Richard A Lovett
Astronomers using small telescopes around the world are unearthing clues to an enigma once dubbed “the most mysterious star in the galaxy.”
The star’s official name is KIC 8462852, but it is better known as Tabby’s star in honour of its discoverer, Tabetha Boyajian, now a professor at Louisiana State University. Tabby’s star is bright enough to appear on many star photos, but it was only recently, after it happened to be in the field of view of the Kepler space telescope, that it came to prominence.
Kepler was designed to study brightness fluctuations in 150,000 stars, hoping to spot the silhouettes of planets passing in front of them. But in the process, it found a number of other astronomical oddities, of which the oddest of all is Tabby’s Star.
An otherwise ordinary star about 50% bigger than the sun, its brightness can change by as much as 20%, seemingly at random. Studies of old images of it have revealed that in addition to these short-term flickers, its brightness has also been undergoing a long-run change since at least the 1890s.
At one time, scientists were seriously considering the possibility that our view was being obstructed by a swarm of alien artefacts, such as a fleet of giant solar collectors milling around it to provide power to an extraterrestrial civilisation.
Today, however, the cause is believed to be natural. In a pair of presentations at a recent meeting of the American Astronomical Society in Denver, Colorado, two teams of scientists, including high school students using a 70-centimetre telescope at The Thacher School in Ojai, California, have found evidence that the obscuring material is actually dust, drifting between us and the star in tendrils of varying thickness.
“The size range [of the particles] is comparable to smoke particles from a campfire,” says Eva Bodman, a postdoctoral researcher at Arizona State University, Tempe, who was part of one of the teams. “So this dust is more like a misty cloud than the dust you find in your home.”
What’s interesting, say Bodman and two high school students from The Thacher School, who helped monitor the star for 135 nights in spring and summer of 2017, is the degree to which the star’s dimming in four major events in May, June, August, and September, varied with the wavelength of the light coming from it.{%recommended 6486%}
“This tells us something about the material obscuring the star,” Bodman says.
If all wavelengths are affected similarly, it means that whatever is blocking the light is composed of large particles, she says. In principle, these could be the once-postulated swarm of alien solar collectors, but more likely they would simply be large grains of dust.
But if blue light and red light are blocked to different degrees, that’s a clue to the size of the particles obscuring our view. The size of those particles can be estimated by comparing how they affect red and blue light. “We do that by comparing to models of how light interacts with dust of various sizes,” Bodman explains.
Based on this, it appears that the dimming of Tabby’s star comes not from large objects such as swarms of asteroids, comets, or alien solar collectors, but from drifting bands of dust particles. But like any good mystery, it’s not quite that simple.
Each of the four dimming events observed in 2017 affected red and blue light differently, suggesting that they involved dust particles of different sizes. And the long-term brightness changes appear to be associated with much larger grains.
“So the dust cloud is extremely complex,” Bodman says. “Each dip is a different kind of dust … What we’re seeing is different parts of the [dust] cloud as they pass in front of the star.”
The source of the dust, however, remains a mystery.
“We don’t know what’s happening,” says Alejandro Wilcox, one of the students studying the star at The Thacher School.
“For all we know there could be microscopic aliens,” he quips. “But realistically, we’re not sure whether it’s really close to the system or far away. It could be closer to us than to Tabby’s star, or it could be closer to the star.”
Bodman doesn’t disagree, but opts for it being something close to the star, rather than a drifting interstellar cloud. “I think it could be clumpy stuff going around the star,” she says.
But if so, she says, that means that the tiny particles associated with the short-term dips would be so close to the star that they should be blown away by stellar winds within a few years. The fact that they’re there to be seen means that they must have been created very recently.
Meanwhile, astronomers are keeping alert for whatever happens next.
“More than 100 astronomers around the world are monitoring the star regularly,” says Yao Yin, another student from The Thacher School team. “If a significant dip happens, we will be able to capture it immediately.”
Also important is to look for similar stars. Tabby’s star was discovered only because its brightness changes were seen by Kepler, which surveyed just a tiny fraction of the sky. Will upcoming sky surveys find additional such objects?
The smart money appears to lie on yes. “[They] already have,” said Robert Stencel, a professor of astronomy at the University of Denver, who declined to give details.
And that means stay tuned. Perhaps even before its mysteries are fully unraveled, Tabby’s star will be unseated as the most mysterious star in the galaxy.
