“Ay, for ‘twere absurd
To think that Nature in the Earth
bred gold
Perfect i’ the instant: something
Went before
There must be remote matter.”
Benjamin “Ben” Johson (1612)1
Where did the Elements Come from?
In the early 19th century the English chemist John Dalton proposed an atomic theory of matter in which chemical compounds are formed from the combination of atoms of definite and characteristic weight.2 Dalton knew that hydrogen was the lightest element, so he gave it an atomic weight of one, and he assigned larger numbers to the other atoms. These elements are now arranged in the periodic table by their atomic number and chemical properties.
Most of the natural chemical elements found on the Earth are exceptionally durable and long-lived. When the substance they occupy wears away, its elements disassemble and become redistributed in something else. Nothing lasts, everything changes, but the constituent elements mostly just get moved around.
So we naturally wonder how the abundant, long-lived, stable chemical elements, like hydrogen, carbon, nitrogen and oxygen, were made? As suggested by their durability, it takes an unusual amount of energy to produce or transform these chemical elements, and that is why ancient alchemists never succeeded in transforming lead into gold within their laboratories. Very high temperatures are required to energize the nuclear reactions that create chemical elements, and they naturally occurred only in the first moments of the expanding Universe or subsequently in the hot interiors of stars.
Clues to the nuclear reactions that gave rise to the elements in these two locations were provided by the discovery that the same chemical elements are found throughout the Cosmos, and by observations of how the relative abundances of these elements depend on their weight.
Universal Chemical Elements
Dark absorption lines or bright emission lines reveal the presence of an element in a cosmic object. Spectroscopic observations of these line features identify the element and only that element. The spacing and relative strengths of the hydrogen lines, first described by Johann Balmer in 1885, is an example.3
About a quarter of a century before that, Gustav Kirchhoff, at the University of Heidelberg, and his chemist colleague Robert Bunsen, were identifying the chemical composition of different substances by the spectral lines they emitted when burned. Since most of the Sun’s spectral lines could be identified with lines known from their laboratory experiments, Kirchhoff and Bunsen concluded in 1860 that the Sun and Earth contain the same chemical elements, with the same relative abundance.4 For a time, astronomers even talked of an iron Sun, owing to the great abundance of iron in the Earth and in the solar line spectra.
Stars of different spectral types showed conspicuous lines of various elements, however, suggesting that some stars might have different chemical compositions. But it was eventually realized that the presence or absence of specific spectral lines depends on the physical conditions in a star’s outer atmosphere and does not necessarily indicate its chemical composition.5
As far as astronomers could tell, the same chemical elements are present in the Earth and the stars, but in different proportions, or relative amounts. The most dramatic differences are in the substantial relative amounts of hydrogen and helium in stars.
Cecilia H. Payne and Stars Dominated by Hydrogen
The English-American astronomer Cecilia H. Payne was born in 1900 at the foot of the Buckingham Chilterns in England, where her ancestors had tilled the soil for at least ten centuries. As a child, she loved the surrounding natural world. Trees were her earliest companions, and the captivating sight of spiders, mimosa, and orchids were still remembered in her later years. Her early education at St. Paul’s Girls’ School also instilled a love of music, when she came under the spell of her shy, charming teacher Gustav Holst and his newly composed The Planets.
Cecilia completed her undergraduate studies on a scholarship to Newnham College at the University of Cambridge as a student in natural science. When recalling her college years, she was fond of quoting the English poet William Wordsworth’s lines: “Nature never did betray the heart that loved her,” as well as the English biologist T. H. Huxley’s advice that the student of Nature should: “Sit down before fact as a little child, be prepared to give up every preconceived notion, follow humbly wherever and to whatever abysses Nature leads, or you shall learn nothing.”6
Upon hearing a lecture by A. S. Eddington near the end of 1919, about the observational confirmation of the curvature of space-time by massive objects, Payne switched her studies from the life sciences to the physical ones, with the zealous enthusiasm of a religious convert. Later, during a public night at the Cambridge Observatory, she met Eddington and blurted out: “I should like to become an astronomer,” which he encouraged.
After completing her undergraduate degree in 1923, Cecilia attended another memorable lecture in London, by the young Harlow Shapley on his discovery of the remote center of the Milky Way. It resulted in her application for graduate study at the Harvard College Observatory, which Shapley directed. When Shapley provided Miss Payne with a fellowship meant to encourage women, she accepted the opportunity and left her family and native country for the Observatory, which became her home for more than 50 years.
