Origins: Fourteen Billion Years of Cosmic Evolution (14 page)

In the same year, the American nuclear chemist William D. Harkins published a paper noting that “elements of low atomic weight [the number of protons plus neutrons in each nucleus] are more abundant than those of high atomic weight and that, on the average, the elements with even atomic numbers [the numbers of protons in each atomic nucleus] 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 fusion rather than on chemical processes such as combustion, 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 will obtain each time you add the two-proton, two-neutron helium nucleus to your previously forged element. These constitute the abundant elements with “even atomic numbers” that Harkins described. But the existence and relative numbers of many other elements remained unexplained. Some other means of element buildup must have been at work in the cosmos.

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 protons naturally repel one another, as do all particles with the same sign of electric charge. To fuse protons, you must bring them sufficiently close (often by way of high temperatures, pressures, and densities) to overcome their mutual repulsion for the strong nuclear force to bind them together. The chargeless neutron, however, repels no other particle, so it can simply march into somebody else’s nucleus and join the other assembled particles, held there by the same force that binds the protons. This step does not create another element, which is defined by a different number of
protons
in each nucleus. By adding a neutron, we make an “isotope” of the nucleus of the original element, which differs only in detail from the original nucleus because its total electric charge remains unchanged. For some elements, the freshly captured neutron proves to be unstable once it joins the nucleus. In that case, the neutron spontaneously converts itself into a proton (which stays put in the nucleus), and an electron (which escapes immediately). In this way, like the Greek soldiers who breached the walls of Troy by hiding inside a wooden horse, protons can sneak into a nucleus in the guise of neutrons.

If the ongoing flow of neutrons stays high, each nucleus can absorb many neutrons before the first one decays. These rapidly absorbed neutrons help to create an ensemble of elements whose origin is identified with the “rapid neutron capture process,” and differ from the assortment of elements that result when neutrons are captured slowly, where each successive neutron decays into a proton before the nucleus captures the next one.

Both the rapid and the slow neutron capture processes are responsible for creating many of the elements not otherwise formed through traditional thermonuclear fusion. The remaining elements in nature can be made by a few other processes, including slamming high-energy photons (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, we may state that each star lives by generating and releasing the energy in its interior that allows the star to support itself against gravity. Without its production of energy through thermonuclear fusion, each stellar ball of gas would simply collapse under its own weight. This fate weighs on stars that exhaust their supplies of hydrogen nuclei (protons) in their cores. As already noted, after converting its hydrogen into helium, the core of a massive star 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 successively higher temperatures for the nuclei to overcome their natural repulsion. Fortunately this happens all by itself, because at the end of each intermediate stage, when the star’s energy source temporarily shuts off, the inner regions collapse, the temperature rises, and the next pathway of fusion kicks in. Since nothing lasts forever, the star eventually confronts one enormous problem: The fusion of iron does not release energy, but instead absorbs it. This brings bad news to the star, which can now no longer support itself against gravity by pulling a new energy-releasing process out of its nuclear fusion hat. At this point, the star suddenly collapses, forcing its internal temperature to rise so rapidly that a gigantic explosion ensues as the star blows its guts to smithereens.

Throughout each explosion, the availability of neutrons, protons, and energy allows the supernova to create elements in many different ways. In their 1957 article, Burbidge, Burbidge, Fowler, and Hoyle combined (1) the well-tested tenets of quantum mechanics; (2) the physics of explosions; (3) the latest collision cross sections; (4) the varied processes that transmute elements into one another; and (5) the basics of stellar evolutionary theory to implicate supernova explosions decisively as the primary source of all the elements heavier than hydrogen and helium in the universe.

With high-mass stars as the source of heavy elements, and supernovae as the smoking gun of element distribution, the fab four acquired the solution to one other problem for free: when you forge elements heavier than hydrogen and helium in stellar cores, you do the rest of the universe no good unless you somehow cast those elements forth into interstellar space, making them available to form worlds with wombats. Burbidge, Burbidge, Fowler, and Hoyle unified our understanding of nuclear fusion in stars with the element production visible throughout the universe. Their conclusions have survived decades of skeptical analysis, so their publication stands as a turning point in our knowledge of how the universe works.

