Stars can twinkle, but they do not just disappear – so when a distant, giant star pulled a disappearing action for about 200 days, it surprised astronomers.
About a decade later, astronomers have reviewed a number of possible explanations – and they still have no idea what is responsible for erasing almost all of the star’s light.
Described in a new study in Monthly announcements from the Royal Astronomical Society, some of the theories still on the table rely on as yet unobserved phenomena, such as a dark slice of material orbiting a nearby black hole, or undiscovered, dust-shrouded companion stars. But over 17 years of observations, the star has only gone dark once in 201
It is clear that no matter what object obscured the distant star is enormous – much larger than the star itself. It also appeared to be completely opaque and blocked much of the starlight completely, and it appeared to have a hard edge.
“The degree of decrease in brightness is really impressive,” says Emily Levesque of the University of Washington, who studies massive stars and was not involved in the observations. “It would be cool to see more observations of this star, no matter what caused this, and to gather how something like this happened.”
Giant stars who behave strangely
The galaxy is full of strangely erected stars, many of which naturally fluctuate in brightness. One of these variable stars, named Betelgeuse, was dramatically dampened in 2019, sparking speculation that it might be about to explode. (It did not.) Instead, the red supergiant at Orion’s shoulder returned to its normal brightness, and astronomers now attribute its fainting to a cool spot in the southern hemisphere and a dusty.
Perhaps even more famously, in 2015, astronomers captured a star that flickered so strangely that some scientists considered the possibility that its light was blocked by an orbiting alien megastructure. The lure of alien technologies launched the star – now known as Tabby’s Star – into the spotlight for years, but observations in 2018 revealed that the culprit was nothing more than dust.
This star, who shone out in the first half of 2012, is equally exciting.
“It’s unusual for a star to dim in brightness by so much and for so long, and it immediately caught my eye as something unusual,” says study author Leigh Smith, an astronomer at the University of Cambridge.
Smith saw the odd eclipse as he sifted through data from the VISTA variables in the Via Lactea or VVV survey. This project, which takes its name from Latin for the “Milky Way”, monitors the southern sky for variable stars in the galaxy’s disk.
The observation was given the star a special name: WIT or “What is this?”, An acronym that astronomers with the plumbing project use to categorize curious objects. The star became known as VVV-WIT-08, and the team marked it for follow-up work. Based on early observations, they estimated that the star was at least 25,000 light-years away in the direction of the galactic bulge, and that it was an eight-billion-year-old giant about 100 times the size of our sun, but smoldered at 1 p.m. cooler temperatures.
In the first half of 2012, the star almost completely disappeared, losing 97 percent of its brightness. Data suggested that whatever had caused such a strong jump was opaque, uniformly obscuring all visible and infrared wavelengths of light throughout the eclipse.
“It’s very difficult to understand,” said Jason Wright of Pennsylvania State University, who was not involved in the sightings. “It’s something bigger than the star that’s completely opaque, and there are not many things that do that.”
Follow-up studies used data from the European Space Agency’s Gaia spacecraft and a ground-based study called OGLE to gather more information about the star. But as these observations came together, so did the questions. It became harder to pinpoint the star’s exact size and distance, and its motion through space looked peculiar – VVV-WIT-08 seemed to travel almost fast enough to escape the Milky Way.
“It is far above anything one would expect in this direction,” says Smith. “So there’s something that’s not quite right here, there’s something wrong with our assumptions.”
An evasive explanation
Confused by the star’s unusual properties, Smith and his colleagues began trying to explain the phenomenon. They considered changes in brightness resulting from pulsations or spasms in the star itself – behaviors that are quite common but do not occur dramatically in stars such as VVV-WIT-08. The researchers also ruled out the idea that the eclipse could be explained by a random adaptation to a dark foreground object closer to Earth that just happened to get in the way, such as a dusty, faint star.
“We need a large number of these dark liquid objects,” says Smith. “It’s a pretty unlikely scenario – we should have seen a lot more of this kind of thing nearby.”
Wright and others think it is more likely that no matter which occluded VVV-WIT-08, gravity is bound to the star. And if that’s true, the authors say, perhaps the best explanation involves a huge, dusty dirt disk swirling around an orbiting companion star. Systems like this already exist, especially the Epsilon Aurigae, where a supergiant star is partially obscured by a giant, dusty companion every 27 years.
But dust filters light, allowing longer, redder wavelengths to pass through – which is not what these observations show. And dirt discs usually taper rather than have hard stops, though Wright points out that small moon shards cut holes in Saturn’s rings that have neat and tidy edges.
It is also not clear what kind of companion object may be in orbit with VV-WIT-08. The team considered a handful of possibilities, including high-ranking stars and dense star bodies such as white dwarfs, but the discs that usually form around these stars do not fully explain observations.
Another potential explanation is a circling black hole surrounded by a dark, dense ring of dirt – something that astronomers believe should exist but have never observed before. It is also possible that the obscured dust is removed from the star by an orbiting companion, but this would not fully explain observations.
Still, Levesque says it makes sense to focus on dust in the system given that astronomers expect giant, evolving stars to throw material that ends up in orbit – even if these systems do not look like this.
“It’s nice not too bizarre; it’s the kind of thing you would expect, ”she says. “But dust does not look so nice, and that would certainly suggest something very unusual about how that dust is distributed.”
And while it may be tempting to wonder if some kind of extraterrestrial megastructure might have flown in front of the star, Wright says the hypothesis is not yet ripe for serious consideration.
“It’s too early at this point,” he says. “There’s so much about this star that we do not know.”