The Harvard astronomers were compiling an enormous collection of stellar spectra that were at Cecilia’s disposal. Her brilliant doctoral thesis and book on Stellar Atmospheres, published in 1925, summarized the existing spectroscopic data for a wide variety of stars, and described how the appearance of their spectral lines is influenced by physical conditions in stellar atmospheres. She showed that the observed line strengths in various stars are mainly due to different temperatures, rather than a difference in their composition, and that the relative abundances of the chemical elements are similar in virtually every bright star. Her results also showed that the Sun and many other stars contain elements commonly found on the Earth, such as carbon, silicon and iron, and in roughly the same proportions.
Cecilia Payne was the first person to earn a doctorate degree in astronomy from Radcliffe College, which is now part of Harvard University. For many subsequent years, she remained upset about the complete lack of recognition from either Harvard or Radcliffe, who both failed to adequately acknowledge her important spectroscopic investigations. But she eventually became the first female to be promoted to Full Professor from within the Faculty of Arts and Sciences at Harvard University, and was later promoted to Chairperson of the Department of Astronomy.
Cecilia benefited from collaborations with other women at the Harvard College Observatory, such as Annie Jump Cannon, Henrietta Swan Leavitt and Antonia C. Maury. They were all unmarried, received no faculty appointments, and suffered from some sort of physical handicap. Annie Cannon and Henrietta Leavitt were both deaf, and Antonia Maury was extremely homely.
In 1931 Cecilia Payne became an American citizen, and in 1934 she married the Russian-born astronomer Sergei Gaposchkin. They settled in Lexington, Massachusetts, where they raised three children. The family was active in the First Unitarian Church in Lexington, where Cecilia taught Sunday School class and advocated the disappearing art of Biblical quotation.
In retrospect, Cecilia’s most significant finding was one she could not herself believe. Although the observed stellar disks contain elements that are commonly found on the Earth, the lightest elements, hydrogen and helium, are vastly more abundant in stars than they are on our planet. As Cecilia described the difference:
“The outstanding discrepancies between the astrophysical and terrestrial abundances are displayed for hydrogen and helium. The enormous abundances derived for these elements in the stellar atmosphere is almost certainly not real. Probably the result may be considered, for hydrogen, as another aspect of its abnormal behavior… The lines of both atoms appear to be far more persistent, at high and low temperatures, than those of any other element.”7
Her analysis indicated that hydrogen was as much as a million times more abundant in the Sun’s outer atmosphere than it is on the Earth, and she just could not understand how that might happen. At the time, all publications at the Harvard College Observatory had to be approved by its director Harlow Shapley, and he most likely encouraged her to play down the glaring hydrogen discrepancy.
In just four years, the influential Princeton astronomer Henry Norris Russell had nevertheless confirmed and extended Cecilia Payne’s conclusion that hydrogen is the overwhelmingly predominant element in the Sun’s outer atmosphere.8 And within a few more years, the Danish astronomer Bengt Strömgren, an expert on stellar structure, showed that the physical conditions within stars requires that the great hydrogen content exists throughout their interiors, not just in their outer atmospheres.9 The enormous abundance of hydrogen then played a central role in discovering how the Sun shines, by the nuclear conversion of hydrogen into helium in its core [See Chapter 9].
Relative Abundances and Synthesis of the Elements
An important clue to understanding where the elements came from is obtained from their relative abundances, initially studied by chemists rather than astronomers. The American chemist William D. Harkins, for example, found an important clue when he noticed that elements of low atomic weight are more abundant in meteorites than those of high atomic weight. These features led Harkins to conjecture in 1917 that the relative abundances of the elements depend on nuclear rather than chemical properties and that heavy elements must have been synthesized from the lighter ones, starting from hydrogen.10
And A. S. Eddington wrote in 1920 that: “I think that the suspicion has been generally entertained that the stars are the crucibles in which the lighter atoms which abound in the nebulae are compounded into more complex elements.”11
Figure 14.1 Abundance and origin of the Sun’s elements The relative abundance of the elements in the solar photosphere, plotted as a function of their atomic number, Z. Hydrogen, the lightest and most abundant element in the Sun, and most of the helium, were formed about 14 billion years ago in the immediate aftermath of the Big Bang that led to the expanding Universe. All the elements heavier than helium were synthesized in the interiors of stars that no longer shine, and then wafted or blasted into interstellar space where the Sun subsequently originated. (Data courtesy of Nicolas Grevesse.)
