Death By Black Hole Part 8

MOST PEOPLE ARE familiar with dust of the household variety, although few know that, in a closed home, it consists mostly of dead, sloughed-off human skin cells (plus pet dander, if you own a live-in mammal). Last I checked, cosmic dust in the interstellar medium contains n.o.body's epidermis. But it does have a remarkable ensemble of complex molecules that emit princ.i.p.ally in the infrared and microwave parts of the spectrum. Microwave telescopes were not a major part of the astrophysicist's tool kit until the 1960s; infrared telescopes, not until the 1970s. And so the true chemical richness of the stuff between the stars was unknown until then. In the decades that followed, a fascinating, intricate picture of star birth emerged. familiar with dust of the household variety, although few know that, in a closed home, it consists mostly of dead, sloughed-off human skin cells (plus pet dander, if you own a live-in mammal). Last I checked, cosmic dust in the interstellar medium contains n.o.body's epidermis. But it does have a remarkable ensemble of complex molecules that emit princ.i.p.ally in the infrared and microwave parts of the spectrum. Microwave telescopes were not a major part of the astrophysicist's tool kit until the 1960s; infrared telescopes, not until the 1970s. And so the true chemical richness of the stuff between the stars was unknown until then. In the decades that followed, a fascinating, intricate picture of star birth emerged.

Not all gas clouds in the Milky Way can form stars at all times. More often than not, the cloud is confused about what to do next. Actually, astrophysicists are the confused ones here. We know the cloud wants to collapse under its own weight to make one or more stars. But rotation as well as turbulent motion within the cloud work against that fate. So, too, does the ordinary gas pressure you learned about in high-school chemistry cla.s.s. Galactic magnetic fields also fight collapse: they penetrate the cloud and latch onto any free-roaming charged particles contained therein, restricting the ways in which the cloud will respond to its self-gravity. The scary part is that if none of us knew in advance that stars exist, frontline research would offer plenty of convincing reasons for why stars could never form.

Like the Milky Way's several hundred billion stars, gas clouds...o...b..t the center of the galaxy. The stars are tiny specks (a few light-seconds across) in a vast ocean of permeable s.p.a.ce, and they pa.s.s one another like ships in the night. Gas clouds, on the other hand, are huge. Typically spanning hundreds of light-years, they contain the ma.s.s equivalent of a million Suns. As these clouds lumber through the galaxy, they often collide with one other, entangling their innards. Sometimes, depending on their relative speeds and their angles of impact, the clouds stick together like hot marshmallows; at other times, adding injury to insult, they rip each other apart.

If a cloud cools to a low enough temperature (less than about 100 degrees above absolute zero), its const.i.tuent atoms will b.u.mp and stick rather than careen off one another, as they do at higher temperatures. This chemical transition has consequences for everybody. The growing particles-now containing tens of atoms-begin to bat visible light to and fro, strongly attenuating the light of stars behind it. By the time the particles become full-grown dust grains, they contain upwards of 10 billion atoms. At that size, they no longer scatter the visible light from the stars behind them: they absorb it, then reradiate the energy as infrared, which is a part of the spectrum that freely escapes the cloud. But the act of absorbing visible light creates a pressure that pushes the cloud opposite the direction of the light source. The cloud is now coupled to starlight.

The forces that make the cloud more and more dense may eventually lead to its gravitational collapse, and that in turn leads to star birth. Thus we face an odd situation: to create a star with a 10-million-degree core, hot enough to undergo thermonuclear fusion, we must first achieve the coldest possible conditions within a cloud.

At this time in the life of a cloud, astrophysicists can only gesticulate what happens next. Theorists and computer modelers face the many parameter problem of inputting all known laws of physics and chemistry into their supercomputers before they can even think about tracking the dynamic behavior of large, ma.s.sive clouds under all external and internal influences. A further challenge is the humbling fact that the original cloud is billions of times wider and a hundred s.e.xtillion times less dense than the star we're trying to create-and what matters on one size scale is not necessarily the right thing to worry about on another.

NEVERTHELESS, ONE THING we can safely a.s.sert is that in the deepest, darkest, densest regions of an interstellar cloud, with temperatures down around 10 degrees above absolute zero, pockets of gas do collapse without resistance, converting their gravitational energy into heat. The temperature in each region-soon to become the core of a newborn star-rises rapidly, dismantling all the dust grains in the immediate vicinity. Eventually the collapsing gas reaches 10 million degrees. At this magic temperature, protons (which are just naked hydrogen atoms) move fast enough to overcome their repulsion, and they bond under the influence of a short-range, strong nuclear force whose technical term is "strong nuclear force." This thermonuclear fusion creates helium, whose ma.s.s is less than the sum of its parts. The lost ma.s.s has been converted into boatloads of energy, as described by Einstein's famous equation we can safely a.s.sert is that in the deepest, darkest, densest regions of an interstellar cloud, with temperatures down around 10 degrees above absolute zero, pockets of gas do collapse without resistance, converting their gravitational energy into heat. The temperature in each region-soon to become the core of a newborn star-rises rapidly, dismantling all the dust grains in the immediate vicinity. Eventually the collapsing gas reaches 10 million degrees. At this magic temperature, protons (which are just naked hydrogen atoms) move fast enough to overcome their repulsion, and they bond under the influence of a short-range, strong nuclear force whose technical term is "strong nuclear force." This thermonuclear fusion creates helium, whose ma.s.s is less than the sum of its parts. The lost ma.s.s has been converted into boatloads of energy, as described by Einstein's famous equation E= mc E= mc2, where E E is energy, is energy, m m is ma.s.s, and is ma.s.s, and c c is the speed of light. As the heat moves outward, the gas becomes luminous, and the energy that had formerly been ma.s.s now makes its exit. And although the region of hot gas still sits womblike within the greater cloud, we may nonetheless announce to the Milky Way that a star is born. is the speed of light. As the heat moves outward, the gas becomes luminous, and the energy that had formerly been ma.s.s now makes its exit. And although the region of hot gas still sits womblike within the greater cloud, we may nonetheless announce to the Milky Way that a star is born.

