For over a century, a star's bizarre behavior has been hiding in plain sight. Now, after unusual fluctuations in its light were spotted in the Kepler data, a researcher has gone back and looked at old photographic plates and found that its behavior has been unusual since some of the earliest images. The new findings make any mundane explanations for the star's erratic behavior even less likely.

"The star KIC8462852 (TYC 3162-665-1) is apparently a perfectly normal star." That's how a new paper from Louisiana State's Bradley Schaefer begins. It's an F-type star, which means it's a bit larger than the Sun but is otherwise boring and stable. If you were to image it (as has happened many times over recent decades), it would look unremarkable.

It took the Kepler telescope to figure out that the star was anything but boring. Kepler was designed to stare at one patch of the sky and watch for signs of planets passing in front of their host stars. By chance, that patch of sky included KIC8462852. Its bizarre behavior—sudden dips in brightness of as much as 20 percent, lasting for seemingly random periods of time—wouldn't have been identified by the software that analyzes Kepler data.

Instead, astronomy enthusiasts noticed the odd behavior, and their findings drew the attention of professional astronomers. Professional astronomers were completely baffled by it. After considering a number of potential explanations, they decided that none are especially likely, but that the least unlikely is a large family of comets.

Schaefer was intrigued by the finding, and he decided to look deeper into KIC8462852's history. In many cases, stars have been inadvertently imaged as part of surveys or studies that targeted something nearby. To find out whether this was the case, Schaefer turned to an archive of photographic plates stored at Harvard. He found a motherlode.

Among the photographic plates that had been digitized, he found more than 1,500 that included KIC8462852. Even after eliminating the lower-quality images, he was left with 1,232 images to analyze, covering an entire century (1890-1989). Schaefer also analyzed 131 by eye (and you could almost hear him telling the young astronomers to get off his lawn as he described doing so).

Two nearby stars that acted as controls saw a bit of variability over the years, but there were no obvious trends. KIC8462852, by contrast, has experienced some pretty extreme variability, including a very large dip in activity at the start of the 20th Century, a partial recovery, and another big drop sometime after 1950. Overall, these changes have resulted in a significant dimming of the star over the past century.

That's not something that this type of star normally does. Schaefer reasons that if one star displays both strange behavior in the short term and strange behavior on the scale of centuries, there's probably a single cause. So, he analyzed some of the leading explanations for the star's short-term behavior to see how they could account for the century-scale dimming.

In general, they can't. Things like comets and dust clouds would require a prodigious source of new material in order to slowly dim the star over these time scales. For what it's worth, the idea of aliens building a Dyson sphere around the star would face the same trouble—the aliens would have to be adding to the structure at an astonishing pace.

So, the mystery of KIC8462852 has gotten a bit deeper. It will undoubtedly get more telescope time in the upcoming years, including at wavelengths not imaged by Kepler. But, at 1,480 light years away, there's not much we can do other than watch closely for the time being.

The arXiv. Abstract number: 1601.03256 (About the arXiv).