Origins: Fourteen Billion Years of Cosmic Evolution (12 page)

Classifying AGNs solely on the basis of their visual appearance alone would provide an incomplete story, so astrophysicists classified AGNs by their spectra and by the full range of their electromagnetic emissions. During the mid- to late 1990s, investigators improved their black hole model, and found that they could characterize nearly all the beasts in the AGN zoo by measuring only a few parameters: the mass of the object’s black hole, the rate at which it’s being fed, and our angle of view on the accretion disk and its jets. If, for example, we happen to look “right down the barrel,” along exactly the same direction as that of a jet emerging from the vicinity of a supermassive black hole, we see a much brighter object than if we happen to have a side view from a much different angle. Variations in these three parameters can account for nearly all the impressive diversity that astrophysicists observe, giving them a welcome de-speciation of galaxy types and a deeper understanding of the formation and evolution of galaxies. The fact that so much can be accounted for—differences in shape, size, luminosity, and color—by so few variables represents an unheralded triumph of late twentieth-century astrophysics. Because it took a lot of investigators and a lot of years and a lot of telescope time, it’s not the sort of thing that gets announced on the evening news—but it’s a triumph nonetheless.

Let us not
conclude, however, that supermassive black holes can explain everything. Even though they have millions or billions of times the Sun’s mass, they contribute almost nothing in comparison with the masses of the galaxies in which they are embedded—typically far less than 1 percent of a large galaxy’s total mass. When we seek to account for the existence of dark matter, or of other unseen sources of gravity in the universe, these black holes are insignificant and may be ignored. But when we calculate how much energy they wield—that is, when we compute the energy that they released as part of their formation—we find that black holes dominate the energetics of galaxy formation. All the energy of all the orbits of all the stars and gas clouds that ultimately compose a galaxy pales when compared with what made the black hole. Without supermassive black holes lurking below, galaxies as we know them might have never formed. The once luminous but now invisible black hole that lies at the center of each giant galaxy provides a hidden link, the physical explanation for the agglomeration of matter into a complex system of billions of stars in orbit around a common center.

The broader explanation for the formation of galaxies invokes not only the gravity produced by supermassive black holes but also gravity in more conventional astronomical settings. What made the billions of stars in a galaxy? Gravity did this too, producing up to hundreds of thousands of stars in a single cloud. Most of a galaxy’s stars were born within relatively loose “associations.” The more compact regions of starbirth remain identifiable “star clusters,” within which member stars orbit the cluster’s center, tracing their paths through space in a cosmic ballet choreographed by the forces of gravity from all the other stars within the cluster, even as the clusters themselves move on enormous trajectories around the galactic center, safe from the destructive power of the central black hole.

Within a cluster, stars move at a broad range of speeds, some so rapidly that they risk escape from the system altogether. This indeed occasionally occurs, as fast stars evaporate from the grip of a cluster’s gravity to roam freely through the galaxy. These free-ranging stars, along with the “globular star clusters” that contain hundreds of thousands of stars each, add to the stars that form the spherical haloes of galaxies. Initially luminous, but today devoid of their brightest, short-lived stars, galaxy haloes are the oldest visible objects in the universe, with birth certificates traceable to the formation of galaxies themselves.

Last to collapse, and thus the last to turn into stars, we encounter the gas and dust that finds itself pulled and pinned into the galactic plane. In elliptical galaxies, no such plane exists, and all of their gas has already turned into stars. Spiral galaxies, however, have highly flattened distributions of matter, characterized by a central plane within which the youngest, brightest stars form in spiral patterns, testimony to great vibrating waves of alternating dense and rarefied gas that orbit the galactic center. Like hot marshmallows that stick together upon contact, all of the gas in a spiral galaxy that did not swiftly participate in making star clusters has fallen toward the galactic plane, stuck to itself, and created a disk of matter that slowly manufactures stars. For past billions of years, and for billions of years to come, stars will continue to form in spiral galaxies, with each generation more enriched in heavy elements than the next. These heavy elements (by which astrophysicists mean all elements heavier than helium) have been cast forth into interstellar space by outflows from aging stars or as the explosive remains of high-mass stars, a species of supernova. Their existence renders the galaxy—and thus the universe—ever more friendly to the chemistry of life as we know it.

