Origins: Fourteen Billion Years of Cosmic Evolution (13 page)

Ten billion years after the Milky Way formed, star formation continues today at multiple locations in our galaxy. Even though most of the star formation that will ever occur in a typical giant galaxy like ours has already taken place, we are fortunate that new stars continue to form, and will do so for many billion years to come. Our good fortune lies in our ability to study the formation process and the youngest stars, seeking clues that will reveal, in all its glory, the complete story of how stars pass from cold gas and dust to luminous maturity.

How old are the stars? No star wears its age on its sleeve, but some show their ages in their spectra. Among the various means that astrophysicists have devised to judge the ages of stars, spectra forms the most reliable hinge for analyzing the different colors of starlight in detail. Every color—every wavelength and frequency of the light waves we observe—tells a story about how matter made the starlight, or affected that light as it left the star, or happened to lie along the line of sight between ourselves and the star. Through close comparison with laboratory spectra, physicists have determined the multitude of ways that different types of atoms and molecules affect the rainbow of colors in visible light. They can apply this fertile knowledge to observations of stellar spectra, and deduce the numbers of atoms and molecules that have affected light from a particular star, as well as the temperature, pressure, and density of those particles. From years of comparing laboratory spectra with the spectra of stars, together with laboratory studies of the spectra of different atoms and molecules, astrophysicists have learned how to read an object’s spectrum like a cosmic fingerprint, one that reveals what physical conditions exist within a star’s outer layers, the region from which light streams directly outward into space. In addition, astrophysicists can determine how atoms and molecules floating in interstellar space at much cooler temperatures may have affected the spectrum of the starlight they observe, and can likewise deduce the chemical composition, temperature, density, and pressure of this interstellar matter.

In this spectral analysis, each different type of atom or molecule has its own story to tell. The presence of molecules of any type, for example, revealed by their characteristic effects on certain colors in the spectrum, demonstrates that the temperature in a star’s outer layers must be less than about 3,000
o
Celsius (about 5,000
o
Fahrenheit). At higher temperatures, molecules move so rapidly that their collisions break them apart into individual atoms. By extending this type of analysis over many different substances, astrophysicists can derive a nearly complete picture of the detailed conditions in stellar atmospheres. Some hard-working astrophysicists are said to know far more about the spectra of stars they love than they do about their own families. This may have its down side for interpersonal relations even as it increases human understanding of the cosmos.

Of all nature’s elements—of all the different types of atoms that can create patterns in a star’s spectrum—astrophysicists recognize and use one in particular to find the ages of the youngest stars. That element is lithium, the third simplest and lightest in the periodic table, and familiar to some on Earth as the active ingredient of some antidepressant medications. In the periodic table of the elements, lithium occupies the position immediately after hydrogen and helium, which are deservedly far more famous because they exist in immensely greater amounts throughout the cosmos. During its first few minutes, the universe fused hydrogen into helium nuclei in great numbers, but made only relatively tiny amounts of any heavier nucleus. As a result, lithium remained a rather rare element, distinguished among astrophysicists by the cosmic fact that stars hardly ever make more lithium, but only destroy it. Lithium rides down a one-way street because every star has more effective nuclear fusion reactions to destroy lithium than to create it. As a result, the cosmic supply of lithium has steadily decreased and continues to do so. If you want some, now would be a good time to acquire it.

For astrophysicists, this simple fact about lithium makes it a highly useful tool for measuring the ages of stars. All stars begin their lives with their fair and proportionate share of lithium, left behind by the nuclear fusion that occurred during the universe’s first half hour—and during the big bang itself. And what is that fair share? About one in every 100 billion nuclei. After a newborn star begins its life with this “richness” of lithium, things go downhill, lithiumwise, as nuclear reactions within the star’s core slowly consume lithium nuclei. The steady and sometimes episodic mixing of matter in the core with matter outside carries material outward, so that after thousands of years, the star’s outer layers can reflect what previously happened in its core.

