Why did a star nearly vanish from the sky?
A star close to the heart of the Milky Way dimmed by 97 per cent for 200 days, and no one quite knows how
A star close to the heart of the Milky Way dimmed by 97 per cent for 200 days, and no one quite knows how
More than 25,000 light years away, there’s an enormous star that’s 100 times the size of the Sun. It’s in the constellation of Sagittarius, close to the heart of the Milky Way. Yet for about 200 days in 2012, it virtually vanished from the night sky. For those studying stars in this dense region of the galaxy, it was understandably something of a surprise. Astronomers had been working their way through data from the VISTA Variables in the Via Lactea Survey in 2019, but they had certainly not expected to come across anything so perplexing.
What they were witnessing was a dimming on an near-unprecedented scale, with the star, called VVV-WIT-08, decreasing in brightness by a mammoth factor of 30: a staggering 97 per cent. “It’s not particularly unusual for stars to dim by a small amount for short periods,” says Dr Leigh Smith from Cambridge’s Institute of Astronomy. “But it’s very unusual for stars to dim as much and for as long as this.”
Smith led the discovery, working with other scientists in the UK, Germany, Brazil, Chile and Poland. The data had been gathered by the European Southern Observatory’s Visible and Infrared Survey Telescope for Astronomy (VISTA), based at the Paranal Observatory in Chile and the largest in the world to survey the sky in near-infrared wavelengths. The study involved creating a database of the light curves of some 1 billion stars and checking for anything which looked unusual. “A light curve is the sequence of brightness measurements of a star at a series of times, and anything unusual tends to be most interesting,” Smith explains. “More often they can indicate problems with the dataprocessing procedures. In our case, we checked on stars that had a large range in brightness, and VVV-WIT-08 was one of the first in the list. After ruling out a data-processing problem, it was clear that the drop in brightness was real.”
Today, VVV-WIT-08 is back at full brightness. The dimming occurred slowly, fading at the start of 2012 to the point of almost disappearing before gradually recovering. Smith says stars change their brightness because they pulsate or are eclipsed by another star in a binary system. Could either of these explain what’s happening?
Prior to the dimming event in 2012, it’s unlikely anyone had ever studied VVV-WIT-08. “The star looks exactly like millions of similar giant stars in the Milky Way,” Smith says. He adds that he wouldn’t be surprised if no one had even looked at the star before his team found it in 2019. “Although the data covering the dimming event existed before then, it needs to be processed, which takes a huge amount of time.”
Even so, VVV-WIT-08 had been captured by several observation campaigns covering large parts of the sky, enabling the astronomers to work with earlier brightness measurements in different wavelengths. “Using these, we were able to roughly estimate what kind of star we were looking at,” Smith says. “The suggestion was that it’s an ordinary giant star, which made the drop in brightness all the more puzzling.”
One explanation is that the dimming was caused by an unknown dark object drifting,
by chance, in front of the giant star. This was dismissed because simulations suggest that an implausibly large number of dark bodies would need to be floating around the galaxy.
It’s more likely it was a planet or a companion star orbiting the giant star, but these alone are unable to account for the depth of the dimming. Whatever was causing the dimming was also surrounded by an opaque disc of dust, and this, it was found, had a hard edge – unusual because debris discs tend to taper, thereby begging the question of how the dust is being distributed to make it appear that neat.
It doesn’t help that no one has been able to observe VVV-WIT-08’s orbital companion, but the data suggests that whatever was hiding the star had to be huge. It managed to block not only visible wavelengths, but infrared ones too. “The obscuring object has to be larger than the giant,” affirms Smith. “That much is known for sure, since if it were smaller it could not have blocked as much light as it did.” This means that the obscuring object has to be at least half as big as the distance between the Earth and Sun, “but likely much bigger,” Smith continues. “Also, because the shape of the dimming is not quite symmetric, we’re able to say for sure the object is elongated – that is, it can’t appear in silhouette as a circle, as a planet might.”
This is why astronomers have considered the presence of a large cloud or a disc, and yet it’s not so straightforward. “A cloud would tend to obscure more visible light than infrared, but for VVV-WIT-08 they are obscured by the same amount,” Smith explains. “That leaves the disc theory remaining, but while large discs are not uncommon and often occur as a result of star formation, they aren’t usually very long-lived. VVV-WIT-08 is relatively old, so the presence of such a disc is quite unusual here.”
