 70803
The Origin of the Chemical
Each of us, a t one time or another, ponders questions such as "Who am I?" "Where have I come from?" "Where am I going to end up?" Certainly, philosophers have toiled through the ages with such queries and yet no rational being has ever seriously suggested that he possessed the correct answers. But, in a sense, we are now fairly certain about a t least a part of the answer to the second question. The startling answer is that ultimately we all came from the core of some burned-out star! It will be the purpose of this and the succeeding article to elaborate on the last statement by presenting the story, as it can be pieced together today, of the origin of the chemical elements, or, in a word nucleosynthesis. We shall leave to the philosophers and metaphysicians concern with the questions surrounding the older word nucleogenesis, a word better reserved for the question of the ultimate origin of matter itself. And we might recall a t this point a favorite line of the famed cosmologist George Gamow. He liked to eliminate the divine from such discussions hy relating what St. Augustine of Hippo was said to have replied when asked, "What was God doing before he created Heaven and Earth?" The reply was, "He was creatine a Hell for ~ e o o l whoask e such auestions!" The problem of the origin of the elements is inextricahlv interwoven with such matters as the oriein of the universe (if it had one) and the structure an2 evolution of galaxies, their stars and other astronomical ohjects, and of course with nuclear physics. So, while the people who work closely with the nucleosynthesis problem are astrophysicists, nuclear physicists, or cosmologists, those of us who call ourselves chemists should have a t least a passive interest in where. when. and how the elements of our trade were produced. Of course some chemists, those who study the abundances and distribution of elements in samples of the earth, moon, and meteorites, make an active contribution to the overall problem. What do we need to know or to postulate before we can hope to formulate a sound theory of nucleosynthesis? 1) The abundances of elements, and more particularly the individual isotopic abundances or ratios for as many elements ss passible. 2) The origin, structure, and evolution of (a) the universe, (b) galaxies, (c) stars, and (d) other astronomical objects (nebulae, quasars, pulsars, supermassive stars, etc.). 3) Stellar properties such as (a) masses; (b) temperatures; (c) composition (and how it depends upon the stellar history); (d) densities; (e) time scales (e.g., haw long do various stellar events take and what times are acutally available for these events); and (0 a rather important quantity known as the "neutron excess," 7 = N - PIN + f', where N = total number of neutrons (bound in atomic nuclei as well as unbound), and P = total number of protons (bound in atomic nuclei as well as unbound). 4) Nuclear properties such as (a) reaction cross sections; (b) reaction rates (for which extrapolation to stellar thermal energies often must be made and trusted); (c) decay schemes and rates, and (dl nuclear reaction networks for reactions such as (n,?), @.n), (P.?), (a,?), (n,p),(a,n)and their inverses.
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The photograph of the Great Nebula in Orion is courtesy of Hale Observatories, Pasadena, California. 306 /Journal of Chemical Education
As this paper is read, i t should be kept in mind that none of the foregoing knowledge is perfectly known, ranging from pure educated speculation about (2), to some degree of certainty about (I), to a relatively high degree of understanding and knowledge of (4). Occurrence and Abundances of Elements Before we examine the questions of where, when, and how the elements were formed, it is appropriate to look a t what we know about the occurrence and abundances of the elements, or more precisely the abundances of the individual nuclides. There are 81 stable elements (from IH to saBi, with only 13Tc and slPm having no stable isotopes), which with their stable isotopes give rise to 280 stable nuclear species. Some elements, such as Be, F, Na, Al, P, Rh, Au, and 14 others possess only one stable isotope, whereas other elements, with mass numbers (A) 92 or ahove, may possess as many as 7 (Mo, Ru, Ba, Nd, Gd, Dy, Yb, and Hg), 8 (Cd, Te), 9 (Xe) and even 10 (Sn) stable isotopes. Beyond bismuth there are eleven naturally occurring radioactive elements (2 = 84-94) which are comprised of a t least 71 naturally occurring isotopes. Since most of these are daughter nuclei of the uranium and thorium decay chains, their abundances and those of their parents P T h , 235U, 23W, 244Pu) are of great value in helping to establish a time scale of certain nucleosynthesis processes and of Gemental existence (vide infro). And then there are more than 1200 artificially produced radioactive nuclei, resulting from the transmutation of the 280 