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4 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 severalsecond 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 Gamma-Ray Burst An artist's conception of a gamma-ray burst. (Credit: Mike Garlick/Photo Researchers) 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 (O 3) 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 landbased plants and animals to swift DNA damage and even extinction. 4 P a g e

5 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!" 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 The European Space Agency's Integral observatory, which can observe objects in gamma rays, X-rays and visible light, was launched from Kazakhstan in 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. 5 P a g e