Astronomers have estimated that the star is 8 billion years old and that it’s burning at cooler temperatures than the Sun. In figuring what may have caused such a dramatic dimming, they are also able to draw on some past studies. The red supergiant Betelgeuse, which is one of the ten brightest stars in the night sky, spewed out a huge cloud of dust in 2019, causing it to dim by two-thirds. “There are a handful of examples of stars obscured by discs that are hosted by a gravitationally bound companion, too,” says Smith.
He points to the multiple star system Epsilon Aurigae, discovered by German astronomer Johann Heinrich Fritsch in 1821. Its brightness drops by
50 per cent every 27 years, with the dimming lasting between 640 and 730 days. That, however, is significantly less dramatic than the 97 per cent drop of VVV-WIT-08.
“VVV-WIT-08 is equalled in the depth of its dimming by a system called TYC 2505-672-1,” Smith says, describing a binary system eclipsed by a disc around its companion star every 69 years. The same goes for two other candidates picked up by Smith’s team, VVV-WIT-10 and VVV-WIT-11, and confidence is high that others will be found.
One thing’s for sure, it would make life easier if astronomers could foresee when VVV-WIT-08 will dim again. However, as can be seen from Epsilon Aurigae and TYC 2505-672-1, it could be many decades before that happens. All that’s known so far is that in 17 years of observations the eclipse of VVV-WIT-08 has only happened once. The next one could happen in ten years. But astronomers go cold at the prospect of not witnessing it again for another 200.
“Having the star return to its former brightness makes it easier to perform follow-up observations. In particular, we’ll be able to more easily monitor the giant star’s velocity along the line of sight – that is, its velocity towards or away from us, the radial velocity,” Smith says. “Having said that, more observations during the dimming event would have been nice. While the giant star is obscured you get more direct observations of the object doing the obscuration.”
When the next eclipse happens will depend heavily on the nature of the unseen companion. “The period of its orbit is dependent on its mass,” Smith continues. “This means that even if we’re never able to directly see the companion, we will be able to infer its mass by measuring the orbital period, and the mass may be all
we need to say for sure what it is.” Thankfully, astronomers may not have to wait that long, because the amount of data that could be used to explain what is happening is growing. It’ll also be possible to monitor the radial velocity to estimate the period of the orbit long before the next dimming event. “We think we now have enough similar objects that we can start to study their properties as a group, which may lead to some breakthroughs in understanding them,” Smith says. “But the origin of the disc material, the nature of the disc hosts and the apparent small size of some of the giant stars are all open questions.”
Another conundrum is why, using data from later observations, including the ground-based Optical Gravitational Lensing Experiment (OGLE) and the European Space Agency’s Gaia spacecraft, VVV-WIT-08 seems to be travelling faster than anyone had imagined. At this speed it looks like it will one day escape the Milky Way, which is strange for a star in a dense region close to the centre of the galaxy.
“Observing the speed made a huge impact on the study,” says Smith. “VVV-WIT-08 moving that fast is highly unlikely. Our estimate of its speed required an estimate of its distance, and for our speed estimate to be too high, our distance estimate must also have been too large.
“For the giant star to be closer than it appeared, it must also be smaller than normal, which might end up being a clue towards the nature of the mystery companion. One theory is that the companion has stripped material from the giant star, shrinking the giant star and forming the disc around the companion.” But this would only partly explain what has been observed.
The astronomers have certainly left nothing off the table. They’ve even considered that the orbiting body is a black hole ringed with debris – something never observed before. What they are saying, however, is that VVV-WIT-08 may well belong to a new class – that it’s a ‘blinking giant’ binary star system. This is one of the possibilities put forward in Smith’s coauthored paper, published in Monthly Notices of the Royal Astronomical Society.
But how did they come to be? No one knows, but Smith and others are determined to get to the bottom of it. VVV-WIT-08 doesn’t appear to be alone, either. There are around six potential star systems that could fit into this theorised ‘blinking star’ category, and the challenge is to work out how they came to be surrounded by discs even though their orbits are so far from the giant star – indicated by the amount of time that is passing for the eclipse to occur again. “We have so much more to discover,” Smith says.