stable isotopes. Sources of Abundance Data Let us now examine briefly the sources of abundance data. First, of course, is (1) the earth itself (core, mantle, and crust), from which we most readily obtain the crustal abundances. (See Table 1, where the most abundant elements of the earth crust are listed, along with abundances from other selected segments of the universe.) Abundance data is also gathered from (2) the surface of the moon (most recent direct source), (3) other planets, (4) meteorites (the "stones" and the "irons"), (5) the sun, (6) variThis is the first of a two-part series on nucleosynthesis. Part n will appear in the June issue. Figures, tables, footnotes, and references will be numbered consecutively throughout the series. A list of general references will be given at the end. Table 1. Elemental Composition, by Per Cenl of Total Atoms, of Selected RstemP
'Theae valuea vary somewhat fmm source tosourno, hut ingeneral theordersof the 4.mentsarecanstant.
ous types of stars (normal stars, believed to have surface ahundances characteristic of the interstellar medium from which they formed, and peculiar stars of various types which have anomalous spectra when compared with the other 99% of "normal" stars), (7)the interstellar medium, (8) cosmic radiation, and (9)gaseous nebulae. The most useful sources, in terms of quantity and quality of data, are (11, (21, (4), (5), and (6). Data from (8) and (9), while more limited, are very valuable. From the sun we get ahundanqe information in several ways: (a) from the line spectrum of the photosphere (outer, relatively cool surface layer of the sun): (h) from solar cosmic rays (particles presumed to he arcelerated hy flares in the photosphere,: ( c ) from the solar wind (particles continuously blown away from the surface of thesun); and (d) from the solar corona (analyzed by ultraviolet spectra ohtained from rocketsahove the earth's atmosphere). With the possible exception of iron ahundance relative to hydrogen in the solar corona and photosphere, there is reasonable agreement among the various solar data. Difficulties encountered in applying the theory of radiation transfer and the formation of spectral lines in a body such as a star, render most abundance data accurate only to withina factor of about two. Meteorites have proven to he of great value since many element ahundances cannot be ohtained from solar line spectra; and hecause they show the least chemical processing of any currently accessible solid object in the solar system, meteorites provide a better sampling of the nonvolatile elements than does the earth itself. Furthermore, radioactive decay in the meteorites shows that they are little changed from 4.6 X 109 yr ago when they solidifiedthis being ahout the age generally accepted for our solar system. What we find from the absorption spectra of stars, after fancy, difficult and inexact interpretation of the line strengths, is that the average composition of many stars is similar to that of the sun, hut not identical. It is the fact of the composition differences which presents us with the meatest challenge and a t the same time the most imnoriant tests of nu~leosynthesistheories. The reason is that basically the composition differences are of two tvpes: those that are caused hy differences in the chemical >omposition of the medium out of which the stars were formed, and those that are produced hy nuclear processes occurring within the stars themselves. Some of the more important specific differences are worth noting. First of all, the oldest known (Population 11)' stars (found in globular clusters) are observed to he substantially deficient, by factors ranging from 100 to 1000, in the heavy elements-meaning here elements heavier than helium hut in practice it is often only Fe which is observed -when compared to younger stars and our sun. Since it is assumed that the surfaces of the stars reflect the composition of the material from which the stars originally formed, the important conclusion is that perhaps 99% of the heavy elements have been synthesized since the formation of the first Population I1 stars, that is, over the course of the past 1019r.2 This is, by the way, taken as support or evidence for a big-hang cosmology, i.e., an evolutionary universe (vide infra). Secondly, the surfaces of many stars exhibit unusual overahundances of certain elements. Because these elements may he related by certain nucleosynthesis processes it is postulated that specific nuclear reactions occurring within the star have modified the surface c~mposition. There is also a possibility that unusual surface ahundances would come from transfer of processed matter from another star. Thirdly, elemental ahundance ratios vary from star to star in ways which suggest that each star has itself modified its chemical composition from that of the galactic gas from which it originally formed. This leads directly to the
Moss number Figure 1. The con = lo6.