Yes, Earth and all its life comes from stardust. No, we have not solved all of our cosmic chemical questions. A curious contemporary mystery involves the element technetium, which, in 1937, was the first element to be created artificially in Earthbound laboratories. (The word “technetium,” along with others that use the prefix “tech-,” derive from the Greek
technetos
, which translates to “artificial.”) We have yet to discover technetium on Earth, but astronomers have found it in the atmospheres of a small fraction of the red giant stars in our galaxy. This would hardly surprise us, were it not for the fact that technetium decays to form other elements, and does so with a half-life of a mere 2 million years, far shorter than the age and life expectancy of the stars in which we observe it. This conundrum has led to exotic theories that have yet to achieve consensus within the community of astrophysicists.

Red giants with these peculiar chemical properties are rare, but sufficiently nettlesome for a cadre of astrophysicists (mostly spectroscopists) who specialize in the subject to generate and distribute the
Newsletter of
Chemically Peculiar Red Giant Stars
. Not available on most newsstands, this publication typically contains conference news and updates on research still in progress. To interested scientists, these ongoing chemical mysteries have an allure as strong as the questions related to black holes, quasars, and the early universe. But you hardly ever read about them. Why? Because, quite typically, the media has predetermined what deserves coverage and what does not. Apparently the news about the cosmic origins of every element in your body and your planet doesn’t make the cut.

Here is your chance to redress the wrongs that contemporary society has inflicted upon you. Let’s take a journey through the periodic table, pausing here and there to note the most intriguing facts about the various elements, and to admire how the cosmos made them all from the hydrogen and helium that emerged from the big bang.

CHAPTER 10

The Elemental Zoo

T
he periodic table of the elements, lovingly created by chemists and physicists during the past two centuries, embodies organizing principles that explain the chemical behavior of all the elements that we know in the universe, or may someday discover. For this reason, we ought to regard the periodic table as a cultural icon, an exemplar of our society’s ability to organize its knowledge. The table testifies to the enterprise of science as an international human adventure, conducted not only in laboratories but also in particle accelerators, and at the space and time frontiers of the entire cosmos.

Amid this well-merited respect, every now and then an entry in the periodic table will strike even a grown-up scientist as a strange beast in a zoo of one-of-a-kind animals conceived and executed by Dr. Seuss. How else can we believe that sodium is a deadly, reactive metal that you can cut with a butterknife, and that pure chlorine is an evil-smelling, deadly gas—yet when we combine sodium and chlorine, we make sodium chloride, a harmless compound essential to life, better known as table salt? What about hydrogen and oxygen, two of the most abundant elements on Earth and in the universe? One is an explosive gas, while the other promotes violent combustion; yet adding the two produces liquid water, which puts out fires.

Amid all the chemical interactions in the periodic table’s little shop of possibilities, we find the elements most significant to the cosmos. These offer the chance to view the table through the lens of an astrophysicist. We shall grasp that chance and dance our way across the table, saluting its most distinguished entries and admiring its little oddities.

The periodic table emphasizes the fact that each of nature’s elements distinguishes itself from all others by its “atomic number,” the number of protons (positive electric charges) in each nucleus of that element. Complete atoms always have a number of electrons (negative electric charges) orbiting the nucleus equal to the element’s atomic number, so the total atom has zero electric charge. Different isotopes of a particular element have the same number of protons and electrons, but different numbers of neutrons.

Hydrogen
, with only one proton in each nucleus, is the lightest and simplest element, made entirely during the first few minutes after the big bang. Out of the ninety-four naturally occurring elements, hydrogen claims more than two thirds of all the atoms in human bodies and more than 90 percent of all the atoms in the cosmos, including the Sun and its giant planets. The hydrogen inside the core of the Sun’s most massive planet, Jupiter, feels so much pressure from the overlying layers that it behaves more like an electromagnetically conductive metal than a gas, and helps to create the strongest magnetic field among the Sun’s planets. The English chemist Henry Cavendish discovered hydrogen in 1766 while experimenting with H
2
O (
hydro-genes
is the Greek word for water-forming, whose
gen
appears in such English words as “genetic”), though his fame among astronomers rests on his having been the first person to calculate Earth’s mass accurately by measuring the gravitational constant
G
that appears in Newton’s famous equation for gravity. Every second of every day and night, 4.5 billion tons of fast-moving hydrogen nuclei (protons) slam together to make helium nuclei within the Sun’s 15-million-degree (Celsius) core. About 1 percent of the mass involved in this fusion transforms itself into energy, leaving the other 99 percent in the form of helium.