We know that stars come in a wide range of ma.s.ses: from a mere one-tenth to nearly a hundred times that of the Sun. For reasons not yet divined, our giant gas cloud contains a mult.i.tude of cold pockets, all of which form at about the same time and each of which gives birth to a star. For every high-ma.s.s star born, there are a thousand low-ma.s.s stars. But only about 1 percent of all the gas in the original cloud partic.i.p.ates in star birth, and that presents a cla.s.sic challenge: figuring out how and why the tail wags the dog.

THE Ma.s.s LIMIT on the low end is easy to determine. Below about one-tenth of the Sun's ma.s.s, the pocket of collapsing gas does not have enough gravitational energy to bring its core temperature up to the requisite 10 million degrees. A star is not born. Instead we get what is commonly called a brown dwarf. With no energy source of its own, it just gets dimmer and dimmer over time, living off the little heat it was able to generate from its original collapse. The outer gaseous layers of a brown dwarf are so cool that many of the large molecules normally destroyed in the atmospheres of hotter stars remain alive and well within it. With such a feeble luminosity, a brown dwarf is supremely difficult to detect, requiring methods similar to those used for the detection of planets. Indeed, only in recent years have enough brown dwarfs been discovered to cla.s.sify them into more than one category. The ma.s.s limit at the high end is also easy to determine. Above about a hundred times that of the Sun's ma.s.s, the star is so luminous that any additional ma.s.s that may want to join the star gets pushed away by the intense pressure of the star's light on the dust grains within the cloud, which carries the gas cloud with it. Here the coupling of starlight with dust is irreversible. So potent are the effects of this radiation pressure that the luminosity of just a few high-ma.s.s stars can disperse nearly all the ma.s.s from the original dark, obscuring cloud, thereby laying bare dozens, if not hundreds, of brand-new stars-siblings, really-for the rest of the galaxy to see. on the low end is easy to determine. Below about one-tenth of the Sun's ma.s.s, the pocket of collapsing gas does not have enough gravitational energy to bring its core temperature up to the requisite 10 million degrees. A star is not born. Instead we get what is commonly called a brown dwarf. With no energy source of its own, it just gets dimmer and dimmer over time, living off the little heat it was able to generate from its original collapse. The outer gaseous layers of a brown dwarf are so cool that many of the large molecules normally destroyed in the atmospheres of hotter stars remain alive and well within it. With such a feeble luminosity, a brown dwarf is supremely difficult to detect, requiring methods similar to those used for the detection of planets. Indeed, only in recent years have enough brown dwarfs been discovered to cla.s.sify them into more than one category. The ma.s.s limit at the high end is also easy to determine. Above about a hundred times that of the Sun's ma.s.s, the star is so luminous that any additional ma.s.s that may want to join the star gets pushed away by the intense pressure of the star's light on the dust grains within the cloud, which carries the gas cloud with it. Here the coupling of starlight with dust is irreversible. So potent are the effects of this radiation pressure that the luminosity of just a few high-ma.s.s stars can disperse nearly all the ma.s.s from the original dark, obscuring cloud, thereby laying bare dozens, if not hundreds, of brand-new stars-siblings, really-for the rest of the galaxy to see.

The Great Nebula in Orion-situated just below Orion's belt, midway down his sword-is a stellar nursery of just that sort. Within the nebula thousands of stars are being born in one giant cl.u.s.ter. Four of the several ma.s.sive ones form the Orion Trapezium and are busy evacuating a giant hole in the middle of the cloud from which they formed. New stars are clearly visible in Hubble Hubble telescope images of the region, each infant swaddled in a nascent, protoplanetary disk made of dust and other molecules drawn from the original cloud. And within each disk a solar system is forming. telescope images of the region, each infant swaddled in a nascent, protoplanetary disk made of dust and other molecules drawn from the original cloud. And within each disk a solar system is forming.

For a long while, newborn stars don't bother anybody. But eventually, from the prolonged, steady gravitational perturbations of enormous pa.s.sing clouds, the cl.u.s.ter ultimately falls apart, its members scattering into the general pool of stars in the galaxy. The low-ma.s.s stars live practically forever, so efficient is their consumption of fuel. The intermediate-ma.s.s stars, such as our Sun, sooner or later turn into red giants, expanding a hundredfold in size as they march toward death. Their outermost gaseous layers become so tenuously connected to the star that they drift into s.p.a.ce, exposing the spent nuclear fuels that powered their 10-billion-year lives. The gas that returns to s.p.a.ce gets swept up by pa.s.sing clouds, only to partic.i.p.ate in later rounds of the formation of stars.

