Cosmos on the table: an astrophysicist looks at chemistry's most famous chart

Natural History, July-August, 2002 by Neil deGrasse Tyson

For many people, the periodic table is a forgotten oddity--a chart full of squares and cryptic letters last encountered on the wall in a high-school chemistry class. As a way of organizing the chemical behavior of all known and yet-to-be-known elements in the universe, the table ought instead 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 menagerie of one-of-a-kind animals conceived by Dr. Seuss. How else could we accept that sodium is a poisonous, reactive metal that you can cut with a butter knife and that pure chlorine is a smelly, deadly gas, yet when added together they make sodium chloride, a harmless, biologically essential compound better known as table salt? Or how about hydrogen and oxygen? One is explosive. The other promotes combustion. Yet the two gases combined make liquid water, which (among other things) puts out fires.

Amid these chemical concoctions we find elements significant to the cosmos, allowing me to offer a view of the periodic table 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. Hydrogen further lays claim to two-thirds of all the atoms in the human body and to about 90 percent of all the atoms in the cosmos, right on down to our solar system. The hydrogen in the core of the massive planet Jupiter is under so much pressure that it behaves more like a conductive metal than like a gas, creating the strongest magnetic field among the planets. The eighteenth-century English scientist Henry Cavendish isolated hydrogen in 1766 during his experiments with [H.sub.2]O (hydro-genes is Greek for "water-forming"). But he is best known among astrophysicists as the first to calculate Earth's mass, after being the first to come up with an accurate value for Newton's gravitational constant.

Every second of every day, 63 billion tons of fast-moving hydrogen atoms are turned into helium as they slam together within the 15,000,000[degrees]K core of the Sun.

Helium is widely known as an over-the-counter gas that, when inhaled, temporarily increases the vibrational frequency of air within your larynx, leaving you sounding like Mickey Mouse. Helium's nucleus contains two protons, and although it's a distant second to hydrogen in cosmic abundance, there's four times more of it than of the rest of the elements in the universe combined. One of the pillars of big bang cosmology is the prediction that in every region of the universe, no less than about 8 percent of all atoms are helium, manufactured in that quantity during the birth of the cosmos. 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 expected, no one has ever found a 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 solar eclipse of 1868. The name "helium" was duly derived from Helios, the Greek sun god. And with more than 90 percent of hydrogen's buoyancy in air but without its explosive demeanor, helium is the gas of choice for the outsize balloon creatures of the Macy's Thanksgiving Day Parade. The department store is rumored to be second only to the U.S. 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. Unlike helium, which can be manufactured in stars, lithium is fragile and easily destroyed by nuclear reactions. Another prediction of big bang cosmology is that we can expect no more than one in a hundred million atoms in any region of the universe to be lithium. No one has yet found a place with more. The combination of the lower limit for helium and the upper limit for lithium imposes a potent dual constraint on tests for big bang cosmology.

The element carbon is found in more than half of all the kinds of molecules in existence. Given the cosmic abundance of carbon--forged in the cores of dying stars, churned up to their surfaces, and released copiously into galaxies--no other element affords a better foundation for the chemistry and diversity of life. Just edging out carbon in abundance is oxygen, also forged and released in stellar remains. 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 combine with all the same elements and to form analogous compounds. In the end, however, we expect carbon to prevail as the main foundation of living things because it's ten times more abundant than silicon in the cosmos and its chemical bonds are substantially stronger. Complex molecules based on carbon are therefore hardy and more likely to survive environmental stress. But that doesn't stop science fiction writers from including silicon-based life in their alien inventory.