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Mysterious dimming of Tabby's star likely due to space dust, not alien superstructures, say scientists

Astronomy On Wednesday, a team of over 200 scientists led by astronomer Tabetha Boyajian announced the mysterious dim-and-recover behavior of stellar object KIC 8462852, also called "Boyajian's Star" or "Tabby's Star," is likely to be due to clouds of color-warping space dust and not a planet, another star or, as some astronomers have hypothesized, a giant structure built by a distant civilization.

"If a solid, opaque object like a megastructure was passing in front of the star, it would block out light equally at all colors," Dr. Boyajian told the press. "This is contrary to what we observe." And, in a separate statement, "Dust is most likely the reason why the star's light appears to dim and brighten. 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."

The research team analyzed 22 months of data collected by an array of ground-based telescopes that covered many wavelengths of light. The study period lasted from March 2016 to December 2017, which included a number of decreases in brightness. The first dip started on the night of Thursday May 18. A robotic 14-inch Celestron telescope at Fairborn Observatory in Arizona watched Tabby's star lose brightness by a dramatic 3% by Friday, confirming a prediction the star would undergo its unusual dimming events once every 750 days. Other major telescope projects and citizen astronomers confirmed these findings. That dip was complex and continued through to the following Sunday, after a brief rise in brightness over the weekend. It ended a few days later, with Boyajian and fellow astronomer and co-author of the recent paper Jason Wright discussing the phenomena with the public via Twitter: "will we have a flurry of dips to come? Stay tuned!"

"Tabby's star[...] went through a lot of very strange dimming events that got up to 22% dimmer during the Kepler Mission, and since then we've been eagerly awaiting another dip. And the reason that we’ve been waiting for that is that whatever is causing the star to get dimmer will leave a spectral fingerprint behind," said Dr. Jason Wright of Pennsylvania State University, after the May event, now a co-author of the current work. "So if it's a lot of dust between us and the star that's passing by, it should block more blue light than red light. If there's gas in that dust, that gas should absorb very specific wavelengths. So we've been eager to see one of these changes, these dips in the star, so we can take the spectra." He also addressed space fans at the time via Twitter: "ALERT:@tsboyajian's star is dipping This is not a drill. Astro tweeps on telescopes in the next 48 hours: spectra please!"

The primary mission of the Kepler project monitored more than 150,000 stars with the primary goal of detecting exoplanets. If a star gets dimmer but then recovers, a planet may have passed in front of it. But Tabby's star has shown something else. An exoplanet can cause its sun to dim by perhaps 1% and not for very long. Some of Tabby's dips have been this mild, but others have hit 20% and lasted weeks. It also tends to recover only partially, slowly showing a net dimming over the years.

The plethora of hypotheses behind Tabby's star has made it a favorite among space fans and both professional and amateur astronomers. Space dust was one of the many hypothesized explanations, but if that's the reason, its origin was and remains a mystery with stargazers speculating many giant comets could be on elliptical orbits or breaking up in front of it or the dips in brightness could be aftereffects of the star consuming a planet. Some even hypothesized alien civilizations might have been harvesting the sun's energy using a Dyson sphere. The Search for Extraterrestrial Intelligence (SETI) devoted some observation time to Tabby's star in the fall of 2016 as part of the Breakthrough Listen project. No radio or laser pulse signals were detected.

"This latest research rules out alien megastructures, but it raises the plausibility of other phenomena being behind the dimming," said Dr. Wright. "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 [...] some astronomers favor the idea that nothing is blocking the star — that it just gets dimmer on its own — and this also is consistent with this summer's data."

This study is a departure from many previous works in that it relies heavily on the work of citizen scientists, volunteers who help examine the great quantities of data produced by NASA's Kepler mission. The follow-up observations after the Kepler project were funded by a $100,000 Kickstarter to continuously monitor Tabby's star. The unusual behavior of Tabby's star was discovered in 2011 by volunteers for the Planet Hunters citizen science project, which was implemented to search for planets in the Kepler data.

"It's exciting. 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," Dr. Boyajian said in a statement.

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