In spite of the rarity of the highest-ma.s.s stars, they hold nearly all the evolutionary cards. They boast the highest luminosity (a million times that of the Sun) and, as a consequence, the shortest lives (only a few million years). And as we will shortly see, high-ma.s.s stars manufacture dozens of heavy elements, one after the other, starting with hydrogen and proceeding to helium, carbon, nitrogen, oxygen, and so forth, all the way to iron in their cores. They die spectacular deaths in supernova explosions, making yet more elements in their fires and briefly outshining their entire home galaxy. The explosive energy spreads the freshly minted elements across the galaxy, blowing holes in its distribution of gas and enriching nearby clouds with the raw materials to make dust of their own. The supernova-blast waves move supersonically through the clouds, compressing the gas and dust, and possibly creating pockets of very high density necessary to form stars in the first place.

As we will see in the next chapter, the supernova's greatest gift to the cosmos is to seed clouds with the heavy elements that form planets and protists and people, so that once again, further endowed by the chemical enrichment from a previous generation of high-ma.s.s stars, another star is born.

TWENTY-ONE.

FORGED IN THE STARS.

Not all scientific discoveries are made by lone, antisocial researchers. Nor are all discoveries accompanied by media headlines and best-selling books. Some involve many people, span many decades, require complicated mathematics, and are not easily summarized by the press. Such discoveries pa.s.s almost unnoticed by the general public.

My vote for the most underappreciated discovery of the twentieth century is the realization that supernovas-the explosive death throes of high-ma.s.s stars-are the primary source for the origin and relative mix of heavy elements in the universe. This unheralded discovery took the form of an extensive research paper published in 1957 in the journal Reviews of Modern Physics Reviews of Modern Physics t.i.tled "The Synthesis of the Elements in Stars," by E. Margaret Burbidge, Geoffrey R. Burbidge, William Fowler, and Fred Hoyle. In the paper they built a theoretical and computational framework that freshly interpreted 40 years of musings by others on such hot topics as the sources of stellar energy and the trans.m.u.tation of elements. t.i.tled "The Synthesis of the Elements in Stars," by E. Margaret Burbidge, Geoffrey R. Burbidge, William Fowler, and Fred Hoyle. In the paper they built a theoretical and computational framework that freshly interpreted 40 years of musings by others on such hot topics as the sources of stellar energy and the trans.m.u.tation of elements.

Cosmic nuclear chemistry is a messy business. It was messy in 1957 and it is messy now. The relevant questions have always included: How do the various elements from the famed periodic table of elements behave when subjected to a.s.sorted temperatures and pressures? Do the elements fuse or do they split? How easily is this accomplished? Does the process liberate or absorb energy?

The periodic table is, of course, much more than just a mysterious chart of a hundred, or so, boxes with cryptic symbols in them. It is a sequence of every known element in the universe arranged by increasing number of protons in their nuclei. The two lightest are hydrogen, with one proton, and helium, with two protons. Under the right conditions of temperature, density, and pressure, you can use hydrogen and helium to synthesize every other element on the periodic table.

A perennial problem in nuclear chemistry involves calculating accurate collision cross-sections, which are simply measures of how close one particle must get to another particle before they interact significantly. Collision cross-sections are easy to calculate for things such as cement mixers or houses moving down the street on flatbed trucks, but it can be a challenge for elusive subatomic particles. A detailed understanding of collision cross-sections is what enables you to predict nuclear reaction rates and pathways. Often small uncertainties in tables of collision cross-sections can force you to draw wildly erroneous conclusions. The problem greatly resembles what would happen if you tried to navigate your way around one city's subway system while using another city's subway map as your guide.

Apart from this ignorance, scientists had suspected for some time that if an exotic nuclear process existed anywhere in the universe, then the centers of stars were as good a place as any to find it. In particular, the British theoretical astrophysicist Sir Arthur Eddington published a paper in 1920 t.i.tled "The Internal Const.i.tution of the Stars" where he argued that the Cavendish Laboratory in England, the most famous atomic and nuclear physics research center of the day, could not be the only place in the universe that managed to change some elements onto others: But is it possible to admit that such a trans.m.u.tation is occurring? It is difficult to a.s.sert, 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 nebulae are compounded into more complex elements. (p. 18) (p. 18) Eddington's paper predates by several years the discovery of quantum mechanics, without which our knowledge of the physics of atoms and nuclei was 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. (p. 18) (p. 18) The observed mix of elements on Earth and elsewhere in the universe was another desirable thing for a model of the trans.m.u.tation of the elements to explain. But first a mechanism was required. By 1931, quantum physics was developed (although the neutron was not yet discovered) and the astrophysicist Robert d'Escourt Atkinson published an extensive paper that he summarizes in his abstract 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" (p. 250).

