The Star That Vanished
Ten years ago a star mysteriously disappeared from the night sky. We still have no idea why.
At first glance this star seems much like any other, one slightly red dot among a thousand others. Stare a little longer, though, and it starts to look a bit odd. It is moving too fast, so fast in fact, that it should have long ago escaped our galaxy and drifted into the vast void of intergalactic space. But it hasn’t - somehow, in defiance of Newton’s laws, it is still here.
Ten years ago something even odder happened to this star; something that defies explanation. It vanished, abruptly disappearing from the night sky. For months almost no trace of it remained. No supernova, no swirling cloud of debris, no black hole violently erupting into existence. Nothing at all, save for a faint blue light.
Then, half a year later, the star reappeared, as suddenly as it had gone. It was, it seems, completely unmarked by the experience. Ever since, the star has remained, peacefully shining in the night sky. Astronomers have searched in vain for signs that the event has repeated, but almost two decades of data show nothing of the kind.
This odd star was dubbed VVV-WIT-08. VVV for the name of the project that discovered its strange behaviour: the VISTA Variables in the Via Lactea survey. WIT - “What Is It?” to reflect its utterly bizarre behaviour, and 08 to distinguish it from the dozen other odd things the VVV survey has picked up.
Intrigued by the strange nature of the star and its even stranger disappearance, astronomers have studied it in greater detail, hoping to get some idea of what could have happened. The result of their work is a paper, published in the Monthly Notices of the Royal Astronomical Society.
There are a few obvious ways a star might disappear. It could have been destroyed - collapsing suddenly into a black hole, for example. The star itself may have drastically dimmed, for some unknown reason. Or something big could have passed between the star and Earth, blocking its light from reaching us.
The team of researchers considered each of these possibilities. The destruction hypothesis can be quickly ruled out - after disappearing for around two hundred days the star returned, carrying on much as before. Dimming is a possibility, they admit, though it would be highly unusual, perhaps impossible, in a star of this kind.
The most likely reason - the one that cleanly explains the pattern of light seen from the star - is the final possibility. Something blocked our view of the star for months on end, and then passed, restoring it to the night sky.
That’s easy to say, but the researchers ran into trouble trying to figure out what that “something” could be. First of all, where is it? Is it something small, close to the Earth, which obscured our view for a few months? Or was it something gigantic, perhaps orbiting the star? Or, instead, did it lie somewhere in the middle?
Take the first option: a small object close to Earth. Thanks to the laws of perspective, this argument might seem reasonable. An asteroid, perhaps, happening to pass in front of the star and blocking its light. The problem is, the Earth and everything else in the Solar System is constantly moving. If an asteroid drifted in front of the star it could block its light, sure, but moments later it would drift away, revealing the star once more.
The laws of orbital mechanics rule out natural motion, but what about artificial motion? An alien spaceship, for example, parked somewhere in our Solar System, invisible except for its effect on a distant star? That theory, while exciting, is also implausible. The spacecraft would have to actively maintain the alignment, deliberately blocking our view of this one star. Why would they bother?
What about the second option, then, a huge object close to the star? A big enough object - some kind of giant planet for example - could easily have blocked out the star for months on end, rather like a long eclipse. The problem, though, is this object would have to be insanely big. So big, in fact, that it is hard to conceive of anything natural of the necessary size.
If that idea seems somewhat challenging, then what about the third option: a big object somewhere in the middle? That, though, also comes with difficulties. First of all, none of the other stars in the region were affected, seemingly indicating that the event was somehow unique to the star in question.
Secondly, no other star in the galaxy shows similar light patterns. If there were giant dark objects floating around interstellar space, we would expect to see several such events - not just one, isolated incident. The event, in other words, is so rare that astronomers believe there must be something special about this particular star.
This means the second option is the most likely. Something unique to this star caused the event - probably something big that happened to be in orbit or passing close by. Having convinced themselves of that, the team of researchers then tried to imagine what that something could be.
The properties they deduced are staggering. To block out the entire star - one far bigger than our own Sun - it must be gigantic. At a minimum, the researchers estimate that it must be at least fifty million miles across - half the distance between the Sun and the Earth. It must be faint, otherwise we would have seen the light from it, and it must be thick enough to completely block out the light of the star.
Few things can reasonably fit this description. Nevertheless, the astronomers behind the paper had a go at coming up with options. Large planets, failed stars, and shattered solar systems are all considered and dismissed. None of them can explain all of the features of the mysterious event.
One possibility is a second star orbiting VVV-WIT-08, stripping gas and dust from it as it passes. The result would be a swirling cloud of debris around that second star, making it almost invisible to our telescopes. When it happened to pass in front of VVV-WIT-08, as seen from Earth, the result would have been an eclipse of kinds, temporarily wiping the star from the heavens.
A second option is a black hole or neutron star spinning around the vanishing star. Such an ultra-dense object could, theoretically, be enough to block the light from the star temporarily. Even so, intense bursts of radiation are normally seen coming from black holes - and none have so far been seen from this system.
That doesn’t necessarily mean there isn’t one. Black holes are known to pass through quiet periods, so the lack of radiation doesn’t kill the idea. What’s more, our surveys of this region of the sky lack detail - which could mean we just missed the radiation.
Nevertheless, the researchers found problems with every solution they considered. That has left some tempted to reach for an old fallback - the idea of alien mega-structures orbiting the star. For now, though, the evidence for that is as weak as it is for every other theory.
If you enjoyed this post then why not subscribe to our One Blue Planet newsletter? You’ll get two free emails a week covering the latest news and findings from physics, astronomy and the space industry. All you need to do is sign up here!
Wow. :)