Substantial, sun-like stars are not supposed to dim. They start with gravity and pressure induced nuclear reactions, and then they burn brighter and brighter until they either explode (go supernova) or burn all their fuel and become small, enormously dense, and not very bright “white dwarfs.”
Of course, the transit technique of searching for exoplanets looks precisely for dimmings — of stars caused by the passage of an exoplanet. But those are tiny reductions in the star’s brightness and short-lived. So if a star is dimming significantly over a much longer period of time, something unusual is going on.
And that is apparently exactly what is happening with the current poster child for mysterious stars — KIC 8462852 or “Tabby’s star,” named after the Yale University postdoc who, with the help of citizen scientists, discovered it, Tabetha Boyajian.
First written up last fall, the big news was data from the Kepler Space Telescope showed that the star had experienced two major and dissimilar dips in brightness — a highly unusual and perplexing phenomenon. The dips appeared much too large to represent the passage of an exoplanet, so explanations tended towards the baroque — a swarm of comets, a vast dust cloud, even an alien megastructure (proposed as a last possible explanation.) The observation was first identified by citizen planet hunters working with Boyajian, making it an even more compelling finding.
Now the mystery has grown stranger still. A paper made public last week based on a different kind of Kepler imaging (full-frame imaging) found not two but one enormous dip in the light curve, as well as a surprising and significant dimming the of star over the four year observing period of the space telescope. The paper has been submitted for publication in American Astronomical Society journals.
Benjamin Montet of Caltech and Joshua Simon of the Observatories of the Carnegie Institution of Washington, analyzed the full-field images taken by Kepler every three months (rather than the hourly images studied by Boyajian et al,) and concluded that something strange was indeed going on.
Their conclusion: “No known or proposed stellar phenomena can fully explain all aspects of the observed light curve.”
Expanding a bit, Montet told Gizmodo: “We spent a long time trying to convince ourselves this wasn’t real. We just weren’t able to.”
A paper describing the results from these full-frame observations went up recently on the prior to printing site arXiv. The site allows members of the astronomy world to offer critiques, and so the results as now released may not be final.
But the story line does seem pretty clear — that Tabby’s star had one very large period of light dimming and had a secular decline in the light it was sending out over the four years of the Kepler mission.
Boyajian, a newly-appointed Louisiana State University researcher and professor, said that she considers the original findings to be entirely compatible with the newest results, with differences based on how the light was being captured (the once-monthly full-frame Kepler images versus the continuous imaging done of more than 100,000 stars.)
What has also become increasingly clear is that the dimming is not the result of an instrument glitch, and that the surrounding stars are not exhibiting the same unusual behavior.
“As far as we know, dimming is not something stars do; they get larger and brighter,” she said. “Especially on these remarkably fast time scales, the dimmings are unprecedented for any kind of star.”
Boyajian had initially favored the theory that the light was being blocked by a large swarm of comets, but she said the new results make that more unlikely. She said it is similarly unlikely that the dimmings are the result of some internal dynamics of the star. So is it all the result of some alien megastructure, the “explanation” that initially brought a lot of attention to Tabby’s star. I think we can assume it is not.
But given the data now available, it has become extremely difficult to find an explanation that checks all the boxes. And that’s why Boyajian and her colleagues began a kickstarter campaign to raise $100,000 for another year of observing through the telescopes of the private Las Cumbres Observatory Global Telescope Network.
As she explained it, one of the telescopes will image the star at least two hours per night for the next year. And if a significant dimming is observed, larger ground-based telescopes will be available to look more closely.
It’s a waiting game now, which is exciting itself,” she said. “It’s only a guess, but based on Kepler light curves, we might see something interesting next spring.”
(My earlier story on Tabby and her star can be found here: Tabby’s Star)