newest moons [see "Sizing Up Pluto," by 

 Charles Liu, May 2006] , Morris and his 

 collaborators looked through astro- 

 nomical archives to confirm their ob- 

 servations. They found that more than 

 a decade ago the Midcourse Space Ex- 

 periment (MSX) had detected the 

 glowing strand, which extends some 

 100 light-years at nearly right angles 

 from the plane of the Milky Way If you 

 squint really hard at the MSX image, 

 you can just make out the fuzzy out- 

 line of a double helix; because MSX 

 didn't have the resolving power of the 

 SST, however, the structure wasn't vis- 

 ible to anyone who didn't already know 

 to look for it. 



Fine. The double-helical coil is out 

 there. Now the obvious question: 

 how'd it get there? 



Whenever we astronomers see twists 

 and braids, we think right away of two 

 things: rotation and magnetic fields. 

 When an electrically charged celestial 

 object spins, it often generates a mag- 

 netic field. Sometimes, cone-shaped 

 magnetic channels lead 

 from the object's north 

 and south poles. If the ro- 

 tation is rapici and the field 

 is strong, those channels 

 can direct the flow of 

 electrically charged mat- 

 ter. That flow manifests 

 itself in many phenome- 

 na — from the gentle cas- 

 cade of solar wind toward 

 Earth's poles, which pro- 

 duces the northern and 

 southern lights, to the 

 superenergetic outflows 

 of quasars, which can 

 generate more energy in 

 a second than the Sun 

 does in a million years. 



Morris and his col- 

 leagues suggest that the 

 double helix could be a 

 magnetic "torsional Alf- 

 ven wave," generated by 

 a flattened, electrically 

 charged, spinning gaseous 

 structure. The structure 

 could be the disklike ring 



of molecular gas that encircles Sgr A*. 

 It's about the right size for the job — a 

 little more than twice the width of the 

 double helix — and it spins around the 

 black hole at a more-than-adequate 

 200,000-plus miles per hour. Dust par- 

 ticles blown upward from the vicinity of 

 the ring would become ionized by ul- 

 traviolet radiation; those particles, now 

 electrically charged, would follow along 

 the magnetic field lines that trace the 

 torsional wave. The particles' infrared 

 glow, picked up by the SST, would ap- 

 pear as a delicate, double-helix structure. 



There are a few bugs in this picture, 

 though. For one thing, between 

 the bottom of the double helix and the 

 molecular ring around the galactic cen- 

 ter is a gap of more than 100 light-years. 

 For another, the double helix doesn't 

 point directly at the ring. Moreover, 

 since there are always two magnetic 

 poles, a second double helix should be 

 protruding downward, perpendicular 

 to the ring in the opposite direction. 

 But there's only one. 



Infrared image of the central region of our galaxy was made by the 

 Spitzer Space Telescope (SST). The detail (top of opposite page) shows 

 the recently discovered Double Helix Nebula, about 100 light-years long; 

 that image was also obtained with the SST, but at a longer infrared 

 wavelength. The origin of the double-helix structure may be a coil-shaped 

 magnetic wave emanating from a flattened, spinning ring of gas near the 

 black hole at the galactic center. Both black hole and ring are hidden from 

 view within the bright white dot near the center of the image above. 



Here's another complication. Can a 

 delicate torsional wave persist long 

 enough to form a double helix that 

 keeps its shape through the rough- 

 and-tumble galactic center and con- 

 tinues twisting evenly hundreds of 

 light-years beyond that? 



One example of how dynamic things 

 can get comes from a recent study by 

 Angelle M. Tanner of the Jet Propul- 

 sion Laboratory in Pasadena and sever- 

 al colleagues (including Morris). Tan- 

 ner's team showed that the stars with- 

 in two light-years of Sgr A* — that is, 

 closer to the black hole than to its en- 

 circling gaseous ring — appear to orbit 

 in roughly the same orientation. But 

 unlike the nearby molecular gas, they're 

 not arranged in a ring. Tanner says the 

 stars' scattered arrangement suggests 

 they formed elsewhere and were pulled 

 into the neighborhood rather recently; 

 their "migration" may well have dis- 

 turbed or disrupted a torsional wave. 



Another contributor to the dy- 

 namism of the galactic center is the 

 unstable nature of the gaseous ring it- 

 self; huge amounts of gas 

 continuously flow in and 

 out of it, which could 

 make the structure of its 

 magnetic field too variable 

 to sustain a prolonged tor- 

 sional wave. 



Until investigators do 

 detailed supercomputer 

 simulations, there probably 

 won't be a complete expla- 

 nation of the Double Helix 

 Nebula. But I wouldn't be 

 surprised if, years from 

 now, its portrait becomes an 

 icon in the gallery of as- 

 tronomy. Like the face on 

 Mars, it's a striking image. 

 Unlike that face, though, 

 this picture may actually 

 shed light on the mysteries 

 of the cosmos. 



Charles Liu is a professor of 

 astrophysics at the Cay ( Universi- 

 ty of New York and an associate 

 with tlw American Museum of 

 Natural History. 



July/August 2006 NATURAi HISTORY 



