Astronomers may have
BY H. Adrian Cho ILLUSTRATIONS BY Pat Latas
H igh above the Earth, a satellite resembling a freezer decked out in hubcaps orbits in silence, looking out into the vast darkness of space, waiting patiently. Suddenly, a wave of gamma raysextremely high energy particles of lightwashes over the satellite. Instruments blink to action and furiously calculate the direction the gamma rays are coming from. In seconds, a radio link feeds the coordinates to computers on the ground, which pass them via the Internet to astronomical observatories across the globe. Telescopes swivel to the specified point in the sky, looking for a flash to accompany the invisible gamma rays. And then, just seconds later, the shower of radiation ceases. Another gamma ray burst has come and gone, leaving astronomers with more data to scrutinize as they struggle to explain the most powerful explosions since the Big Bang.
Gamma ray bursts captivate astronomers for a simple reason: they are almost inexplicably powerful. A gamma ray burst radiates more energy in 10 seconds than the sun will during its entire 10-billion-year lifetime. For a moment, the burst will outshine all the other stars in the universe combined. If a gamma ray burst occurred at the center of our Milky Way galaxy, 27,000 light years away, it would shine like a second sun in our skies. If one occurred around the celestial corner, it might, some scientists believe, wipe out life on the planet. (Dont worrythe chances of this happening any time soon appear to be astronomically small.) Moreover, as observations improve, estimates of the power of gamma ray bursts keep going up, daunting even seasoned astronomers.
Ive been boggled for about 30 years, says Stan Woosley, an astronomer at the University of California, Santa Cruz. Every time I get unboggled, they up the bar in energy by another factor of 10.
Astronomers have pressed the limits of physics and imagination trying to explain these mysterious explosions. In the 30 years since gamma ray bursts were discovered, researchers have concocted more than 150 theories of what they might be. Today, as specialized satellites search the sky and new data come in nearly every day, researchers know that gamma ray bursts must be incredibly far away and staggeringly powerful, and only a handful of the purported explanations remain plausible. One of the surviving theories belongs to Woosley, a soft-spoken 53-year-old Texas native with a teenagers mop of brown curls, a gentle smile, and a passion for explosions.
The reason that anyone goes into gamma ray bursts is they like to blow things up, says Chris Fryer, a researcher working with Woosley. I meet supernova theorists and they always have someplace they use for blowing things up. It sounds farfetcheduntil you hear a little about his bosss past.
When Woosley was in high school, he used to have a small laboratory in a storage shed in the apartment complex where he lived with his family. Id do little chemistry experiments, he says. Mostly, I liked to make things that would explode. He never did any damage with the things he made, but one day while he was at school the shed caught fire. When firemen opened the door to the shed, there was a small explosion. The butane that Woosley had been using to run his Bunsen burner had built up in the confined area, and it ignited with the inrush of fresh air. Although no one was hurt, the shed was destroyed, and thenceforth Woosleys enthusiasm for pyrotechnics was frowned on. Lets just say my chemistry career was discouraged, he says.
But Woosley never lost his taste for things that go bang. After obtaining a bachelors degree in physics in 1966 and a Ph.D. in astrophysics in 1971 from Rice University in Houston, he launched into a career studying supernovae and stellar eruptions. Having published more than 200 papers on these subjects, Woosley has spent his adult life figuring out how to make ever-bigger explosions. Now he thinks hes figured out how to make the most powerful explosions of all.
Woosley believes gamma ray bursts originate in the death throes of huge starsthose that are 30 to 50 times more massive than the sun. Such massive stars burn out quicklyin millions, instead of billions, of yearsand collapse under the pull of their own tremendous gravity. The collapse triggers a huge explosion. Woosley thinks that if such a star spins fast enough, the explosion will produce a pair of back-to-back jets, one over each pole of the spinning, collapsing star. The two jets will, he believes, shoot into space to produce gargantuan flashes of gamma rays that can be detected literally across the universe.
