New data on Boyajian's Star show that dust — not an alien megastructure — is probably the cause of this star's mysterious behavior.
After years of speculation, astronomers have announced observations of the mysterious KIC 8462852, also known as Boyajian’s Star. It’s even been popularly referred to as the “alien megastructure” star, but the new measurements show one thing definitively: there’s no alien megastructure. Instead, cosmic dust is probably the culprit behind this star’s strange behavior. Still, plenty of questions remain about how and why this star is dimming.
Why is Boyajian’s Star So Mysterious?
In 2016 Tabetha Boyajian (Louisiana State University) and collaborators, including citizen scientists on the Planet Hunters website, announced the discovery of the “most mysterious star in the Galaxy.” Boyajian’s Star exhibited lots of strange behavior: In data collected by Kepler satellite, the star’s brightness appeared to dip in ways that were not easily explained. Some dips diminished the stars brightness by up to 22% for several days at a time, and they weren’t periodic or symmetric.
The discovery set off a flurry of investigations into the star. Scientists trawling through astronomical archives found that Boyajian’s star had dimmed over much longer timelines, slowly fading over the last century.
Yet despite its strange behavior, the star itself looked remarkably ordinary. It was a bit more massive than the Sun, a little younger, but the star showed no extra infrared radiation that would be expected if it hosted a large debris disk or planets. The spectrum also didn’t show any strange spectral lines that would have indicated material accreting onto the star (or being ejected from it).
So Boyajian and collaborators proposed an unlikely but not impossible scenario: that something had kicked large amounts of dust, potentially from swarms of comets, into orbit around the star. Other astronomers proposed that interstellar dust was obscuring our view of Boyajian’s star, while still others suggested the star had recently ingested a planet. Some astronomers even proposed that the star itself could be misbehaving.
One thing was clear: more data were necessary. Kepler’s steady gaze garnered incredibly detailed observations of the star over four years, but the spacecraft’s main mission had come to an end in 2013, before the discovery paper was even published. Ground-based observations since Kepler had glimpsed the star only on good behavior. Moreover, the dips that Kepler had caught provided limited information, since they were only observed at a single wavelength band.
What astronomers really needed was round-the-clock observations.
So Boyajian set off on a first for astronomy: crowdfunding observations of a star. She used a Kickstarter campaign to secure the funding for observations using the Las Cumbres Observatory (LCO), a network of robotic telescopes around the globe. LCO could monitor the star 24/7, providing all the data needed to catch the star’s fleeting dips and trigger follow-up observations.
The network began observing the star in March 2017, and in May 2017 the team caught the first dip of the new campaign. The dip was nicknamed “Elsie” (a play on LC, which could stand for “light curve” or “Las Cumbres”), and was relatively small, with just about 1% of the star’s light obscured. The telescope network observed three more dips, each separated by a few weeks, until the campaign ended in August 2017. When the dips began, Boyajian alerted the astronomical community, resulting in a deluge of data across the spectrum. Dozens of teams secured data at visible, infrared and even radio wavelengths. Others took detailed spectra and measured the star’s polarized light.
What the New Data Say
Analysis of the new observations will appear in the Astrophysical Journal Letters (full text available here).
Most notably, the data indicate that the star didn’t dim the same way at all wavelengths. Blue light was slightly more obscured than redder wavelengths, indicating that the dimming must be caused by material that’s not solid. That’s a strike against the unlikely idea that there might be “alien megastructures” forming a Dyson sphere (or swarm) around the star. The star’s infrared output also didn’t change during the dip, another strike against this idea.
The fact that more blue light is blocked or scattered suggests that dust is the culprit. To match the observations, the dust grains must be quite small, about 1000 times smaller than the smallest grains of sand. This size tells us that the dust is probably circumstellar, as interstellar dust grains tend to be even smaller.
Stars themselves easily disrupt small dust grains nearby. The star’s light exerts pressure, which can knock the grains around, blowing the dust out of orbit. So the dust is probably short-lived — how it got there is the next mystery the team hopes to solve.
One possible explanation is that the a swarm of comets, asteroids, or a dusty planetesimal could generate dust. That dust could briefly dim the star’s light before being ejected to greater distances, where it might gradually dim the star over much longer timescales.
Once again, more observations may be all that’s needed to unravel the mysteries of the dust. In their 2016 paper, Boyajian and colleagues predicted that the next dips would start in May 2017, and they were correct. If this signal is indeed periodic, it should repeat again in June 2019. Any periodicity would help further explain the dust’s source and how it’s interacting with the star’s light.
Years after its identification by a citizen scientist, Boyajian’s Star continues to be a source of wonder for the public and astronomers alike. Tabby and her team led the first successful crowdfunding campaign for telescope time, and she has done an amazing job at keeping her supporters informed. (Full disclosure: I am one of her Kickstarter supporters). While the current observations rule out the splashy alien headlines, it underscores this object’s unique nature, and the need for continuing observations for years to come.