At about the same time, the nuclear chemist William D. Harkins published a paper noting that "elements of low atomic weight are more abundant than those of high atomic weight and that, on the average, the elements with even atomic numbers are about 10 times more abundant than those with odd atomic numbers of similar value" (Lang and Gingerich 1979, p. 374). Harkins surmised that the relative abundances of the elements depend on nuclear rather than on conventional chemical processes 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 get each time you add the two-proton helium nucleus to your previously forged element. These const.i.tute the abundant elements with "even atomic numbers" that Harkins refers to. But the existence and relative mix of many other elements remained unexplained. Another means of element buildup must have been at work.

The neutron, discovered in 1932 by the British physicist James Chadwick while working at the Cavendish Laboratory, plays a significant role in nuclear fusion that Eddington could not have imagined. To a.s.semble protons requires hard work because they naturally repel each other. They must be brought close enough together (often by way of high temperatures, pressures, and densities) for the short-range "strong" nuclear force to overcome their repulsion and bind them. The chargeless neutron, however, repels no other particle, so it can just march into somebody else's nucleus and join the other a.s.sembled particles. This step has not yet created another element; by adding a neutron we have simply made an "isotope" of the original. But for some elements, the freshly captured neutron is unstable and it spontaneously converts itself into a proton (which stays put in the nucleus) and an electron (which escapes immediately). Like the Greek soldiers who managed to breach the walls of Troy by hiding inside the Trojan Horse, protons can effectively sneak into a nucleus under the guise of a neutron.

If the ambient flow of neutrons is high, then an atom's nucleus can absorb many in a row before the first one decays. These rapidly absorbed neutrons help to create an ensemble of elements that are identified with the process and differ from the a.s.sortment of elements that result from neutrons that are captured slowly.

The entire process is known as neutron capture and is responsible for creating many elements that are not otherwise formed by traditional thermonuclear fusion. The remaining elements in nature can be made by a few other means, including slamming high-energy light (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-ma.s.s star, it is sufficient to recognize that a star is in the business of making and releasing energy, which helps to support the star against gravity. Without it, the big ball of gas would simply collapse under its own weight. A star's core, after having converted its hydrogen supply into helium, 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 higher and higher temperatures for the nuclei to overcome their natural repulsion. Fortunately this happens naturally because at the end of each intermediate stage, the star's energy source temporarily shuts off, the inner regions collapse, the temperature rises, and the next pathway of fusion kicks in. But there is just one problem. The fusion of iron absorbs energy rather than releases it. This is very bad for the star because it can now no longer support itself against gravity. The star immediately collapses without resistance, which forces the temperature to rise so rapidly that a t.i.tanic explosion ensues as the star blows its guts to smithereens. During the explosion, the star's luminosity can increase a billionfold. We call them supernovas, although I always felt that the term "super-duper novas" would be more appropriate. of oversimplifying the life cycle of a high-ma.s.s star, it is sufficient to recognize that a star is in the business of making and releasing energy, which helps to support the star against gravity. Without it, the big ball of gas would simply collapse under its own weight. A star's core, after having converted its hydrogen supply into helium, 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 higher and higher temperatures for the nuclei to overcome their natural repulsion. Fortunately this happens naturally because at the end of each intermediate stage, the star's energy source temporarily shuts off, the inner regions collapse, the temperature rises, and the next pathway of fusion kicks in. But there is just one problem. The fusion of iron absorbs energy rather than releases it. This is very bad for the star because it can now no longer support itself against gravity. The star immediately collapses without resistance, which forces the temperature to rise so rapidly that a t.i.tanic explosion ensues as the star blows its guts to smithereens. During the explosion, the star's luminosity can increase a billionfold. We call them supernovas, although I always felt that the term "super-duper novas" would be more appropriate.

Throughout the supernova explosion, the availability of neutrons, protons, and energy enable elements to be created in many different ways. By combining (1) the well-tested tenets of quantum mechanics, (2) the physics of explosions, (3) the latest collision cross-sections, (4) the varied processes by which elements can trans.m.u.tate into one another, and (5) the basics of stellar evolutionary theory, Burbidge, Burbidge, Fowler, and Hoyle decisively implicated supernova explosions as the primary source of all elements heavier than hydrogen and helium in the universe.

With supernovas as the smoking gun, they got to solve one other problem for free: when you forge elements heavier than hydrogen and helium inside stars, it does the rest of the universe no good unless those elements are somehow cast forth to interstellar s.p.a.ce and made available to form planets and people. Yes, we are stardust.

I do not mean to imply that all of our cosmic chemical questions are solved. A curious contemporary mystery involves the element technetium, which, in 1937, was the first element to be synthesized in the laboratory. (The name technetium, along with other words that use the root prefix "tech-," derives from the Greek word technetos technetos, which translates to "artificial.") The element has yet to be discovered naturally on Earth, but it has been found in the atmosphere of a small fraction of red giant stars in our galaxy. This alone would not be cause for alarm were it not for the fact that technetium has a half-life of a mere 2 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 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. 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.

