The Periodic Table of the Cosmos

Natural History Magazine

by Neil deGrasse Tyson

From Natural History Magazine, July/August 2002

Published under the title Cosmos on the Table.

For many people, the Periodic Table of Chemical Elements is a forgotten oddity—a chart of boxes filled with cryptic letters last encountered on the wall in high-school chemistry class. As the organizing principle for the chemical behavior of all known and yet-to-be-known elements in the universe, the Table instead ought to be a cultural icon: testimony to the enterprise of science as an international human adventure conducted in laboratories, particle accelerators, and on the frontier of the cosmos itself.

Yet every now and then, even a scientist can't help thinking of the Periodic Table as a zoo of one-of-a-kind animals conceived by Dr. Seuss. How else could we believe that sodium is a poisonous, reactive metal that you can cut with a butter-knife, while pure chlorine is a smelly, deadly gas, yet when added together make sodium chloride, a harmless, biologically essential compound better known as table salt? Or how about hydrogen and oxygen? One is an explosive gas, and the other promotes violent combustion, yet the two combined make liquid water, which puts out fires.

Amid these chemical confabulations we find elements significant to the cosmos, allowing me to offer the Periodic Table as viewed through the lens of an astrophysicist.

With only one proton in its nucleus, hydrogen is the lightest and simplest element, made entirely during the big bang. Out of the 94 naturally occurring elements, hydrogen lays claim to more than two-thirds of all the atoms in the human body, and more than ninety percent of all atoms in the cosmos, right on down to the solar system. The hydrogen in the core of the massive planet Jupiter is under so much pressure that it behaves more like an electromagnetically conductive metal than a gas, creating the strongest magnetic field among the planets. The English chemist Henry Cavendish discovered hydrogen in 1766 during his experiments with H2O (hydro-genes is Greek for water-forming), but he is best known among astronomers as the first to calculate Earth's mass after having measured an accurate value for the gravitational constant in Newton's famous equation for gravity. Every second of every day, 4.5 billion tons of fast-moving hydrogen atoms are turned into energy as they slam together to make helium within the 15-million degree core of the Sun.

Helium is widely recognized as an over-the-counter gas that, when inhaled, temporarily increases the vibrational frequency of your windpipe and larynx, leaving you to sound like Mickey Mouse. Helium is the second simplest and second most abundant element in the universe. Although a distant second to hydrogen in abundance, there's four times more of it than all other elements in the universe combined. One of the pillars of big bang cosmology is the prediction that in every region of the cosmos, no less than about 8 percent of all atoms are helium, manufactured in that percentage in the well-mixed primeval fireball that was the birth of our universe. Since the thermonuclear fusion of hydrogen within stars gives you helium, some regions of the cosmos could easily accumulate more than their 8 percent share of helium, but as predicted, no one has ever found a region of the galaxy with less.

Some thirty years before it was discovered and isolated on Earth, helium was detected in the Sun's spectrum during the total eclipse of 1868. The name helium was duly derived from Helios, the Greek sun god. And with 98 percent of hydrogen's bouyancy in air, but without its explosive characteristics, helium is the gas of choice for the outsized balloon-characters of the Macy's Thanksgiving Day parade, making the department store second only to the US military as the Nation's top consumer of helium.

Lithium is the third simplest element in the universe, with three protons in its nucleus. Like hydrogen and helium, lithium was made in the big bang, but unlike helium, which can be manufactured in subsequent nuclear reactions, lithium is destroyed by every known nuclear reaction. Another prediction of big bang cosmology is that we can expect no more than 1 percent of the atoms in any region of the universe to be lithium. No one has yet found a galaxy with more lithium than this upper limit supplied by the big bang. The combination of helium's upper limit and lithium's lower limit gives a potent dual-constraint on tests for big bang cosmology.

The element carbon can be found in more kinds of molecules than the sum of all other kinds of molecules combined. Given the abundance of carbon in the cosmos—forged in the cores of stars, churned up to their surfaces, and released copiously into the galaxy—a better element does not exist on which to base the chemistry and diversity of life. Just edging out carbon in abundance rank, oxygen is common too, forged and released in the remains of exploded stars. Both oxygen and carbon are major ingredients for life as we know it.

But what about life as we don't know it? How about life based on the element silicon. Silicon sits directly below carbon on the Periodic Table allowing it to create the same chemical compounds as carbon except with silicon in its place. In the end, we expect carbon to win because it's not only ten times more abundant than silicon in the cosmos, it forms chemical bonds that are substantially stronger. In particular, complex molecules based on silicon lack the hardiness to survive ecological stress. But that doesn't stop science fiction writers, who keep exobiologists on their toes, wondering what the first truly alien life form will be like.

