The object's brightness might have dipped because it is partially obscured by dust.
This illustration shows what the luminous blue variable star in PHL 293B could have looked like before its mysterious disappearance. If it's the type of star that researchers think it is (or was), it's possible that it underwent a rapid shrinking as it shed mass but grew brighter in astronomers' telescopes. In 2019, Allan and colleagues hoped to use the European Southern Observatory's Very Large Telescope to learn more about the distant LBV's mysterious evolution, only to discover that the star had seemingly completely vanished from its host galaxy.
The traces were absent from the data the team collected in 2019, leaving them to wonder what had happened to the star.
Trinity College Dublin astronomer Andrew Allan and colleagues wanted to find out more about how very massive stars end their lives, and the object in PHL 293B seemed like the ideal target.
"One of the most memorable moments was when we noted the absence of the massive star signature in our first observation which was obtained with the ESPRESSO instrument of ESO's Very Large Telescope".
Giant star pulls off vanishing act. Did it become a black hole or was it all an illusion?
But there is another possibility, the study in Monthly Notices of the Royal Astronomical Society reports. The massive star that had been previously documented was just plain gone.
Co-author Jose Groh, also of Trinity College Dublin, commented: "We may have detected one of the most massive stars of the local Universe going gently into the night". The outburst would have produced a large rate of mass loss and would have caused the star's luminosity to temporarily spike.
They said that if the star collapsed into a black hole without producing a supernova explosion, it would be "a rare event", as "our current understanding of how massive stars die points to majority ending their lives in a supernova".
Scientists are not sure why the star can no longer be seen but believe there may be two possible explanations.
The outburst may have resulted in the luminous blue variable being transformed into a less luminous star, which could also be partly hidden by dust.
The researchers said further studies are needed to understand what happened to the star. The extremely massive star was of particular interest because scientists still don't know much about how such objects behave towards the end of their lifetimes, especially in metal-poor environments such as the Kinman Dwarf galaxy. Typically, stars in its class end their life cycle with a bang, in a supernova - the most powerful explosion in the universe.
The astronomers plan to follow up on their work with a more powerful observatory, when it is ready.
The archival search also revealed new information. ESO's Extremely Large Telescope will have a single 127-foot (39 meters) mirror, compared with the VLT's combined aperture of 107-foot (32 m) mirror across four telescopes.