Woosley has dubbed this pirouetting star explosion a collapsar. With the assistance of researcher Fryer and graduate student Andrew MacFadyen, he is honing his idea, adjusting variables such as the mass of the star and the rate of rotation to see just how much energy he can get out of the system. Of course, no storage shed is big enough to contain such enormous explosions. Woosley and company set off their fireworks in the virtual universe inside their computers.
D espite their incredible size, gamma ray bursts flashed across the heavens without our knowing it until just a few decades ago. Gamma rays are particles of lightor photonsmuch like the ones we detect with our eyes. But gamma rays have much higher energy and cannot be seen. They also cannot pass through Earths atmosphere. Instead, they interact with the air and fizzle out in invisible showers of low-energy electrons, positrons (the antimatter twins of electrons), and other subatomic particles. So gamma ray bursts remained hidden until the Space Age began and scientists could send their instruments up above Earths life-giving blanket of gases.
In 1967, military satellites discovered the bursts inadvertently. The Vela satellites were supposed to look for radiation from space-based nuclear bomb tests, which were banned under a treaty between the United States and the Soviet Union. But the satellites detected occasional flashes of gamma rays that seemed to come from deep space instead. These flashes, which occurred every few months and lasted only a few seconds, intrigued astronomers, who conjectured that they originated on the surfaces of neutron starsthe cores of stars 10 to 30 times more massive than the sun that have burned outwithin our own galaxy.
This hypothesis met a serious challenge with the launch of a more sensitive satellite that could better show where the bursts were coming from. The Burst and Transient Source ExperimentBATSE for shortwent up in 1991. With its greater sensitivity, the new satellite began recording roughly one gamma ray burst per day. It also improved upon the military satellites by showing the position of each burst in the sky within 10 degrees. If gamma ray bursts originated within the Milky Way galaxy, scientists reasoned, then more of them should appear in the direction of the center of the galaxy. But by the end of 1992, BATSE had collected enough data to show that gamma ray bursts occurred in equal numbers in all directions. This distribution left no doubt: gamma ray bursts originate outside the galaxy.
But how far outside? The obvious interpretation of the data was that, like the galaxies themselves, the bursts were distributed across the entire universe. This cosmological distribution, as astronomers call it, posed one serious problem. It meant that gamma ray bursts are billions of light years away. To be detectable from such distances, the bursts would have to be staggeringly huge.
To get around this problem, some astronomers suggested the bursts, whatever they might be, originated in a thin spherical cloud, dubbed a halo, that surrounds the galaxy the way the peel surrounds an orange. This idea ran into a snag immediately. Since Earth is closer to the edge of the galaxy than the center, gamma ray bursts would not be seen in equal numbers in all directions unless the diameter of the halo greatly exceeded the 100,000-light-year diameter of the galaxy itself. Still, if gamma ray bursts originated in a galactic halo, the explosions could be hundreds of thousands of times closer and billions of times smaller than if the bursts originated in the deepest recesses of space. Consequently, some astronomers and astrophysicists embraced the halo theory despite its unseemly complexity.
That embrace has weakened over the last two years with the discovery that gamma ray bursts have optical counterparts. That is, bursts leave behind spots of visible light in the sky that can be seen with ordinary telescopes.
In 1997, a Dutch and Italian team launched a satellite that could determine the direction of a burst to within a tenth of a degree. On February 28, 1997, the satellite, called BeppoSAX, observed a gamma ray burst, and quickly relayed its coordinates to computers on Earth, which passed them to astronomical observatories. Within a day, telescopes had found a weak, fading light in the same spot in the sky.
A bigger break came less than three months later. The Dutch-Italian satellite observed another burst on May 8, 1997. This time, the 10-meter Keck telescope at Mauna Kei, Hawaii, not only spotted the optical counterpart, but also measured the color of the light coming from it. Keck researchers found the light was much more red than blue, a sure sign the thing they were looking at was billions of light years away.
