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Boyajian’s Star, also known as Tabby’s Star, has attracted attention from astronomers and invited speculation. One theory even says the star could exist within a vast structure that an alien civilization built to harvest its energy.
NASA’s Kepler space telescope has even observed dimming by the object—formally known as KIC 8462852—of up to 20 percent over a matter of days. Further, the star has had much subtler but longer-term enigmatic dimming trends, with one continuing today. None of this behavior is standard for normal stars slightly more massive than the sun.
A new study using NASA’s Spitzer and Swift missions, as well as the Belgian AstroLAB IRIS observatory, suggests that the cause of the dimming over long periods is likely an uneven dust cloud moving around the star.
This pretty much rules out the alien megastructure theory…
But, there’s a smoking gun: There is less dimming in the infrared light from the star than in its ultraviolet light. Any object larger than dust particles would dim all wavelengths of light equally when passing in front of Tabby’s Star.
“This pretty much rules out the alien megastructure theory, as that could not explain the wavelength-dependent dimming,” says Huan Meng, who did the research as part of a postdoctoral fellowship at the University of Arizona. Meng is lead author of the study in the Astrophysical Journal.
“We suspect, instead, there is a cloud of dust orbiting the star with a roughly 700-day orbital period.”
We experience the uniform dimming of light often in everyday life. If you go to the beach on a bright, sunny day and sit under an umbrella, the umbrella reduces the amount of sunlight hitting your eyes in all wavelengths. But if you wait for the sunset, the sun looks red because tiny particles scatter away the blue and ultraviolet light.
The new study suggests the objects causing the long-period dimming of Tabby’s Star can be no more than a few micrometers in diameter (about one ten-thousandth of an inch).
From January to December 2016, the researchers observed Tabby’s Star in ultraviolet using Swift and in infrared using Spitzer. Supplementing the space telescopes, they also observed the star in visible light during the same period using AstroLAB IRIS, a public observatory with a 27-inch-wide (68-centimeter) reflecting telescope located near the Belgian village of Zillebeke.
Based on the strong ultraviolet dip, they determined the blocking particles must be bigger than interstellar dust, small grains that could be located anywhere between Earth and the star. Such small particles could not remain in orbit around the star because pressure from its starlight would drive them farther into space.
Dust that orbits a star, called circumstellar dust, is not so small it would fly away, but also not big enough to uniformly block light in all wavelengths. This is currently considered the best explanation, although others are possible.