When astrophysicists look for the youngest stars, they therefore follow a simple rule: Look for the stars with the
greatest
abundance of lithium. Each star’s number of lithium nuclei in proportion to, for example, hydrogen (determined from careful study of the star’s spectrum), will locate the star at some point along a graph that shows how stars’ ages correlate with lithium in their outer layers. By using this method, astrophysicists can identify, with confidence, the youngest stars in a cluster, and can assign each of those stars a lithium-based age. Because stars are efficient destroyers of lithium, older stars show little if any of the stuff. Hence the method works well only for stars less than few hundred million years old. But for these younger stars, the lithium approach works wonders. A recent study of two dozen young stars in the Orion nebula, all of which have masses close to the Sun’s, show ages that range between 1 and 10 million years. Some day astrophysicists may well identify still younger stars, but for now, 1 million years represents about the best they can do.

Except for dispersing
the cocoons of gas from which they formed, groups of newborn stars bother nobody for a long time, as they quietly fuse hydrogen into helium in their cores and destroy their lithium nuclei as part of their fusion reactions. But nothing lasts forever. Over many million years, in response to the continual gravitational perturbations from enormous clouds that pass by, most would-be star clusters “evaporate,” as its members scatter into the general pool of stars in the galaxy.

Nearly 5 billion years after our star formed, the identity of the Sun’s siblings has vanished, whether or not those stars remain alive. Of all the stars in the Milky Way and other galaxies, those with low masses consume their fuel so slowly that they live practically forever. Intermediate-mass stars such as our Sun eventually turn into red giants, expanding their outer gas layers a hundredfold in size as they slide toward death. These outer layers become so tenuously connected to the star that they drift into space, exposing a core of spent nuclear fuels that powered the stars’ 10-billion-year lives. The gas that returns to space will be swept up by passing clouds, to participate in later rounds of star formation.

Despite their rarity, the highest-mass stars hold nearly all the evolutionary cards. Their high masses give them the greatest stellar luminosities—some of them can boast a million times the Sun’s—and because they consume their nuclear fuel far more rapidly than low-mass stars do, they have the shortest lives of all stars, only a few million years, or even less. Continued thermonuclear fusion within high-mass stars allows them to manufacture dozens of elements in their cores, starting with hydrogen and proceeding to helium, carbon, nitrogen, oxygen, neon, magnesium, silicon, calcium, and so on, all the way to iron. These stars forge still more elements in their final fires, which can briefly outshine a star’s entire home galaxy. Astrophysicists call each of these outbursts a supernova, similar in appearance (though quite different in their origin) to the Type Ia supernovae described in Chapter 5. A supernova’s explosive energy spreads both the previously made and the freshly minted elements through the galaxy, blowing holes in its distribution of gas and enriching nearby clouds with the raw materials to make new dust grains. The blast moves supersonically through these interstellar clouds, compressing their gas and dust, possibly creating some of the high-density pockets needed to form stars.

The greatest gift to the cosmos from these supernovae consists of all the elements other than hydrogen and helium—elements capable of forming planets and protists and people. We on Earth live on the product of countless stars that exploded billions of years ago, in epochs of Milky Way history long before our Sun and its planets, condensing within the dark and dusty recesses of an interstellar cloud—itself endowed with chemical enrichment furnished from previous generations of high-mass stars.

How did we
come to taste this delicious kernel of knowledge, the fact that all the elements beyond helium were forged within stars? The authors’ award for the most underappreciated scientific discovery of the twentieth century goes to the recognition that supernovae—the explosive death throes of high-mass stars—provide the primary source for the origin and abundances of heavy elements in the universe. This relatively unsung realization appeared in a lengthy research article, published in 1957 in the U.S. journal
Reviews of Modern Physics
under the title
“The Synthesis of the Elements in Stars,” and written by E. Margaret Burbidge, Geoffrey R. Burbidge, William Fowler, and Fred Hoyle. In this paper, the four scientists created a theoretical and computational framework that freshly interpreted and melded together forty years of musings by other scientists on two key topics: the sources of stellar energy and the transmutation of chemical elements.