Red giants with peculiar chemical properties are rare, but nonetheless common enough for there to be a cadre of astrophysicists (mostly spectroscopists) who specialize in the subject. In fact, my professional research interests sufficiently overlap the subject for me to be a regular recipient of the internationally distributed Newsletter of Chemically Peculiar Red Giant Stars Newsletter of Chemically Peculiar Red Giant Stars (not available on the newsstand). It typically contains conference news and updates on research in progress. To the interested scientist, these ongoing chemical mysteries are no less seductive than questions related to black holes, quasars, and the early universe. But you will hardly ever read about them. Why? Because once again, the media has predetermined what is not worthy of coverage, even when the news item is something as uninteresting as the cosmic origin of every element in your body. (not available on the newsstand). It typically contains conference news and updates on research in progress. To the interested scientist, these ongoing chemical mysteries are no less seductive than questions related to black holes, quasars, and the early universe. But you will hardly ever read about them. Why? Because once again, the media has predetermined what is not worthy of coverage, even when the news item is something as uninteresting as the cosmic origin of every element in your body.

TWENTY-TWO.

SEND IN THE CLOUDS.

For nearly all of the first 400 millennia after the birth of the universe, s.p.a.ce was a hot stew of fast-moving, naked atomic nuclei with no electrons to call their own. The simplest chemical reactions were still just a distant dream, and the earliest stirrings of life on Earth lay 10 billion years in the future.

Ninety percent of the nuclei brewed by the big bang were hydrogen, most of the rest were helium, and a trifling fraction were lithium: the makings of the simplest elements. Not until the ambient temperature in the expanding universe had cooled from trillions down to about 3,000 degrees Kelvin did the nuclei capture electrons. In so doing, they turned themselves into legal atoms and introduced the possibility of chemistry. As the universe continued to grow bigger and cooler, the atoms gathered into ever larger structures-gas clouds in which the earliest molecules, hydrogen (H2) and lithium hydride (LiH), a.s.sembled themselves from the earliest ingredients available in the universe. Those gas clouds sp.a.w.ned the first stars, whose ma.s.ses were each about a hundred times that of our Sun. And at the core of each star raged a thermonuclear furnace, h.e.l.l-bent on making chemical elements far heavier than the first and simplest three.

When those t.i.tanic first stars exhausted their fuel supplies, they blew themselves to smithereens and scattered their elemental entrails across the cosmos. Powered by the energy of their own explosions, they made yet heavier elements. Atom-rich clouds of gas, capable of ambitious chemistry, now gathered in s.p.a.ce.

Fast forward to galaxies, the princ.i.p.al organizers of visible matter in the universe-and within them, gas clouds pre-enriched by the flotsam of the earliest exploding stars. Soon those galaxies would host generation after generation of exploding stars, and generation after generation of chemical enrichment-the wellspring of those cryptic little boxes that make up the periodic table of elements.

Absent this epic drama, life on Earth-or anywhere else-would simply not exist. The chemistry of life, indeed the chemistry of anything at all, requires that elements make molecules. Problem is, molecules don't get made, and can't survive, in thermonuclear furnaces or stellar explosions. They need a cooler, calmer environment. So how in the world did the universe get to be the molecule-rich place we now inhabit?

RETURN, FOR A MOMENT, to the element factory deep within a first-generation high-ma.s.s star.

As we just saw, there in the core, at temperatures in excess of 10 million degrees, fast-moving hydrogen nuclei (single protons) randomly slam into one another. The event sp.a.w.ns a series of nuclear reactions that, at the end of the day, yield mostly helium and a lot of energy. So long as the star is "on," the energy released by its nuclear reactions generates enough outward pressure to keep the star's enormous ma.s.s from collapsing under its own weight. Eventually, though, the star simply runs out of hydrogen fuel. What remains is a ball of helium, which just sits there with nothing to do. Poor helium. It demands a tenfold increase in temperature before it will fuse into heavier elements.

Lacking an energy source, the core collapses and, in so doing, heats up. At about 100 million degrees, the particles speed up and the helium nuclei finally fuse, slamming together fast enough to combine into heavier elements. When they fuse, the reaction releases enough energy to halt further collapse-at least for a while. Fused helium nuclei spend a bit of time as intermediate products (beryllium, for instance), but eventually three helium nuclei end up becoming a single carbon nucleus. (Much later, when carbon becomes a complete atom with its complement of electrons in place, it reigns as the most chemically fruitful atom in the periodic table.) Meanwhile, back inside the star, fusion proceeds apace. Eventually the hot zone runs out of helium, leaving behind a ball of carbon surrounded by a sh.e.l.l of helium that is itself surrounded by the rest of the star. Now the core collapses again. When its temperature rises to about 600 million degrees, the carbon, too, starts slamming into its neighbors-fusing into heavier elements via more and more complex nuclear pathways, all the while giving off enough energy to stave off further collapse. The factory is now in full swing, making nitrogen, oxygen, sodium, magnesium, silicon.

Down the periodic table we go, until iron. The buck stops at iron, the final element to be fused in the core of first-generation stars. If you fuse iron, or anything heavier, the reaction absorbs energy instead of emitting it. But stars are in the business of making energy, so it's a bad day for a star when it finds itself staring at a ball of iron in its core. Without a source of energy to balance the inexorable force of its own gravity, the star's core swiftly collapses. Within seconds, the collapse and the attendant rapid rise in temperature trigger a monstrous explosion: a supernova. Now there's plenty of energy to make elements heavier than iron. In the explosion's aftermath, a vast cloud of all the elements inherited and manufactured by the star scatters into the stellar neighborhood. And consider the cloud's top ingredients: atoms of hydrogen, helium, oxygen, carbon, and nitrogen. Sound familiar? Except for helium, which is chemically inert, those elements are the main ingredients of life as we know it. Given the stunning variety of molecules those atoms can form, both with themselves and with others, they are also likely to be the ingredients of life as we don't don't know it. know it.