Helium
, the second most abundant element in the universe, can be found on Earth only in a few underground pockets that trap this gas. Most of us know only helium’s whimsical side, available for testing through over-the-counter purchases. When you inhale helium, its low density in comparison with atmospheric gases increases the vibrational frequency within your windpipe, causing you to sound like Mickey Mouse. The cosmos contains four times more helium than all other elements combined (not counting hydrogen). One of the pillars of big bang cosmology is the prediction that throughout the cosmos, no fewer than about 8 percent of all atoms are helium, which the well-mixed primeval fireball manufactured during its immediate post-birth pangs. Since the thermonuclear fusion of hydrogen within stars produces additional helium, some regions of the cosmos can accumulate more than their initial 8 percent share of helium, but—just as the big bang model predicts—no one has ever found a region of our galaxy or anybody else’s galaxy with less.

Some thirty years before they discovered and isolated helium on Earth, astrophysicists had detected helium in the Sun by the telltale features that they saw in the Sun’s spectrum of light during the total eclipse of 1868. They naturally named this previously unknown material helium after Helios, the Greek sun god. With 92 percent of hydrogen’s buoyancy in air, but without the explosive characteristics of hydrogen that destroyed the German
Hindenburg
dirigible, helium provides the gas of choice for the outsized balloon characters of the Macy’s Thanksgiving Day parade, making the famed department store second only to the U.S. military as the world’s top consumer of helium.

Lithium
, the third simplest element in the universe, has three protons in each nucleus. Like hydrogen and helium, lithium was made soon after the big bang, but unlike helium, which is often made in subsequent nuclear reactions, lithium will be
destroyed
by every nuclear reaction that occurs in stars. Hence we expect to find no object or region with lithium present in more than the relatively small relative abundances—no more than 0.0001 percent of the total—produced in the early universe. As predicted by our model of element formation during the first half hour, no one has yet found a galaxy with more lithium than this upper limit. The combination of the upper limit on helium and the lower limit on lithium furnishes us with a potent dual constraint to apply in testing the theory of big bang cosmology. A similar test of the big bang model of the universe, which it has passed with flying colors, compares the abundance of deuterium nuclei, each of which has one proton and one neutron, with the amount of ordinary hydrogen. Fusion during the first few minutes produced both of these nuclei, but made far more of simple hydrogen (just one proton).

Like lithium, the next two elements in the periodic table,
beryllium
and
boron
(with four and five protons, respectively, in each nucleus) owe their origin mainly to thermonuclear fusion in the early universe, and they appear only in relatively modest numbers throughout the cosmos. The scarcity on Earth of the three lightest elements after hydrogen and helium makes them bad news for those who accidentally ingest them, since evolution has proceeded essentially without encountering them. Intriguingly, controlled doses of lithium do seem to relieve certain types of mental illness.

With
carbon
, element number six, the periodic table springs into glorious efflorescence. Carbon atoms, with six protons in every nucleus, appear in more kinds of molecules than the sum of all non-carbon-containing molecules combined. The cosmic abundance of carbon nuclei—forged in the cores of stars, churned to their surfaces, and released in copious amounts into the Milky Way galaxy—joins with carbon’s ease in forming chemical combinations to make carbon the best element on which to base the chemistry and diversity of life. Just edging out carbon in abundance,
oxygen
(eight protons per nucleus) also offers a highly reactive and abundant element, similarly forged within and released from aging stars and stars that explode as supernovae. Both oxygen and carbon constitute major ingredients for life as we know it. The same processes made and distributed
nitrogen
, element number seven, which again appears in great quantities throughout the universe.

But what about life as we don’t know it? Could other life forms use a different element as the heart of their complex shapes? How about life based on
silicon
, element number 14? Silicon sits directly below carbon on the periodic table, which means (see how useful the table can be to those who know its secrets) that silicon can create the same sorts of chemical compounds that carbon does, with silicon taking the place of carbon. In the end, we expect carbon to prove superior to silicon, not only because carbon has ten times the abundance of silicon in the cosmos but also because silicon forms chemical bonds that are either substantially stronger or noticeably weaker than those that carbon makes. In particular, the strength of the bonds between silicon and oxygen makes tough rocks, whereas complex molecules based on silicon lack the hardiness to survive ecological stresses that carbon-based atoms exhibit. These facts don’t stop science fiction writers from championing silicon, thus keeping exobiological speculation on its toes and allowing us to wonder what the first truly alien life form will be like.

