Astronomers have discovered a huge star in the center of the Milky Way that is puzzling them: Observations with the Vista telescope at the European Southern Observatory (ESO) in Chile, which has been monitoring around a billion stars for changes in brightness in the infrared range for almost a decade, showed that celestial body VVV-WIT-08 dimmed by 97 percent over a few hundred days and then slowly returned to its former brightness.
At the beginning of 2012, the star, which is more than 25,000 light-years away from Earth, began to fade, the researchers from the Institute for Astronomy at the University of Cambridge report in the “Monthly Notices of the Royal Astronomical Society”. In April of the same year, it had almost disappeared before its radiance returned within the next 100 days. Since then, VVV-WIT-08 has been shining with its original power without any further fluctuations. “It seemed to come out of nowhere,” said Dr. Leigh Smith of the British newspaper “The Guardian” the sudden darkening of the star.
Such a duration and intensity of dimming is extremely unusual and rare, reports the science magazine “Scinexx”. Because the telescope observations only go back 17 years, it is not even clear whether this is a one-time or regular event. So far, astronomers have only observed two stars with extremely long return periods of eclipses. The Epsilon Aurigae, which is obscured by its companion’s dust disk every 27 years, and the red giant TYC 2505-672-1, which even has 69 years between events.
Stars that change their brightness in short cycles, on the other hand, are more common. Cepheids, for example, shrink or grow every one to a hundred days and also change their temperature and luminosity in the same rhythm. Even stars that only pulsate on one side have been observed. In addition, there are double stars whose partners alternately cover each other.
None of this applies to VVV-WIT-08. The WIT also explains this in its name. “Sometimes we come across variable stars that don’t fit into any established category,” Scinexx quoted Philip Lucas of the University of Hertfordshire, co-author of the report. “We then call them WIT for ‘What is this?’.”
Because they spotted the giant star in such a dense region of the galaxy, the researchers wondered if an unknown dark object could have accidentally slipped in front of it. Computer simulations would have almost ruled out such a random darkening. Still, astronomers believe that something obscured VVV-WIT-08 from the outside. “The occulting object must have some core characteristics: it must be gravitationally bound to the giant star, be very faint, have a radius greater than 50 Earth radii, and appear elliptical,” Smith and his colleagues explained.
A companion star surrounded by a dense disk of dust is most likely. However, the typical features of such disks did not match the observations, the scientists wrote. Another possibility would be a companion star, which sucks material from the giant star and thus forms an extended gas disk. The problem, however, is that in view of the long duration of the occultation, the companion would have to be very far away from the giant star – and therefore too far away to suck material from it.
The astronomers conclude that none of the envisaged scenarios can fully explain why VVV-WIT-08 was obscured for so long and to such a large percentage. “The duration, depth and achromaticity of the blackout makes this event extraordinarily unusual – its mystery is yet to be revealed.”
Quellen: “Monthly Notices of the Royal Astronomical Society”, “The Guardian”, “Scinexx”, ORF