universal
abundances 01 atomic species, based upon
sili-
pertinent conclusions that new nuclei are being synthesized all the time in stellar interiors and that over the eons and by various mechanisms stars are ejecting the new material hack into interstellar space. Finally, an exciting discovery was made in 1952 by P. Merrill when he found atomic lines of the element technetium in the surfaces of S-tvne ". stars. T c does not occur naturally on earth because all of its isotopes (14 of them!) are radioactive with half-lives of 2.6 x 106 vr or less. (It is the 97 isotope that has the longest half-life, followed by , X lo5 yr; hut i t is the 99 9 * T ~1.5 , X lo6 yr and 9 9 T ~2.1 isotope that is synthesized by the process of slow time scale neutron capture that we shall describe later.) This then is powerful astronomical evidence that a t least Stype stars arepresently synthesizing Tc. More recent observations of other radioactive elements in stars reinforces the evidence. The relative ahundances of the various nuclear isotopes of a given element cannot, in general, be obtained from solar or stellar ohservations.3 These isotopic ratios are ohtained from mass spectrometer experiments with samples of the solar system available to us, such as the earth's
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Our sun is believed to be a recent (second or later generationpopulation I) star of age 5 x lo9 yr. The best estimates of the age of the universe (or more confidently, of our galaxy)-and many different age calculations agree quite impressively on tbisis (12 2) X 108yr. 2Althougbit is believed that in the majority of stars there has been no mixing between the surface and the interior over the lifetime of the star, there are exceptions. Indeed, certain stars give strong evidence of deep mixing so that their surface material is contaminated with elements produced in their interiors by nuclear reactions. When high resolution spectral-line studies become possible, more information about isotope ratios may be gotten from molecular spectra of cool-surface stars and of the interstellar medium. For example, optical bands of Cz molecules have already been studied in the surfaces of coal stars and 12C/13C ratios examined. Other stellar surface molecules include CN, CH, CH+, and CS. Molecules detected in interstellar space (the first in 1963, but most of them very recently) have included OH (also recently found in another galaxy!); NHI (1968); HzO, H2C0 (1969); Ha, HCN, HCBN (cyanoacetylene), CHIOH, CO, HCOOH (1970); C2Hz, HCONHz (formamide, the first 4-different element molecule),HCNO, CHn,DCN (1911-72).