In addition to forming an active ingredient in table salt,
sodium
(eleven protons per nucleus) glows across this great land as hot sodium gas in most municipal street lamps. These lamps “burn” brighter, longer, and use less energy than conventional incandescent bulbs do. They come in two varieties: the common high-pressure lamps, which look yellow-white, and the rarer, low-pressure lamps, which look orange. It turns out that while all light pollution hurts astronomy, low-pressure sodium lamps inflict less harm because their contamination, much more narrowly confined in color, can be easily accounted for and removed from telescope data. In a model of town-telescope cooperation, the entire city of Tucson, Arizona, the closest large municipality to the Kitt Peak National Observatory, has, by agreement with the local astronomers, converted all its streetlights to low-pressure sodium lamps—which also turn out to be more efficient, and therefore save energy for the city.

Aluminum
(twelve protons per nucleus) provides nearly 10 percent of Earth’s crust, yet remained unknown to the ancients and unfamiliar to our grandparents because it combines so effectively with other elements. Its isolation and identification occurred only in 1827, and aluminum did not enter common household use until the late 1960s, when tin cans and tin foil yielded to aluminum cans and aluminum foil. Because polished aluminum makes a near-perfect reflector of visible light, astronomers today coat nearly all their telescope mirrors with a thin film of aluminum atoms.

Although
titanium
(thirteen protons per nucleus) has a density 70 percent greater than aluminum’s, it’s more than twice as strong. Its strength and relative lightness make titanium—the ninth most abundant element in Earth’s crust—a modern darling for many applications, such as military aircraft components, that require a light, strong metal.

In most cosmic locations, oxygen atoms outnumber carbon. In stars, once every carbon atom has latched onto one of the available oxygen atoms to form carbon monoxide or carbon dioxide molecules, the leftover oxygen atoms bond with other elements, such as titanium. The spectra of the light from red-giant stars are riddled with features created by titanium oxide (molecules of TiO), which itself is no stranger to stars on Earth: star sapphires and rubies owe their radiant asterisms to titanium oxide impurities within their crystal lattices, with aluminum oxide impurities adding extra color. Furthermore, the white paint used for telescope domes features titanium oxide, which happens to radiate infrared with high efficiency, a fact that greatly reduces the daytime heat accumulated within the dome. At nightfall, with the dome open, the air temperature near the telescope falls more rapidly to the temperature of the nighttime air, reducing atmospheric refraction and allowing the light from stars and other cosmic objects to arrive with greater sharpness and clarity. Although not directly named for a cosmic object, titanium derives its handle from the Titans of Greek mythology, as does Titan, Saturn’s largest moon.

Carbon may be the most significant element in life, but by many measures,
iron
, element number 26, ranks as the most important of all the elements in the universe. Massive stars manufacture elements in their core, marching through the periodic table in the sequence of increasing number of protons per nucleus, from helium to carbon to oxygen to neon, and so forward all the way to iron. With twenty-six protons and at least as many neutrons in its nucleus, iron has a distinctive quality that derives from the quantum mechanics rules that govern how protons and neutrons interact: Iron nuclei have the highest binding energy per nuclear particle (proton or neutron). This means something quite simple. If you seek to split iron nuclei (in what physicists call “fission”), you must provide them with additional energy. On the other hand, if you combine iron atoms (a process called “fusion”), they will also absorb energy. It takes energy to fuse iron nuclei and it takes energy to split them apart. For all other elements, only one or the other half of this dual description applies.

Stars, however, are in the business of using
E = mc
2
to turn mass into energy, which they must do to oppose their tendency to collapse under their own gravity. When stars fuse nuclei in their cores, nature demands, and obtains, nuclear fusion that releases energy. By the time that a massive star fuses most of the nuclei in its core into iron, it has exhausted all its options for using thermonuclear fusion to generate energy, because any further fusion will require rather than release energy. Deprived of a source of energy from thermonuclear fusion, the star’s core will collapse under its own weight, then instantly rebound in a titanic explosion known as a supernova, outshining a billion suns for more than a week. Such supernovae occur because of the special property of iron nuclei—their refusal either to fuse or to split without an input of energy.

By describing hydrogen, helium; lithium, beryllium, and boron; carbon, nitrogen, and oxygen; and aluminum, titanium, and iron, we have surveyed nearly all of the key elements that make the cosmos—and life on Earth—go round.