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Robofly
Killer Surf!
Tracking the Bloom
This Won’t Hurt a Bit
Echoes from the Past
A Ride on the Wild Side
Tongue Twister
KC and the Ground Sludge Band
Twinkle, Twinkle Collapsing Star
One if by Land, One if by Sea
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TWINKLE
,TWINKLE,
COLLAPSING STAR
Cosmologists
try to explain
most powerful EXPLOSIONS
in the universe
BY
H.
Adrian Cho
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 rays—extremely high energy particles
of light—washes 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. (Don’t worry—the 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.
“I’ve 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 teenager’s 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 farfetched—until
you hear a little about his boss’s 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. “I’d 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 Woosley’s
enthusiasm for pyrotechnics was frowned on. “Let’s
just say my chemistry career was discouraged,” he says.
But Woosley never lost his
taste for things that go bang. After obtaining a bachelor’s
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
he’s figured out how to make the most powerful explosions
of all.
Woosley believes gamma ray
bursts originate in the death throes of huge stars—those
that are 30 to 50 times more massive than the sun. Such massive
stars burn out quickly—in millions, instead of billions,
of years—and 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
light—or photons—much like the ones we detect with
our eyes. But gamma rays have much higher energy and cannot be
seen. They also cannot pass through Earth’s 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 Earth’s 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 stars—the cores of
stars 10 to 30 times more massive than the sun that have burned
out—within 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 Experiment—BATSE for short—went
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 Nobel’s
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 MacFadyen’s 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 science’s 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 won’t 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 parts—protons
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 star—a
ball of pure neutrons only tens of miles in diameter, but several
times more massive than the sun—amid a vast wispy plume
of gas and radiation.
Woosley’s hypothetical
collapsar begins with an even bigger star—one 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 hole—literally 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 disk—which is known
as an accretion disk—slurping 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
you’ve 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 consumed—in
just seconds—energy 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
Woosley’s 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.
Woosley’s 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 Woosley’s idea
remains untested, it has already gained adherents. “It looks
like he’s 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 NASA’s
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 bursts—the ones lasting less than a second—may,
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. “I’ve had
more wrong models than anyone in the world,” he says. “And
now I think I’ve got one of the right ones.”
If he is right—if he
has figured how to make the biggest explosions in all of creation—he
will be an inspiration to future astronomers, especially those
now playing with explosives in their hide outs.
BATSE satellite | accretion
disc | gamma jets | gamma
burst | computer model
-
- BIO
-
- WRITER Hyunduk Adrian Cho
- B.A., physics,
University of Chicago; Ph.D., physics, Cornell University.
Internship: Idaho National Engineering and Environmental Laboratory.
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