Light from distant stars and galaxies is redder than light from those nearby because the universe is expanding. This expansion implies stars and galaxies in every direction are moving away from us, and objects that are farther away are receding faster. (Think of the universe as a cookie baking in an oven and the stars as chocolate chips within the cookie. In twenty minutes, the blob of cookie dough spreads dramatically. Chocolate chips that start on opposite sides of the blob go from one inch apart to three inches apart. Chips that start right next to each other also move apart, but only by about a half an inch in twenty minutes. The farther apart the chips, the more quickly they recede from one another.) The light from an object becomes redder as the object recedes faster, just as the pitch of a train whistle drops lower as the train pulls away faster. So by measuring the redness of starlight, astronomers can tell how fast an object is receding and, hence, how far away it is. By such reasoning, astronomers at Keck concluded the optical counterpart they had spotted resided in the deepest recesses of the universe.
Just recently, ground-based observers have managed to top even this feat. On January 23, 1999, BATSE detected a burst and quickly dispatched the coordinates to observers on the ground. Twenty-two seconds later, an automated array of 35-mm camera lenses and digital cameras stationed in Los Alamos, N.M., snapped a picture of the specified patch of the night sky. A few hours after that, researchers working with the array used the more accurate coordinates provided by BeppoSAX to find the point in the picture that the gamma rays had come from. And at the very spot, the picture showed a star. The researchers had managed to capture an image of the optical counterpart while the gamma ray burst was still happening. To their surprise, the flash from the explosion billions of miles away was bright enough to be seen with a decent pair of binoculars.
Given the evidence, most astronomers now believe gamma ray bursts originate in the farthest reaches of space. But this conclusion leaves them with a fundamental problem, namely, how do you make an explosion hundreds of times more powerful than a supernova?
W hen Alfred Nobel was experimenting with nitroglycerine in the 1860s, the city elders of Stockholm ordered him to move his lab onto a barge in the middle of a lake after an explosion killed several people, including Nobels brother Emil. The designers of the first atomic bomb tested their creation in the New Mexico desert, far away from cities and towns and the prying eyes of outsiders. Today, astronomers and astrophysicists have found a more convenient place to mess around with calamitous blasts &endash; inside their computers.
Within the workstations in the astronomy department at U.C. Santa Cruz, stars collapse, huge sheets of matter slip into oblivion, elusive subatomic particles collide and annihilate one another, and gigantic plumes of energy and matter shoot into space. All the while, Woosley, Fryer, and MacFadyen, sit in safety on the other side of the terminal screens.
On MacFadyens workstation screen, a large circular green spot stands against a bluish background. Suddenly, a tiny black dot appears in the middle of the green circle. On either side of the dot, wings of red, orange, and yellow stretch out horizontally into green, then blue. Above and below the black spot, forks of blue reach out into purple. The pattern could be a tie-dye pattern, a full-color Rorschach inkblot test, or perhaps a Daliesque butterfly. In fact, it is the portrait of a dying star. The colors stand for different densities of stellar matter. The image is sciences best understanding of a gamma ray burst, one of the most violent events in the universe.
The obvious way to try to make a gamma ray burst would be to blow up a huge star, one more than 30 times more massive than the sun, in a gigantic supernova. But this wont work because the matter blown out of the star absorbs the energy of the explosion and slows it down. Because of such impedance, supernovae last for weeks or even months. Gamma ray bursts, on the other hand, last only a few seconds, or at most a couple of minutes. A burst must therefore release an enormous amount of energy and only a smattering of matter.
Woosley has dreamt up a new kind of star explosion that should do the trick. His invention, the collapsar, is essentially a glorified spinning supernova. The spin is all important, for it pulls much of the stellar matter out of the way and allows two back-to-back jets of super-energized electrons and positrons to shoot into space at enormous speeds. These jets, also known as fireballs, produce the gamma ray burst.
In a normal supernova, a star 10 to 30 times more massive than the sun blows itself apart. The explosion begins when the star runs out of its nuclear fuel. Stars burn by combining nuclei of lighter elements to make nuclei of heavier elements in a process called nuclear fusion. At first, a star burns hydrogen to make helium. If the star is sufficiently hot, it burns helium to make carbon, and so on. Eventually, if the star is large enough and hot enough, its core fills up with iron, at which point the star falters because burning iron consumes more energy than it produces. The core of the star then collapses under its own gravity and the outer layers of the star crash in on top of it.
