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GAMMA RAYS

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Death Stars

Do the most powerful blasts of energy in the universe threaten Earth?

You don't want to be anywhere near a giant star when it dies. In just a few seconds, the fiery giant

collapses and forms a black hole, a region of space so densely packed with matter that the pull of gravity

overpowers everything. Nothing, not even light, can escape it.

Just before the dying star vanishes forever into the black hole, however, it gives off a several-

second burst of gamma rays so energetic, it flashes all the way across the universe. Gamma rays are

the most intense form of radiation. Gamma-ray bursts, scientists now think, are the most sudden and

violent events in the cosmos. If one struck Earth from close by, we could

be in trouble.

Satellite Vision

Gamma-ray bursts were discovered in the 1960s. Scientists didn't

know about them before that because Earth's atmosphere most

gamma rays. Even in space, they are invisible to the human eye. To find

them, scientists had to launch satellites, such as Swift, an orbiting

telescope equipped with a gamma-ray detector, which is operated by

NASA, the U.S. space agency. Circling Earth every 90 minutes, Swift can

detect gamma rays from any direction in the universe. When it does, it

quickly swings its main telescopes in that direction for a closer look.

"For decades, we had many theories for what caused gamma-ray

bursts, but we couldn't see enough of them quickly enough to study them

and test the theories," says Neil Gehrels, a NASA astronomer. "It's

amazing that such giant astronomical events can take place in just a few

seconds." Now, with quick-reacting telescopes, scientists have amassed

enough data to support the collapsing-star theory. They call such

stars collapsars.

Two Puzzles - Two puzzles about gamma-ray bursts remain. The first is,

How do collapsars emit their gamma rays? The gamma radiation from a

collapsar is so intense that scientists doubt it is given off in all directions. If it were, the total energy

output of the event would be too much for physics to explain. What's more likely, says Gehrels, is that a

collapsar emits gamma rays in focused "jets." If that is true, then far more collapsars exist than we

can see. The ones we don't see don't have their beams pointed toward us and are invisible to us.

Puzzle number two is posed by a type of gamma-ray burst called a short burst that lasts less then

two seconds. Collapsars don't implode that quickly, so some other catastrophic event must be the cause

of short bursts. Perhaps the cause is the collision of two neutron stars falling into each other and

creating a black hole. Neutron stars are "corpses" of even older stars that have become dense balls of

neutrons.

Most collapsars occur in distant galaxies many millions or billions of light-years away from Earth.

But gamma-ray bursts are so powerful, they have no trouble covering such distances. "We can see them

from anywhere in the universe"--at least the ones pointed at us, says Gehrels.

If a gamma-ray burst originated closer than a few thousand lightyears away from Earth, it could do

real damage to the planet. It could destroy the ozone layer, a layer of ozone gas (O3) in the

stratosphere. The ozone layer protects the planet from many types of harmful cosmic radiation, highly

energized particles emitted by the sun, other stars, and even distant galaxies. Life on Earth evolved

under the shield of the ozone layer. A nearby gamma ray burst could deplete the ozone, exposing land-

based plants and animals to swift DNA damage and even extinction.

Gamma-Ray Burst

An artist's

conception of

a gamma-ray burst.

(Credit: Mike

Garlick/Photo

Researchers)

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Death Stars cont.

Don't worry about having to buy gamma-ray burst insurance. Although astronomers

detect gamma-ray bursts about once a day, the universe is a super-vast place with billions of galaxies.

On average, each galaxy has just one gamma-ray burst every 100,000 years, and it usually occurs in one

of the galaxy's star-forming regions, says Gehrels.

Earth is nowhere near any major star-forming region in our galaxy. And among our local population

of stars, none is big enough to be a collapsar. A local gamma-ray burst is still possible, but the danger

posed to each of us by drunken drivers is trillions of times higher.

NASA Goddard Space Flight Center: Gamma-Ray Bursts

MSNBC: "Killer Gamma-Ray Bursts? Forget About It!"

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NASA to Launch Gamma-Ray Telescope Sept. 20, 2007, n.p.

By Alex Dominguez Associated Press Writer

GREENBELT, Md. (AP)--A new NASA space telescope will give scientists a peek at some of the most

energetic objects and events in the universe. The new Gamma-ray Large Area Space Telescope to be

launched next spring doesn't see visible light like our eyes, but gamma rays, the most energetic photons

in the electromagnetic spectrum. They are produced by black holes, supernovae, neutron stars and other phenomena. GLAST will

be the first gamma ray observatory to survey the entire sky. Scientists are hoping it will provide clues

about dark matter, the early universe and allow them to test fundamental principles of physics. "These are the things we can think of, it's hard to say what you're going to find," said Steve Ritz, a GLAST

project scientist at NASA's Goddard Space Flight Center, which is leading the project. Gamma rays don't survive the trip through Earth's atmosphere which is why NASA is launching GLAST into

orbit from Cape Canaveral. The GLAST observatory consists of two instruments, the Large Area Telescope

and the GLAST Burst Monitor. Gamma ray photons have so much energy they create matter after striking a tungsten plate in the

telescope--producing an electron and its exact opposite, a positron. The tracks of the two particles tell

which direction the gamma ray photon came from, said GLAST team member Dave Thompson. GLAST

follows previous gamma ray observatories, including NASA's Swift spacecraft, which was launched in 2004

and the Compton Gamma Ray Observatory was placed into orbit by the space shuttle Atlantis in 1991 and

deorbited in 2000. The European Space Agency's Integral observatory, which can observe objects in gamma

rays, X-rays and visible light, was launched from Kazakhstan in 2002. Despite the number of observatories studying gamma rays, a lot remains to be discovered. For

example, the source of more than half of the gamma rays detected by the EGRET observatory is unknown,

Thompson said. "That's why we certainly need to know more about the gamma ray sky," Thompson said.

"We've only scratched the surface of the how and why." Dark energy and dark matter are particularly intriguing because they are two of the biggest

mysteries of modern science. Scientists believe some gamma ray bursts may be created by dark matter

collisions. Dark matter, thought to be atomic particles left over from the Big Bang, doesn't give off light or

heat, but does have mass and affects the gravity of galaxies it inhabits. While it can't be seen, scientists

believe it accounts for much of the mass of the universe. GLAST will also allow scientist to study black holes, collapsed stars with extremely strong

gravitational pull that suck matter in and create jets of gas and gamma rays. Cosmologists also hope to learn

about the birth and early evolution of the Universe.