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What Is a Black Hole?

First things first. Did you know that black holes are made of warped space and warped time? They have mass, but no matter. Their gravitational pull is so great that nothing, not even light, can escape their grasp. They sound simple, until you try to wrap your brain around them. Even Albert Einstein, a physicist whose general theory of relativity (1915) predicted the existence of black holes, refused to believe that they were real. In a 1939 paper, Einstein said, “The essential result of this investigation is a clear understanding as to why the ‘Schwarzschild singularities’ [as black holes were known at the time] do not exist in physical reality.” Or nice idea, pal, but hey, we live in the real world.

To really understand black holes—and maybe to feel smarter than Einstein when you realize they really do exist—it helps to take a trip back in time. All the way back, in fact, to the late seventeenth century. That’s when an apple falling from a tree is said to have led English physicist and mathematician Sir Isaac Newton to formulate his law of universal gravitation.

Data from four different types of telescopes created this image of the spiral galaxy Messier 106, also known as NGC 4258. At the center of this galaxy is a massive black hole.

Gravity: It’s Not Just a Good Idea, It’s the Law

By the mid-sixteenth century, scientists had discovered that the planets in our universe travel around the sun in an elliptical (oval) pattern. Nobody knew why the planets moved that way or what kept them in orbit. Newton tackled that problem in his 1687 book Philosophiæ Naturalis Principia Mathematica (Mathematical Principals of Natural Philosophy)—known as the Principia for short. In it, he described three laws of motion. Although it took Newton more than five hundred pages to explain his laws, they boil down to three sentences:

Every object stays in its state of rest or uniform motion in a straight line unless some kind of force acts upon it.
The force acting on an object is equal to the mass of that object times its acceleration. In mathematical form, it is expressed like this: F=ma, where F is force, m is mass, and a is acceleration.
For every action, there is an equal and opposite reaction.

In everyday conversation, people tend to use the word theory to describe a hunch or guess. “Evolution?” say creationists (people who believe in the biblical explanation for the creation of the universe). “That’s just a theory.”

In fact, evolution is a theory. To scientists, a theory is not a half-baked idea. A theory explains a collection of facts about some aspect of the natural world. It can be tested and used to make predictions. Peter Godfrey-Smith, a philosopher of science at the City University of New York and at the University of Sydney in Australia, compares theories to maps. Just as a map represents an area of land, a theory represents a territory of science. A road map shows highways, rivers, and cities that we know exist. In the same way, a theory is made up of observable facts. A good theory, like a good map, is an accurate depiction of the physical world.

It’s important to understand the difference between facts, laws, hypotheses, and theories. When an apple drops from a tree, it falls down, not up. That’s a fact. Newton’s law of universal gravitation describes the observation that the apple falls down. It allows us to calculate all sorts of things about a falling apple: the strength of the gravitational pull between it and Earth, its acceleration as it falls, how long it will take to hit a daydreaming Newton on the head, and so on.

What Newton’s law doesn’t tell us is why gravity exists or how it works. Scientists develop testable hypotheses, or smart guesses, to explain observations. Until the late nineteenth century, scientists believed that gravity was due to what scientists call ether, tiny invisible particles all around us that pushed things down. But the evidence didn’t support that hypothesis, so scientists rejected it.

Einstein proposed that what we perceive as gravity is actually the result of a distortion in the fabric of space and time. Einstein’s hypothesis held up through repeated observation and experimentation. So it became a scientific theory: the general theory of relativity. Not only did Einstein’s theory explain gravitational attraction, but it also predicted gravitational waves and black holes. Theories may change as scientists make new discoveries. They can be adapted to include that new knowledge.

Isaac Newton (1642–1727) was a physicist and mathematician. He laid out what he understood about laws of motion, the behavior of fluids, and gravitation in his book Philosophiae Naturalis Principia Mathematica. The book established Newton’s reputation as one of the greatest minds of science.

Newton used those laws in a thought experiment to try to understand the motion of the planets. It went something like this: What if he were to shoot a cannonball from the top of a very high mountain where, just as in outer space, there is no air resistance? Without a force to act on it, the cannonball would continue in a straight line forever. But, in reality, Newton knew that the cannonball would move forward and eventually drop back to Earth. Newton came up with the idea of gravity to name the downward-pulling force acting upon the cannonball.

