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Tabetha Boyajian
Astronomer Tabetha Boyajian discusses Tabby’s Star, “the most mysterious star in the universe,” during a TED talk in February 2016 in Vancouver, B.C. (TED via YouTube)

The astronomers who once speculated that an alien megastructure might be responsible for the weirdly fluctuating light from a distant star have now fully ruled out that way-out explanation.

Their conclusion, reported in a paper published today by Astrophysical Journal Letters, is based on a crowdfunded analysis of the light patterns in a wide range of wavelengths.

The authors of the paper include Louisiana State University’s Tabetha Boyajian, who led the discovery team for the star known as KIC 8462852 or “Tabby’s Star”; and Penn State’s Jason Wright, who first proposed the alien-megastructure hypothesis.

Tabby’s Star was discovered about 1,500 light-years away in the constellation Cygnus, thanks to an analysis of data from NASA’s Kepler space telescope. Soon after the discovery was announced in 2015, it was dubbed the “most mysterious star in the universe,” because its brightness underwent dramatic dips over intervals ranging from five to 80 days.

Wright suggested that the variations in brightness might be due to the blocking effect of a huge structure that was built around the star. He acknowledged that the presence of an alien megastructure, such as a Dyson sphere, was highly unlikely. Nevertheless, he and his colleagues thought the hypothesis was worth checking out.

“We were hoping that once we finally caught a dip happening in real time, we could see if the dips were the same depth at all wavelengths,” he explained today in a news release. “If they were nearly the same, this would suggest that the cause was something opaque, like an orbiting disk, planet, or star, or even large structures in space.”

Dips in brightness
This chart shows variations in the brightness of the star KIC 8462852 between May 2017 and December 2017, as recorded by telescopes in Texas (ELP), Hawaii (OGG) and the Canary Islands (TFN). Significant dips have been nicknamed Elsie, Celeste, Skara Brae and Angkor. (Boyajian et al.)

To help pay for the observations and analysis, Wright and his colleagues raised more than $100,000 in a Kickstarter campaign.

A network of telescopes around the world, coordinated through the Las Cumbres Observatory in California, detected four sustained dips in the starlight between March 2016 and December 2017. The team went so far as to name each of the dips. The first two were called Elsie and Celeste, while the latter two were named after lost cities (Skara Brae in Scotland and Angkor in Cambodia).

The authors wrote that as far as they were concerned, the dips in brightness had some things in common with those lost cities.

“They’re ancient; we are watching things that happened more than 1,000 years ago,” they said. “They’re almost certainly caused by something ordinary, at least on a cosmic scale. And yet that makes them more interesting, not less. But most of all, they’re mysterious. What the heck was going on there, all those centuries ago?”

When the light levels in various wavelengths were measured, Tabby’s Star became a little less mysterious. The team found that the dimming was much less pronounced in some wavelengths than in others.

“Dust is most likely the reason why the star’s light appears to dim and brighten,” Boyajian said. “The new data shows that different colors of light are being blocked at different intensities. Therefore, whatever is passing between us and the star is not opaque, as would be expected from a planet or alien megastructure.”

A healthy amount of mystery still remains.

“The latest research rules out alien megastructures, but it raises the probability of other phenomena being behind the dimming,” Wright said. “There are models involving circumstellar material — like exocomets, which were Boyajian’s team’s original hypothesis — which seem to be consistent with the data we have.”

There’s also a chance that the dimming is being caused by an as-yet-unknown stellar phenomenon rather than a band of dust or a swarm of comets that’s blocking the starlight.

Boyajian said the discovery and study of KIC 8462852 should be seen as a certifiable win for citizen-backed astronomy, even though it turns out aliens aren’t involved. She pointed out that the star would never have been studied if it weren’t for its discovery by volunteers in the Planet Hunters citizen-science campaign, the interest from more than 200 astronomers, and the support of more than 1,700 Kickstarter donors.

“It’s exciting,” she said. “I am so appreciative of all of the people who have contributed to this in the past year — the citizen scientists and professional astronomers. It’s quite humbling to have all of these people contributing in various ways to help figure it out.”

KIC 8462852 may be known as Tabby’s Star, but it turned out to be a star the whole world could claim.

Boyajian is the principal author of the paper in Astrophysical Journal Letters, titled “The First Post-Kepler Brightness Dips of KIC 8462852.” Co-authors include James Davenport of Western Washington University, who is also a DIRAC Fellow at the University of Washington. To trace the history of the mystery (or donate to the cause), check out the project website, which is called “KIC 8462852: Where’s the Flux?”

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