Culprit named in starlight theft
Big carbon molecules known as buckyballs are suspected in old interstellar mystery.
Mysterious molecules floating through interstellar space have been siphoning away shards of the starlight that reaches Earth, and scientists have long tried — and failed — to identify the culprits.
Now researchers say they may have found some of the suspects: buckyballs.
If they’re right about the enormous spherical carbon molecules, it could offer clues about the potential for pre-life chemistry in other parts of the Milky Way, experts said.
It may also help explain the nature of the diffuse interstellar bands that have been puzzling scientists for 90 years.
These bands of missing light were first noticed in 1919 by Mary Lea Heger Shane, then a graduate student working at the Lick Observatory outside San Jose. Shane was studying the light spectrum of binary stars.
Astronomers spread out the wavelengths of starlight as if splitting it into a rainbow with a prism, then look for dark lines that punctuate the otherwise perfect colors. Those lines represent missing wavelengths of light, which are absorbed by specific chemicals within the star. That’s why a star’s particular pattern of dark lines serves as its light fingerprint.
But when Shane looked at the binary stars’ fingerprints, or spectra, she noticed something was amiss. There seemed to be dark bands of missing light that could not be caused by the chemical composition of the stars. Scientists can tell the difference because the lines in a star’s light fingerprint shift on the spectrum as the star physically moves back and forth and its light is stretched and squeezed on its way to Earth.
The mysterious dark bands, however, did not move.
If the missing light wasn’t absorbed by chemicals within the star, it must have been absorbed by molecules in space that lie along the starlight’s path to Earth. But what could those molecules be?
Scientists believe carbon-rich molecules are a good candidate, because it’s easy for carbon to link up with itself and create long chains. That is an exciting prospect, given that life on Earth is carbon-based.
The problem is, no one has been able to confirm whether this is true, nor say with certainty what the specific light-stealing molecules are.
“Many people have come up with all sorts of ideas,” said Michael McCarthy, an astrochemist at the Harvard-Smithsonian Center for Astrophysics. Some of these ideas “have been borderline crazy … killer bacteria in space, some wild porphyrin molecule.”
Scientists have probed the nature of these lines by trying to create molecules in the laboratory that reproduce the same dark lines that astronomers see in starlight.
That’s how the molecule buckminsterfullerene, a group of 60 carbon atoms arranged in a sphere, was first discovered. The trio of scientists who synthesized it were probing the nature of these diffuse interstellar bands
and ended up creating the massive molecules almost by accident. (They named the molecules after designer/inventor Buckminster Fuller, though they’re better known as buckyballs. The discovery was recognized with a Nobel Prize in chemistry in 1996.)
Scientists have long thought that buckyballs could be one cause of the missing light. Some focused on positively charged ions of the molecule (called “C60plus”) because space radiation can easily knock off a negatively charged electron.
But it’s been difficult to
prove because researchers test their spectra by lodging the molecules in a matrix of solid neon, which has the potential to warp the light fingerprint.
In new experiments, spectroscopist John Paul Maier of the University of Basel in Switzerland and his colleagues tested the buckyballs in extremely cold, dense helium gas whose temperature was within 8 degrees Celsius of absolute zero.
In space, Maier noted, a buckyball might collide with a hydrogen atom about once a year. But by using the cold helium gas in the lab, researchers could observe a million collisions a second.
“Basically we do exactly the same measurement that’s going on in space … except we are speeding it up,” Maier said.
Sure enough, the resulting absorption lines for C60plus ions matched the dark lines spotted at two distinct wavelengths — 9,632 and 9,577 angstroms — that were first described in 1994.
The results were reported this month in the journal Nature.
The discovery could be a “real breakthrough for the field,” said McCarthy, who wasn’t involved in the new study. “They’re telling us about a significant reservoir of, in all likelihood, organic matter the universe.”
Buckyballs are so intricate, the thinking goes, that they could lead to other complex carbon chains.
“I think it’s telling us that organic chemistry is kind of universal, both on Earth and in many potential astronomical objects,” McCarthy said.
Benjamin McCall, a researcher at the University of Illinois at Urbana-Champaign who was not involved with the study, called it “the most convincing identification of the molecule responsible for [diffuse interstellar bands] in the almost century-long history of the problem.”
However, he added, scientists would need to make more observations to lock down their case.
“What we have now might win a conviction in civil court (‘preponderance of the evidence’), but not in criminal court (‘proof beyond a reasonable doubt’),” McCall wrote in an email.