Twentieth-century predictions that black holes might exist naturally raised the question of how such superdense objects could form. Over time, scientists came to realize that there might be more than one answer to this question, depending on the size of the black hole. Ever since the days of Michell and Laplace, astronomers and physicists had focused their attention on star-sized objects with extreme gravity. So the quest to understand how such bodies form concentrated on the life cycles of and physical processes within stars.
However, the mathematical equations of Einstein's general theory of relativity allow for the existence of black holes of any size, including very small ones. After John Wheeler coined the term black hole in 1967, a number of scientists began theorizing about miniature black holes. A mini–black hole might be the size of an atom. Yet its matter would be so densely compacted that it would weigh something like 100 trillion tons! An even tinier black hole—say the size of an atom's nucleus—would still tip the scales at about a billion tons.
Such mini–black holes would have no obvious connection with star-sized, or stellar, black holes. So the formation of the smaller version likely has nothing to do with the life and death of stars. What force or process, then, could have created mini–black holes? In the early 1970s, noted British physicist Stephen Hawking offered a believable answer, namely that these tiny superdense objects came into being during the Big Bang—the enormous explosion in which, most scientists believe, the known universe was created. "With vast quantities of matter exploding all over the place," Isaac Asimov explains,
some different sections of the expanding substance [i.e., matter] might collide. Part of this colliding matter might then be squeezed together under enormous pressure from all sides. The squeezed matter might shrink to a point where the mounting gravitational intensity would keep it shrunk forever. 14
Hawking and others think that millions of such mini–black holes might still exist in various parts of the universe. If so, sooner or later a few might come close to an asteroid, planet, or other large solid body and be drawn to it by its gravity. Such a cosmic meeting would probably be neither dangerous nor catastrophic, however. According to Asimov:
If a mini–black hole collides with a larger body, it will simply bore its way through. It will engulf the first bit of matter with which it collides, liberating enough energy in the process to melt and vaporize the matter immediately ahead. It will then pass through the hot vapor, absorbing it as it goes and adding to the heat, emerging at last as a considerably larger black hole than it was when it entered. 15
The channel cut through a planet by a mini–black hole would be so small and narrow that it would be far less noticeable or consequential than a tunnel dug through a garden by an ant. If mini–black holes do exist, therefore, their physical effects on large bodies are minimal and they are of little concern to human beings and any other living beings inhabiting the universe.
By contrast, black holes of stellar and larger masses have far more important potential consequences for the universe and life. And this is why scientists have devoted so much time and effort in recent years to understanding how they form, as well as their properties. They realized that it would require a tremendously violent process or event to compress such gigantic quantities of matter into an extremely small space. Moreover, the energy produced would have to be millions of times larger than in those events people normally deem catastrophic—including earthquakes, volcanic eruptions, and the crash of asteroids onto planetary surfaces. It became clear to scientists that only the phenomenally violent death of a large star could account for the creation of a stellar black hole.
Understanding how stars die and create superdense bodies requires some basic knowledge of how stars live. In the same way that people undergo an inevitable life cycle, stars are born, live out their lives, and finally die. The nursery of a typical star, including one like the Sun, is an extremely large cloud of gases and dust floating through space. Such clouds come into being when "winds" created by exploding stars blow scattered molecules of gas and particles of dust around; some become even more scattered, while others become more concentrated. When such a cloud becomes concentrated enough, gravity causes it to contract still further over time. This contraction also produces heat, which makes the gases and dust grow steadily hotter. Soon, the center of the cloud becomes hot enough to cook a steak; then it reaches the temperature of a blast furnace; and finally, after a few million years, the temperature at the cloud's core becomes hot enough to fuse hydrogen atoms and thereby ignite nuclear reactions. At that instant, the core emits a huge burst of blinding light and other energy that blows away the cooler outer layers of the cloud, leaving behind a giant ball of white-hot gases—a newborn star.
