Supernovas light up the sky like few other celestial bodies. Their sudden appearance, at odds with the slow seasonal rotation of the constellations, has arrested astronomers for more than a thousand years. The Chinese called them guest stars. Others saw them as signs from God.

Those fleeting lights come from massive explosions, the violent ends of the lives of stars. Before each explosion, the star becomes so hot and so dense that individual atoms collide and merge to forge new elements. The blast hurls the new elements into space, forming a great glowing cloud.

This is the stardust that later coalesced to form Earth and other rocky planets. The raw materials from which we are made — the calcium in our bones, the iron in our blood — initially formed in the hearts of supernovas. At the most basic level, that of individual atoms, supernovas represent our origins.

They may also foretell our fate. Supernovas shine so brightly astronomers can see them, through telescopes, clear across the universe. A specific variety of supernova, called Type Ia, always blazes with a uniform brightness. Not only that, the quality of their light distinguishes them from all other kinds. They are the long-sought "standard candles," luminous objects so consistent that astronomers can be sure they are observing the same thing whether viewed from near or far.

Seven years ago, cosmologists used these supernovas as benchmarks to measure distances to remote galaxies. The galaxies are so far away that their light has taken billions of years to reach us. What the scientists found astonished them all.

They found that the expansion of the universe is gathering speed. If this outward rush continues, other galaxies will eventually retreat beyond our sight. With their stars too dim for our eyes to see the future night sky beyond the Milky Way will become an inky void. This realization came as a shock. At the time cosmologists were debating when the expansion would halt and whether gravity would take over, pulling everything back together in a catastrophic collapse. They hadn’t imagined a runaway universe.

Nothing from the current understanding of astrophysics could explain it. The only way they could account for an accelerating expansion of the universe was to propose a completely novel physical force — a repulsive force that counteracts the attractive force of gravity. They call it "dark energy."

The nature of dark energy remains a mystery. Figuring it out depends on assembling a more complete history of the universe, something that could be accomplished by observing many more supernovas.

The exact mix of elements flung into the universe depends on how much of the star’s matter fuses before it blows up. And that precise blend of atoms imparts the specific light signature astronomers use to recognize a Type Ia supernova.

So, understanding our atomic origins and our cosmic fate are linked. Both depend on knowing how supernovas explode.

How to explode a star

Understanding how stars explode is a job for Stan Woosley. "I love explosions," Woosley says, "as long as they are far away and don’t hurt anyone."

We’re safe, because Woosley’s explosions are contained in computers. Woosley is an astrophysicist at UC Santa Cruz who uses fundamental physical principles to try to explain what cosmologists observe. He recently won two national astronomy prizes: the Bruno Rossi prize from the American Astronomical Society and the Hans Bethe prize from the American Physical Society. Both recognize his work on celestial blasts.

"A large fraction of the elements of life have come from exploding stars," he says. "It is important to know the physics of explosions — that determines which elements are made."

To do that, Woosley’s group is initially focusing on the first few microseconds of the explosion. Post-doctoral fellow Mike Zingale is using one of the fastest computers on Earth, the new supercomputer Columbia at NASA’s Ames Research Center in Mountain View, California, to determine just how a supernova explosion gets started.

"Part of the problem is, right now, there’s lots of ways you can envision these things exploding," Zingale says. "What we are trying to do is whittle them down to just one model that’s robust enough to explain why these things appear as standard candles."

Almost everyone agrees on the starting point: a white dwarf star.

White dwarfs are the carbon and oxygen cores left behind when stars like our sun exhaust their supply of nuclear fuel. They are too small to explode on their own. Left alone, they cool and dim and eventually fade into darkness.

But some white dwarfs spin with a swollen companion star in a stellar do-si-do. They steal matter, sucking it away from the larger star. When the dwarf accumulates enough stolen matter to reach a critical mass, 38 percent more than the mass of the Sun, it begins to collapse. Pressure and temperature soar, and the nuclei of individual atoms crash together and fuse to form new elements.

Each round of fusion generates energy that rushes outward with increasing force. The energy builds until suddenly it blows the entire star into space, where it shines so radiantly astronomers can see it billions of light years away.

Because the inward crush always begins when the star reaches the same mass, the explosions are all alike. It’s like starting off with identical bombs, each with the same type and amount of fuel.

No one knows how the fusion gets started, or how the reaction crosses that threshold to explode as a supernova. That’s where the supercomputer comes in.

Star Cubes

Zingale starts with a "spark," a single point of fusion, and predicts what will happen next. He follows the expanding layer of fusion, called a "flame," as it moves through the surrounding stellar material.

"What you do is you take the star, or some region of the star, and you break it up into cubes," Zingale says. He assigns a starting state to each cube: temperature, density, velocity, and more. Then he applies basic laws of physics to understand how each cube will change, moment to moment, based on what happens in the adjacent cubes. "Those laws tell you how the whole system evolves."