Newton reasoned that the faster the cannonball was moving, the farther forward it would travel before dropping to Earth. If the cannonball were moving fast enough, it would fall into an orbit around Earth instead of into it, much as the moon orbits Earth. If it moved really fast, it would reach escape velocity, leaving Earth and its orbit altogether.

He realized that the same force—gravity—that pulls an apple to Earth when it falls from a tree could also explain the orbits of the moon around Earth and the planets around the sun. Newton’s law of universal gravitation, published as part of the Principia, states that the gravitational force between any two objects in the universe is proportional to the product of their masses. The gravitational force is inversely proportional to the square of the distance between the center of each mass. So more massive objects have greater gravitational pull than less massive objects. And that gravitational pull gets weaker with distance.

Fast-forward to 1783, in a village in West Yorkshire, England. The head of the parish in the village, a brilliant clergyman named John Michell, had an unusually keen interest in science. He met with some of the leading scientists of the time, including Benjamin Franklin, and published influential papers about the nature of magnetism, astronomy, and earthquakes. He was trying to figure out a way to measure the mass of distant stars. He concluded that the gravity of some really massive stars might be so great that not even light could escape them. Michell published his calculations in a paper that described a universe with many invisible “dark stars.” The only way to detect them, he said, would be to observe their gravitational effects on nearby objects.

French mathematician and astronomer Pierre-Simon Laplace independently came to the same conclusion just over a decade later. In his 1796 book Exposition du système du monde (Explanation of the System of the World), Laplace referred to black holes as black stars, writing, “It is . . . possible that the greatest luminous [light-filled] bodies in the universe are invisible.”

You’ve got to give Michell and Laplace major props for predicting the existence of dark (or black) stars, but they made one critical mistake. They believed, as did Newton, that light was made up of tiny particles called corpuscles. These corpuscles were thought to have mass, and because they had mass, gravity would pull on them. Scientists such as James Clerk Maxwell soon discovered, however, that visible light is actually a form of electromagnetic radiation. Scientists understood that electromagnetic radiation has no mass. As the corpuscle theory of light lost favor, so too did the idea of a dark, black star.

Light, electromagnetic waves, and radiation all refer to the same thing: electromagnetic energy (EM). Electromagnetic energy is created when a charged particle, such as an electron, is accelerated through an electric field. This movement produces oscillating electric and magnetic fields, traveling back and forth at right angles to one another in a packet of energy called a photon. Photons travel in a wavelike pattern at the speed of light. Unlike sound waves, electromagnetic waves can travel through the vacuum of space.

Scientists describe EM according to wavelength, frequency, and energy. The wavelength, measured in meters, is the distance between peaks (tops) of a wave. Frequency is the number of waves that form in a given length of time. One wave, or cycle, per second is called a hertz. Each photon varies in energy, measured in electron volts. We say that a higher energy photon is more energetic. All three characteristics are related: the shorter the wavelength, the higher the frequency of the waves and the more energetic the photons. The opposite is true too: the longer the wavelength, the lower the frequency of the waves and the less energetic the photons.

This range of wavelengths, frequencies, and energy is known as the electromagnetic spectrum, from radio waves (low energy) to gamma rays (high energy). In between are the wavelengths of the visible spectrum—the light that our eyes can see.

What’s the Matter?

As scientists were beginning to understand the nature of light, they were also gaining new insight on matter—the “stuff” that makes up the universe (at least the stuff that we can detect). The concept of the atom had been around since the ancient Greek philosopher Democritus (ca. 460–370 bce) upended the accepted notion that the universe was made of four elements: earth, air, fire, and water. He described the universe this way: “by convention sweet and by convention bitter, by convention hot, by convention cold, by convention color, but in reality atoms and void.” Atoms (from the Greek word atomos, meaning “indivisible”), Democritus said, were different sizes and shapes, and they all moved about in a void of nothingness.

Centuries later, English chemist and physicist John Dalton came up with a scientific theory about atoms based on his experiments with gases. In 1803 he proposed that each element—such as hydrogen, carbon, or iron—had its own kind of atom and that these atoms varied in size and mass. He was right, but like Democritus, he believed that atoms were indivisible little spheres of matter.

It wasn’t until 1897 that English physicist J. J. Thomson found a particle even smaller that the atom—the electron. Thomson proposed that an atom was actually made of a ball of positively charged matter, studded with negatively charged electrons. It became known as the plum-pudding model, in which the bulk of the atom—the positively charged pudding—was studded with negatively charged electrons.

