The stars may twinkle, but they don’t just disappear. So when a distant giant star performed an act of disappearance for about 200 days, it took astronomers by surprise.
Now, about a decade later, astronomers have sifted through a variety of possible explanations – and they I still have no idea what’s to blame to erase almost all of the star’s light.
Described in a new study by Monthly notices from the Royal Astronomical Society, some of the theories still on the table are based on as yet unobserved phenomena, such as a dark disc of matter orbiting a nearby black hole, or undiscovered, dust-covered companion stars. But in 17 years of sightings, the star has only died out once, in 2012, making it harder for teams to identify a plausible culprit.
It is clear that whatever object has eclipsed the distant star, it is huge, much bigger than the star itself. It also appeared to be completely opaque, blocking out much of the starlight entirely, and it appeared to have a hard edge.
“The degree of brightness drop is really impressive,” says Emilie Lévesque from the University of Washington, which studies massive stars and did not participate in the sightings. “It will be cool to see more sightings of this star, what caused it, and piece together how something like this happened.”
Giant stars acting strangely
The galaxy is full of strangely behaving stars, many of which naturally fluctuate in brightness. One of those variable stars, named Betelgeuse, faded significantly in 2019, sparking speculation that it could be on the verge of exploding. (It doesn’t.) Instead, Orion’s shoulder-mounted red supergiant has returned to its normal brightness, and astronomers are now attribute his fainting to a cool place in its southern hemisphere and a puff of dust.
Perhaps even more famous, in 2015 astronomers caught a star sparkling so strangely that some scientists have considered the possibility that its light is blocked by an alien megastructure in orbit. The allure of alien tech 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.
Equally intriguing is this star that flashed for the first half of 2012.
“It is unusual for a star to decrease in brightness so much and for such a long time, and this immediately caught my attention as something unusual,” says the author of the study. Leigh smith, astronomer at the University of Cambridge.
Smith spotted the bizarre eclipse as he sifted through data from the VISTA variables in Via Lactea, or VVV, survey. This project, which takes its name from the Latin “Milky Way”, monitors the southern sky for variable stars in the galaxy’s disk.
The sighting earned the star a special designation: WIT, or “What is this?” », An acronym that the astronomers of the VVV project use to categorize curious objects. The star became known as VVV-WIT-08, and the team reported it for follow-up work. Based on the first 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 larger than our sun, but smoldering in cooler temperatures.
During the first half of 2012, the star almost completely disappeared, losing 97% of its brightness. The data suggested that whatever caused such a precipitous plunge was opaque, uniformly obscuring all visible and infrared wavelengths of light throughout the eclipse.
“It’s very difficult to understand,” said Jason wright from Pennsylvania State University, which was not involved in the sightings. “It’s something bigger than the star that’s completely opaque, and there aren’t a lot of things that do that.”
Follow-up surveys used data from the European Space Agency Gaia Spaceship and a ground survey called LORGNER to glean more information on the star. But as these observations accumulated, so did the questions. It became more difficult to determine the precise size and distance of the star, and its movement in space seemed peculiar – VVV-WIT-08 almost seemed to travel fast enough to escape the Milky Way.
“It’s way above anything you would expect in this direction,” Smith said. “So there is something wrong here, there is something wrong with our assumptions. “
An elusive explanation
Puzzled by the star’s unusual characteristics, Smith and his colleagues began to try to explain the phenomenon. They took into account changes in brightness that come from pulsations or spasms within the star itself – a fairly common behavior, but one that does not occur to a dramatic degree in stars like VVV-WIT-08. . Scientists have also ruled out the idea that the eclipse could be explained by a random alignment with a dark object in the foreground closer to Earth that just got in the way, like a dusty, dark star.
“We would need a lot of these dark floating objects,” says Smith. “It’s a pretty unlikely scenario – we should have seen a lot more of this sort of thing nearby.”
Wright and others believe that the occluded VVV-WIT-08 is more likely to be gravitationally bound to the star. And if that’s true, the authors say, perhaps the best explanation involves a huge disc of dusty debris swirling around a companion star in orbit. Such systems already exist, in particular Epsilon Charioteer, where a supergiant star is partially eclipsed by a giant dust-covered companion every 27 years.
But dust filters the light, letting longer, redder wavelengths through, which is not what these observations show. And the debris disks generally decrease rather than have hard stops, although Wright points out that the little moons cut the holes in the rings of Saturn which have neat and tidy edges.
It’s also unclear what kind of companion object might 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 disks that normally form around these stars do not fully explain the sightings.
Another potential explanation is an orbiting black hole surrounded by a ring of dark, dense debris, something astronomers believe should exist, but have never observed before. It is also possible that the obscuring dust was removed from the star by a companion in orbit, but that would not fully explain the sightings.
Still, Levesque says it makes sense to focus on the dust in the system, given that astronomers expect evolving giant stars to release matter that ends up in orbit, even if those systems don’t. not quite look like this one.
“It’s nicely not too weird; that’s the kind of thing you’d expect, ”she says. “But the dust doesn’t look that crisp, and that would certainly imply something very unusual about the way that dust is distributed.”
And while it may be tempting to wonder if some type of alien megastructure could have flown past the star, Wright says that hypothesis is not yet ripe for serious consideration.
“It’s premature at this point,” he says. “There is so much about this star that we don’t know.”
