Death by Black Hole: And Other Cosmic Quandaries - Neil deGrasse Tyson (2014)



Chapter 20. DUST TO DUST

Acasual look at the Milky Way with the unaided eye reveals a cloudy band of light and dark splotches extending from horizon to horizon. With the help of simple binoculars or a backyard telescope, the dark and boring areas of the Milky Way resolve into, well, dark and boring areas—but the bright areas resolve into countless stars and nebulae.

In a small book entitled Sidereus Nuncius (The Starry Messenger), published in Venice in 1610, Galileo gives an account of the heavens as seen through a telescope, including the first-ever description of the Milky Way’s patches of light. Referring to his yet-to-be-named instrument as a “spyglass,” he is so excited he can 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. (Van Helden 1989, p. 62)

Surely “immense number of stars” is where the action is. Why would anybody be interested in the dark areas where stars are absent? They are probably cosmic holes to the infinite and empty beyond.

Three centuries would pass before anybody figured out that the dark patches are thick, dense clouds of gas and dust, which obscure the more distant star fields and hold stellar nurseries deep within. Following earlier suppositions of the American astronomer George Cary Comstock, who wondered why faraway stars were much dimmer than their distance alone would indicate, it was not until 1909 when the Dutch astronomer Jacobus Cornelius Kapteyn (1851–1922) would name the culprit. In two research papers, both titled “On the Absorption of Light in Space,” Kapteyn presented evidence that clouds, his newfound “interstellar medium,” not only scatter the overall light of stars but do so unevenly across the rainbow of colors in a star’s spectrum, attenuating the blue light more severely than the red. This selective absorption makes the Milky Way’s faraway stars look, on average, redder than the near ones.

Ordinary hydrogen and helium, the principal constituents of cosmic gas clouds, don’t redden light. But larger molecules do—especially those that contain the elements carbon and silicon. And when the molecules get too big to be called molecules, we call them dust.

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 nobody’s epidermis. But it does have a remarkable ensemble of complex molecules that emit principally 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 class. 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 orbit the center of the galaxy. The stars are tiny specks (a few light-seconds across) in a vast ocean of permeable space, and they pass one another like ships in the night. Gas clouds, on the other hand, are huge. Typically spanning hundreds of light-years, they contain the mass 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 constituent atoms will bump 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, massive 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 sextillion 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 assert 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 mass is less than the sum of its parts. The lost mass has been converted into boatloads of energy, as described by Einstein’s famous equation E= mc2, where E is energy, m is mass, and c is the speed of light. As the heat moves outward, the gas becomes luminous, and the energy that had formerly been mass 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 masses: 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 multitude of cold pockets, all of which form at about the same time and each of which gives birth to a star. For every high-mass star born, there are a thousand low-mass stars. But only about 1 percent of all the gas in the original cloud participates in star birth, and that presents a classic challenge: figuring out how and why the tail wags the dog.

THE MASS LIMIT on the low end is easy to determine. Below about one-tenth of the Sun’s mass, 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 classify them into more than one category. The mass limit at the high end is also easy to determine. Above about a hundred times that of the Sun’s mass, the star is so luminous that any additional mass 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-mass stars can disperse nearly all the mass 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 cluster. Four of the several massive 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 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 passing clouds, the cluster ultimately falls apart, its members scattering into the general pool of stars in the galaxy. The low-mass stars live practically forever, so efficient is their consumption of fuel. The intermediate-mass 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 space, exposing the spent nuclear fuels that powered their 10-billion-year lives. The gas that returns to space gets swept up by passing clouds, only to participate in later rounds of the formation of stars.

In spite of the rarity of the highest-mass 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-mass 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-mass stars, another star is born.