In a series of ingenious experiments in 1909, Thomson’s student, a New Zealander named Ernest Rutherford, showed that at the center of each atom is a small, dense, and positively charged nucleus. He published his results in 1911. In the years that followed, scientists established that the nucleus is actually made of smaller particles—positively charged protons and neutrons, which have no charge. A cloud of negatively charged electrons surrounds the nucleus.

Over time, scientists found even more basic building blocks of matter. In 1964, for example, physicists Murray Gell-Mann and George Zweig proposed that neutrons and electrons are made of even smaller particles. As further proof that physicists have a good sense of humor, they named the particles quarks, after a phrase—“Three quarks for Muster Mark!” —in James Joyce’s classic book Finnegans Wake (1939).

Einstein Rocks the World

In the early twentieth century, a brilliant young German physicist named Albert Einstein rocked the scientific world. He realized that Newtonian physics and Maxwell’s theory about the nature of light couldn’t both be right. In 1905 the twenty-six-year-old patent office clerk was living in Bern, Switzerland. That year he published his first theory of relativity. Known as the special theory of relativity, this theory overturned Newton’s long-accepted ideas about space and time—and eventually led to the renewal of the conversation about the existence of black holes.

Newton had written that the space of the everyday world is made up of three dimensions: east-west, north-south, and up-down. Space, according to Newton, is absolute. In his view, distances never change. For example, the distance from your bedroom to the kitchen is always the same. Newton also believed in the absolute nature of time. He thought that people all experience time in the same way. For example, whether your fifty-minute physics class takes place in a classroom or on a moving train, on Earth or in outer space, it will always last the same fifty minutes.

Einstein’s special theory of relativity rejected the concepts of absolute space and absolute time. He said that we live in a four-dimensional world of space-time in which time and distance (space) vary with an observer’s frame of reference, or point of view. Your frame of reference is based on where you happen to be and how fast you are moving.

This photo of Albert Einstein (1879–1955) was taken in 1921, the year he was awarded the Nobel Prize in Physics.

Let’s say you’re on a cross-country trip on a train going west at 98 feet (30 meters) per second. You and your friend pass the time by playing Ping-Pong in the club car. You’re both pretty evenly matched, so the two of you hit the ball back and forth, east to west, at 6.6 feet (2 m) per second each way. That’s your frame of reference.

Observers standing by the railroad tracks watching your train speed by have a different frame of reference. If they used a radar gun to clock the speed of your Ping-Pong balls, they would measure the speed of the westbound ball at 105 feet (32 m) per second (98 feet, or 30 m, per second for the train plus 6.6 feet, or 2 m, per second for the ball). They would clock the eastbound Ping-Pong ball at 92 feet (28 m) per second (30 m per second minus 2 m per second). So the speed of the ball depends on the observer.

Let’s speed things up a bit. According to Einstein’s theory, the faster an object is moving, the more time will appear to slow down for the object from the perspective of someone who is not moving. Einstein called this phenomenon time dilation. He explained it using two identical twins as examples. Let’s say that the twins, who are sixteen years old, wear identical watches. The adventuresome twin travels into space on a rocket traveling near the speed of light (186,000 miles, or 299,338 kilometers, per second) before returning home. His twin, a homebody, decides to stay on Earth. According to the traveling twin’s watch, he was gone for two years. He is now eighteen years old. But for the homebody twin, thirty years have passed. He is now forty-six years old!

Each twin has a foot-long hot dog. Because distances grow shorter in relation to motion, the traveling twin’s foot-long hot dog would be a little shorter than his Earth-bound twin’s foot-long. The only thing that remains constant in space and on Earth, according to Einstein’s theory, is the speed of light—and nothing can go faster than light.

If Einstein’s theory of relativity was true—and he was quite sure that it was—then he recognized that Newton’s law of gravity must be flawed. According to Newton, the force of the gravitational pull between two objects—say, Earth and the sun—depends on the distance separating them. In Newtonian physics, the gravitational effects—regardless of force—would be felt instantly no matter the distance between objects.