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Volume 50, Number 5, May 1973 / 307
crust, the oceans, the atmosphere, meteorites or lunar soil. The constancy of these isotopic abundance ratios lends validity t o the assumption that all of these materials were once part of a homogeneous gas cloud from which our solar system formed and of which parts (earth, moon, meteorites) solidified about 4.6 X 109vr aeo. The universal elemental abundance zata is plotted in Fimre 1 (notice the logarithmic ordinate scale) and the 10 most abundant elements are listed in Table 1. The most striking features of this data, and those which, of course, must be spoken to most clearly by any theory of nucleosynthesis, are that: (a) hydrogen is the most abundant element in the universe, accounting for about 93% by number of atoms, or about 75% by mass; (b) helium is the next most abundant element, accounting for about 7% by number and 24% by mass-there is a big mystery surrounding the helium content of the universe; in particular, just how the helium was formed and why its abundance varies so greatly from object to object; and many believe this mystery to be the most important unsolved problem in stellar astronomy, so we shall have more to say about this later od; (c) all the remaining elements account for something less than 1%of the atoms and something greater than 1% by mass; (d) after H and He, the next 8 most abundant elements (see Table 1) include those with 6, 8, 10, 12, 14, and 16 protons (but not 4) and, curiously, N with 7 and Fe with 26 protons; (e) among the general features of the Figure 1 plot is the prominent peak a t = T e ; there are several less prominent peaks including those in the regions of 50, 82, and 126 neutron numbers (these numbers alone - with 2.. 8.. 20.. and 28 are so-called maeic numbers since nuclei with these numbers of either protons or neutrons are "closed shell" and stable): (fl Li. Be. and B are particularly rare compared with the' ndighl;oring elements H, He, C, N, and 0; (a) - nuclides with atomic mass numbers which are multiplets of 4 are more numerous than their immediate neighbors; (h) in general, atoms of even mass number are mire numerousthan those of odd. Another concise way of picturing the relative abundance of the elements is as follows
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Nevertheless, the weight of recent observational evidence seems clearly to support the evolutionary "bigbang" universe championed in recent times by Gamow, Alpher and Herman. This theory proposes that the universe was "created" a t a time to (about 1010 years ago) when a hyperdense sphere of matter (dubbed "ylem" by Gamow), or "primeval atom" or "cosmic egg" (Lemaitre), or giant neutron hall of perhaps or neutrons packed into a volume roughly that of our solar system, with incredihle density and temperature exploded in a "hig-bang" (Gamow) a t or near the speed of light. The temperature dropped rapidly, from perhaps 1012'K in the first micro-second to 109°K after 5 min, to 40 X 106"K after one day, then more slowly to 5000'K after 300,000 years. Astrophysicist D. D. Clayton now regards the age pattern for galaxies, established a t (12 2) x lo9 years by so many independent methods, but particularly by remnant radioactivity (radioactive decay rates and schemes and mother/daughter isotopic ratios), to be the prime fact of an evolutionary cosmology. In the late 1940's, Gamow, Alpher, and Herman attempted to explain (3) the synthesis of the heavy elements from the initial dense hot cloud of baryons, leptons, and radiation in the first 30-60 minutes of existence of our universe-some 10 billion years ago! The effort failed, due to the difficulty of building nuclides beyond mass 4 (there being no stable mass 5 or 8 nuclides). However, there is still reason to believe (assuming of course that the big bang theory of the universe is baiically correct) that the nuclides D, 3He, 4He, and perhaps ?Li were produced in nearly their currently observed abundances in that hectic primordial hour and are, therefore, with H, the oldest nuclides running about. The problem is unsettled and is ultimately tied to the helium mystery mentioned earlier.
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Helium Problem
Theories of the Universe
The current far-from-settled controversv over helium is basically that the present galactic luminosity, which is generated mainlv bv H burning to He (vide irzfra). is calculated to convert -only 3 - 5 % f H t o ~ in.101~ e years, whereas the universal abundance of He appears to be 2427% by mass. (It is interesting that the simplest big bang theory produces about 27% He a t the time of origin.) There is a recent proposal (due to Wagoner in 1969) that the collapse, and explosive re-expansion (or "bounce") of very massive (>lo4 sun masses) stars might have produced significant amounts of helium by an explosive process eons ago. There are those in the field who tentatively allow that He production may actually come from all three mechanisms, viz., cosmological, supermassive stellar explosions, and normal hydrogen-burning. Compounding the problem is the fact that it is not possible to measure directly the concentrations of D, 3He, 4He, and 7Li in the surfaces of the oldest "main-seq ~ e n c e "stars, ~ which are the very ones which should re-
Models of the universe, with emphasis on its nature and origin, fall generally into two classes: the static or steadystate universe, and the evolutionary or "hig-hang" universe. The former comes on a line from Einstein and DeSitter and the latter from Friedmann, Lemaitre, and Eddington. In more recent times, the steady-state universe, championed by Bondi, Gold, and Hoyle starting in 1948 ( I ) , assumes not only the cosmological principle: "the universe looks very much the same from anv location and in all directions,'' but the perfect cosmolo&al principle which adds to the foregoing . .the requirement ". . . and a t all times." This theory requires the continuous creation of matter so that the observed expansion of the universe (or a t least the red-shift of lieht from distant ealaxies) does not alter the requirements of the perfect-cosmological principle. Most nhvsicists and cosmoloeists find this objectionable, but ;here are some (see, e.gl ref. (2)) who believe that the question is far from settled.
