Natural History Magazine
Not all scientific discoveries are made by lone, anti-social researchers. Nor are all discoveries accompanied by media headlines and best-selling books. Some involve many people, span many decades, require complicated mathematics, and are not easily summarized by the press. Such discoveries pass almost unnoticed by the general public.
My vote for the most under-appreciated discovery of the twentieth century is the realization that supernovae—the explosive death throes of high-mass stars—are the primary source for the origin and relative mix of heavy elements in the universe. This unheralded discovery took the form of an extensive research paper published in 1957 in the journal Reviews of Modern Physicstitled “The Synthesis of the Elements in Stars,” by E. Margaret Burbidge, Geoffrey R. Burbidge, William Fowler, and Fred Hoyle. In the paper they built a theoretical and computational framework that freshly interpreted forty years of musings by others on such hot topics as the sources of stellar energy and the transmutation of elements.
Cosmic nuclear chemistry is a messy business. It was messy in 1957 and it is messy now. The relevant questions have always included: How do the various elements from the famed periodic table of elements behave when subjected to assorted temperatures and pressures? Do the elements fuse or do they split? How easily is this accomplished? Does the process liberate or absorb energy?
The periodic table is, of course, much more than just a mysterious chart of a hundred, or so, boxes with cryptic symbols in them. It is a sequence of every known element in the universe arranged by increasing number of protons in their nuclei. The two lightest are hydrogen, with one proton, and helium, with two protons. Under the right conditions of temperature, density, and pressure, you can use hydrogen and helium to synthesize every other element on the periodic table.
A perennial problem in nuclear chemistry involves calculating accurate collision cross-sections, which are simply measures of how close one particle must get to another particle before they interact significantly. Collision cross-sections are easy to calculate for things such as cement mixers or houses moving down the street on flat-bed trucks, but it can be a challenge for elusive subatomic particles. A detailed understanding of collision cross-sections is what enables you to predict nuclear reaction rates and pathways. Often small uncertainties in tables of collision cross-sections can force you to draw wildly erroneous conclusions. The problem greatly resembles what would happen if you tried to navigate your way around one city’s subway system while using another city’s subway map as your guide.
Apart from this ignorance, scientists had suspected for some time that if exotic nuclear process existed anywhere in the universe, then the centers of stars were as good a place as any to find it. In particular, the British theoretical astrophysicist Sir Arthur Eddington published a paper in 1920 titled the “The Internal Constitution of the Stars” where he argued that the Cavendish Laboratory in England, the most famous atomic and nuclear physics research center of the day, could not be the only place in the universe that managed to change some elements onto others:
But is it possible to admit that such a transmutation is occurring? It is difficult to assert, but perhaps more difficult to deny, that this is going on and what is possible in the Cavendish Laboratory may not be too difficult in the sun. I think that the suspicion has been generally entertained that the stars are the crucibles in which the lighter atoms which abound in the nebulæ are compounded into more complex elements.
Eddington’s paper predates by several years the discovery of quantum mechanics, without which, our knowledge of the physics of atoms and nuclei was feeble, at best. With remarkable prescience, Eddington began to formulate a scenario for star-generated energy via the thermonuclear fusion of hydrogen to helium and beyond:
We need not bind ourselves to the formation of helium from hydrogen as the sole reaction which supplies the energy [to a star], although it would seem that the further stages in building up the elements involve much less liberation, and sometimes even absorption, of energy. The position may be summarised in these terms: the atoms of all elements are built of hydrogen atoms bound together, and presumably have at one time been formed from hydrogen; the interior of a star seems as likely a place as any for the evolution to have occurred.
The observed mix of elements on Earth and elsewhere in the universe was another desirable thing for a model of the transmutation of the elements to explain. But first a mechanism was required. By 1931, quantum mechanics was developed (although the neutron was not yet discovered) and the astrophysicist Robert d’Escourt Atkinson, published an extensive paper that he summarizes in his abstract as a
synthesis theory of stellar energy and of the origin of the elements in which the various chemical elements are built up step by step from the lighter ones in stellar interiors, by the successive incorporation of protons and electrons one at a time.
At about the same time, the nuclear chemist William D. Harkins published a paper noting that
elements of low atomic weight are more abundant than those of high atomic weight and that, on the average, the elements with even atomic numbers are about 10 times more abundant than those with odd atomic numbers of similar value. Harkins surmised that the relative abundances of the elements depend on nuclear rather than on conventional chemical processes and that the heavy elements must have been synthesized from the light ones.
The detailed mechanism of nuclear fusion in stars could ultimately explain the cosmic presence of many elements, especially those that you get each time you add the two-proton helium nucleus to your previously forged element. These constitute the abundant elements with
even atomic numbers that Harkins refers to. But the existence and relative mix of many other elements remained unexplained. Another means of element buildup must have been at work.