However, relativity predicts that the distance between objects differs, depending on the observer’s frame of reference. This would mean, therefore, that the force of gravity will differ and that the impact of gravity would not be felt at all. Einstein wrote, “If a person falls freely, he will not feel his own weight.” So objects in free fall do not feel the effects of gravity, even as they are plummeting toward the ground. In the theory of relativity, the effects of gravity and acceleration are the same. Think about it: Let’s say you jumped off a cliff, hopefully with a nice cushion at the bottom. You’re in free fall—falling only under the influence of gravity. As you fall, you won’t feel your own weight. It will seem as though gravity has disappeared—even though you are accelerating toward Earth at an alarming rate! If you drop a rock midway down, you’ll fall together, side by side, as if in space.

This led Einstein to the conclusion that objects in free fall follow the shortest path through space-time. But space-time itself is curved—and the thing that curves space-time is mass. Think of placing a bowling ball in the center of a trampoline. The bowling ball causes the trampoline to sag in the center, just as a massive object in space bends the space-time around it. Then think about placing a Ping-Pong ball on the edge of the same trampoline. The ball will follow the curved path directly down to the bowling ball. In this same way, the motion of everything—from light waves to entire galaxies—is influenced by the curves in space-time.

Einstein concluded that gravity is the warping of the geometry of space-time based on the presence of matter. He published this theory—known as the general theory of relativity—in 1915.

Einstein based the theory entirely on mathematics. He suggested a way of putting it to the test. He knew that the sun has a strong gravitational field. Its mass (about 1.99 × 1030 kilograms) should not only affect the orbits of its planets but anything nearby. According to relativity, the sun’s gravitational field should bend light traveling to Earth from distant stars. Knowing the mass of the sun, scientists should be able to calculate the apparent gravitational pull the sun should have on the path of starlight. The only time scientists would be able to prove the bending effect of light would be during a total solar eclipse, when the moon passes between the sun and Earth. During a solar eclipse, the glare of the sun no longer blocks the view of the stars from Earth.

This illustration shows a simplified version of the curvature of space-time. The warping caused by each colored sphere is proportional to its mass. The curvature of space-time influences the motion of massive bodies. As they move, the curvature changes. Gravity describes this relationship between matter and space-time.

In 1917 the astronomer Sir Frank Watson Dyson proposed the perfect experiment to test Einstein’s theory. He knew that there would be a total solar eclipse in 1919. It would occur just as the sun crossed the bright Hyades star cluster, in the constellation Taurus, in the sky. The light from the stars would have to pass through the sun’s gravitational field on its way to Earth. Because of the eclipse, the light would be visible during the day. This would be the big chance to prove the theory.

In May 1919, British astrophysicist Arthur Eddington sailed down to Principe, an island off the western coast of Africa, in time for the solar eclipse. In preparation, he spent January and February observing and photographing the Hyades star cluster at night from Oxford, England. That established his baseline. Then, on May 29, he photographed the same stars during the solar eclipse from his vantage point in Principe. From Earth the light from the stars and the light of the sun were visible. Sure enough, in comparing the photos, Eddington saw that the positions of the stars were different in each image. They appeared to have shifted! What’s more, they shifted exactly as Einstein’s mathematical theory predicted, based on the mass of the sun. Eddington understood that the stars themselves had not actually shifted. The sun’s mass warped the space-time path of their light so that on Earth, the position of the stars seemed to have moved. Einstein was pleased, but not at all surprised. When asked how he would have felt if the eclipse experiment had not confirmed his theory, he replied, “Then I would feel sorry for the good Lord. The theory is correct anyway.”

A large mass such as the sun deflects the light of a distant star that passes close by. An observer on Earth perceives the star to be in a different position. The shift is tiny, but it can be measured during a total eclipse of the sun.

Einstein, for all his cheeky confidence, would nevertheless acknowledge Newton’s contribution to physics. In his autobiographical notes, he wrote the following:

Newton, forgive me; you found the only way which in your age was just about possible for a man with the highest powers of thought and creativity. The concepts, which you created, are guiding our thinking in physics even today, although we now know that they will have to be replaced by others farther removed from the sphere of immediate experience, if we aim at a profounder understanding of relationships.

Einstein was referring to the relationships between the sun and other celestial objects, but he had long been mindful of the relationships among scientists. And he felt it was important to acknowledge the ways in which they construct theories based on the accomplishments of their predecessors.