As the core caves in, its temperature soars to tens of billions of degrees Celsius and its density climbs to 100 billion times that of water. Under such conditions, atomic nuclei break down into their basic partsprotons and neutrons. These particles, along with healthy doses of electrons and positrons, incessantly ricochet off one another and, by dint of a fundamental nuclear interaction, produce copious quantities of subatomic particles called neutrinos. These wimpy little particles have almost no mass and interact only very weakly with other subatomic particles. (Neutrinos by the billions pass through Earth every second without so much as bumping elbows with a proton, neutron, or electron.) However, they eventually blow the rest of the star to bits.
The slippery nature of the neutrinos allows them to escape the incredibly dense, hot core while particles that interact too strongly with matter, such as photons, cannot. The neutrinos fly outward in a shock wave that interacts with the in-falling material just enough to catch it, stop it, and blow it out into space. The star bounces, as Woosley likes to say. The explosion leaves a neutron stara ball of pure neutrons only tens of miles in diameter, but several times more massive than the sunamid a vast wispy plume of gas and radiation.
Woosleys hypothetical collapsar begins with an even bigger starone 30 to 50 times more massive than the sun. When the core of such a star burns out, it also collapses. But in this case the core is far too massive to stop collapsing once it has squeezed down to a neutron star. Its gravity continues to pull it in on itself, and it collapses to zero radius, leaving behind a black holeliterally a spherical hole in the fabric of space and time that reaches out with an enormous gravitational pull and gobbles up anything that comes too near, including light. The outer layers of the star fall toward the black hole, but now the pull of gravity is so strong the shock wave of neutrinos cannot push the in-falling matter back out.
If the huge star is not spinning, the explosion simply fizzles. All the matter and its energy tumble into the black hole and disappear. But if the star is spinning fast enough, something more complicated happens. Matter near the equator of the star gets thrown out into space in an attenuated supernova explosion, while matter from the rest of the star forms a thin disk of gas swirling around the equator of the black hole. The black hole sits in the middle of the diskwhich is known as an accretion diskslurping up the hot, dense soup of protons and neutrons. The black hole and the accretion disk form the motor that drives a gamma ray burst, Woosley believes.
The trick is to generate energy with the disk and transfer it out to the regions over the poles of the spinning black hole where there is little matter to get in the way. Making the energy is easy. As the matter in the disk flows toward the black hole, it is squeezed and stirred by the enormous pull of gravity. The compressed and agitated gas heats up to 20 billion degrees Celsius, and the heat provides the energy for the gamma ray burst.
Getting the energy out of the disk is harder. To get energy to flow to the relatively empty regions above the poles, Woosley and company rely on neutrinos. Only the wimpy little neutrino and its antimatter partner are able to slip out of the incredibly dense disk.
Once out of the disk, neutrinos and antineutrinos fly off in all directions. Many fall into the black hole, but huge numbers remain outside of it. The geometry of the situation ensures many of the surviving neutrinos will collided with antineutrinos above the poles of the black hole. To see why, think of a swarm of fleas jumping around on a doughnut. Occasionally, a flea on one side of the doughnut will jump across the hole just as a flea from the other side is coming the opposite way. The two will collide above the hole. Swap fleas for neutrinos, the doughnut for the accretion disk, and the doughnut hole for the black hole, and youve demonstrated that neutrinos and antineutrinos tend to collide above the poles of the black hole. Of course, as any Star Trek fan knows, particle and antiparticle will annihilate each other and release their energy.
With the energy flowing into the polar regions, the fireworks can begin. As the disk is consumedin just secondsenergy collects above the poles and shoots out into space in the form of two opposing narrow jets, one over each pole. These jets, a fiery mixture of electrons, positrons, and photons, slice through space at nearly the speed of light, 186,000 miles per second.