One of the most significant graphs ever plotted by astronomers (over half a century ago) is the so-called Hertzsprung-Russell (H-R) diagram. This is simply a plot of the absolute visual magnitude (or luminosity) of stars af known parallaxes (hence distance from us), plotted vertically, versus their spectral type, plotted horizontally. The spectral type or class is a function of the surface temperature of the star. About 99% of the observable stars fall on or near a roughly diagonal line (smoothly curved) running from lower right (red dwarfs, surface temperature 3000°K) to the upper left (blue giants, surface temperatures up to 30,WO "K) of this H-R diagram. These are the mainsequence stars, of which our sun is a prominant member (spectral class G, surface temperature 5800°K).The mast conspicuous exceptions t o the main sequence group, and lying above it, are the red giants (there are supergiants and sub-giants and other odd-balls to the upper right of the main sequence). The smaller number of subdwarfs and white dwarfs fall below the mainsequence.
Elements Hydrogen: 'H; pD) Helium: 4He;($He) Li, Be, B C, N, 0 , Ne Silicon group: Na-Sc Iron moun: 50 5 A 5 62 ~ i d i l e w e i ~group: h t 63 5 A 5 100 Heavyweight group: A > 100
308 /Journal of Chemical Education
Fractional Abundance by Mass 0.73; (lo-') 0.24; (6 x 10-9}("98%) 10-8 1.8 X (-2%) 2 x lo-J (-0.2%) 2 x lo-' 10-6
}
lo-'
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flect the composition of the galaxy near the time of its formation. The reasons are that the concentrations of D and ?Li are too small for their lines to be ohserved, and the surface temperatures of these stars are too low to excite the atomic lines of He. Furthermore, the helium contents measured in recent years in both the solar wind (H/He 20) and solar cosmic rays (H/He 16) are in reasonable agreement with each other but substantially different from the value inferred or observed in most stars 10). It seems and assumed for so many years (H/He safe to say that a great deal more will be known about the universe when the helium problem is finally solved.
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Recent History of Nucleosynthesis Theories The development of recent theories of nucleosynthesis begins in about the mid-1940's. when hydrogen thennonuclear reactions were fairly well developed and understood. Gamow and his colleagues in 1948 proposed a theory of nucleosynthesis based upon their big-hang hypothesis. Assuming that the primordial matter in the cosmic egg was no more complex (at t = 0 ) than neutrons (which break down spontaneously into protons, electrons, and neutrinos with a half-life of about 12 min), elements were to he built u p from this simple starting point during the time, nerhans the first 30 min or so. after the initial cosmic explosidn. Because of the rapid& dropping temperature and densitv - reauired . hv the bie-bane hv~othesis.hut mainlv hecause it requiredneutron; whiih would be decaying fa& (only about 5 2 as many left after one hour, just from decay), their theory had to snythesize the elements in that short a time. Basically their theory pictured neutrons breaking down forming protons, which then, on collision with other neutrons, would produce stable deuterium (ZD), which then could capture another neutron to become tritium (3T), which is radioactive and breaks down into stable 3He. The 3He could capture a neutron to become stable 4He. There are, of course, many other particle-particle reactions which can occur in such a system leading from 1H to 4He, and we shall detail these later on as we look a t the primary stellar energy source. Now, the main process of neutron-particle, as well as other particle-particle reactions and radioactive decay are supposed to continue until the whole list of elements is built up. There was some interesting support for the neutron-capture theory. For example, it is clear that the ahundance of an isotope resulting from this process will vary inversely with the neutron capture cross section of that isotope. Thus, if an isotope captures neutrons readily i t will disappear fast in a neutron flux and little of