Even using NASA’s Columbia supercomputer, there’s a limit to how many star cubes Zingale can follow at once. "The flame is thinner than a sheet of paper," he says, "and the star is about the size of Earth." It’s like zooming in on a digital image of the Earth to view a single leaf. You would need many pixels for the leaf to resolve its veins, edges, and other details. But if you photographed the entire planet at that same high resolution, the image file would never fit on your flash memory card.

Zingale faces the same dilemma with his flame. He would need a billion trillion (1021) cubes to resolve the fine-scale flame throughout the whole star. Even a supercomputer can’t hold all those cubes, so for now he is following the expanding pocket of fusion that starts with a single spark.

Zingale follows the reaction on a video created by the supercomputer. The video displays the shape of the flame, with temperature indicated in false color and the time scale slowed to within the limits of human visual perception. The movie shows small bubbles of fused carbon, formed when the star ignites. Within microseconds, the bubbles deform and mushroom outward. That was expected.

What came next was a surprise. The burning bubble transformed into a shape no researchers had ever seen. "It quickly developed into this hot little whirly ring going around 1,500 times a second, and that was quite amazing," Woosley says.

Woosley demonstrates the shape and motion of the moving pocket of nuclear fusion with a pink plastic gun that shoots smoke rings. Each puff of vapor twirls and expands, but holds its essential halo shape as it floats up to the ceiling. "Mike’s beautiful green whirly torus is very similar," Woosley says. "We could have saved a lot of money if we’d just bought this toy."

All of this dramatic activity happens in a piece of the star less than 40 inches wide. Following even that small segment required 200,000 processor hours. Columbia is a parallel computer, able to run analyses on hundreds of processors simultaneously. Zingale’s program codes run as many as 504 processors at once. Even with the computer running continuously, the calculations take nearly a month to complete.

Other modeling groups are using broader simulations to blow up the whole star. But to do that, they have to make assumptions about what is happening on smaller scales. Wolfgang Hillebrandt’s group at the Max Planck Institute for Astrophysics in Munich, Germany, models whole-star explosions by assuming that the thermonuclear fusion flows like ordinary burning — the billowing of flames in a fireplace. "Our models are based on the assumption that thermonuclear burning inside a white dwarf is similar to turbulent chemical combustion," Hillebrandt says. He notes that their simulated explosions closely match astronomical observations of real supernovas.

But Zingale and Woosley prefer the opposite approach. They are working to understand the fundamental physics underlying the blast. "Some people magically put it in their model," Zingale says.

They want to start small, be sure the details are right, and build outward from there. Once they are sure they haven’t made an error that would be magnified if scaled up, they will model the entire explosion. "Eventually we’ll play the big game too," Woosley says.

From spark to explosion

Fusion occurs along the expanding surface of the bubble or ring leaving behind lighter burned material called ash. Light, hot ash rising into colder denser material is inherently unstable. It eddies and churns like smoke from a hot fire rising into cool air. "The whole plasma begins to boil and bubble and swirl around chaotically," Woosley says. "These chaotic motions carry the flame around with them."

It’s an energetic process, but it doesn’t immediately detonate the star. "For about a hundred years, the star boils from all this energy that’s being generated by the carbon runaway," Woosley says. "It doesn’t just get hot and blow up. There’s a long ramp-up of a century when the temperature is getting hotter and hotter." When the temperature finally reaches 700 million degrees Centigrade, the fusion starts to occur so fast that convection can no longer carry the energy away. Individual hot spots begin to form. These are sparks, but they do not yet ignite an explosion.

Much of the star must burn to generate the massive energy of a supernova. "We’re going to need to burn at least two-thirds of the star to get a healthy explosion," Woosley says. The reaction is occurring only at the leading edges of the flame. As turbulence shears and wrinkles the shape, the surface area of the flame expands, allowing fusion to consume the star faster.

To understand how this works, imagine a round dumpling dropped in a pot of hot oil. The dumpling’s surface quickly cooks to a satisfying crunch, but the inside remains gooey. The batter dribbled into hot oil to make a funnel cake at the state fair cooks more thoroughly. The lacy shape exposes much more of the dough to the heat, resulting in a more satisfying crunch-to-goo ratio. The rough, wrinkled surface of Zingale’s whirling ring has the same effect. It rapidly fuses increasing amounts carbon and oxygen fuel to generate yet more energy, as well as new elements.

When the turbulent flames consume enough carbon, the star rapidly inflates. The fusion front rips through the star and obliterates it, leaving a blazing cloud of gas. The light of the supernova beams outward in all directions — including a long journey toward telescopes on Earth.

Cosmic mismatch

However, astronomers rarely see the flash. A Type Ia supernova only shines for a few weeks. Catching a star in the act of exploding is no small feat. Only a few Type Ia supernovas per millennium appear in any one galaxy, and most are tiny specks — easy to miss in the haze of far-off galaxies. Saul Perlmutter of the Lawrence Berkeley Laboratory leads a team that is hunting for those distant, dim lights.

