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Come to appreciate the wonderful Mayan legacy. 

 Immerse yourself in the colonial Merida, 

 its archaeology, cuisine, traditions and all 

 the cultural heritage Yucatan has to offer on this 

 Smithsonian Journeys Travel Adventures tour. 



For more information, call 

 Smithsonian Journeys Travel Adventures at 

 1-800-528-8147 and mention promotion 

 code: A409-AX1 -918 or visit our web site at 

 www.smithsonianjourneystraveladventures.org 



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BeirtMD nun anciAnuNS 



forth at 12,000 miles a second, creat- 

 ing shock waves whose Mach numbers 

 are in the thousands. And, as with iron 

 in the solar corona, the maelstrom cre- 

 ates strange new versions of common- 

 place elements. 



Astrophysicists call that short-lived 

 spectacle a supernova. During its first 

 several weeks it can outshine billions 

 of suns. Nowadays, investigators iden- 

 tify hundreds of new supernovas each 

 year. Those discoveries come about 

 not because of the explosion but be- 

 cause of the shock waves that pass 

 through the star's own outer layers and 

 render the event visible across billions 

 of light-years. 



It you think a supernova shock front 

 is big and bad, picture what happens 

 when an entire galaxy crashes into its 

 neighbors. Take the cluster of large 

 galaxies called Stephan's Quintet,a col- 

 lision of four galaxies plus one inter- 

 loper that just happens to show up in 

 the foreground. The colliding mem- 

 bers of the group have torn gas clouds 

 from their host galaxies and strewn 

 them hither and yon, making a real mess 

 of the environs. One protagonist, plum- 

 meting toward its three neighbors at 

 speeds exceeding Mach 100, has creat- 

 ed a bow shock so immense that its 

 leading edge is larger than the entire 

 expanse of our own Milky Way galaxy. 



Speaking of the Milky Way, it's 

 falling toward the Andromeda galaxy. 

 Shock waves are forecast for a few bil- 

 lion years from now. 



Back on Earth, if you want to blow 

 something up, you need a bomb, a 

 medium, and the shock wave generat- 

 ed by exploding the bomb within the 

 medium. A conventional bomb doesn't 

 just puff a gust of air, like the wolf in 

 "The Three Little Pigs." The intense 

 heat generated at the instant of detona- 

 tion creates a ferocious pocket of ex- 

 panding air that moves faster than the 

 local speed of sound — a shock wave. 

 Not only does the shock wave make a 

 loud boom, but it also causes a cata- 

 strophic imbalance of pressure between 

 the sides of nearby structures (and peo- 

 ple) facing the explosion and the sides 



20 naiiirm ins i ory September 2006 



facing away. Such are the forces that 

 blow things apart into unrecognizable 

 fragments. The reason conventional 

 bombs are generally useless in space, and 

 are thus a poor defense against incom- 

 ing asteroids, is that most of space is an 

 airless vacuum. If you really want to 

 destroy an asteroid, you'll have to plant 

 the bomb deep inside, turning the 

 asteroid itself into the medium. 



Unlike a conventional bomb, an 

 atomic bomb doesn't need a medium 

 to make it lethal: the high-energy light 

 of the explosion itself passes straight 

 through the transparent air. If you're a 

 slab of concrete with a high melting 

 point, you can easily survive this phase. 

 But if you're an organic life-form and 

 happen to be positioned close to 

 ground zero, you vaporize. Any surviv- 

 ing structures in the vicinity are then 

 leveled by the stupendous shock wave 

 that is carried by the medium. 



When it comes to shock waves, how- 

 ever, neither atomic bombs nor hydro- 

 gen bombs can compete with gamma- 

 ray bursts, the greatest blasts in the 

 universe. Although not yet fully under- 

 stood, they may be the death throes of 

 supermassive stars under particular cir- 

 cumstances of rotation, environment, 

 and orientation to our line of sight. 



Bright enough to be seen from Earth 

 no matter where in the universe it takes 

 place, a cosmic gamma-ray burst is akin 

 to a supernova on steroids. Between 

 the burst and Earth lies the airless vac- 

 uum of space, and so there's a gap in 

 the medium that might otherwise car- 

 ry the burst's annihilating sound and 

 fury down here to us. The resulting si- 

 lence confirms that in space, not only 

 will no one hear you scream, as Holly- 

 wood occasionally reminds us, but no 

 one will hear you explode either. 



Astrophysicist Neil deGrasse Tyson is the 

 director of the Hayden Planetarium at the Ameri- 

 can Museum of Natural History. In October,catch 

 him as he hosts the season premier of the PBS 

 television series NOVA scienceNOW. Tyson's 

 latest book, Death by Black Hole: And 

 Other Cosmic Quandaries — an anthology of 

 his favorite Natural History essays — will be 

 published this fall by WW Norton. 