it will accumulate. If it does not readily capture neutrons, it would tend to accumulate. Well, the abundance curves do indeed reveal that (with few exceptions) the isotopes with low neutron capture cross sections are the most abundant ones. However, there are also some minor problems with this successive build-up picture; for example, the very low ahundance of the early nuclei of Li, Be, and B, and the very considerable abundance of Fe. Although the theory could make a stab a t explaining away these problems, there is a far more serious, indeed perhaps fatal, flaw in the model. Of all the mass numbers from 1 to about 260, only two are not stable a t all, regardless of nucleon composition, and those are mass 5 and mass 8.5 How are these mass numbers to he surmounted in a successive build-up model which does not permit ternary collisions of particles? An old one-line joke among cosmologists goes, "The Gamow theory huilds all of the elements up to helium." There have been some recent efforts to see if the mass 5 and 8 gaps could be bridged by going via (a,y)reactions from 3He to ?Be to "C, but we shall not pursue this here. The prohlem of hig-hang nucleosyntheses has been reexamined recently by Peebles in 1966 (4) and by Wagoner, Fowler, and Hoyle in 1967 (5), using the much more nu-
merous empirical nuclear reaction rates which became available in the intervening years. The results of the new calculations show that helium can be produced in the bigbang, hut no significant amounts of heavier elements. There is a set of data (called "an interesting universe" by the latter authors) which allows the hig-bang to synthesize 2D, 3He, 4He, and perhaps 7Li in proportions characteristic of the solar system. However, spallation processes in stellar atmospheres or production by cosmic rays can also synthesize the necessary 2D, 3He and VLi, and a t the same time the observed amounts of GLi, 9Be, 'OB, and 'IB, which are not made in the big-hang. The suggestion by Hoyle and his colleagues that all of the elements (except hydrogen) are synthesized in the intensely hot interiors of stars gained tremendous momentum in the 1950's, and if stellar explosions are included, it is universally accepted today that this is the synthesis site for all elements from C on (and including a t least part of the He). (The idea of nuclear processes operating in stars goes back over 50 years to Eddington and others.) We should note that nucleosynthesis in stars is consistent with all cosmologies, and in particular with either the expolsive-evolutionary or the steady-state cosmology. In 1956, Suess and Urey published their important elemental abundance paper (67,hased mainly on meteorite measurements, which set the stage for the famous 1957 paper (7) of Burbridee. Burbridee. Fowler. and Hovle (BZFH). Thev outlined a< lkast eigh 2) by some other means. Therefore. it is proposed t h a t element production occurred in what we see now as quasi-stellar sources (quasars) and that these objects ejected heavy elements ( a t a rate of about 1 solar mass per year) into the intergalactic medium, from which the protogalaxies aecreted significant amounts of material which ultimately condensed into the first, Population 11, stars.
We have already discussed the theory of cosmological element production which must occur in the first hour of the big-bang universe, and we saw that this theory is no longer tenable for elements beyond helium. But the 1966 discovery by Penzias and Wilson that the universe is apparently bathed in an isotropic hlack-body radiation corresponding to 3"K, which had been predicted by gar no^,^ gave not only a strong boost to the big-bang theory but also to the idea of cosmological production of helium. In the next article we shall follow the details of element production throughout the evolutionary lifetimes of stars, examining superficially the nucleosynthesis sites and then, in some detail, the nuclear processes believed responsible for the presently observed nuclear abundances of all the chemical elements. Literature Cited (11 Rondi. H . . and Gold. T..Mon Not. Roy. Arfron. Sac. 108. 25'1 (19481: Hoylr. F.. Mon Not. Ro.s.Ar