The universe is now ready, willing, and able to form the first molecules in s.p.a.ce and construct the next generation of stars.

IF GAS CLOUDS are to make enduring molecules, they must hold more than the right ingredients. They must also be cool. In clouds hotter than a few thousand degrees, the particles move too quickly-and so the atomic collisions are too energetic-to stick together and sustain molecules. Even if a couple of atoms manage to come together and make a molecule, another atom will shortly slam into them with enough energy to break them apart. The high temperatures and high-speed impacts that worked so well for fusion now work against chemistry. are to make enduring molecules, they must hold more than the right ingredients. They must also be cool. In clouds hotter than a few thousand degrees, the particles move too quickly-and so the atomic collisions are too energetic-to stick together and sustain molecules. Even if a couple of atoms manage to come together and make a molecule, another atom will shortly slam into them with enough energy to break them apart. The high temperatures and high-speed impacts that worked so well for fusion now work against chemistry.

Gas clouds can live long, happy lives as long as the turbulent motions of their inner pockets of gas hold them up. Occasionally, though, regions of a cloud slow down enough-and cool down enough-for gravity to win, causing the cloud to collapse. Indeed, the very process that forms molecules also serves to cool the cloud: when two atoms collide and stick, some of the energy that drove them together is captured in their newly formed bonds or emitted as radiation.

Cooling has a remarkable effect on a cloud's composition. Atoms now collide as if they were slow boats, sticking together and building molecules rather than destroying them. Because carbon readily binds with itself, carbon-based molecules can get large and complex. Some become physically entangled, like the dust that collects into dust bunnies under your bed. When the ingredients favor it, the same thing can happen with silicon-based molecules. In either case, each grain of dust becomes a happening place, studded with hospitable crevices and valleys where atoms can meet at their leisure and build even more molecules. The lower the temperature, the bigger and more complex the molecules can become.

AMONG THE EARLIEST and most common compounds to form-once the temperature drops below a few thousand degrees-are several familiar diatomic (two-atom) and triatomic (three-atom) molecules. Carbon monoxide (CO), for instance, stabilizes long before the carbon condenses into dust, and molecular hydrogen (H and most common compounds to form-once the temperature drops below a few thousand degrees-are several familiar diatomic (two-atom) and triatomic (three-atom) molecules. Carbon monoxide (CO), for instance, stabilizes long before the carbon condenses into dust, and molecular hydrogen (H2) becomes the prime const.i.tuent of cooling gas clouds, now sensibly called molecular clouds. Among the triatomic molecules that form next are water (H2O), carbon dioxide (CO2), hydrogen cyanide (HCN), hydrogen sulfide (H2S), and sulfur dioxide (SO2). There's also the highly reactive triatomic molecule H3+, which is eager to feed its third proton to hungry neighbors, instigating further chemical trysts.

As the cloud continues to cool, dropping below 100 degrees Kelvin or so, bigger molecules arise, some of which may be lying around in your garage or kitchen: acetylene (C2H2), ammonia (NH3), formaldehyde (H2CO), methane (CH4). In still cooler clouds you can find the chief ingredients of other important concoctions: antifreeze (made from ethylene glycol), liquor (ethyl alcohol), perfume (benzene), and sugar (glycoaldehyde), as well as formic acid, whose structure is similar to that of amino acids, the building blocks of proteins.

The current inventory of molecules drifting between the stars is heading toward 130. The largest and most structurally intricate of them are anthracene (C14H10) and pyrene (C16H10), discovered in 2003 in the Red Rectangle Nebula, about 2,300 light-years from Earth, by Adolf N. Witt of the University of Toledo in Ohio and his colleagues. Formed of interconnected, stable rings of carbon, anthracene and pyrene belong to a family of molecules that syllable-loving chemists call polycyclic aromatic hydrocarbons, or PAHs. And just as the most complex molecules in s.p.a.ce are based on carbon, so, of course, are we.

THE EXISTENCE OF MOLECULES in free s.p.a.ce, something now taken for granted, was largely unknown to astrophysicists before 1963-remarkably late, considering the state of other sciences. The DNA molecule had already been described. The atom bomb, the hydrogen bomb, and ballistic missiles had all been "perfected." The Apollo program to land men on the Moon was in progress. Eleven elements heavier than uranium had been created in the laboratory. in free s.p.a.ce, something now taken for granted, was largely unknown to astrophysicists before 1963-remarkably late, considering the state of other sciences. The DNA molecule had already been described. The atom bomb, the hydrogen bomb, and ballistic missiles had all been "perfected." The Apollo program to land men on the Moon was in progress. Eleven elements heavier than uranium had been created in the laboratory.