The new star has enough hydrogen in its interior to keep its self-sustaining nuclear reactions going for billions of years. And throughout this longest portion of its life cycle, it continues producing light and heat. If certain other factors in the star's solar system are favorable—such as the formation of a planet at the right distance from the star and the presence of water—this abundant light and heat makes the rise of life possible in that solar system. No significant danger is posed to such life as long as the star remains stable.
The reason a typical star can remain stable for so long is that two enormous forces occurring within the body of the object oppose each other, creating an equilibrium, or balance. One of these forces is gravity, which makes the massive quantities of matter in the star's outer layers fall inward, creating great pressure. The Sun "contains a thousand times more mass than Jupiter," Begelman and Rees point out. If the Sun were a cold body, "gravity would compress it to a million times the density of an ordinary solid. It would be … about the same size as the Earth, but 330,000 times more massive." 16
But as everyone can easily see and feel, the Sun is not a cold body. Stars like the Sun produce enormous amounts of energy, accounting for the second major force at work within them. The nuclear reactions taking place in a star's core release immense amounts of heat, light, and tiny particles that travel outward toward the surface. In the Sun, for example, each and every second the core produces the same amount of energy as 100 million nuclear bombs exploding simultaneously. As this terrific stream of energy moves outward from the core, it exerts a huge amount of outward pressure. And that pressure balances the force of gravity pushing inward. The Sun's center, Begelman and Rees summarize,
has a temperature of about 15 million degrees … thousands of times hotter even than its glowing surface. At these high temperatures, the atomic nuclei inside the sun are moving randomly at speeds of hundreds of kilometers per second. It is the pressure of this hot interior … that counteracts the [internal] effect of gravity in all stars like the sun. 17
Thanks to this balance between outward and inward pressures, stars like the Sun maintain their structures and remain stable for long periods of time. Astronomers estimate that the Sun, which has been in this stable state for several billion years, will remain in it for several billion years to come. Like an animal or a person, however, the great luminous ball cannot live forever. Eventually, a star must use up all of its fuel and enter its death throes, producing a catastrophe in which most of its matter is forced into an extremely dense state. This state can take one of three different forms, depending on the star's initial mass; in each case, a superdense object is created. Two of these objects—a white dwarf and a neutron star—are in a sense immediate precursors of and steps on the road to the black hole. The third is the black hole itself.
The Sun is destined to be transformed into the first of the three superdense bodies created when a star dies, a white dwarf. Billions of years from now, our star will begin to run out of the hydrogen that fuels the nuclear reactions in its core. When most of the hydrogen is gone, the core will get both denser and hotter. This extra heat will cause the Sun's outer layers to expand outward, transforming it into an enormous star hundreds of times bigger than it is now; in fact, its surface will engulf the orbits of Mercury and Venus, destroying those planets, and the surface of Earth (which will then be the innermost planet in the solar system) will be scorched as if in a blast furnace. However, because the Sun's new surface will be stretched and more spread out than before, any given portion of it will be a bit cooler. So the star's color will change from a hot yellow to a cooler red. For this reason such expanded, cooler stars are called red giants.
Eventually, the core of the red giant Sun will completely run out of hydrogen, at which point it will start burning the next heaviest element, helium. Of course, the helium will soon get used up, too, and in time the star will no longer burn fuel to produce nuclear reactions. At that point, the delicate balance that keeps the star stable will be undermined. The outward pressure of escaping energy will decrease, allowing it to be overcome by the inward pressure of
Once formed by stellar collapse, solitary white dwarfs slowly cool and fade from view. If they are part of binary (double-star) systems, however, white dwarfs can periodically produce explosions called novas, as explained here by University of Amherst astronomer Thomas T. Arny in his noted astronomy text Explorations .
