Stars can do a lot of things. They can get warmer, expand, explode, and even collapse into a black hole. They can not, as a rule, simply disappear. However, that’s what appears to have happened to an ultra-bright star in the constellation Aquarius. Astronomers went looking for this well-known star in late 2019 only to find that it was missing. The team has devised several possible explanations, but this is a real head-scratcher.
Astronomers studied this object in great detail between 2001 and 2011 because it’s no ordinary star. It is (or was?) a very rare type of ancient star called a massive luminous blue variable (LBV). And “luminous” is putting it mildly. The light output varied (as the name implies), but it was about 2.5 million times brighter than the sun on average. That’s the only reason we were able to see it — it resides in the Kinman Dwarf galaxy some 75 million light-years away.
Last year, the team from Trinity College Dublin had hoped to use the European Southern Observatory’s Very Large Telescope to check in on this object, but it was no longer visible. Had it died? This star was getting on toward the end of its life cycle, which is why it was so interesting in the first place. Yet, a supernova from a dying star would leave evidence, and there was no sign of such an event.
The researchers have gone back to look at older records of the LBV to come up with some possible explanations. Based on this older data, the team speculates that the star may have been experiencing a strong outburst period the last time astronomers glanced in its direction. That may have ended around 2011, causing the star to become dim enough that we can no longer pick it out from the background that far away. That suggests the LBV could flare up again at any time and become visible.
A more exciting and speculative hypothesis is that the LBV has indeed bought the farm, but in a way we’ve never seen before. It may have somehow burned itself out and collapsed into a black hole without all the usual telltale signs. If this is true, there should be a black hole with around 100 solar masses lurking in the Kinman Dwarf galaxy. This has the potential to rewrite our understanding of the solar life cycle.
For now, we’re stuck with speculation. The team hopes to use the ESO’s upcoming Extremely Large Telescope (ELT) to take a closer look and solve this mystery once and for all. That project is currently slated to begin in 2025.