It is not the jets themselves that appear as a gamma ray burst. Rather, gamma rays are produced when parts of a jet traveling at different speeds bump into and rub against one another. The shocks from such shifting release energy in the form of gamma rays, much as the shocks from the shifting of tectonic plates during an earthquake release energy in the form of seismic waves.
Borrowed from the study of the giant black holes at the centers of galaxies, the black hole and accretion disk idea requires a bit of fine-tuning to produce a gamma ray burst. For example, the black hole must spin. Spin pulls the matter away from the poles of the black hole. It also throws the matter in the disk away from the black hole in the same way that spin in a washing machine throws the clothes against the side of the drum. This makes the disk slip into the black hole more slowly, over the course of seconds, and gives it a chance to transfer its energy to the poles. Without enough spin, the black hole swallows the disk and all of its energy, too, in a fraction of a second.
If correct, the black hole and disk idea also reduces the amount of energy needed to make a gamma ray burst. Because the jets shoot out in narrow cones filling about one percent of the sky, the gamma ray bursts they produce require 100 times less energy than they would if the explosion shot out symmetrically in all directions. On the other hand, the fact that the burst of gamma rays comes out in such narrow beams implies we see only one of every 100 bursts, that is, the ones that point right at Earth.If Woosleys idea is correct, the universe teems with gigantic explosions.
Woosely and company are still working out the details of the collapsar model of gamma ray bursts. But they would like to test it against astronomical data as soon as possible.
In physics, MacFadyen says, you can set up an experiment, like dropping a feather and a bowling ball at the same time. In astronomy, you can only make observations.
Fortunately for the researchers, new and better observations should come fast as research in gamma ray bursts intensifies. (Astronomers are now publishing more than a paper per day on the subject.) Within the next few years, new satellites will replace the aging BATSE and BeppoSAX. The new satellites will provide astronomers with coordinates for bursts more precisely and rapidly, making it easier to spot optical counterparts. They will also be more sensitive to gamma rays and, hence, detect more bursts. Beyond that, researchers hope to launch satellites featuring both gamma ray detectors and optical telescopes, which will obviate the hassle of coordinating observations with telescopes on the ground.
To test the collapsar theory, Woosley and coworkers are particularly interested in getting a peek at the neighborhoods that gamma ray bursts come from. The collapsar theory predicts gamma ray bursts will originate inside galaxies, while other theories, which mostly involve pairs of stars cartwheeling through the cosmos, predict they may come from the space between galaxies.
Woosleys team would also like to go back and double check the spots in the sky where gamma ray bursts have been seen. We predict that if you go back two weeks after a gamma ray burst and look with a high-powered telescope you should see a supernova, he says. Only the collapsar theory predicts a supernova will accompany the burst, so seeing one in the same spot as a burst would provide strong evidence that the model is correct.
While Woosleys idea remains untested, it has already gained adherents. It looks like hes really on the right track, says Kevin Hurley, an astronomer at the University of California, Berkeley. He may have solved one of the more important cosmic mysteries.
Others are more skeptical. Others are more skeptical. Alice Harding, an astronomer at NASAs Goddard Space Flight Center in Greenbelt, Md., is reserving judgment until the details of the collapsar theory become clearer. One strength of the theory is that the energy is very high, she says. But the problem is the mechanism is not well understood. Harding notes the latest evidence suggest some gamma ray burststhe ones lasting less than a secondmay, in fact, originate within the galaxy. The collapsar theory would not fit this particular type of gamma ray burst, she says.
For his part, Woosley remains guardedly optimistic. In the nearly 30 years that he has puzzled over gamma ray bursts, Woosley has seen many theories come and go, including several of his own devising. Ive had more wrong models than anyone in the world, he says. And now I think Ive got one of the right ones.
If he is rightif he has figured how to make the biggest explosions in all of creationhe will be an inspiration to future astronomers, especially those now playing with explosives in their hide outs.
Text © 1999 H. Adrian Cho
Illustrations © 1999 Pat Latas