This astrophysical shortfall came about because an entire window of the electromagnetic spectrum-microwaves-hadn't yet been opened. Turns out, as we saw in Section 3, the light absorbed and emitted by molecules typically falls in the microwave part of the spectrum, and so not until microwave telescopes came online in the 1960s was the molecular complexity of the universe revealed in all its splendor. Soon the murky regions of the Milky Way were shown to be churning chemical factories. Hydroxyl (OH) was detected in 1963, ammonia in 1968, water in 1969, carbon monoxide in 1970, ethyl alcohol in 1975-all mixed together in a gaseous c.o.c.ktail in interstellar s.p.a.ce. By the mid-1970s, the microwave signatures of nearly forty molecules had been found.

Molecules have a definite structure, but the electron bonds that hold the atoms together are not rigid: they jiggle and wiggle and twist and stretch. As it happens, microwaves have just the right range of energies to stimulate this activity. (That's why microwave ovens work: a bath of microwaves, at just the right energy, vibrates the water molecules in your food. Friction among those dancing particles generates heat, cooking the food rapidly from within.) Just as with atoms, every species of molecule in s.p.a.ce identifies itself by the unique pattern of features in its spectrum. That pattern can readily be compared with patterns catalogued in laboratories here on Earth; without the lab data, often supplemented by theoretical calculations, we wouldn't know what we were looking at. The bigger the molecule, the more bonds have been deputized to keep it together, and the more ways its bonds can jiggle and wiggle. Each kind of jiggling and wiggling has a characteristic spectral wavelength, or "color"; some molecules usurp hundreds or even thousands of "colors" across the microwave spectrum, wavelengths at which they either absorb or emit light when their electrons take a stretch. And extracting one molecule's signature from the rest of the signatures is hard work, sort of like picking out the sound of your toddler's voice in a roomful of screaming children during playtime. It's hard, but you can do it. All you need is an acute awareness of the kinds of sounds your kid makes. Therein is your laboratory template.

ONCE FORMED, a molecule does not necessarily lead a stable life. In regions where ferociously hot stars are born, the starlight includes copious amounts of UV, ultraviolet light. UV is bad for molecules because its high energy breaks the bonds between a molecule's const.i.tuent atoms. That's why UV is bad for you, too: it's always best to avoid things that decompose the molecules of your flesh. So forget that a gigantic gas cloud may be cool enough for molecules to form within it; if the neighborhood is bathed in UV, the molecules in the cloud are toast. And the bigger the molecule, the less it can withstand such an a.s.sault.

Some interstellar clouds are so big and dense, though, that their outer layers can shield their inner layers. UV gets stopped at the edge of town by molecules that give their lives to protect their brethren deep within, thereby retaining the complex chemistry that cold clouds enjoy.

But eventually the molecular Mardi Gras comes to an end. As soon as the center of the gas cloud-or any other pocket of gas-gets dense enough and cool enough, the average energy of the moving gas particles gets too weak to keep the structure from collapsing under its own weight. That spontaneous gravitational shrinkage pumps the temperature back up, turning the erstwhile gas cloud into a locus of blazing heat as thermonuclear fusion gets underway.

Yet another star is born.

INEVITABLY, INESCAPABLY, one might even say tragically, the chemical bonds-including all the organic molecules the cloud so diligently made en route to stardom-now break apart in the searing heat. The more diffuse regions of the gas cloud, however, escape this fate. Then there's the gas close enough to the star to be affected by its growing force of gravity, but not so close as to be pulled into the star itself. Within that coc.o.o.n of dusty gas, thick disks of condensing material enter a safe orbit around the star. And within those disks, old molecules can survive and new ones can form with abandon.

What we have now is a solar system in the making, soon to comprise molecule-rich planets and molecule-rich comets. Once there's some solid material, the sky's the limit. Molecules can get as fat as they like. Set carbon loose under those conditions, and you might even get the most complex chemistry we know. How complex? It goes by another name: biology.

TWENTY-THREE.

GOLDILOCKS AND THE THREE PLANETS.

Once upon a time, some four billion years ago, the formation of the solar system was nearly complete. Venus had formed close enough to the Sun for the intense solar energy to vaporize what might have been its water supply. Mars formed far enough away for its water supply to be forever frozen. And there was only one planet, Earth, whose distance was "just right" for water to remain a liquid and whose surface would become a haven for life. This region around the Sun came to be known as the habitable zone.

Goldilocks (of fairy-tale fame) liked things "just right," too. One of the bowls of porridge in the Three Bears' cottage was too hot. Another was too cold. The third was just right, so she ate it. Also in the Three Bears's cottage, one bed was too hard. Another was too soft. The third was just right, so Goldilocks slept in it. When the Three Bears came home, they discovered not only missing porridge but also Goldilocks fast asleep in a bed. (I forget how the story ends, but if I were the Three Bears-omnivorous and at the top of the food chain-I would have eaten Goldilocks.) The relative habitability of Venus, Earth, and Mars would intrigue Goldilocks, but the actual story of these planets is somewhat more complicated than three bowls of porridge. Four billion years ago leftover water-rich comets and mineral-rich asteroids were still pelting the planetary surfaces, although at a much slower rate than before. During this game of cosmic billiards, some planets had migrated inward from where they had formed while others were kicked up to larger orbits. And among the dozens of planets that had formed, some were on unstable orbits and crashed into the Sun or Jupiter. Others were ejected from the solar system altogether. In the end, the few that remained had orbits that were "just right" to survive billions of years.

