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.
Citizen scientists have had an integral part in exploring Tabby’s Star since its discovery. Participants in the Planet Hunters project, which allows anyone to search for planets in the Kepler data, first identified this object as “bizarre” and “interesting.”
That led to a 2016 study formally introducing the object, which gets its nickname from Tabetha Boyajian, now at Louisiana State University, who was the lead author of the original paper and is a coauthor of the new study. The recent work on long-period dimming involves amateur astronomers who provide technical and software support to AstroLAB.
Siegfried Vanaverbeke, an AstroLAB volunteer who holds a PhD in physics, became interested in Tabby’s Star after reading the 2016 study, and persuaded other citizen scientists to use AstroLAB to observe it.
“I said to my colleagues: ‘This would be an interesting object to follow,’” Vanaverbeke says. “We decided to join in.”
Astronomer George Rieke, a coauthor of the new study, contacted the AstroLAB group when he saw its data on Tabby’s Star posted in a public astronomy archive. The US and Belgium groups teamed up to combine and analyze their results.
“It could be a swarm of comets, a recent planetary impact, or residuals from billions of years of planet formation.”
While study authors have a good idea why Tabby’s Star dims on a long-term basis, they didn’t address the shorter-term dimming events that happened in three-day spurts in 2017. They also didn’t confront the mystery of the major 20 percent dips in brightness that Kepler observed while studying the Cygnus field of its primary mission.
Previous research with Spitzer and NASA’s Wide-field Infrared Survey Explorer suggested a swarm of comets may be to blame for the short-period dimming. Comets are also one of the most common sources of dust that orbit stars, so they also could be related to the long-period dimming.
“From our study, we can say that the cause of the long-term dimming should be circumstellar dust grains larger than those in interstellar dust, but we cannot say what the source of the dust is,” Meng says. “It could be a swarm of comets, a recent planetary impact, or residuals from billions of years of planet formation.”
Now that Kepler is exploring other patches of sky in its current mission, called K2, it can no longer follow up on Tabby’s Star. But future telescopes may help unveil more secrets of this mysterious object.
“Tabby’s Star could have something like a solar activity cycle,” Vanaverbeke says. “This is something that needs further investigation and will continue to interest scientists for many years to come.”