An unconventionally-bright star has disappeared in a stellar mystery of cosmic magnitude, according to new research published Tuesday in Monthly Notices of the Royal Astronomical Society.
Giant star disappears without a trace
A stellar object inside the Kinman dwarf galaxy has vanished from view. A gigantic and extraordinarily bright blue star was hypothesized to exist, based on astronomical observations that happened from 2001 to 2011.
But beginning sometime in 2019, no one else could detect it.
The study's authors — headed by Andrew Allan of Trinity College Dublin, have come up with two explanations: The star has either experienced a sharp drop in luminosity and is momentarily hiding behind stellar dust, or it has collapsed into a new black hole without exploding into a supernova.
If the latter is the case, it would mark the second-known failed supernova.
The Kinman dwarf galaxy is 75 million light-years from Earth — in other words: far, far away. At this range, astronomers can't typically differentiate one star from another. But the hypothesized star (now missing) is a luminous blue variable (LBV), which are observable even at extreme distances. LBVs are massive stars whose behavior is difficult to predict near the end of their lifespan.
The unpredictability of this star as it moves through varying brightness and spectra can be seen from Earth. Additionally, the star is a maddeningly 2.5 million times brighter than the Sun, reports Gizmodo.
Or at least, it used to be.
Possible fates of the vanishing star
Astronomers gathered observations from 2001 to 2011, and concluded this star was likely a late-stage LBV in the Kinman dwarf galaxy. In 2019, they checked-in with the bright star using the European Southern Observatory's Very Large Telescope. But to their surprise, there appeared to be nothing there — a discovery that was at once both exciting and discouraging.
"We were all pleasantly surprised to find that the star's signature was not present in our observation taken in August 2019 using the ESPRESSO instrument of ESO's Very Large Telescope," said Allan to Gizmodo. "We initially hoped for a higher-resolution observation that resembled the past observations, which we would use for our models."
At first, the astronomers thought something must be wrong with ESPRESSO — so Allan and his colleagues took another gander with the telescope's X-shooter instrument.
"We rechecked the ESPRESSO observation a number of times but were unable to detect the star's signature," said Allan to Gizmodo. "As the conditions were not perfect on the day this observation was made, we wanted to make sure the signature was truly absent. This time we used the X-Shooter instrument of the Very Large Telescope and were happy to find that this also pointed towards the star disappearing."
MIA after major stellar outburst
Seeing nothing there, the team suddenly had in its possession a major cosmic mystery that begged to be solved. So they waded into the archives, seeking earlier observations of the dwarf galaxy that might lend context to the observation.
Sure enough, the suspected giant star experienced a major outburst period that ended around 2011. LBVs often throw tremendous temper tantrums, causing sudden losses in mass and sharp drops in brightness.
In the wake of the 2011 outburst period, the astronomers think "we are seeing the end of an LBV eruption of a surviving star, with a mild drop in luminosity, a shift to hotter effective temperatures, and some dust obscuration." So, while the star may still be active, it may simply be too dim for Earth-bound astronomers to observe.
Phantom black hole
It could also be a newly-formed black hole, missing the typical supernova explosion typically preceding such transformations. This is called a failed supernova.
"This would be consistent with some of the current computer simulations that predict that some stars will not produce a bright supernova when they die," said Allan, to Gizmodo. "This happens when a massive black hole is formed, and it is not spinning very fast. However a collapse to a black hole without producing a supernova has only been observed once in the past, in the galaxy NGC 6946 where a smaller massive star seemed to disappear without a bright supernova explosion."
If this is the case, it would be the first known black hole formed without a supernova from a massive star in a low-metallicity galaxy. This is significant because it represents a finding that "could hold important clues as to how stars could collapse to a black hole without producing a bright supernova," Allan added.