We have outlined
the birth of a classical spiral galaxy, in an evolutionary sequence that has played out tens of billions of times, yielding galaxies in a host of different arrangements: In clusters of galaxies. In long strings and filaments of galaxies. And in sheets of galaxies.

Because we look back in time as we look outward into space, we possess the ability to examine galaxies not only as they are now but also as they appeared billions of years ago, simply by looking up. The problem with turning this concept into observational reality resides in the fact that galaxies billions of light-years away appear to us as extremely small and dim objects, so even our best telescopes can barely resolve their outlines. Nevertheless, astrophysicists have made great progress in this effort during the past few years. The breakthrough came in 1995, when Robert Williams, then the director of the Space Telescope Science Institute at Johns Hopkins University, arranged for the Hubble Telescope to point toward a single direction in space, near the Big Dipper, for ten days’ worth of observation. Williams deserves the credit because the telescope’s Time Allocation Committee, which selects the observing proposals most worthy of actual telescope time, judged it unworthy of support. After all, the region to be studied was deliberately chosen for having nothing interesting to look at, and thus to represent a dull and boring patch of sky. As a result, no ongoing projects could benefit directly from such a large commitment of the telescope’s highly oversubscribed observing time. Happily, Williams, as the director of the Space Telescope Science Institute, had the right to assign a few percent of the total—his “director’s discretionary time”—and invested his clout on what became known as the Hubble Deep Field, one of the most famous astronomical photographs ever taken.

The ten-day exposure, coincidentally made during the government shutdown of 1995, produced by far the most researched image in the history of astronomy. Studded with galaxies and galaxylike objects, the deep field offers us a cosmic palimpsest, in which objects at different distances from the Milky Way have written their momentary signatures of light at different times. We see objects in the deep field as they were, say, 1.3 billion, 3.6 billion, 5.7 billion, or 8.2 billion years ago, with each object’s epoch determined by its distance from us. Hundreds of astronomers have seized upon the wealth of data contained in this single image to derive new information about how galaxies have evolved with time, and about how galaxies looked soon after they formed. In 1998, the telescope secured a companion image, the Hubble Deep Field South, by devoting ten days of observation to another patch of sky in the direction opposite to that of the first deep field, in the celestial southern hemisphere. Comparison of the two images allowed astronomers to assure themselves that the results from the first deep field did not represent an anomaly (for example, if the two images had been identical in every detail, or statistically unlike each other in every way, one might have concluded that the devil was at work), and to refine their conclusions about how different types of galaxies form. After a successful servicing mission, in which the Hubble Telescope was outfitted with even better (more sensitive) detectors, the Space Telescope Science Institute just couldn’t resist and, in 2004, authorized the Hubble Ultra Deep Field, laying bare the ever more distant cosmos.

Unfortunately, the earliest stages of galaxy formation, which would be revealed to us by objects at the greatest distances, confound even the Hubble Telescope’s best efforts, not least because the cosmic expansion has shifted most of their radiation into the infrared region of the spectrum, not accessible to the telescope’s instruments. For these most distant galaxies, astronomers await the design, construction, launch, and successful operation of the Hubble’s successor, the James Webb Space Telescope (JWST), named after the head of NASA during the Apollo era. (Cynics say that this name, rather than one that honors a famous scientist, was chosen to assure that the telescope project will not be canceled, since this would involve deleting an important official’s legacy.)

The JWST will have a mirror larger than Hubble’s, designed to unfurl itself like an intricate mechanical flower, opening in space to provide a reflective surface much larger than any that can fit inside one of our rockets. The new space telescope will also possess a suite of instruments far superior to those of the Hubble Telescope, which were originally designed during the 1960s, built during the 1970s, launched in 1991, and—even though significantly upgraded during the 1990s—still lack such fundamental abilities as the capacity to detect infrared radiation. Some of this ability now exists in the Spitzer InfraRed Telescope Facility (SIRTF), launched in 2003, which orbits the Sun much farther from Earth than the Hubble does, thereby avoiding interference from the copious amounts of infrared radiation produced by our planet. To achieve this goal, JWST will likewise have an orbit much farther from Earth than the Hubble Telescope does, and will therefore be forever inaccessible to servicing missions as they are currently conceived—NASA had better get this one right the first time. If the new telescope goes into operation in 2011, as currently planned, it should then provide spectacular new views of the cosmos, including images of galaxies more than 10 billion light-years away, seen much closer to their time of origin than any revealed by the Hubble Deep Fields. Working in tandem with the new space telescope, as they have with the old, large ground-based instruments will study in detail the wealth of objects to be revealed by our next great step in space-borne instrumentation.

