OUT THERE 



Deceptive 

 Nebulous 

 Apparition? 



The double helix 



at the center of the galaxy 



By Charles Liu 



Appearances can be deceiving — 

 especially in outer space. Thir- 

 ty years ago, the Viking 1 or- 

 biter took thousands of photographs of 

 the Martian surface: craters, canyons, 

 mountains, and more. The peaks and 

 shadows of one mountain evoked a 

 fuzzy, mile- wide human face. For the 

 next quarter century, folks with vivid 

 imaginations — including some who 

 were pretty handy with image-enhanc- 

 ing software — insisted that this geolog- 

 ical oddity was evidence of intelligent 

 life on Mars. Finally, in 2001, high- 

 resolution images made by the Mars 

 Global Surveyor laid their speculations to 

 rest: the first "face" was a coincidental 

 confluence of light and shadow. The 

 mountain was really just a mountain. 



But you can't blame people whose 

 eyes led them to an anthropomorphic 

 conclusion. They were far from the 

 first, even among Mars-watchers. A 

 century ago the American astronomer 

 Percival Lowell thought he saw a vast 

 network of artificial canals on the Mar- 

 tian surface, and his sketches and spec- 

 ulations created a sensation. 



Such misinterpretations happen all 

 the time, according to psychologists. 

 The human tendency to translate ran- 

 dom sensory input into familiar shapes 

 is an important cognitive process. It 

 helped early humans recognize preda- 

 tors and other threats in the environ- 

 ment. It also helps explain why people 



of molecular gas — picture 

 the rings of Saturn, 

 but a lot bigger and 

 even more chaot- 

 ic. The second, 

 outer disk is 

 about twenty 

 light-years 

 across. With- 



have given con- 

 stellations, star clusters, 

 and nebulae the names of gods, humans, 

 animals, and other terrestrial objects. 



So it's understandable that today, 

 when genetics and biotechnology loom 

 large in our coDective consciousness, as- 

 tronomers have dubbed one recent dis- 

 covery the Double Helix Nebula. That 

 object, a coiling jet of glowing gas — 

 spied with an infrared eye by a team of 

 astronomers led by Mark R. Morris of 

 the University of California at Los An- 

 geles — looks a lot like a strand of DNA. 

 This striking corkscrew structure is a 

 mere 200 light-years away from the nu- 

 cleus of our galaxy, and it may well em- 

 anate from that enigmatic middle of the 

 Milky Way — where a gaseous spiral, a 

 disky gaseous ring, a swarm of hot 

 young stars, and a supermassive black 

 hole all lurk behind a deep, dense screen 

 of dusty gas. 



Even without a glowing double 

 helix sticking out of it, the galac- 

 tic center is a weird place. For starters, 

 its black hole, known as Sgr A* (yes, 

 the asterisk is part of the name), weighs 

 in at a hefty 3.5 million times the mass 

 of our Sun. A swirling whirlpool-like 

 disk of hot, ionized gas, complete with 

 spiral arms that sorta-kinda mimic the 

 larger galaxy in which it resides, is cen- 

 tered on the black hole. Beyond this 

 disk lies another thin disk primarily 



in, among, 

 and above 

 those gaseous 

 structures are 

 surprisingly 

 large numbers 

 of hot, massive 

 stars that are much 

 younger than the stars 

 you'd expect to find near 

 a supermassive black hole. 

 How does the Double Helix Neb- 

 ula fit into this picture? Well first, con- 

 sider how it was found, a good clue to 

 its identity. It was captured with the aid 

 of the Spitzer Space Telescope (SST), 

 which detects infrared light. Infrared, 

 which is invisible to human eyes but de- 

 tected by human skin as heat, has longer 

 wavelengths than visible light and can 

 penetrate substantially thicker screens of 

 dust than visible light can. That makes 

 the SST an ideal tool for studying the 

 Milky Way's galactic center, whose vis- 

 ible light is almost completely quenched 

 by intervening, obscuring dust. 



Morris and his colleagues had been 

 looking for infrared emissions at a wave- 

 length of about twenty-four microns. 

 That's the wavelength at which inter- 

 stellar dust emits its greatest heat, if it's 

 at a temperature of —240 degrees 

 Fahrenheit. To their surprise, they no- 

 ticed a pattern that looked a lot like a 

 double helix. Given the temperature of 

 the pattern, it had to be a stream of dusty 

 gas, more than 1 00 light-years long and 

 about 200 light-years from Sgr A*. 



But could the double helix be an- 

 other face-on-Mars illusion, perhaps 

 created by a chance superposition of 

 glowing gas? Probably not. There's 

 good reason to think the object has a 

 coherent physical structure. For one 

 thing, it wasn't an isolated discovery. 

 Like the discoverers of Pluto's two 



NATURAL HISTORY July/August 2006 