But it wasn’t until 1956 that Hans E. Suess and Harold Clayton Urey, at the University of Chicago, published detailed abundance data for meteorites that suggested that most elements were made inside stars (Fig. 14.1).12 The systematic decline in abundance of elements with increasing atomic weight was explained by the fact that the nuclear reactions in the most common stars produced only a small amount of heavy elements.
These element-building processes are now termed nucleosynthesis, which describes the synthesis of atomic elements from their sub-atomic, nuclear components, the neutrons and protons rather than whole atoms. Some of the elements were made within stars, and thrust back into space. This is called stellar nucleosynthesis, and it accounts for the production of heavy elements.
But where did the abundant lightest elements hydrogen and helium come from? They were made in the earliest stages of the expanding Universe, before the stars were formed. It is known as big-bang nucleosynthesis.
Creating Elements in the Big Bang
Support for big-bang nucleosynthesis arose as a byproduct of the development of the atomic bomb during World War II (1939–1945). George Gamow knew how elements were being produced by chain reactions during the detonation of these bombs, and shortly after the war, in 1946, he concluded that all of the elements originated by similar explosive nuclear reactions during the Big Bang, the most energetic and hottest explosion of them all.13
Working with Ralph A. Alpher, his graduate student at George Washington University, Gamow proposed that the original substance of the Universe was a hot, highly compressed nuclear gas, which they named ylem, the material from which the elements formed. This ylem was supposed to consist only of neutrons at a very high temperature of about 10 billion degrees kelvin.
The cosmic ylem cooled as the Universe expanded, and some of the neutrons would have decayed into protons. According to Gamow and Alpher successive captures of neutrons by protons led to the formation of the elements when the temperature dropped. The probability of a neutron being captured had been measured in the Los Alamos Atomic Energy (Manhattan) Project, and using these data, which were declassified after World War II, they obtained a reasonable fit to the observed decline of element abundance with increasing weight.
Their novel idea was published in 1948, in a paper entitled “The Origin of the Chemical Elements,” with Hans Bethe added as an author by the funloving Gamow, even though Bethe contributed nothing to the research or the writing. This was a pun on the first letters of the Greek alphabet — alpha, beta and gamma or a, b, and g — for Alpher, Bethe and Gamow.14 Bethe, who was one of the reviewers of the article when it was submitted, enjoyed the joke. To add to the fun, the paper was published on April Fool’s Day.
For several years, Alpher teamed up with Robert Herman, a fellow employee at the Johns Hopkins University, to continue these calculations. With immodest humor, they used “divine creation curves” to specify the decrease in temperature and density as the expanding Universe cooled and dispersed.
Modern computations have conclusively demonstrated that all of the hydrogen and deuterium, and most of the helium, that are found in the Cosmos today were synthesized in the immediate aftermath of the Big Bang.15 These calculations also indicated how much matter is now in the Universe in both visible and invisible form.
So Gamow and his colleagues were partly right. The lightest and most abundant element, hydrogen, and therefore the majority of atoms that are around today, was indeed formed at the dawn of time before the stars even existed, in the immediate aftermath of the Big Bang that produced the expanding Universe. All of the hydrogen that is now found in stars and interstellar space was created back then, about 13.7 billion years ago, and so was most of the helium, the second-most abundant element. Although helium is synthesized within stars, the amounts of helium now observed in the Cosmos are much greater than the amounts that could have been produced in all the stars in the Universe over its entire lifetime. Most of it had to be created in the early stages of the Big Bang. So every time you buy a floating balloon, which has been inflated with helium, you are getting atoms made many billions of years ago.
To put it another way, the close fit between the calculated and measured cosmic abundances of the light elements provides strong evidence that the Big Bang occurred and that the observed Universe had such a beginning.
But Gamow was also partly wrong, for all of the chemical elements were not created in the Big Bang, only the abundant lighter ones were. The creation of carbon, the next most abundant atom after helium, requires the simultaneous collision of three helium nuclei, known as a three-body reaction. By the time that the expanding Universe became sufficiently cool for atomic elements to form, at about 1 billion degrees kelvin, the material was so dispersed, thinned out, and cooled down that only helium could be produced in abundance.
The cores of stars are the only places that are dense enough and hot enough to synthesize elements heavier than helium, over long intervals of time.
