One of the most mysterious stellar objects may be revealing some of its secrets at last. With its unusual dips in brightness, Boyajian's Star, also called Tabby's Star, has attracted attention from professional and amateur astronomers and invited speculation as to whether it could be surrounded by a vast structure built by an alien civilization to harvest the star's 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. In addition, the star has had much subtler but longer-term enigmatic dimming trends, with one continuing today. None of this behavior is expected for normal stars slightly more massive than the sun.
Speculation has included the idea that the star swallowed a planet that is unstable, and a more imaginative theory involves a giant contraption or "megastructure" built by an advanced civilization, which could be harvesting energy from the star and causing its brightness to decrease.
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 discounts the "alien megastructure" notion and other exotic speculations.
The smoking gun: Researchers found 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," said Huan Meng, who did the research as part of a postdoctoral fellowship at the University of Arizona. Meng, who graduated with a Ph.D. from the UA's Lunar and Planetary Laboratory in 2014, is lead author of the new study published in the Astrophysical Journal. "We suspect, instead, there is a cloud of dust orbiting the star with a roughly 700-day orbital period."
Why Dust Is Likely
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 the blue and ultraviolet light is scattered away by tiny particles. 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, researchers 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, the researchers 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.
Collaboration With Amateur Astronomers
Citizen scientists have had an integral part in exploring Tabby's Star since its discovery. Light from this object was first identified as "bizarre" and "interesting" by participants in the Planet Hunters project, which allows anyone to search for planets in the Kepler data. That led to a 2016 study formally introducing the object, which is nicknamed for Tabetha Boyajian, now at Louisiana State University, who was the lead author of the original paper and is a co-author of the new study. The recent work on long-period dimming involves amateur astronomers who provide technical and software support to AstroLAB.
Several AstroLAB team members who volunteer at the observatory have no formal astronomy education. Franky Dubois, who operated the telescope during the Tabby's Star observations, was the foreman at a seat belt factory until his retirement. Ludwig Logie, who helps with technical issues on the telescope, is a security coordinator in the construction industry. Steve Rau, who processes observations of star brightness, is a trainer at a Belgian railway company.
Siegfried Vanaverbeke, an AstroLAB volunteer who holds a Ph.D. in physics, became interested in Tabby's Star after reading the 2016 study, and persuaded Dubois, Logie and Rau to use AstroLAB to observe it.
"I said to my colleagues: 'This would be an interesting object to follow,'" Vanaverbeke recalled. "We decided to join in."
UA astronomer George Rieke, a co-author on the new study, contacted the AstroLAB group when he saw its data on Tabby's Star posted in a public astronomy archive. The U.S. and Belgium groups teamed up to combine and analyze their results.
While study authors have a good idea why Tabby's Star dims on a long-term basis, they did not address the shorter-term dimming events that happened in three-day spurts in 2017. They also did not 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 orbits stars, and so they also could be related to the long-period dimming studied by Meng and colleagues.
"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 said. "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. This is something that needs further investigation and will continue to interest scientists for many years to come," Vanaverbeke said.
NASA's Goddard Space Flight Center in Greenbelt, Maryland, manages the Swift mission in collaboration with Penn State University, the Los Alamos National Laboratory in New Mexico, and Orbital Sciences Corp. in Dulles, Virginia. Other partners include the University of Leicester and Mullard Space Science Laboratory in the United Kingdom, Brera Observatory and the Italian Space Agency in Italy, with additional collaborators in Germany and Japan.
NASA's Jet Propulsion Laboratory in Pasadena, California, manages the Spitzer Space Telescope mission for NASA's Science Mission Directorate in Washington. Science operations are conducted at the Spitzer Science Center at Caltech in Pasadena. Spacecraft operations are based at Lockheed Martin Space Systems in Littleton, Colorado. Data are archived at the Infrared Science Archive housed at the Infrared Processing and Analysis Center at Caltech. Caltech manages JPL for NASA.