Enjoying EarthSky? Subscribe.

270,227 subscribers and counting ...

Is dust what’s dimming Tabby’s Star?

Astronomers say they’ve found a smoking gun that rules out the idea of an energy-harvesting megastructure or Dyson sphere – built by an alien civilization – surrounding Tabby’s Star.

Artist’s concept of a hypothetical uneven ring of dust causing the mysterious dimming of the star KIC 8462852, also known as Boyajian’s Star or Tabby’s Star. Image via NASA/JPL-Caltech.

Have you been following the strange case of KIC 8462852, also known as Tabby’s Star? It’s a star that behaves like no other ever seen, whose weird and dramatic dips in brightness have caused some astronomers to speculate about a possible Dyson sphere or megastructure around the star, built by an advanced civilization with the goal of large-scale energy harvesting. The Kepler space telescope has seen Tabby’s Star dim up to 20 percent over a matter of days, but the star also has much subtler, longer-term enigmatic dimming trends, with one continuing today. A new study announced on October 4, 2017 suggests an uneven dust cloud moving around the star as the cause of the long-term dimming. No Dyson sphere needed! But is it true?

The astronomers who published the study point to a smoking gun. They say their observations – using the Spitzer and Swift space missions, as well as the Belgian AstroLAB IRIS observatory – found less dimming in the infrared light from the star than in its ultraviolet light. Their statement explained:

Any object larger than dust particles [such as the great beams and girders of a Dyson sphere] would dim all wavelengths of light equally when passing in front of Tabby’s Star.

Huan Meng at the University of Arizona, Tucson is lead author of the new study, which also has as co-author Tabetha Boyajian at Yale University. She is the astronomer for whom Tabby’s Star is named, and also made this star famous in a 2016 TEDTalk about it. The study is published in the peer-reviewed Astrophysical Journal. Meng said:

This pretty much rules out the alien megastructure theory, as that could not explain the wavelength-dependent dimming. We suspect, instead, there is a cloud of dust orbiting the star with a roughly 700-day orbital period.

Andrew Siemion, director of the Berkeley SETI Research Center, explains the megastructure theory of Tabby’s Star. In this theory, the dimming of the star is caused by an alien-built structure. Image via UC Berkley/EarthSky.

These astronomers statement explained why they consider their explanation – the idea that the dimming is caused by dust – the best one:

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).

They explained that they observed Tabby’s Star in ultraviolet using Swift, and in infrared using Spitzer, from January to December 2016. 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. Their statement said:

Based on the strong ultraviolet dip, [we] 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.

In their study, these astronomers are addressing the long-term dimming of Tabby’s Star. 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.

In other words, more mysteries remain. But isn’t that always the case?

Artist’s concept of cascading comets around a distant star. Previous research suggested a swarm of comets may be to blame for the short-period dimming of Tabby’s Star. Comets are also one of the most common sources of dust that orbits stars, and so could also be related to the long-period dimming studied by Meng and colleagues. Image via NASA/JPL/Caltech/Vanderbilt University.

Bottom line: A new study published in the Astrophysical Journal on October 4 suggests dust as the source for the long-term dimming of Tabby’s Star. Other dimming trends remain unexplained.

Deborah Byrd

MORE ARTICLES