As first reported in this magazine, there is a star in the constellation Cygnus that behaves like no other. During the years-long stare of the Kepler space observatory, this otherwise run-of-the mill star—a bit hotter and more massive than the Sun—exhibited an extraordinary series of dimming events, as though briefly and occasionally eclipsed by an irregular series of large, opaque objects, the nature of which can only be guessed.

Discovered by citizen scientists scouring data publicly available through the crowdsourced Planet Hunters effort, these dimming events kicked off a years-long effort by a team led by Tabetha Boyajian at Yale to figure out what was going on with the star. After working with NASA to rule out technical issues that might have caused the oddities, they scrutinized the star itself for evidence that it might be unusually young, as very young stars have disks of warm dust, gas, and rocks orbiting them that can create all sorts of strange behavior.

The star proved to be completely pedestrian: Not only does it appear to be mature and lack any disk of material, it showed no other signs of peculiarity, either. If not for the Kepler data, the star would attract no attention at all.

But attract attention it has. The star with its weird behavior has not only captured the imagination of professional astronomers determined to solve the puzzle, but has also gained notoriety in the eyes of the public. The star found fame—from Saturday Night Live to Late Night with Stephen Colbert—because of a suggestion by astronomers (alright, we admit it was us) that radio astronomers partaking in the search for extraterrestrial intelligence (SETI) should point their telescopes that way.

SETI astronomers have long suggested that advanced alien civilizations might construct planet-sized “megastructures” to harvest massive amounts of starlight.  Such objects might be detected when they happened to pass between Earth and the star and might resemble similar signatures caused by natural objects.

Meanwhile, astronomers continued to study the star for signs that its strange eclipses could have an easily explainable natural cause. Boyajian postulated that the eclipses might be caused by a family of comets around the star. While it is still the best among many contrived explanations, comets cannot explain each and every event observed by Kepler. The search for a more convincing natural explanation remains ongoing.

In October 2015, shortly after the alien megastructure hypothesis went public, Bradley Schaefer, an astronomer specializing in careful stellar brightness measurements, decided to look for previous episodes of odd behavior by examining a historical astronomical treasure: photographs of the sky stored in the plate stacks at the Harvard Astronomical Plate Collection in Cambridge, Massachusetts.

For about 100 years, small telescopes have imaged the entire sky, bit by bit, on photographic plates. These plates are now carefully archived and stored by curators at the Harvard-Smithsonian Center for Astrophysics. The center has slowly digitized and made the plates available to the public through a program known as Digital Access to a Sky Century @ Harvard (DASCH).

Schaefer constructed a historical record of Boyajian’s star using online DASCH data. While Schaefer found no evidence of obvious prior eclipse events, the DASCH data did seem to show that the star had slowly dimmed by nearly 20 percent over the past 100 years.

Intrigued, but aware of the numerous systematic errors that plague photographic plate astronomy, Schaefer visited the plate stacks in Cambridge and repeated the measurements in person. His conclusion remained unchanged and unprecedented: No other star had shown such dimming before, and no natural explanation seemed forthcoming. This provided the first evidence beyond the Kepler data that something strange was going on with the star.

But other astronomers were skeptical of Schaefer’s claim. Two teams began simultaneous efforts to verify Schaefer’s surprising result with their own analyses of the DASCH data. One team was a collaboration between German amateur astronomer Michael Hippke and NASA Postdoctoral Fellow Daniel Angerhausen, and the other a team of professional astronomers from Vanderbilt University and Lehigh University lead by doctoral student Michael Lund.

The first rebuttal of Schaefer’s claim came from Hippke’s team who concluded that the long-term dimming of Boyajian’s star was not really there, but only an artifact of the hodge-podge nature of the photographic plates themselves.

However, as Schaefer was quick to point out in an immediate online response, Hippke had failed to account for many of the quirks that make working with photographic plates a “lost art” of astronomy, effectively refuting Hippke’s contradictory claim.

In reply to Schaefer’s precise but caustic counterargument, Hippke’s team and the Vanderbilt/Lehigh collaboration combined their efforts to re-examine the DASCH data on Boyajian’s star with more expertise. They conclude that Boyajian’s star indeed does not exhibit any extraordinary dimming over the past century. This time, their results have been peer-reviewed and recently accepted for publication in the Astrophysical Journal.  

So where does this all leave us? If Hippke is correct (and his paper is persuasive) then we are back where we were before Schafer’s claim, and the Kepler data remain the only strange thing about this star. But that still brings us no closer to a resolution of the mystery—that will require additional observations.

Efforts at the Allen Telescope Array to detect obviously artificial radio signatures came up empty, showing us that there were no very strong alien signals being sent when they looked. Our team still hopes to be awarded time on the much more powerful Green Bank Telescope to take a closer look.

On the side of natural explanations, if a recent cataclysmic event in the system created a large cloud of dust responsible for the dimming events Kepler saw, then it’s possible a heat signature will show up at some point in the next couple of years; another team of astronomers is using NASA’s Spitzer infrared space telescope to watch for that possibility.  

Boyajian herself is coordinating a worldwide effort of professional telescopes to monitor the star regularly for the next several years, to catch the star in the act. Citizen scientists are still on the case, too: the American Association of Variable Star Observers is tracking brightness measurements of the star made by amateur astronomers. When the star dims again, we will use telescopes around the world to measure how much the star dims at different wavelengths. Since different substances have characteristic absorption patterns, this will tell us the composition of the intervening material. For instance, if it dims much more at ultraviolet wavelengths than in the infrared, we will know dust is to blame. If we see the characteristic pattern of cometary gases, that will help confirm the cometary hypothesis.   

And if we see the same brightness changes at all wavelengths? That would indicate that whatever is blocking the starlight is big and opaque—inconsistent with comets, but consistent with the alien megastructure hypothesis.

The public attention on the slow process of understanding this strange star has cast a light on how science really looks from the trenches. The exchange between Schaefer and Hippke, while dramatic at times, shows how a single study is not the end of the road for scientific conclusions, merely the start of a longer journey of testing, verification, and improvement.

Indeed, in Hippke’s thoughts on this controversy he explains that the choice for a fast-paced and public dialog was intentional, “so that the community, the involved media and the interested public did not have to wait many months” for the next development.

As Boyajian herself reminds us in her TED talk, “extraordinary claims require extraordinary evidence,” and as the dust settles on the Schaefer/Hippke controversy, Boyajian’s star remains the most mysterious star in our galaxy.