Cosmic nuclear chemistry, the quest to understand how nuclear fusion makes and destroys different types of nuclei, has always been a messy business. The crucial questions have always included: How do the various elements behave when various temperatures and pressures act upon them? Do the elements fuse or do they split? How easily do they do this? Do these processes liberate new kinetic energy or absorb existing kinetic energy? And how do the processes differ for each element in the periodic table?

What does the periodic table of the elements mean to you? If you are like most former students, you will remember a giant chart on the wall of your science class, tricked out with mysterious boxes in which cryptic letters and symbols murmured tales of dusty laboratories to be avoided by young souls in transition. But to those who know its secrets, this chart tells a hundred stories of cosmic violence that brought its components into existence. The periodic table lists every known element in the universe, arranged by the increasing number of protons in each element’s nuclei. The two lightest elements are hydrogen, with one proton per nucleus, and helium, with two. As the four authors of the 1957 paper saw, under the right conditions of temperature, density, and pressure, a star can use hydrogen and helium to create all the other elements in the periodic table.

The details of this creation process, and of other interactions that destroy nuclei rather than create them, provide the subject matter for nuclear chemistry, which involves the calculation and use of “collision cross sections” to measure how closely one particle must approach another before they are likely to interact significantly. Physicists can easily calculate collision cross sections for cement mixers, or double-wide mobile homes moving down the street on flatbed trucks, but they face greater challenges in analyzing the behavior of tiny, elusive subatomic particles. A detailed understanding of collision cross sections enables physicists to predict nuclear reaction rates and pathways. Often small uncertainties in their tables of cross sections lead them into wildly erroneous conclusions. Their difficulties resemble what would happen if you tried to navigate your way through one city’s subway system with another city’s subway map as your guide: your basic theory would be correct, but the details could kill you.

Despite their ignorance of accurate collision cross sections, scientists during the first half of the twentieth century had long suspected that if exotic nuclear processes exist anywhere in the universe, the centers of stars seemed likely places to find them. In 1920, the British theoretical astrophysicist Sir Arthur Eddington published a paper entitled the “The Internal Constitution of the Stars,” in which he argued that the Cavendish Laboratory in England, the leading center for atomic and nuclear physics research, could not be the only place in the universe that managed to change some elements into others:

But is it possible to admit that such a transmutation is occurring? It is difficult to assert, but perhaps more difficult to deny, that this is going on . . . and what is possible in the Cavendish Laboratory may not be too difficult in the sun. I think that the suspicion has been generally entertained that the stars are the crucibles in which the lighter atoms which abound in the nebulæ are compounded into more complex elements.

Eddington’s paper, which foreshadowed the detailed research of Burbidge, Burbidge, Fowler, and Hoyle, appeared several years before the discovery of quantum mechanics, without which our understanding of the physics of atoms and nuclei must be judged feeble at best. With remarkable prescience, Eddington began to formulate a scenario for star-generated energy via the thermonuclear fusion of hydrogen to helium and beyond:

We need not bind ourselves to the formation of helium from hydrogen as the sole reaction which supplies the energy [to a star], although it would seem that the further stages in building up the elements involve much less liberation, and sometimes even absorption, of energy. The position may be summarised in these terms: the atoms of all elements are built of hydrogen atoms bound together, and presumably have at one time been formed from hydrogen; the interior of a star seems as likely a place as any for the evolution to have occurred.

Any model of the transmutation of the elements ought to explain the observed mix of elements found on Earth and elsewhere in the universe. To do this, physicists needed to find the fundamental process with which stars generate energy by turning one element into another. By 1931, with theories of quantum mechanics rather well developed (although the neutron had not yet been discovered), the British astrophysicist Robert d’Escourt Atkinson published an extensive paper, summarized as a “synthesis theory of stellar energy and of the origin of the elements . . . in which the various chemical elements are built up step by step from the lighter ones in stellar interiors, by the successive incorporation of protons and electrons one at a time.”