The neutron, discovered in 1932 by the British physicist James Chadwick while working at the Cavendish Laboratories, plays a significant role in nuclear fusion that Eddington could not have imagined. To assemble protons requires hard work because they naturally repel each other. They must be brought close enough together (often by way of high temperatures, pressures, and densities) for the short-range strong nuclear force to overcome their repulsion and bind them. The chargeless neutron, however, repels no other particle, so it can just march into somebody else’s nucleus and join the other assembled particles. This step has not yet created another element; by adding a neutron we have simply made an isotope of the original. But for some elements, the freshly captured neutron is unstable and it spontaneously converts itself into a proton (which stays put in the nucleus), and an electron (which escapes immediately). Like the Greek soldiers who managed to breach the walls of Troy by hiding inside the Trojan Horse, protons can effectively “sneak” into a nucleus under the guise of a neutron.
If the ambient flow of neutrons is high, then an atom’s nucleus can absorb many in a row before the first one decays. These rapidly absorbed neutrons help to create an ensemble of elements that are identified with the process and differ from the assortment of elements that result from neutrons that are captured slowly.
The entire process is known as neutron capture and is responsible for creating many elements that are not otherwise formed by traditional thermonuclear fusion. The remaining elements in nature can be made by a few other means, including slamming high-energy light (gamma rays) into the nuclei of heavy atoms, which then break apart into smaller ones.
At the risk of oversimplifying the life cycle of a high-mass star, it is sufficient to recognize that a star is in the business of making and releasing energy, which helps to support the star against gravity. Without it, the big ball of gas would simply collapse under its own weight. A star’s core, after having converted its hydrogen supply into helium, will next fuse helium into carbon, then carbon to oxygen, oxygen to neon, and so forth up to iron. To successively fuse this sequence of heavier and heavier elements requires higher and higher temperatures for the nuclei to overcome their natural repulsion. Fortunately this happens naturally because at the end of each intermediate stage, the star’s energy source temporarily shuts off, the inner regions collapse, the temperature rises, and the next pathway of fusion kicks in. But there is just one problem. The fusion of iron absorbs energy rather than releases it. This is very bad for the star because it can now no longer support itself against gravity. The star immediately collapses without resistance, which forces the temperature to rise so rapidly that a titanic explosion ensues as the star blows its guts to smithereens. During the explosion, the star’s luminosity can increase a billion-fold. We call them supernovae, although I always felt that the term “super-duper-nova” would be more appropriate.
Throughout the supernova explosion, the availability of neutrons, protons, and energy enable elements to be created in many different ways. By combining 1) the well-tested tenets of quantum mechanics, 2) the physics of explosions, 3) the latest collision cross sections, 4) the varied processes by which elements can transmutate into one another, and 5) the basics of stellar evolutionary theory, Burbidge, Burbidge, Fowler, & Hoyle decisively implicated supernova explosions as the primary source of all elements heavier than hydrogen and helium in the universe.
With supernovae as the smoking gun, they got to solve one other problem for free: when you forge elements heavier than hydrogen and helium inside of stars then it does the rest of the universe no good unless those elements are somehow cast forth to interstellar space and made available to form planets and people. Yes, we are stardust.
I do not mean to imply that all of our cosmic chemical questions are solved. A curious contemporary mystery involves the element technetium, which, in 1937, was the first element to be synthesized in the laboratory. (The name technetium, along with other words that use the root prefix tech- derives from the Greek word technetos, which translates to “artificial.”) The element has yet to be discovered naturally on Earth, but it has been found in the atmosphere of a small fraction of red giant stars in our galaxy. This alone would not be cause for alarm were it not for the fact that technetium has a half-life of a mere two-million years, which is much, much shorter than the age and life expectancy of the stars in which it is found. In other words, the star cannot have been born with the stuff, for if it were, there would be none left by now. There is also no known mechanism to create technetium in a star’s core and have it dredge itself up to the surface where it is observed, which has led to exotic theories that have yet to achieve consensus in the astrophysics community.
Red giants with peculiar chemical properties are rare, but nonetheless common enough for there to be a cadre of astrophysicists (mostly spectroscopists) who specialize in the subject. In fact, my professional research interests sufficiently overlap the subject for me to be a regular recipient of the internationally distributed Newsletter of Chemically Peculiar Red Giant Stars, not available on the newsstand, it typically contains conference-news and updates on research in progress. To the interested scientist, these ongoing chemical mysteries are no less seductive than questions related to black holes, quasars, and the early universe. But you will hardly ever read about them. Why? Because once again, the media has predetermined what is not worthy of coverage, even when the news item is something as uninteresting as the cosmic origin of every element in your body.