Schwarzschild to the Rescue

Not long after Einstein published his general theory of relativity, he received two letters from Karl Schwarzschild, a physicist, mathematician, and astronomer. Born to Jewish parents in Germany, Schwarzschild had served in the German army on the Russian front in World War I (1914–1918). While in the army, he had found the exact solutions to Einstein’s field equations for the general theory of relativity. These are a set of ten equations that describe the interaction of gravity as a result of the curvature of space-time by matter and energy. Schwarzschild wrote to Einstein to tell him of the solutions, knowing that Einstein himself had only come up with approximate solutions to the mathematical equations. The equations and their solution describe the space-time surrounding a spherical, nonrotating object. The solution is known as the Schwarzschild metric.

Karl Schwarzschild solved ten mathematical equations that Einstein developed to describe the interaction of gravity as a result of space-time warping by matter and energy. In this undated photo, Schwarzschild works at his desk in Potsdam, Germany.

From the math, Schwarzschild made some astounding predictions. If a giant star with huge mass could be compressed into a tiny sphere with a radius R (the distance from the center of the sphere to its outer edge, or the Schwarzschild radius), the mass would collapse into a point that mathematicians call a singularity. The singularity—infinitely small, infinitely dense—would create a gravitational field that would curve space-time to the point that nothing could escape—not even light. Space-time would curve infinitely too, and all the rules of physics as we know them would break down. Crazy! Michell and Laplace’s dark, black star had returned, at least as a mathematical possibility.

While a black hole itself is a singularity, the Schwarzschild radius describes the ultimate point of no return–the event horizon. Beyond that point, nothing can escape the pull of a black hole. Its escape velocity would have to be greater than the speed of light, which is impossible. And that radius depends on the mass of the black hole. Theoretically, if you wanted to turn Earth into a black hole, you would have to squash its event horizon to the size of a pea. If you could squeeze the sun into about thirty-one football fields, you’d have another black hole.

In the following years, physicists confirmed Schwarzschild’s calculations. In 1939, at the University of California, Berkeley, J. Robert Oppenheimer and his graduate student Hartland Snyder wrote a paper that convincingly described the fate of a very massive dying star. They concluded that once nuclear fusion shuts down, gravity would cause the star to collapse to an infinitely dense point. (Nuclear fusion is an atomic reaction that powers stars. As stars age and die, nuclear fusion slows down and eventually stops.) Nothing could escape from a black hole, they wrote, not even light. “The [dying] star . . . tends to close itself off from any communication with a distant observer,” they wrote. “Only its gravitational field persists.”

It wasn’t until 1967 that another physicist, John Archibald Wheeler of Princeton University, gave these collapsing stars a name. The term—black holes—captured the imagination of the public. Wheeler also famously said that a black hole has no hair, meaning that it doesn’t reveal any information about what’s inside. The material properties of any object are unknowable once it falls into a black hole. Wheeler went on to explain, “I was thinking of a room full of bald-pated people who were hard to identify individually because they showed no differences in hair length, style, or color.” (Some physicists propose that black holes may have “soft” hair after all. So far, it’s an unresolved issue.)

We have no way, Wheeler said, of knowing whether black holes were created from “neutrinos [tiny subatomic particles with almost no mass and with no charge], or electrons and protons, or old grand pianos.” All the same, black holes do reveal their mass, how fast they spin, and their charge. These are the three elements that scientists use to determine the existence and description of black holes. (The charge is so small, however, that most physicists ignore it when they are describing a black hole.)

Signature of a Black Hole

In 1970 Italian engineers launched an X-ray satellite

telescope into orbit from the African nation of Kenya. Named Uhuru (the Swahili word for “freedom”), the satellite was part of the Explorer satellite program of the National Aeronautics and Space Administration (NASA) to detect high-energy X-ray sources in space. Because Earth’s atmosphere blocks most X-rays, previous efforts to study astronomical sources of this form of radiation relied on X-ray telescopes launched on rockets. They climbed a few hundred miles above Earth before falling back to the ground. The rocket-based telescopes gave scientists only about five minutes to study X-ray sources before they plunged back to Earth. Scientists were especially interested in a closer look at a strong X-ray source they had already detected in the direction of the constellation Cygnus, or the Swan. (Cygnus is also known as the Northern Cross.) Scientists had dubbed the X-ray source Cygnus X-1. Uhuru would give astronomers a chance to study Cygnus X-1 for longer periods of time and in more detail. Uhuru orbited Earth for more than two years, opening a new and much wider window for astronomers to look at the universe.

