For a period of several months, a huge star located more than 25,000 light-years from Earth got increasingly dimmer before eventually returning to its usual brightness. An obstruction of some kind likely caused this strange effect, but astronomers aren’t exactly sure what it is.

In early 2012, a red giant star known as VVV-WIT-08 began to show a gradual drop in luminosity (the “WIT” in the star’s name stands for “What Is This?” — more on that soon). This dimming continued until April of that year, when the star declined to 97% of its original brightness, making it virtually invisible. Gradually, the star, 100 times larger than the Sun, returned to its former glory. The whole thing from start to finish lasted about 200 days.

New research published late last week in Monthly Notices of the Royal Astronomical Society chronicles this strange event in detail, in which VVV-WIT-08 exhibited a “smooth, eclipse-like drop” in its usual luminosity. Astronomer Leigh Smith from Cambridge University’s Institute of Astronomy co-led the work.

Now, it is not uncommon for stars to experience dips in brightness. Causes are typically attributed to eclipsing companion stars or stars that naturally pulsate. What is unusual, however, is for a single blinking episode to last for such a long time.

Interestingly, there are analogues in the scientific literature. Giant star Epsilon Aurigae experiences a partial eclipse, in which it dims by about 50% every 27 years. Another star detected a few years ago, called TYC 2505-672-1, does something similar, with an eclipse occurring every 69 years. At 3.45 years, TYC 2505-672-1 features the longest stellar eclipse known to astronomers. In both cases, astronomers attribute a massive disc of orbiting dust as the likeliest cause.

There are other stars that seems to undergo regular dimming, so this might actually be a thing. And by thing, the astronomers are referring to a “population of long-period eclipsing binaries composed of late-type giant stars and opaque-disc-hosting companions,” as it’s described in the study. The nature of this massive disc of opaque dust, however, remains a mystery.

“It’s amazing that we just observed a dark, large and elongated object pass between us and the distant star and we can only speculate what its origin is,” Sergey Koposov, a co-author of the study and an astronomer with the University of Edinburgh, explained in a statement.

The team, which includes astronomers from Carnegie Mellon University, University College London, and several other institutions, detected the probable long-period eclipsing binary in data gathered by VISTA Variables in the Via Lactea survey (VVV) run by the European Southern Observatory. The dimming was also spotted in data collected by the Optical Gravitational Lensing Experiment (OGLE), a campaign managed by the University of Warsaw.

“Occasionally we find variable stars that don’t fit into any established category, which we call ‘what-is-this?’, or ‘WIT’ objects,” Philip Lucas, co-leader of the study and a professor at the University of Hertfordshire, said in the statement. “We really don’t know how these blinking giants came to be. It’s exciting to see such discoveries from VVV after so many years planning and gathering the data.”

Models made from the data point to an elliptical object with a uniform transparency. It’s also thick — somewhere around 37.3 million kilometres deep, or about a quarter the average distance from Earth to the Sun (AU). Uncertainties about the object’s orbit mean the scientists can’t be sure of its total size, with estimates ranging from tens of AU to hundreds of AU across (1 AU is 150 million kilometres).

In terms of an explanation, the team considered a chance encounter with an unidentified passing object, but they ruled that out, noting that “a chance alignment with the giant star requires an improbably large space density of dark foreground objects,” according to the study. Indeed, as their simulations showed, an impossibly large number of rogue, dark objects have to exist in the Milky Way for this theory to work. It’s far more likely, they argue, that the mystery object is orbitally bound to VVV-WIT-08.

Other theories proposed by the scientists include a debris disc produced by a white dwarf or black hole or a disc composed of matter stripped from the star itself. None of the proposed theories are great, but the authors said a possible mass transfer from the giant star is an “attractive” possibility.

“Despite intensive efforts, it is clear that we have left room for further work on this intriguing object!,” conclude the astronomers in their study. Should this population of binary objects exist, the Legacy Survey of Space and Time (LSST) campaign at the upcoming Vera Rubin Observatory is very likely to spot them. LSST was delayed by the covid-19 pandemic, and it likely won’t get started until late 2023.