Earth settled into an orbit with an average distance of 93 million miles from the Sun. At this distance, Earth intersects a measly one two-billionth of the total energy radiated by the Sun. If you a.s.sume that Earth absorbs all incident energy from the Sun, then our home planet's average is about 280 degrees Kelvin (50 degrees F), which falls midway between winter and summer temperatures. At normal atmospheric pressures, water freezes at 273 degrees and boils at 373 degrees Kelvin, so we are well-positioned for nearly all of Earth's water to remain in a happy liquid state.

Not so fast. Sometimes in science you can get the right answer for the wrong reasons. Earth actually absorbs only two-thirds of the energy that reaches it from the Sun. The rest is reflected back into s.p.a.ce by Earth's surface (especially the oceans) and by the clouds. If reflectivity is factored into the equations, then the average temperature for Earth drops to about 255 degrees Kelvin, which is well below the freezing point of water. Something must be operating in modern times to raise our average temperature back to something a little more comfortable.

But wait once more. All theories of stellar evolution tell us that 4 billion years ago, when life was forming out of Earth's proverbial primordial soup, the Sun was a third less luminous than it is today, which would have placed Earth's average temperature even further below freezing.

Perhaps Earth in the distant past was simply closer to the Sun. But after the early period of heavy bombardment, no known mechanisms could have shifted stable orbits back and forth within the solar system. Perhaps the greenhouse effect was stronger in the past. We don't know for sure. What we do know is that habitable zones, as originally conceived, have only peripheral relevance to whether there may be life on a planet within them.

The famous Drake equation, invoked in the search for extraterrestrial intelligence, provides a simple estimate for the number of civilizations one might expect to find in the Milky Way galaxy. When the equation was conceived in the 1960s by the American astronomer Frank Drake, the concept of a habitable zone did not extend beyond the idea that there would be some planets at the "just right" distance from their host stars. A version of the Drake equation reads: Start with the number of stars in the galaxy (hundreds of billions). Multiply this large number by the fraction of stars with planets. Multiply what remains by the fraction of planets in the habitable zone. Multiply what remains by the fraction of those planets that evolved life. Multiply what remains by the fraction that have evolved intelligent life. Multiply what remains by the fraction that might have developed a technology with which to communicate across interstellar s.p.a.ce. Finally, when you introduce a star formation rate and the expected lifetime of a technologically viable civilization you get the number of advanced civilizations that are out there now, possibly waiting for our phone call.

Small, cool, low-luminosity stars live for hundreds of billions and even possibly trillions of years, which ought to allow plenty of time for the planets around them to evolve a life-form or two, but their habitable zones fall very close to the host star. A planet that forms there will swiftly become tidally locked and always show the same face toward the star (just as the Moon always shows the same face to Earth) creating an extreme imbalance in planetary heating-all water on the planet's "near" side would evaporate while all water on the planet's "far" side would freeze. If Goldilocks lived there, we would find her eating oatmeal while turning in circles (like a rotisserie chicken) right on the border between eternal sunlight and eternal darkness. Another problem with the habitable zones around these long-lived stars is that they are extremely narrow; a planet in a random orbit is unlikely to find itself at a distance that is "just right."

Conversely, large, hot, luminous stars have enormous habitable zones in which to find their planets. Unfortunately these stars are rare, and live for only a few million years before they violently explode, so their planets make poor candidates in the search for life as we know it-unless, of course, some rapid evolution occurred. But animals that can do advanced calculus were probably not the first things to slither out of the primordial slime.

We might think of the Drake equation as Goldilocks mathematics-a method for exploring the chances of getting things just right. But the Drake equation as originally conceived misses Mars, which lies well beyond the habitable zone of the Sun. Mars displays countless meandering dry riverbeds, deltas, and floodplains, which const.i.tute in-your-face evidence for running water in the Martian past.

How about Venus, Earth's "sister" planet? It falls smack dab within the Sun's habitable zone. Covered completely by a thick canopy of clouds, the planet has the highest reflectivity of any planet in the solar system. There is no obvious reason why Venus could not have been a comfortable place. But it happens to suffer from a monstrous greenhouse effect. Venus's thick atmosphere of carbon dioxide traps nearly 100 percent of the small quant.i.ties of radiation that reach its surface. At 750 degrees Kelvin (900F) Venus is the hottest planet in the solar system, yet it orbits at nearly twice Mercury's distance from the Sun.

If Earth has sustained the continuous evolution of life through billions of years of storm and drama, then perhaps life itself provides a feedback mechanism that maintains liquid water. This notion was advanced by the biologists James Lovelock and Lynn Margulis in the 1970s and is referred to as the Gaia hypothesis. This influential, yet controversial idea requires that the mixture of species on Earth at any moment acts as a collective organism that continuously (yet unwittingly) tunes Earth's atmospheric composition and climate to promote the presence of life-and by implication, the presence of liquid water. I am intrigued by the idea. It has even become the darling of the New Age movement. But I'd bet there are some dead Martians and Venusians who advanced the same theory about their own planets a billion years ago.