If a white dwarf has a nearby companion, gas expelled from the companion may fall onto the dwarf…. Coming from the companion's outer layers, such gas is rich in hydrogen and may briefly replenish the white dwarf's fuel supply. The new fuel forms a layer on the white dwarf's surface, where gravity compresses and heats it. The gas layer eventually reaches the ignition temperature for hydrogen, but … nuclear burning in a degenerate gas can be explosive. The detonating hydrogen is blasted into space and forms an expanding shell of hot gas … that radiates far more energy than the white dwarf itself. Sometimes these stellar explosions are visible to the naked eye. When earlier astronomers saw such events, they called them novas, from the Latin word for "new," because the explosion would make a bright point of light appear in the sky where no star was previously visible.
gravity. "Gravitation has been waiting," Asimov writes, "pulling patiently and tirelessly for many billions of years, and finally resistance to that pull has collapsed." 18
As gravity takes over, the red giant Sun will start to shrink. Some of its outer material will escape into space. But most will remain inside the shrinking star, which, compelled by gravity's mighty hand, will finally crush this matter into a white dwarf—a small, hot, but only dimly luminous ball about the size of Earth. A white dwarf is so dense that a mere tablespoon of its material weighs a thousand tons. Thus it comes as no surprise that such a body possesses a very deep gravity well and therefore a powerful gravitational pull. To escape a white dwarf, a spaceship would need to reach a speed of about three thousand miles per second! (Of course, it would be foolhardy to land on a white dwarf in the first place, since its gravity would quickly crush the ship and its occupants into flattened deposits of debris.)
Astronomers have determined that the ultimate fate of average-sized stars—those having up to 1.4 times the mass of the Sun (or 1.4 solar masses)—is to become white dwarfs. But what about stars that start out with more than 1.4 solar masses? They obviously have stronger gravities. So it is only logical that their end will be more violent and result in the formation of an object even more dense than a white dwarf.
Indeed, a star possessing between 1.4 and perhaps 8 solar masses bypasses the white dwarf stage and proceeds to the next stop on the road to the black hole. (Scientists still differ on the mass of stars that will become neutron stars. Other estimates include 1.4 to 3.2 and 1.4 to 5 solar masses.) This heavier star goes through the same initial steps as a Sun-sized star—depletion of hydrogen, expansion into a red giant, and the burning of helium. But after that, things happen very differently, as Begelman and Rees explain:
Massive stars are powered in later life by a sequence of nuclear reactions involving heavier and heavier elements. As each nuclear fuel is exhausted—hydrogen fused into helium, then helium into carbon and oxygen, etc.—the inner part of the star contracts becoming even hotter…. This process would proceed all the way up to iron. At every stage up to this point, the creation of heavier atomic nuclei releases energy that staves off gravitational collapse. But there are no nuclear reactions that can release energy from iron; iron is the end of the nuclear road for a star. What happens next is one of the most spectacular events known in astronomy…. Since there are no nuclear reactions that can extract energy from iron, the supply of fuel is shut off and the core suffers sudden and catastrophic collapse … in a fraction of a second…. The density of the collapsing core becomes so great that the protons and electrons [the charged particles of its atoms] are fused together to form neutrons, electrically neutral subatomic particles. 19
Because such an object is made up almost entirely of neutrons (forming a substance many scientists call neutronium), it is called a neutron star.
The collapse that creates a neutron star is so violent that it triggers a secondary catastrophe—a stupendous explosion. In this spectacular outburst, called a supernova, significant portions of the star's outer layers blast away into space. This material forms a gaseous shell, often referred to as a supernova remnant, that expands outward for thousands or even millions of years, growing increasingly thinner. (It grows fainter, too, except when lit up by the glow of any stars it passes.)
The rest of the star's original mass is now concentrated in a ball of neutronium about ten to twenty miles across, roughly the size of a large city. So dense is the material in a neutron star that a tablespoon of it weighs at least several trillion tons. Furthermore, such a star's escape velocity is nearly 125,000 miles per second, about two-thirds the speed of light.