Creating Elements in Stars
Hydrogen is converted into helium within the cores of stars, and these nuclear reactions provide the energy that supports these stars and makes them shine.16 These reactions are known as hydrogen burning. The important remaining question was where the heavier abundant elements, such as carbon, nitrogen and oxygen, came from. They were produced in the late stages of the lives of exceptionally massive and luminous stars.
Once the hydrogen in a star’s central regions has been converted into helium, the core contracts and heats up, releasing energy that causes the outer envelope of the star to expand into the vast giant structure. The helium ash produced by hydrogen burning serves as nuclear fuel in the hot, compact cores of giant stars.
The Estonian astronomer Ernst Öpik and the American astronomer Edwin E. Salpeter nearly simultaneously showed how carbon nuclei could be created in this situation during triple collisions of helium nuclei.17 The release of energy by fusing helium into carbon within a star is known as helium burning.
The English astrophysicist and cosmologist Fred Hoyle was one of the first to propose that many other chemical elements were forged under several different situations inside stars, and he collaborated with the American nuclear physicist William Fowler in investigating these possibilities.18 The duo also found time to climb the Munros in the Scottish Highlands, which kept them fit and according to Fowler renewed his soul.
William A. Fowler and Stellar Nucleosynthesis
William “Willy” Fowler was raised in the railroad center of Lima, Ohio, where he acquired a lifelong fascination with steam locomotives. His paternal grandfather was a coal miner who came to Pittsburgh from Scotland, his maternal grandfather was a grocer who immigrated to Pittsburgh from Ireland, his father was an accountant, and his family was not affluent.
On graduation from high school, Willy studied Engineering Physics at Ohio State University, where he waited on tables, washed dishes, and stoked furnaces for meals. He was admitted to graduate school at the California Institute of Technology in 1933 with an assistantship of room, board and tuition, and was assigned to work at the Kellogg Radiation Laboratory where he and other graduate students made experimental measurements of nuclear reactions at very low energies. [The laboratory was named for the American “corn flakes king” Will Keith Kellogg who funded it.]
These investigations were stopped during the Second World War (1939–1945), when the people at Kellogg developed, tested and produced small rockets for the Navy and constructed non-nuclear components of the first atomic bombs. Heavy elements were created during the tests of these bombs, which indicated that these elements did not have to be produced at the Big-Bang beginning of the observable Universe.
After the war, Fowler and his colleagues at Kellogg stayed away from the competitive and lucrative construction of high-energy accelerators used to study elementary particles, and instead resumed laboratory measurements of nuclear processes involved in stars. These experiments and their interpretation continued at the California Institute of Technology for half a century, with a minimum teaching load, little academic bureaucracy, and the support of the Office of Naval Research and the National Science Foundation.
Fowler’s team carried out bombardments of nuclei at relatively low energies, which excited the nuclei but did not tear them into pieces. The group also converted the laboratory nuclear interaction cross sections into reaction rates inside stars, which no theorist could predict.19 In this way, they helped determine how energy is generated within stars by converting light elements into heavier ones, and also accounted for the cosmic creation of the elements. These discoveries were considered so important that William Fowler was awarded the 1983 Nobel Prize in Physics for them.
We Belong to the Stars
Fowler and Fred Hoyle teamed up with Margaret and Geoffrey Burbidge to write an overview of the synthesis of elements in stars. It emphasized the details of the cosmic abundance of the elements and explained both the abundant elements and those that are relatively under-abundant or just not there. This influential review became widely known by the acronym B2FH.20
The B2FH paper showed how all of the elements from carbon, of atomic number six, to uranium of number ninety-two, can be created by nuclear processes in stars starting with hydrogen and helium, which were produced in the Big Bang. The heavier elements were forged during slow, successive nuclear burning stages in stars and the abrupt explosions of stars near the ends of their lives.
So stars are the crucibles in which all the chemical elements heavier than hydrogen and helium were formed. These elements have been created by nuclear reactions in stars during their ongoing lives and then blown or blasted into space by dying stars, gathered by gravity, and collected into the next generation of stars and their planets.
We are here today because former stars forged the chemical elements necessary for life within their nuclear furnaces. You are composed of these elements, assemblages of former star stuff. So Walt Whitman was right to say: “I believe a leaf of grass is no less than the journey work of stars,”21 and Joni Mitchell was right on when she sang:
“We are stardust
We are golden.”22
There is one remaining mystery that no astronomer has solved. We just do not know exactly how the observable Universe came into being in the first place or where it came from. A precise knowledge of this very beginning is hidden behind a closed door.