The satellite Uhuru is shown here during preflight tests at NASA’s Goddard Space Flight Center in Maryland. Engineers launched the X-ray telescope from Kenya on December 12, 1970, as part of a large project to explore the universe. It was the first X-ray telescope to be launched for this purpose.

Uhuru’s telescopes soon revealed that the energy from the X-ray photons from Cygnus X-1 flickered in short, rapid bursts, up to one thousand times a second. This was a clue that the source was small. At first, astronomers thought it might be a neutron star, which is a powerful X-ray source. Rapidly rotating neutron stars produce a beam of X-rays that we see as pulses of radiation, much like a sweeping beam of light from a lighthouse. But the pulses of neutron stars are regular—and the flickering from Cygnus X-1 didn’t seem to have a pattern.

Soon astronomers realized that they might very well be looking at a black hole—or at least at its signature. In fact, scientists can’t really see black holes. They’re a bit like tornadoes—you don’t actually see the furiously spinning column of air. Instead, you see water droplets, dust, trees, or houses caught up in the funnel. Likewise, astronomers “see” black holes by observing the intense radiation they emit as material falls into them and by observing the motions of nearby stars.

Follow-up studies using radio and optical telescopes showed that Cygnus X-1 was not one but two objects. One was a giant blue star that swung around an invisible partner once every 5.6 days. From the speed of the blue star’s orbit, astronomers determined that the mystery partner’s mass was at least ten times greater than the sun—and only 55 miles (90 km) in diameter. No neutron star could be that massive and support itself without collapsing.

This was persuasive evidence of a black hole.

And those mysterious flickering X-ray signals? Astronomers found that the black hole was pulling gas from the giant blue star. The gas wasn’t falling straight into the black hole. It was orbiting the black hole’s space-time in tighter and tighter spirals, like water circling a drain. As the gas moved closer to the black hole, the temperature of the gas rose tens of millions of degrees. The gas emitted enormous amounts of X-ray energy in pulses. Great clumps of gas broke off and fell into the black hole, never to be seen again.

Scientists had discovered physical proof of their first black hole.

In 1973 the astronomer and author Carl Sagan wrote, “All of the rocky and metallic material we stand on, the iron in our blood, the calcium in our teeth, and the carbon in our genes were produced billions of years ago in the interior of a red giant star. We are made of star-stuff.”

So what are stars made of? A star begins its life as a cloud of gas and dust that comes together and collapses under its own gravitational attraction. As the gas and dust collapse, the material in the center heats up. That hot core is called a protostar. As the protostar’s gravity pulls in more mass, the core becomes even hotter and denser. When temperatures at the core reach about 15 million kelvin (K), hydrogen atoms in the core begin to fuse (a process known as nuclear fusion), forming helium. (Kelvin is a temperature scale designed with absolute zero as the temperature at which all molecules stop moving. Absolute zero is a hypothetical number, as it is impossible to completely stop all molecular movement.) This is a full-fledged star. Stars continue fusing hydrogen into helium throughout their lives, which can last millions or even billions of years. Throughout the life of the star, radiation and heat at the core prevent the star from collapsing.

Stars at this stage—main sequence stars—vary in size, mass, and brightness. But they are all powered by the conversion of hydrogen to helium, which releases a tremendous amount of energy.

Eventually—in millions or billions of years, depending on the size of the star—the core of the star will run out of hydrogen. Then gravity begins to take over and the inner layers of the star begin to collapse. This again raises the temperature and pressure at the core. But the outer layers expand outward—to as much as one hundred times the star’s original size. The star, now a red giant, begins to burn hydrogen in its outer core. When this happens to our sun—in about 6.5 billion years or so—it will engulf Earth and fry it to a crisp.

Any star up to about seven times the size of the sun is considered a medium-size star and is destined to become a white dwarf. As the core continues to collapse, the heat and pressure cause helium to fuse and create a very dense core of carbon and oxygen nuclei. When the star no longer has enough energy to fuse those nuclei, the star will cool and shrink. The star will end its life as a white dwarf star, about the size of Earth but a million times denser. Over billions of years, it will theoretically cool down to something resembling a lump of coal—a black dwarf. It’s theoretical, because the time scientists calculate it would take a white dwarf to reach this state is longer than the current age of the universe, about 13.8 billion years.