Perlmutter’s group, the Supernova Cosmology Project, is searching for supernovas that exploded so long ago that the light is just now reaching their telescopes after traveling for 5 to 10 billion years. Perlmutter led one of the two teams that discovered the accelerating expansion of space. He believes further observations of many more Type Ia supernovas will reveal a more complete history of the expansion. "We are making very detailed measurements using supernovas as distance indicators," Perlmutter says. "We need to know how the universe has grown, how the expansion has changed."

In 1998 Perlmutter’s group and a rival group led by Brian Schmidt at the Australian National University made their startling discovery by comparing the distance and speed of about four dozen supernovas. For the first 7 billion years the expansion of space gradually slowed, with gravity acting as the brake. But as the universe aged, something else took over, superceding gravity and pushing the universe to expand faster. That force is dark energy.

The mystery of dark energy may be revealed when physicists understand precisely when it took over, how strong it is, and whether it’s constant or episodic. Answering those questions depends on the precision of the astronomer’s measurements. For that, they need to know exactly what to look for in the supernova light — information Woosley and Zingale hope their models of exploding stars will provide.

Fingerprints of a supernova

The glowing cloud of a supernova radiates vast floods of light. But individual atoms within the cloud also deflect some photons, preventing them from reaching the astronomers’ telescopes. They effectively absorb some of the light, like droplets of fog around a streetlamp.

Astronomers can determine the chemical composition of the cloud by looking at which wavelengths of light are absorbed. If you beamed a light through a cloud of pure hydrogen and then through a prism, you would see a rainbow, called a spectrum, broken up by several dark lines. Those lines are the shadows of hydrogen atoms. In the same way, elements such as silicon, oxygen and iron each cast a recognizable imprint on a supernova’s light.

The characteristic pattern of spectral shadows distinguishes Type Ia supernovas from other celestial lights. Astronomers can use this spectral fingerprint to recognize a Type Ia supernova, even if it’s far away and very dim.

Light reveals distance and speed of celestial objects

Lacking the ability to travel vast distances, or back in time, all astronomers have to go on when they observe celestial objects is light. They can tell how far away an object is if they know its true brightness. It is like seeing the faint headlights of an approaching car far away on a highway. As the car comes closer, the lights appear brighter, but they haven’t actually brightened at all. The light from a "standard candle" works the same way. Astronomers can tell how far away a Type Ia supernova is by how much its light has faded compared to the radiance of closer supernovas.

Distance is also a measure of how long ago the supernova exploded. Light from remote stars takes so long to reach us that the stars provide a view of ancient space. An inventory of stars at many different distances from us reveals the history of the universe, like the time slices in an archeological dig.

Less obvious is the method for telling how rapidly a celestial body is speeding away from us. Space itself expands while the light travels toward us, and the waves of light expand along with it. Imagine a Slinky stretched between you and a hot air balloon. The coils of the Slinky represent waves of light. As the balloon rises the Slinky stretches, pulling the coils further apart. Stretching space does the same thing, extending the light waves and shifting them closer to the red end of the light spectrum. This distension of wavelengths is called the red shift - a measure of the velocity of the source of the light.

The mystery of dark energy

It seems the fate of the universe was riding on a struggle between the restraining force of gravity and propelling force of dark energy. And dark energy is winning. Gravity diminishes with distance, and distances between objects have increased as the universe expands. "Matter dilutes while the universe expands," University of Chicago physicist Sean Carroll says. "Dark energy stays constant. In the past, matter was winning. As the universe expands and dilutes, dark energy takes over more and more."

But what dark energy really is remains a mystery. It may be the energy of empty space, the power in a vacuum. Or it may not exist at all. The need to invoke it to account for the supernova observations may arise from a primary flaw in our understanding of physics.

"At this point, all bets are off," says astronomer Adam Reiss of the Space Telescope Science Center in Baltimore. He thinks dark energy may compel scientists to find new ideas. "People are backing up more and more trying to get a bigger perspective," he says, "and allowing for as much as a revolution in fundamental physics to explain dark energy."

ABOUT THE WRITER

Susan D. Brown
B.A. (music and psychology), DePauw University
M.S. (biopsychology), University of Maryland, College Park

ABOUT THE ILLUSTRATOR

Having an artistic mother and a scientist as a father, Emma Burns was continually torn between the two principles that ruled her life. That was until she discovered the Science Illustration program during her year abroad, and was instantly relieved of her dilemma. It was while reading the list of classes that she realized her true dream, and knew she would be returning to Santa Cruz to participate in the program. Having fully immersed herself in the world of science illustration, she is looking forward to the adventures that will follow now that she is a full-fledged science illustrator.