In addition to being an active ingredient in table salt, sodium is the most common glowing gas in most municipal street lamps across the Nation. They burn brighter and longer than conventional incandescent bulbs. Two varieties exist, the common high-pressure lamps, which look yellow-white, and the rarer, low-pressure lamps, which look orange. Turns out, while all light pollution is bad for astronomy, the low-pressure sodium lamps are least bad because their contamination can be easily subtracted from telescope data. In a model of cooperation, the entire city of Tucson Arizona, the nearest large municipality to the Kitt Peak National Observatory, has, by agreement with the local astronomers, converted all its street lights to low-pressure sodium lamps.

Aluminum is nearly ten percent of Earth's crust yet was unknown to the ancients and unfamiliar to our grandparents. The element was not isolated and identified until 1827 and did not enter common, household use until the late 1960s, when tin cans and tin foil yielded to aluminum cans and aluminum foil. Polished aluminum makes a near-perfect reflector of visible light and is the coating of choice for nearly all telescope mirrors today.

Although titanium is 70 percent denser than aluminum, it's more than twice as strong. So titanium, the ninth most abundant element in Earth's crust, has become a modern darling for many applications, such as military aircraft components, that require a light, strong metal. For most places in the universe, the number of oxygen atoms exceeds that of carbon. After every carbon atom has latched onto the available oxygen atoms (forming carbon monoxide or carbon dioxide), the leftover oxygen bond with other things, like titanium. The spectra of red giant stars are riddled with features traceable to titanium oxide, which itself is no stranger to stars on Earth: star sapphires and rubies owe their radiant asterisms to titanium oxide impurities in their crystal lattice. Furthermore, the white paint used for telescope domes features titanium oxide, which happens to be highly reflective in the infrared part of the spectrum, greatly reducing the heat accumulated from sunlight in the air surrounding the telescope. At nightfall, with the dome open, the air temperature near the telescope rapidly equals the temperature of the nighttime air, allowing light from stars and other cosmic objects to be sharp and clear. And while not directly named for a cosmic object, Titanium derives from the Titans of Greek mythology, as was Titan, Saturn's largest moon.

By many measures, iron ranks as the most important element in the universe. Massive stars manufacture elements in their core, in sequence from helium to carbon to oxygen to nitrogen, and so forth, all the way up the Periodic Table to iron. With twenty-six protons and at least as many neutrons in its nucleus, iron's odd distinction comes from having the least total energy (highest binding energy) per nuclear particle of any element. This means something quite simple: If you split iron atoms (fission), they will absorb energy. And if you combine iron atoms (fusion) they will also absorb energy. Stars, however, are in the business of making energy. Without a source of energy, a star collapses under its own weight and instantly rebounds in a titanic explosion known as a supernova, outshining a billion suns for more than a week.

The soft metal gallium has such a low melting point that it will liquefy on contact with your hand. Apart from this parlor demo, gallium is not interesting to astrophysicists, except as one of the ingredients in the gallium chloride experiments used to detect elusive neutrinos from the Sun. In such an experiment, a huge (100-ton) vat of liquid gallium chloride is monitored for any collisions between neutrinos and gallium nuclei, turning it them germanium. The encounter emits a spark of x-ray light that gets measured every time a nucleus gets slammed. The solar neutrino problem, where fewer neutrinos were detected than predicted by solar theory, was only recently solved, using telescopes such as this.

Every form of the element technetium is radioactive. Not surprisingly, it's found nowhere on Earth, except in particle accelerators, where we make it on demand. Technetium exhibits this distinction in its name, which derives from the Greek technetos meaning artificial. For reasons not yet fully understood, technetium lives in the atmospheres of a select subset of red giant stars. This alone would not be cause for alarm except 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.

Along with osmium and platinum, iridium is one of the three heaviest elements on the Table—two cubic feet of weighs as much as a Buick, which makes iridium one of the world's best paperweights—able to defy all known office fans. Iridium is also the world's most famous smoking gun. A thin layer of it can be found worldwide at the famous KT boundary in geological strata, dating from 65-million years ago. Coincidentally, that's when every land species larger than a bread box, including the legendary dinosaurs, went extinct. Iridium is rare on Earth's surface, but relatively common in metallic asteroids. Whatever might have been your favorite theory for destroying the dinosaurs, a killer-asteroid from outer space now seems quite compelling.

I don't know how Albert would have felt about this, but an unknown element was discovered in the debris of the first hydrogen bomb test in the Pacific (November 1952) and was named Einsteinium, in his honor. I might have named it Armageddium, instead.

Ten entries in the Periodic Table get their names from objects that orbit the Sun:

Phosphorus derives its name from the Greek for light-bearing, and was the ancient name for the planet Venus when it appeared before sunrise in the dawn sky.

Selenium, comes from selene, which is Greek for the Moon, named so, because the element was always associated with the element tellurium, which had already been named for Earth, from the Latin tellus.

