Deceptive nebulous apparition? The double helix at the center of the galaxy

Appearances can be deceiving-especially in outer space. Thirty years ago, the Viking 1 orbiter 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-enhancing software--insisted that this geological 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 Martian surface, and his sketches and speculations created a sensation.

Such misinterpretations happen all the time, according to psychologists. The human tendency to translate random sensory input into familiar shapes is an important cognitive process. It helped early humans recognize predators and other threats in the environment. It also helps explain why people have given constellations, 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 collective consciousness, astronomers have dubbed one recent discovery 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 Angeles--looks a lot like a strand of DNA. This striking corkscrew structure is a mere 200 light-years away from the nucleus of our galaxy, and it may well emanate 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 alllurk behind a deep, dense screen of dusty gas.

Even without a glowing double helix sticking out of it, the galactic 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 centered on the black hole. Beyond this disk lies another thin disk primarily of molecular gas--picture the rings of Saturn, but a lot bigger and even more chaotic, The second, outer disk is about twenty light-years across. Within, among, and above surprisingly 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 Nebula fit into this picture? Well first, consider 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 detected 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 visible light is almost completely quenched by intervening, obscuring dust.

Morris and his colleagues had been looking for infrared emissions at a wavelength of about twenty-four microns. That's the wavelength at which interstellar dust emits its greatest heat, if it's at a temperature of -240 degrees Fahrenheit. To their surprise, they noticed 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 100 light-years long and about 200 light-years from Sgr A*.

But could the double helix be another 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 newest moons [see "Sizing Up Pluto," by Charles Liu, May 2006], Morris and his collaborators looked through astronomical archives to confirm their observations. They Found that more than a decade ago the Midcourse Space Experiment (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 outline of a double helix; because MSX didn't have the resolving power of the SST, however, the structure wasn't visible 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 magnetic field. Sometimes, cone-shaped magnetic channels lead from the object's north and south poles. If the rotation is rapid and the field is strong, those channels can direct the flow of electrically charged matter. That flow manifests itself in many phenomena--from the gentle cascade of solar wind toward Earth's poles, which produces 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.