WHEN I WAS A CHILD and asked my parents where I came from, they referred me to the copy of The Stork Didn’t Bring You nestled in the den library, and that was that. While still in awe of the biological details, I became a physicist. Physicists, who by profession think in the simplest terms possible, have their own version of the story. Brushing aside questions of egg and cell development, evolution of the species, and so on, they get down to atoms in the body. A physicist’s answer to where we came from is an investigation of the origins of the chemical elements. As it turns out, we were all made in stars, some five to ten billion years ago.
All living matter we know about is composed mainly of hydrogen, carbon, oxygen, nitrogen, phosphorus, and sulfur. Carbon, with its rich variety of chemical bonds, is particularly suited for forming the complex molecules that life thrives on. Atoms of all these elements are continuously cycled and recycled over many generations through our planet’s biosphere, incorporated into plants from the soil, swallowed by animals, inhaled and exhaled, evaporated from oceans, and returned to soil, air, and sea. We’ve been trading atoms with other living things since life began.
But where did the atoms come from? One rather bland possibility is that they were always here, in their observed proportions, thus putting a stop to further delicate questions. A great deal of scientific evidence, however, suggests this is not the case. First of all, the Earth is radioactive. Atoms of various elements are constantly aging and changing into other atoms by the ejection and transformation of protons and neutrons in their nuclei. For example, uranium 238, consisting of 92 protons and 146 neutrons, changes into thorium 234 by the simultaneous emission of two protons and two neutrons. Thorium is itself unstable and decays into another element and then another, until lead 206 is produced. Lead, at last, is mature, and the transformation process comes to a halt.
For years, chemists and physicists have been taking census reports of these busy families of atoms, with their noisy infants, teenagers, and quiet senior citizens. It seems perfectly natural that the relative proportions of the elements had to be different in the past. How far in the past? Analyses of the observed numbers of uranium versus lead atoms, for example, have determined that the Earth is 4.5 billion years old. Our atomic roots go back this far and further.
The best guide to what was happening so long ago is found in the vast reaches of space, beyond our solar system, beyond our galaxy of one hundred billion stars, beyond our neighboring galaxies. When we peer out through our telescopes at distant galaxies, hundreds of millions of light-years away, we find them receding from us. The universe is and has been in a state of expansion, with the galaxies rushing away from each other like painted dots on an expanding balloon. Running this scene backward in time suggests the universe began about ten billion years ago, in an initial explosion called the Big Bang. At this point even the people who work on these things start getting wide-eyed, despite the logic of equations, computers, and telescopes.
The early universe was much denser than today’s. And it was much hotter, just as squeezing an ordinary gas tends to raise its temperature. When the universe was sufficiently young and hot, none of the chemical elements except hydrogen 1 (whose nuclei are single protons) could have existed. The constituent protons and neutrons of any compound atomic nucleus would have simply evaporated under the intense heat. For example, carbon and nitrogen atoms would have disintegrated into unattached protons and neutrons at temperatures exceeding about 2,000 billion degrees Fahrenheit. According to cosmological theory, this was the case until a ten-thousandth of a second after the Big Bang. As far as we can tell, the infant universe held only a shapeless gas of subatomic particles. Atoms, stars, planets, and people came later.
With only some introductory thermodynamics, a little cosmology, and some whisperings from nuclear physics, we have narrowed down the origin of the elements to sometime after the universe began but before the formation of the Earth. Where and when did this occur?
Results in nuclear physics indicate that, starting from a hot gas of unattached protons and neutrons, synthesis of complex atoms proceeds along a family tree, in which heavier atoms grow from lighter ones. Since the temperature and density of the expanding, primeval universe were dropping rapidly with time, there was only a brief period, ending a few minutes after the Big Bang, when conditions were right for creating elements. Before this period, every partnership of two or more particles evaporated; after this period, the subatomic particles did not have the energy and were too far apart for fusion to occur easily. According to theoretical calculations, element formation in this delicate interval got only as far along as helium 4 (two neutrons and two protons), the lightest element after hydrogen. The predicted amount of helium produced, about twenty-five percent of the mass in initial protons and neutrons, is in delightful agreement with current-day observations of the cosmic helium abundance. Nice, but what about carbon, oxygen, and other elements?
The answer, as we now understand it, began emerging in 1920 when the eminent British astronomer Sir Arthur Eddington first proposed that the sun and other stars are powered by nuclear fusion reactions. This is the same source of energy that is unleashed, for ghastly purposes, in our hydrogen bombs. In the deep interior of stars, densities and temperatures can become sufficiently high to fuse lighter elements into heavier ones, going far beyond helium. Such observed features of stars as their masses, temperatures, and luminosities accord well with the theoretical models and provide indirect confirmation of the hypothesized nuclear reactions. These are the facts of life that adult physicists and astronomers will tell you.
More direct evidence for the element-producing activity of stars comes from analysis of the debris ejected by exploding stars. In such explosions, called supernovae, nuclear reactions proceed at an extremely rapid rate; both the hastily produced elements and those manufactured in the preceding, more leisurely evolution of the star are spewed out into space, where we can have a good look at them. Analyzing the telltale colors of light emitted by stellar ejecta reveals a host of heavy elements, in the relative proportions predicted by nuclear physicists.
The first stars could have begun forming long ago, when the universe was only a million years old. In fact, we see evidence for a great spread in the ages of stars. New stars are continually being born. Relatively young stars, like our sun, and its surrounding planetary system have condensed out of gas enriched with the drifting fragments of ancestral stars, gas thus enriched with heavy elements.
As we go about our daily business on this small planet, we have little feeling for the bond between us and those distant points of light. Excepting hydrogen and helium, all the atoms in us and our biosphere were bred somewhere in space, in the nuclear reactions of some now defunct star.