More massive stars come to a more spectacular ending. The beginning of the end is similar to that of smaller stars, with the outer layers expanding into a red supergiant. The core has so much mass that it has enough energy to continue fusing atoms until the core is filled with iron atoms. Then the core lacks the energy to fuse any more atoms, and it loses the battle against gravity.

The core collapses, becoming even hotter and denser, crushing the iron atoms together. As the positively charged nuclei are squeezed together, the repulsive force between the nuclei causes the whole thing to explode in a massive shock wave. Material from the star spews out in space. The star loses about 75 percent of its mass (this is the star stuff we are made of), but the rest collapses into a core.

If the remaining core is about 1.4 to 3 times the mass of the sun, it will become a neutron star. Neutron stars are not quite massive enough to become black holes, but they’re impressive. They are one hundred trillion times denser than Earth. They may have magnetic fields more than ten million times stronger than we have on Earth. Still more massive stars—those that started with more than twenty times the mass of the sun—will collapse into the infinite singularity of a black hole.

The Birth of a Black Hole

About 3.6 billion years ago, around the time the first single-celled organisms began to emerge on Earth, a black hole was born one-quarter of the way across the universe, somewhere in the constellation Leo. This particular black hole had once been a huge star, at least thirty to forty times the mass of our sun—and much hotter. Like other stars, this huge star ran on fuel created by the natural fusing of atomic nuclei of hydrogen and helium gases. The fusion formed even heavier elements. The enormous amounts of energy released by these reactions created an outward pressure. That pressure counteracted the inward pull of gravity. At some point, the huge star inevitably began to run out of fuel.

Once the star used up all its fuel, gravity won out. The star began its inevitable collapse to a single point—and it didn’t go with a whimper.

As the star collapsed, it sent jets of gaseous material outward at nearly the speed of light. Those jets slammed into still-collapsing incoming gaseous material, creating an enormous shock wave. The jets continued out into space, where they collided with gas the dying star had already released. These interactions created extremely high-energy photons called gamma rays—about thirty-five billion times more energetic than visible light—as well as other frequencies of electromagnetic radiation.

Gamma Ray Bursts

Billions of years later, those gamma rays reached Earth and the humans observing the universe. One of those humans was Tom Vestrand, an astrophysicist at the Los Alamos National Laboratory in New Mexico. Vestrand was interested in studying the cosmic explosions that produce black holes and their calling cards—gamma ray bursts (GRBs). “Those gamma ray bursts are intense flashes of light that have energies all the way from radio frequencies . . . to gamma ray energies,” Vestrand explained. “And they can last anywhere from a fraction of a second up to a minute.”

The Swift X-Ray Telescope took this 0.1-second exposure of GRB 130427A. Gamma ray bursts are the most powerful explosion in the universe. They last seconds or minutes. Astronomers associate GRBs with the death of a star and the birth of a new black hole.

Fans of science fiction know that wormholes are tunnels that allow travelers to journey from one point in space and time to another. For example, astronomer Carl Sagan’s 1985 book Contact (also a 1997 film) featured a wormhole that allowed astronauts to travel to the star system Vega. The Doctor in television’s Doctor Who travels through a wormhole in his TARDIS spacecraft and time machine to visit different points in space and time. In the 2014 movie Interstellar, an astronaut travels through a wormhole to search for habitable planets.

To visualize a wormhole, think of an ant on an apple. The surface of the apple is that ant’s entire universe. The top of the apple represents one point in the space-time of that universe. The bottom of the apple represents another point in space-time. If the ant wants to get from the top of the apple to the bottom, it has to walk along its curved surface. But if you were to take an apple corer and remove the apple’s core, the ant would have a shortcut—or wormhole—for getting from the top of the apple to the bottom.

Are wormholes even possible, or are they just convenient ways for science fiction writers to shuttle their characters around in space and time? According to Einstein’s theories, they are possible, but not at all likely. In 1916 Austrian physicist Ludwig Flamm was studying Schwarzschild’s solution to Einstein’s equations, which predicted black holes. Flamm came up with another solution, which predicted the opposite of a black hole. He predicted a white hole. While a black hole draws matter into its event horizon, a white hole spews matter out of its event horizon. Flamm thought that a white hole might be on the other side of a black hole, in a totally different part of the universe—or even in a different universe altogether! In 1935 another physicist, Nathan Rosen, working at Princeton University with Einstein, came up with the same idea. Their idea became known as the Einstein-Rosen bridge.