All of this sounds convincing in theory. But astronomers had no direct proof of the existence of neutron stars until the late 1960s, when objects called pulsars began to be found. In 1968, for example, astronomers discovered a strange object at the center of the Crab Nebula. Located in the constellation of Taurus, the bull, this bright, rapidly expanding cloud of gases is the remnant of a supernova that occurred in 1054 and was recorded by Chinese and Japanese observers. Modern astronomers noted that the object at the center of the nebula gives off regular, intense bursts, or pulses, of radiation at the rate of thirty per second. Appropriately, they named this and other similar objects pulsars.
Neutron stars have enormous gravity, which would cause a living creature to be crushed out of existence in a fraction of a second, as explained by the great science explainer Isaac Asimov in his book The Collapsing Universe .
Suppose that an object with the mass of the sun collapses to the neutron-star stage and is only 14 kilometers [8.7 miles] in diameter. An object on its surface will now be only 1/100,000 the distance to its center as it would be if it were on the surface of the sun. The tidal effect on the neutron star's surface is therefore 100,000 × 100,000 × 100,000 times that on the sun's surface, or a million billion times that on the sun's surface and a quarter of a million billion times that on the Earth's surface. A two-meter-tall human being standing on a neutron star and immune to its radiation, heat, or total gravity would nevertheless be stretched apart by a force of 18 billion kilograms in the direction toward and away from the neutron star's center, and of course the human being, or anything else, would fly apart into dust-sized particles.
It soon became clear that pulsars are neutron stars, which rotate (spin) at incredible speeds. This rapid rotation is caused by the enormous inward rush of energy that occurs during the star's collapse into a superdense ball. As for why a neutron star pulsates energy, noted astronomer Herbert Friedman writes:
When a neutron star collapses, it also drags with it the original stellar magnetic field until it is concentrated one billion-fold at the surface of the neutron star. In the tight grip of such a strong field, plasma [hot gases] at the magnetic poles would be whipped around with the spinning star. This whirling plasma could generate [a] highly directional radio emission [i.e., radiation shooting out of a specific location on the star] that would beam into space like the light of a rotating searchlight beacon atop a lighthouse. As the radio beam sweeps over the Earth, our radio telescopes record repeated flashes. 20
Scientists now know that neutron stars like the one in the Crab Nebula are not the last word, so to speak, in the awesome story of stellar collapse. That distinction belongs to the black hole. Light is just barely able to escape the deep gravity well of a neutron star, so in a sense it almost qualifies for black hole status. In fact, says John Gribbin, "A neutron star sits on the very threshold of being a black hole." 21 One major factor that sets black holes apart from neutron stars, however, is that no light can escape from a black hole; light and everything else that gets too close to a black hole becomes trapped inside its gravity well forever.
A stellar black hole forms from the collapse of a star having more than eight times the mass of the Sun. So powerful is the force of the inrushing matter that it bypasses both the white dwarf and neutron star stages and compresses that matter into an even denser state. In fact, the matter keeps on falling down the star's gravity well in a sort of neverending death spiral. This is because the gravity well of a black hole is like a bottomless pit, from which nothing can escape.
Not surprisingly, this densest of superdense objects jams an extremely large amount of material into a very small volume of space. A stellar black hole is surprisingly small, therefore. One formed during the death of a star having eight solar masses would probably be only about the size of a small house. It is important to remember that most of the former star's original matter is still inside the black hole. (Some of its matter was ejected into space during the supernova accompanying the star's collapse.) That means that the object's gravitational pull will be roughly the same as that of the original star. Any planets orbiting the star before its collapse would continue orbiting the black hole, which would not capture and consume them unless they strayed too close to it.
The survival of a planet and the survival of living things that might inhabit it are two different things, however. A majority of life forms that happen to exist on planets orbiting a star that becomes a black hole will die from powerful radiation released during the catastrophic collapse and supernova. And any life that has the misfortune to survive this disaster will quickly freeze to death after the star stops radiating light and heat. Clearly, the formation of a stellar black hole is one of the most awesome and potentially lethal events that can occur in nature.