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Tabby's Star
This illustration depicts a hypothetical uneven ring of dust orbiting KIC 8462852, also known as Boyajian’s Star or Tabby’s Star. (NASA / JPL-Caltech Illustration)

Are aliens building a huge energy-generating megastructure around a weirdly dimming star? That way-out hypothesis has suffered another blow, thanks to a study that draws upon infrared as well as ultraviolet observations.

The star, known as KIC 8462852 or Tabby’s Star, first came to attention two years ago when citizen scientists sifting through data from NASA’s Kepler space telescope noticed some unusually drastic dips in its brightness. The star’s nickname comes from Tabetha “Tabby” Boyajian, the Yale astronomer who oversaw those observations.

Another astronomer, Penn State’s Jason Wright, mused that the data could be explained by the construction of a huge orbital structure known as a Dyson sphere — although he cautioned that “aliens should always be the very last hypothesis you consider.”

Since then, however, the alien hypothesis has been very much considered — along with more mundane explanations such as swarms of comets, stellar variability or clouds of gas and dust. Further observations found that KIC 8462852, which is about 1,500 light-years away in the constellation Cygnus, has been going through a long-term dimming trend.

The latest findings, published in the Astrophysical Journal, indicate that the long-term dimming is due to the effect of obscuring streams of dust.

Astronomers analyzed data from NASA’s Spitzer Space Telescope and Swift spacecraft, as well as from the Belgian AstroLAB IRIS observatory, and compared the levels of dimming in infrared vs. ultraviolet wavelengths.

They found that the ultraviolet light dimmed significantly more than the infrared light. That fits the pattern for starlight shining through a haze of dust particles no bigger than about a ten-thousandth of an inch.

The effect is similar to the way the sun reddens in a smoky sky, as was the case in the Pacific Northwest during this summer’s wildfire season. Tiny particles scatter more light in the bluer, shorter-wavelength part of the spectrum, and less light on the redder, longer-wavelength side.

The researchers said a similar effect appears to be at work around Tabby’s Star.

“This pretty much rules out the alien megastructure theory, as that could not explain the wavelength-dependent dimming,” lead author Huan Meng of the University of Arizona said in a NASA news release. “We suspect, instead, there is a cloud of dust orbiting the star with a roughly 700-day orbital period.”

Tabby’s Star is just one among millions in this image from the All-Sky Automated Survey for Supernovae. (Carnegie Institution for Science / ASAS-SN Image / Benjamin Shappee)

The explanation applies to the long-term dimming, but not necessarily to the shorter-term changes in brightness. Those changes could be due to a swarm of comets, or variations in stellar activity, or the breakup of a planet orbiting the star … or even, heaven help us, alien shenanigans.

Meng and his colleagues favor the comet-swarm hypothesis, because passing comets could contribute fine-grained material to orbiting dust clouds.

In any case, Boyajian and her colleagues are continuing to monitor the ups and downs of Tabby’s Star in multiple wavelengths, aided in part by a crowdfunding campaign. The latest confirmed dip in brightness was registered just last month.

Yet another study, led by astronomers Josh Simon and Benjamin Shappee at the Carnegie Institution for Science, adds to the intrigue.

The researchers analyzed more than a decade’s worth of observations from the All Sky Automated Survey and the high-precision All-Sky Automated Survey for Supernovae — and found that Tabby’s Star has gone through periods of significant brightening as well as dimming.

Simon and Shappee said their findings support the view that Tabby’s Star is worth keeping watch on.

“We haven’t solved the mystery yet,” Simon said in a news release.  “But understanding the star’s long-term changes is a key piece of the puzzle.”

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