Stars, to be fair to them, are pretty reliable. Look up at them one day and, assuming you have a human lifespan, you can look again some time in the future and you'll likely still find them there, twinkling away from years to billions of years in the past.
Every now and then though, astronomers find that they have disappeared, sometimes with no readily apparent explanation for where they went. One such star was discovered to have vanished in 2019, leaving scientists stumped as to what happened.
The massive star was first spotted in 1962 in the metal-poor Kinman Dwarf galaxy around 75 million light-years away. From its spectra, it was determined to be a "luminous blue variable" star, giant stars that produce unpredictable variations in their light.
The star was seen repeatedly in observations of the Kinman Dwarf galaxy in the following decades. Then in 2019, astronomers looked using the European Southern Observatory's (ESO's) Very Large Telescope and found that the star – which had previously been 2.5 million times brighter than our Sun – had completely vanished.
Looking back through data, they found that it had disappeared from view sometime between 2011 and 2016. Usually, stars like this one would not disappear without a trace, instead ending their lives as bright supernovas.
"It would be highly unusual for such a massive star to disappear without producing a bright supernova explosion," Andrew Allan, then a PhD student at Trinity College Dublin and lead author on the paper, said in a statement. So where did it go?
Data indicated to the team that the star was undergoing a particularly strong outburst period before its disappearance, which can make blue variable stars lose mass and decrease in luminosity.
The team had two hypotheses. One, that the period of high activity led to a decrease in luminosity, which could explain the star's disappearance if coupled with the star being partially obscured by dust. Alternatively, the team say it's possible that the star collapsed into a black hole without going supernova, a rare event that would challenge what we know about the end of the lives of massive stars.
Unfortunately, an answer will be hard to come by, given that the galaxy is too far away to get a good look at with current technology. ESO’s Extremely Large Telescope, bigger than its Very Large Telescope, may provide more evidence when it launches in 2028.