The Creation of the Elements

Creation of hydrogen and helium in the Big Bang

We have discussed in book 3 of this series the evidence for the Big Bang theory of the origin of the universe. Did all the chemical elements we're made of come into being in the Big Bang? The answer is definitely no, since the temperatures in the first microseconds after the Big Bang were so high that atoms and nuclei could not hold together at all. Even after things had cooled down enough for nuclei and atoms to exist, theorists are sure that the only elements created were hydrogen and helium.

We are stardust

In that case, where did all the other elements come from? Astronomers came up with the answer. By studying the combinations of wavelengths of light, called spectra, emitted by various stars, they had been able to deter-mine what kinds of atoms they contained. (We will have more to say about spectra in book 6.) They found that the stars fell into two groups. One type was nearly 100% hydrogen and helium, while the other contained 99% hydrogen and helium and 1% other elements. They interpreted these as two generations of stars. The first generation had formed out of clouds of gas that came fresh from the big bang, and their composition reflected that of the early universe. The nuclear fusion reactions by which they shine have mainly just increased the proportion of helium relative to hydrogen, without making any heavier elements.

The members of the first generation that we see today, however, are only those that lived a long time. Small stars are more miserly with their fuel than large stars, which have short lives. The large stars of the first generation have already finished their lives. Near the end of its lifetime, a star runs out hydrogen fuel and undergoes a series of violent and spectacular reorganizations as it fuses heavier and heavier elements. Very large stars finish this sequence of events by undergoing supernova explosions, in which some of their material is flung off into the void while the rest collapses into an exotic object such as a black hole or neutron star.

The second generation of stars, of which our own sun is an example, condensed out of clouds of gas that had been enriched in heavy elements due to supernova explosions. It is those heavy elements that make up our planet and our bodies.

Top left: Construction of the UNILAC accelerator in Germany, one of whose uses is for experiments to create very heavy artificial elements. Top right: This formidable-looking apparatus, called SHIP, is really nothing more than a glorified version of the apparatus used by Thomson to determine the velocity and mass-to-charge ratios of a beam of unknown particles. Nuclei from a beam of ions produced by UNILAC strike a metal foil target, and the nuclei produced in the result-ing fusion reaction recoil into ship, which is connected to the "down-stream" end of the accelerator. A typical experiment runs for several months, and out of the billions of fusion reactions induced during this time, only one or two may result in the production of superheavy atoms. In all the rest, the fused nucleus breaks up immediately. SHIP is used to identify the small number of "good" reactions and separate them from this intense background.

Artificial synthesis of heavy elements

Elements up to uranium, atomic number 92, were created by these astronomical processes. Beyond that, the increasing electrical repulsion of the protons leads to shorter and shorter half-lives. Even if a supernova a billion years ago did create some quantity of an element such as Berkelium, number 97, there would be none left in the Earth's crust today. The heaviest elements have all been created by artificial fusion reactions in accelerators. The heaviest element that has been reported in a published scientific paper is 112, but as of 1999 scientists at Berkeley and Dubna have announced the creation of 114 and 118 as well.

Although the creation of a new element, i.e. an atom with a novel number of protons, has historically been considered a glamorous accomplishment, to the nuclear physicist the creation of an atom with a hitherto unobserved number of neutrons is equally important. The greatest neutron number reached so far is 179. One tantalizing goal of this type of research is the theoretical prediction that there might be an island of stability beyond the previously explored tip of the chart of the nuclei shown in section 2.8. Just as certain numbers of electrons lead to the chemical stability of the noble gases (helium, neon, argon, ...), certain numbers of neutrons and protons lead to a particularly stable packing of orbits. Calculations dating back to the 1960's have hinted that there might be relatively stable nuclei having approximately 114 protons and 184 neutrons. Proton number 114 has been achieved, and indeed displays an amazingly long half-life of 30 seconds. This may not seem like very long, but lifetimes in the microsecond range are more typical for the superheavy elements that have previously been discovered. There is even speculation that certain superheavy elements would be stable enough to be produced in quantities that could for instance be weighed and used in chemical reactions.