Rich in possibility
though the future may be, we should not neglect the astrophysicists’ impressive accomplishments during the past three decades, which spring from their abilities to create new instruments to observe the universe. Carl Sagan liked to say that you had to be made from wood not to stand in awe of what the cosmos has done. Thanks to our improved observations, we now know more than Sagan did about the amazing sequence of events that led to our existence: the quantum fluctuations in the distribution of matter and energy on a scale smaller than the size of a proton that spawned superclusters of galaxies, thirty million light-years across. From chaos to cosmos, this cause-and-effect relationship crosses more than thirty-eight powers of ten in size and forty-two powers of ten in time. Like the microscopic strands of DNA that predetermine the identity of a macroscopic species and the unique properties of its members, the modern look and feel of the cosmos was writ in the fabric of its earliest moments, and carried relentlessly through time and space. We feel it when we look up. We feel it when we look down. We feel it when we look within.

Part III

The Origin
of Stars

CHAPTER 9

Dust to Dust

I
f you look at the clear night sky far from city lights, you can immediately locate a cloudy band of pale light, broken in places by dark splotches, that runs from horizon to horizon. Long known as the (lower-case) “milky way” in the sky, this milk-white haze combines the light from a staggering number of stars and gaseous nebulae. Those who observe the milky way with binoculars or a backyard telescope will see the dark and boring areas resolve themselves into, well, dark and boring areas—but the bright areas will turn from a diffuse glow into countless stars and nebulae.

In his small book
Sidereus Nuncius
(The Starry Messenger)
, published in Venice in 1610, Galileo Galilei provided the first account of the heavens as seen through a telescope, including a description of the milky way’s patches of light. Referring to his instrument as a spyglass, since the name telescope (“far-seer” in Greek) had yet to be coined, Galileo could barely contain himself:

The milky way itself, which, with the aid of the spyglass, may be observed so well that all the disputes that for so many generations have vexed philosophers are destroyed by visible certainty, and we are liberated from wordy arguments. For the Galaxy is nothing else than a congeries of innumerable stars distributed in clusters. To whatever region of it you direct your spyglass, an immense number of stars immediately offer themselves to view, of which very many appear rather large and very conspicuous but the multitude of small ones is truly unfathomable.
*

Surely Galileo’s “immense number of stars,” which delineate the most densely packed regions of our Milky Way galaxy, must locate the real astronomical action. Why, then, should anybody be interested in the intervening dark areas with no visible stars? Based on their visual appearance, the dark areas are probably cosmic holes, openings to the infinite and empty spaces beyond.

Three centuries would pass before anyone figured out that the dark patches in the milky way, far from being holes, actually consist of dense clouds of gas and dust that obscure more distant star fields and hold stellar nurseries deep within themselves. Following earlier suggestions by the American astronomer George Cary Comstock, who wondered why faraway stars are much dimmer than their distances alone would indicate, the Dutch astronomer Jacobus Cornelius Kapteyn in 1909 identified the culprit. In two research papers, both titled “On the Absorption of Light in Space,”
†
Kapteyn presented evidence that the dark clouds—his newfound “interstellar medium”—not only block the light from stars but also do so unevenly across the rainbow of colors in a star’s spectrum: they absorb and scatter, and therefore attenuate, light at the violet end of the visible spectrum more effectively than they act on red light. This selective absorption preferentially removes more violet than red light, making faraway stars appear redder than nearby ones. The amount of this interstellar reddening of starlight increases in proportion to the total amount of material that the light encounters on its journey to us.

Ordinary hydrogen and helium, the principal constituents of cosmic gas clouds, don’t redden light. But molecules made of many atoms do so—especially those that contain the elements carbon and silicon. When interstellar particles grow too large to be called molecules, with hundreds of thousand or millions of individual atoms in each of them, we call them dust. Most of us know dust of the household variety, although few of us care to learn that, in a closed home, dust consists mostly of dead, sloughed-off human skin cells (plus pet dander, if you have one or more live-in mammals). As far as we know, cosmic dust contains nobody’s epidermis. However, interstellar dust does include a remarkable ensemble of complex molecules, which emit photons primarily in the infrared and microwave regions of the spectrum. Astrophysicists lacked good microwave telescopes until the 1960s, and effective infrared telescopes until the 1970s. Once they had created these observational instruments, they could investigate the true chemical richness of the stuff that lies between the stars. During the decades that followed these technological advances, a fascinating, intricate picture of star birth emerged.