On January 1, 1801, the Italian astronomer Giuseppe Piazzi discovered a new planet orbiting the Sun in the suspiciously large gap between Mars and Jupiter. Keeping with the tradition of naming planets after Roman gods, the object was named Ceres, after the goddess of harvest. Ceres is, of course, the root of the word cereal. At the time, there was much excitement in the scientific community, and the first element to be discovered after this date was named cerium in its honor. Two years later, another planet was discovered, orbiting the Sun in the same gap as Ceres. This one was named Pallas for the Roman goddess of wisdom, and, like cerium before it, the first element discovered thereafter was named palladium in its honor. The naming party ended a few decades later, after dozens more of these planets were discovered in the same place, and after closer analysis revealed that these objects were much, much smaller than the smallest known planets. A new swath of real estate had been discovered in the solar system, populated by small, craggy chunks of rock and metal. Ceres and Pallas were not planets, they are asteroids, and they live in the asteroid belt, now known to contain upwards of 15,000 objects—somewhat more than the number of elements in the Periodic Table.

The metal mercury, liquid and runny at room temperature, and the planet Mercury, the fastest of all planets in the solar system, are both named for the speedy Roman messenger god of the same name.

Thorium, while named for Thor, the lightning bolt-wielding Scandinavian god, corresponds with lightning bolt-wielding Jupiter in Roman mythology. By jove, recent Hubble Space Telescope images of Jupiter's polar regions reveal extensive electrical discharges deep within its turbulent cloud layers.

Alas, Saturn, my favorite planet, has no element named for it, but Uranus, Neptune and Pluto are famously represented. The element Uranium was discovered in 1789, and named in honor of William Herschel's planet, discovered by him just eight years earlier. All isotopes of uranium are unstable, spontaneously decaying to lighter elements, accompanied by the release of energy. The first atomic bomb ever used in warfare had uranium as its active ingredient, and was dropped by the United States, incinerating the Japanese City of Hiroshima. With 92 protons packed in its nucleus, Uranus is widely described as the largest naturally occurring element, although trace amount of larger elements can be found naturally where uranium ore is mined.

If Uranus deserved an element named in its honor, then so did Neptune. Unlike uranium, however, which was discovered shortly after the planet, neptunium was discovered in 1940 in the Berkeley Cyclotron, a full 97 years after the German astronomer John Galle found Neptune in a spot on the sky predicted by the French mathematician Joseph Le Verrier after studying Uranus' unexplained orbital behavior. Just as Neptune comes right after Uranus in the solar system, so too does neptunium come right after uranium in the Periodic Table of Elements.

The Berkeley cyclotron discovered many elements not found in nature, including plutonium, which directly follows neptunium in the Table and was named for Pluto, which Clyde Tombaugh discovered at Arizona's Lowell Observatory in 1930. Just as with the discovery of Ceres 129 years earlier, excitement prevailed. Pluto was the first planet discovered by an American and, in the absence of better data, was widely regarded as a planet of commensurate size and mass to Uranus and Neptune. As our measurements of Pluto's size got better and better, it kept getting smaller and smaller. Our knowledge of Pluto's dimensions did not stabilize until the late 1970s during the Voyager missions to the outer solar system. We now know that cold, icy Pluto is, by far the smallest planet, with the embarrassing distinction of being littler than the solar system's six largest moons. Like the asteroids, hundreds more objects were later discovered in the outer solar system, with orbits similar to that of Pluto, signaling the existence of a heretofore undocumented reservoir of small icy bodies called the Kuiper Belt of comets. In this regard, one could argue that Ceres, Pallas, and Pluto slipped into the periodic table under false pretenses.

Unstable weapons grade plutonium was the active ingredient in the atomic bomb used in over the Japanese city of Nagasaki, just three days after the uranium bomb was dropped over Hiroshima, bringing a swift end to the Second World War. Small quantities of non-weapons-grade radioactive plutonium can be used to power radioisotope thermoelectric generators (sensibly abbreviated at RTGs) for spacecraft that travel to the outer solar system, where the intensity of sunlight has diminished below the level usable by solar panels. One pound of plutonium will generate ten million kilowatt-hours of heat energy, which is enough to power a household light bulb for eleven thousand years, or a human being for just as long.

And so ends our cosmic journey through the Periodic Table of Chemical Elements, right to the edge of the solar system. For reasons I have yet to determine, many people don't like chemicals, which might explain the perennial movement to rid foods of them. Perhaps sesquepedalious chemical names just sound dangerous. But in that case we should blame the chemists, and not the chemicals. Personally, I am quite comfortable with chemicals. My favorite stars, as well as my best friends are made of them.

Neil deGrasse Tyson, an astrophysicist, is the Frederick P. Rose Director of New York City’s Hayden Planetarium and a visiting research scientist at Princeton University.