Researchers Line Up to Get DIBs on Solving Outer Space Mystery
The diffuse interstellar bands haven’t broken the top 40 yet, but their light show is amazing.
They’re not a musical act, but a cosmic one: Astronomers see the bands when they smear the light from some distant stars out into a spectrum, the same way a prism breaks sunlight into a rainbow.
The rainbow they create has the familiar vertical swaths of color, but it also has about 200 thin, dark bands shot through it. The dark bands are like a forest of trees partially obscuring every color of the rainbow, and varying in width, intensity and other characteristics.
The light show is amazing to astronomers because it has proven an extremely difficult nut to crack. More than 70 years after the bands’ discovery, nobody can explain what could be producing such an intricate pattern of light and dark.
Although it is clear that the bands are basically a fancy kind of shadow created as starlight filters through something floating in interstellar space, the identity of that material remains a mystery.
“It’s just an extremely irritating thing that a major puzzle like this has continued to elude us,” says George Herbig, an astronomer at the University of Hawaii in Honolulu.
Even more frustrating, whatever is making the diffuse interstellar bands, a.k.a. the DIBs, is also part of what suns, planets and people are made of. So solving the DIBs mystery may reveal secrets about ourselves and our origins.
Researchers have proposed various causes for the bands, from alien bacteria floating in space to clouds of the 60-carbon “buckyballs” that recently won a Nobel prize for two U.S. scientists and one Briton. In fact, this year’s Nobel winners were trying to simulate the DIBs in their laboratory when they stumbled upon the unusual molecules.
So far, none of the DIBs candidates have proven satisfactory.
But now a laser pioneer who has spent decades studying atomic physics thinks he has a simple explanation for the DIBs. Peter Sorokin, a researcher at IBM’s Thomas Watson Research Center in Yorktown Heights, N.Y., has theorized that DIBs are created by clouds of hydrogen--the most abundant gas in the universe and an obvious DIBs candidate.
“There’s a lot of hydrogen out there. It’s surprising, really, that no one thought of something like this,” says Sorokin, who is quick to acknowledge that he knows next to nothing about astronomy.
According to Sorokin’s theory, the DIBs are made by interstellar hydrogen that is absorbing light in a complicated double-whammy process. Like Bach’s “Goldberg Variations,” which create 30 tunes from a single theme, that basic process can be adjusted ever so slightly to make the dozens of DIBs.
Most astronomers find Sorokin’s theory difficult to understand--and even harder to believe.
“It’s got some serious astrophysical flaws,” says Ted Snow, an astronomer at the University of Colorado at Boulder. “It doesn’t appear to work so well to anybody but Peter Sorokin.”
They may not think he’s right, but Snow and his colleagues encourage Sorokin’s efforts nonetheless. They find themselves in the odd situation, actually fairly common in science, of praising an outsider for wandering into a field where he has no background.
“I applaud the interest of people who are outside the field who might bring something to the subject that astronomers have missed,” Herbig says. “It’s about time somebody came along and straightened this out.”
Sorokin first presented his theory in 1994 at a Boulder, Colo., meeting dedicated to the DIBs problem. At the time, Snow and his colleagues had no trouble showing why the new guy’s theory wouldn’t hold water.
So Sorokin went back to his New York laboratory, where he fiddled with the idea a bit. Then he showed it to Snow again. And again. And again.
Sorokin’s persistence has paid off. He and IBM researcher James Glownia will finally get an official hearing from the astronomy community in a paper they’ve written for the Dec. 20 issue of the Astrophysical Journal, one of the most prestigious publications in the field.
In their paper, Sorokin and Glownia argue that hydrogen molecules could account for at least 70 of the DIBs through a complicated process that requires the gas be bathed not just in visible starlight but also in invisible, high-energy ultraviolet light.
There is lots of evidence to support his theory, Sorokin said in Baltimore recently at a meeting of the Council for the Advancement of Science Writing. For example, the most impressive DIBs come from bright stars that make plenty of the ultraviolet light that his theory requires.
Sorokin also showed how his theory could account for several distinctive features of the DIBs, and cited laboratory experiments that have reproduced a few of the bands using the same double-whammy process he describes.
But dimmer stars also make DIBs, and they couldn’t possibly be producing the high-energy ultraviolet rays required by Sorokin’s theory, Snow points out. And if there were hydrogen clouds bathed with ultraviolet light around distant stars, astronomers would have seen them by now.
Snow has compiled a litany of rebuttals to Sorokin’s argument and is willing to share them with anybody who asks. He and most astronomers favor a family of complex carbon-based molecules, known as polycyclic aromatic hydrocarbons, as the cause of the DIBs.
The highly variable chemical composition of the molecules, which are found on Earth in car exhaust and other combustion products, makes it possible that they could absorb light at 200 different wavelengths, Herbig says.
In space, the carbon molecules are probably generated in aged stars, and then blown all over the place when the stars die.
Settling the debate will require more observations, perhaps by using instruments such as the Hubble Space Telescope and the far-ultraviolet spectroscopy explorer, a satellite scheduled for launch in 1998.
But until the right observations are made, Snow adds, he’s glad to hear anybody’s ideas.
“If you’ve got a self-consistent argument,” Snow said, “it’s not a bad idea to put it out there and get people thinking about it.”