Not all gas clouds will form stars at all times. More often than not, a cloud finds itself confused about what to do next. Actually, astrophysicists are the confused ones here. We know that an interstellar cloud “wants” to collapse under its own gravity to make one or more stars. But the cloud’s rotation, as well as the effects of turbulent gas motions within the cloud, oppose that result. So, too, does the gas pressure that you learned about in high school chemistry class. Magnetic fields can also fight collapse. They penetrate the cloud and constrain the motions of any free-roaming charged particles contained therein, resisting compression and thus impeding the ways in which the cloud can respond to its own gravity. The scary part of this thought-exercise comes from the realization that if no one knew in advance that stars exist, front-line research would offer plenty of convincing reasons why stars could never form.

Like the several hundred billion stars in our Milky Way galaxy, named after the band of light that the galaxy’s most densely populated regions paint across our skies, giant clouds of gas orbit our galaxy’s center. The stars amount to tiny specks, only a few light-seconds across, that float in a vast ocean of nearly empty space, occasionally passing close by one another like ships in the night. Gas clouds, on the other hand, are huge. Typically spanning hundreds of light-years, they each contain as much mass as a million Suns. As these giant clouds lumber through the galaxy, they often collide with one another, entangling their gas- and dust-laden innards. Sometimes, depending on their relative speeds and their angles of impact, the clouds stick together; at other times, adding injury to the insult of collision, they rip each other apart.

If a cloud cools to a sufficiently low temperature (less than about 100 degrees above absolute zero), its constituent atoms will stick together when they collide, rather than careening off one another as they do at higher temperatures. This chemical transition has consequences for everybody. The growing particles—now containing tens of atoms each—begin to scatter visible light to and fro, strongly attenuating the light of the stars behind the cloud. By the time that the particles become full-grown dust grains, they each contain billions of atoms. Aging stars manufacture similar dust grains and blow them gently into interstellar space during their “red-giant” phases. Unlike smaller particles, dust grains with billions of atoms no longer scatter the visible light photons from the stars behind them; instead, they absorb those photons and then reradiate their energy as infrared, which can easily escape from the cloud. As this occurs, the pressure from the photons, transmitted to the molecules that absorb it, pushes the cloud in the direction opposite to the direction of the light source. The cloud has now coupled itself to starlight.

Star birth occurs when the forces that make a cloud progressively denser eventually lead to its gravitationally induced collapse, during which each part of the cloud pulls all the other parts much closer. Since hot gas resists compression and collapse more effectively than cool gas does, we face an odd situation. We must cool the cloud before it can ever heat itself by producing a star. In other words, the creation of a star that possesses a 10-million-degree core, sufficiently hot for thermonuclear fusion to begin, requires that the cloud must first achieve its coldest possible internal conditions. Only at extremely cold temperatures, a few dozen degrees above absolute zero, can the cloud collapse and allow star formation to begin in earnest.

What happens within a cloud to turn its collapse into newborn stars? Astrophysicists can only gesticulate. Much as they would like to track the internal dynamics of a large, massive interstellar cloud, the creation of a computer model that includes the laws of physics, all the internal and external influences on the cloud, and all the relevant chemical reactions that can occur within it still lies beyond our abilities. A further challenge resides in the humbling fact that the original cloud has a size billions of times larger than that of the star we are trying to create—which in turn has a density 100 sextillion times the average density within in the cloud. In these situations, what matters most on one scale of sizes may not be the right thing to worry about on another.

Nevertheless, relying on what we see throughout the cosmos, we can safely assert that within the deepest, darkest, densest regions of an interstellar cloud, where temperatures fall to about 10 degrees above absolute zero, gravity does cause pockets of gas to collapse, easily overcoming the resistance offered by magnetic fields and other impediments. The contraction converts the cloud pockets’ gravitational energy into heat. The temperature within each of these regions—soon to become the core of a newborn star—rises rapidly during the collapse, breaking apart all the dust grains in the immediate vicinity as they collide. Eventually, the temperature in the central region of the collapsing gas pocket reaches the crucial value of 10 million degrees on the absolute scale.

