Mystery object blotted out a giant star for 200 days

Something huge and opaque caused a star in our galaxy to dim, and astronomers are struggling to explain the phenomenon.
An artist’s impression shows the star VVV-WIT-08, which mysteriously disappeared during the first half of 2012.
Photograph by Illustration by Amanda Smith
Published 17 Jun 2021, 12:42 BST

Stars may twinkle, but they don’t just vanish—so when a distant, giant star pulled a disappearing act for about 200 days, it took astronomers by surprise.

Now, roughly a decade later, astronomers have sifted through a variety of possible explanations—and they still have no idea what’s responsible for blotting out nearly all of the star’s light.

Described in a new study in the Monthly Notices of the Royal Astronomical Society, some of the theories still on the table rely on as-yet unobserved phenomena, such as a dark disk of material orbiting a nearby black hole, or undiscovered, dust-enshrouded companion stars. But over 17 years of observations, the star has only gone dark once, in 2012, making it more difficult for teams to nail down a plausible culprit.

It’s clear that whatever object eclipsed the distant star is huge—much bigger than the star itself. It also appeared to be completely opaque, blocking much of the starlight entirely, and it seemed to have a hard edge.

“The degree of drop in brightness is really impressive,” says Emily Levesque of the University of Washington, who studies massive stars and wasn’t involved in the observations. “It'll be cool to see more observations of this star, of whatever caused this, and to piece together how something like this happened.”

Giant stars acting strange

The galaxy is full of weirdly behaving stars, many of which naturally fluctuate in brightness. One of these variable stars, named Betelgeuse, dramatically dimmed 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 are now attributing its fainting spell to a cool spot in its southern hemisphere and a puff of dust.

Perhaps even more famously, in 2015, astronomers caught a star flickering so oddly that some scientists considered the possibility its light was being blocked by an orbiting alien megastructure. The allure of alien technologies launched the star—now known as Tabby’s Star—into the limelight for years, but observations in 2018 revealed that the culprit was nothing more than dust.

This star that winked out for the first half of 2012 is similarly intriguing.

“It’s unusual for a star to dim in brightness by this much and for this long, and it immediately caught my eye as something unusual,” says study author Leigh Smith, an astronomer at the University of Cambridge.

Smith spotted the odd eclipse while he was sifting through data from the VISTA Variables in the Via Lactea, or VVV, survey. This project, which draws its name from the Latin for “milky way,” monitors the southern sky for variable stars in the galaxy’s disc.

The observation earned the star a special designation: WIT, or “What is this?,” an acronym that astronomers with the VVV project use to categorise curious objects. The star became known as VVV-WIT-08, and the team flagged 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 some 100 times larger than our sun, but smouldering at cooler temperatures.

During the first half of 2012, the star almost completely disappeared, losing 97 percent of its brightness. Data suggested that whatever had caused such a precipitous plunge was opaque, uniformly obscuring all visible and infrared wavelengths of light throughout the entire eclipse.

“That’s very hard to understand,” says Jason Wright of Pennsylvania State University, who wasn’t involved in the observations. “It’s something bigger than the star that’s completely opaque, and there aren’t many things that do that.”

Follow-up investigations used data from the European Space Agency’s Gaia spacecraft and a ground-based survey called OGLE to glean more information about the star. But as those observations piled up, so did the questions. It became tougher to pin down the star’s precise size and distance, and its motion through space looked peculiar—VVV-WIT-08 appeared to almost be travelling fast enough to escape from the Milky Way.

“It’s well above anything you would expect in this direction,” Smith says. “So there’s something not quite right here, there’s something wrong with our assumptions.”

An elusive explanation

Perplexed by the star’s unusual characteristics, Smith and his colleagues began trying to explain the phenomenon. They considered changes in brightness that originate from pulsations or spasms within the star itself—behaviour that is quite common, but which doesn’t occur to a dramatic degree in stars like VVV-WIT-08. The scientists also ruled out the idea that the eclipse could be explained by a chance alignment with a dark foreground object closer to Earth that just happened to get in the way, such as a dusty, dim star.

“We’d need a huge number of these dark floating objects,” Smith says. “That’s a pretty unlikely scenario—we should have seen many more of this kind of thing nearby.”

Wright and others think it’s more likely that whatever occluded VVV-WIT-08 is gravitationally bound to the star. And if that’s true, the authors say, perhaps the best explanation involves a huge, dusty debris disc swirling around an orbiting companion star. Systems like this already exist, notably Epsilon Aurigae, where a supergiant star is partially eclipsed by a giant, dust-enshrouded companion every 27 years.

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But dust filters light, allowing longer, redder wavelengths to pass through—which is not what these observations show. And debris discs usually taper off rather than having hard stops, although Wright points out that small moonlets mow gaps into Saturn’s rings that have neat, tidy edges.

It’s also not clear what kind of companion object could be in orbit with VV-WIT-08. The team considered a handful of possibilities, including main sequence stars and dense stellar corpses such as white dwarfs, but the discs that normally form around those stars don’t fully explain the observations.

Another potential explanation is an orbiting black hole surrounded by a dark, dense debris ring—something that astronomers think should exist, but have never observed before. It’s also possible that the obscuring dust is being stripped from the star by an orbiting companion, but that wouldn’t fully explain the observations.

Still, Levesque says that focusing on dust in the system makes sense, given that astronomers expect giant, evolving stars to shed material that ends up in orbit—even if those systems don’t look quite like this one.

“It’s nicely not too bizarre; it’s the sort of thing that you would expect,” she says. “But dust does not look this neat, and it would certainly imply something very unusual about how that dust is distributed.”

And while it might be tempting to wonder whether some type of extraterrestrial megastructure could have flown in front of the star, Wright says that hypothesis isn’t ripe for serious consideration yet.

“It’s premature at this point,” he says. “There’s so much about this star that we don’t know.”

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