Some theoretical physicists have proposed that wormholes are entangled black holes. Or maybe radiation given off by one black hole is captured by another black hole. If you were to separate the entangled black holes, you’d find a wormhole linking one to the other.

No matter what, physicists think that wormhole tunnels would only exist fleetingly. Once they formed, they would be so unstable that they would collapse. Not even light could make it through the tunnel. So don’t hold your breath about traveling through a wormhole.

Gamma ray bursts can come from any direction. They can also come without any warning. So in 2002, Vestrand and his team designed a network of small robotic telescopes, known as RAPid Telescopes for Optical Response (RAPTOR) telescopes. The RAPTOR telescopes, networked and controlled by a powerful computer, scan the skies for unusual events. When one telescope finds something, the other telescopes swivel to the same spot in the sky in less than three seconds to take digital images of the event. “[RAPTOR] can drive itself, like a Google car,” said Vestrand. “It can detect when an important event is happening, and I don’t have to stay up all night to see it.”

Vestrand’s hard work on the telescopes paid off. On April 27, 2013, the RAPTOR telescopes detected a shockingly bright flash of visible light, along with a powerful burst of gamma ray emissions. The initial burst was followed by an afterglow that lasted for nearly a day. GRB 130427A (named for the date it was detected) was one of the biggest and brightest GRBs ever detected and the longest lasting by far.

What’s more, the RAPTOR telescopes weren’t the only witnesses to GRB 130427A. NASA’s satellite X-ray and gamma ray telescopes recorded the same event. One of the really surprising things about GRB 130427A is that its energy is higher than what scientists believed to be possible. What does this mean for how scientists think about electromagnetic radiation, black holes, and the structure of the universe? Does it change their ideas? We don’t know yet. That’s what makes scientists like Vestrand so excited about researching GRBs.

Tom Vestrand’s interest in science began one day in junior high school in Livona, Michigan, when the science teacher was absent. The substitute teacher played an old educational film, The Strange Case of the Cosmic Rays (1957), directed by Frank Capra. It told the story of fifty years of research into cosmic rays and the nature of the universe, with all the red herrings, dead ends, and clues of a scientific mystery. “The narrator challenged the viewer to get involved in this mystery story and explore the nature of cosmic rays,” Vestrand said. “And it said, ‘Come back in 50 years, and we’ll see how things have progressed.’”

Vestrand was briefly interested in oceanography in high school. “I thought [French explorer] Jacques Cousteau was cool, because he had this TV show, and he went out on adventures and built things like submarines and other stuff,” Vestrand said. The message of the Capra film made a deeper impression on young Vestrand. Ten years later, in graduate school at the University of Maryland at College Park, he wrote a PhD dissertation on the role of cosmic rays and objects in galaxies beyond the Milky Way. He was especially focused on objects near supermassive black holes. That paper led him to search for cosmic explosions and ultimately to his current passion: the gamma ray bursts that signal the biggest explosions since the creation of our universe through the big bang.

Billions of Black Holes

Twenty-first-century astronomers believe that large galaxies likely have hundreds or even thousands of small black holes. The entire universe may have billions. “Small” is relative, of course, when you’re looking at galaxies. A small black hole could have mass that is a few times greater than that of our sun. Even more astounding, scientists have come to believe that at the center of most galaxies is a hulking giant of a black hole, perhaps as massive as billions of suns.

Astrophysicist Neil deGrasse Tyson is the director of the Hayden Planetarium in New York City. Among other things, he studies black holes, wormholes, and star formation (opposite page). Check him out on the Internet. He explains black holes in easy-to-understand language in this short video clip at https://www.youtube.com/watch?v=PtA7O3AOCPU.

Scientists are still not sure how supermassive black holes form, but they have a pretty good hypothesis. About 13.8 billion years ago, our universe was created in a massive explosion of space and time that we call the big bang. Less than one billion years later, giant clouds of superhot gas cooled enough to collapse and become stars. These stars burned out quickly, becoming black holes. Those black holes attracted more matter that formed a new galaxy. Those baby galaxies orbited around one another, sometimes pulling so close they collided. When the galaxies merged, their black holes did too. Astrophysicist Neil deGrasse Tyson, director of the Hayden Planetarium in New York City, says, “There’s no other way to say it: galactic cannibalism. . . . It’s just that simple: the big galaxies get bigger, the little ones get eaten.”