At this magic temperature, some of the protons (which are simply naked hydrogen atoms, shorn of the electron that orbits them) move fast enough to overcome their mutual repulsion. Their high speeds allow the protons to approach one another closely enough for the “strong nuclear force” to make them bond. This force, which operates only at extremely short distances, binds together the protons and neutrons in all nuclei. The thermonuclear fusion of protons—“thermo” because it occurs at high temperatures, and “nuclear fusion” because it fuses particles into a single nucleus—creates helium nuclei, each of which has a mass slightly less than the sum of the particles from which it fused. The mass that disappears during this fusion turns into energy, in a balance described by Einstein’s famous equation. The energy embodied in mass (always in an amount equal to the mass times the square of the speed of light) can be converted into other forms of energy, such as additional kinetic energy (energy of motion) of the fast-moving particles that emerge from nuclear fusion reactions.

As the new energy produced by nuclear fusion diffuses outward, the gas heats and glows. Then, at the star’s surface, the energy formerly locked in individual nuclei escapes into space in the form of photons, generated by the gas as the energy released through fusion heats it to thousands of degrees. Even though this region of hot gas still resides within the cosmic womb of a giant interstellar cloud, we may nonetheless announce to the Milky Way that . . . a star is born.

Astronomers know that stars range in mass from a mere one tenth of the Sun’s to nearly one hundred times our star’s mass. For reasons not well understood, a typical giant gas cloud can develop a multitude of cold pockets that all tend to collapse at about the same time to give birth to stars—some puny and others giants. But the odds favor the puny: for every high-mass star, a thousand low-mass stars are born. The fact that no more than a few percent of all the gas in the original cloud participates in star birth presents a classic challenge in explaining star formation: What makes the star-forming tail wag the largely unchanged dog of an interstellar gas cloud? The answer probably lies in the radiation produced by newborn stars, which tends to inhibit further star formation.

We can easily explain the lower bound on the masses of newborn stars. Pockets of collapsing gas with masses less than about one tenth of the Sun’s have too little gravitational energy to raise their core temperatures to the 10 million degrees required for the nuclear fusion of hydrogen. In that case, no nuclear-fusing star will be born; instead, we obtain a failed, would-be star—an object that astronomers call a “brown dwarf.” With no energy source of its own, a brown dwarf fades steadily, shining from the modest heat generated during the original collapse. The gaseous outer 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 them. Their feeble luminosities make brown dwarfs immensely difficult to detect, so to find them, astrophysicists must employ complex methods similar to those they occasionally use to detect planets: searching for the faint infrared glow from these objects. Only in recent years have astronomers discovered brown dwarfs in numbers sufficient to classify them into more than one category.

We can also easily determine the upper mass limit to star formation. A star with a mass greater than about a hundred times the Sun’s will have a luminosity so great—such an enormous outpouring of energy in the form of visible light, infrared, and ultraviolet—that any additional gas and dust attracted toward the star will be pushed away by the intense pressure of starlight. The star’s photons push on the dust grains within the cloud, which in turn carry the gas away with them. Here starlight couples irreversibly to dust. This radiation pressure operates so effectively that just a few high-mass stars within a dark, obscuring cloud will have luminosities sufficient to disperse nearly all its interstellar matter, laying bare to the universe dozens, if not hundreds, of brand-new stars—all siblings, really—for the rest of the galaxy to see.

Whenever you gaze
at the Orion nebula, located just below the three bright stars of Orion’s Belt, midway along the Hunter’s somewhat fainter sword, you can see a stellar nursery of just this sort. Thousands of stars have been born within this nebula, while thousands more await their birth, soon to create a giant star cluster that becomes more and more visible to the cosmos as the nebula dissipates. The most massive new stars, forming a group called the Orion Trapezium, are busy blowing a giant hole in the middle of the cloud from which they formed. Hubble Telescope images of this region reveal hundreds of new stars in this zone alone, each infant swaddled within a nascent protoplanetary disk made of dust and other molecules drawn from the original cloud. And within each of these disks, a planetary system is forming.