A massive star 75 million light years away ‘vanished’ and astronomers think it may be the first time they’ve seen one collapse into a black hole without going supernova first.
The blue variable star is in the Kinman Dwarf galaxy and its absence was spotted by astronomers using the European Southern Observatory Very Large Telescope (VLT).
It is an ‘unstable’ type of star that is up to 2.5 million time brighter than the Sun – it vanished from view some time between 2011 and 2019, astronomers say.
One explanation is that the distant star has been obscured by dust, but another theory is that the star collapsed into a black hole without producing a supernova.
If this theory is true then it would be the first direct detection of ‘such a monster star ending its life in this manner’, according to team leader Andrew Allan.
The researchers from Trinity College Dublin said further studies were needed to understand what happened to the star but they may have to wait until the European Souther Observatory Extremely Large Telescope comes online in 2025.
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 most of them ending their lives in a supernova’.
Between 2001 and 2011, various teams of astronomers studied the mysterious massive star, and their observations indicated it was in a late stage of its evolution.
Allan and his collaborators in Ireland, Chile and the US wanted to find out more about how very massive stars end their lives, and the object in the Kinman Dwarf seemed like the perfect target – as it was expected to go supernova soon.
But when they pointed ESO’s VLT to the distant galaxy in 2019, they could no longer find the telltale signatures of the star.
‘Instead, we were surprised to find out that the star had disappeared!’ says Allan.
Located some 75 million light-years away in the constellation of Aquarius, the Kinman Dwarf galaxy is too far away for astronomers to see its individual stars, but they can detect the signatures of some of them.
From 2001 to 2011, the light from the galaxy consistently showed evidence that it hosted a ‘luminous blue variable’ star some 2.5 million times brighter than the Sun.
Stars of this type are unstable, showing occasional dramatic shifts in their spectra and brightness, the research team explained.
Even with those shifts, luminous blue variables leave specific traces scientists can identify, but they were absent from the data the team collected in 2019, leaving them to wonder what had happened to the star.
‘It would be highly unusual for such a massive star to disappear without producing a bright supernova explosion,’ says Allan.
They used a number of different instruments available through the VLT including pointing all four 26ft telescopes at the star at the same time.
‘We may have detected one of the most massive stars of the local Universe going gently into the night,’ says team-member Jose Groh, also of Trinity College Dublin.
The team then turned to older data collected using the telescope that is based in the Chilean Atacama Desert, and other data from alternative telescopes worldwide.
‘The ESO Science Archive Facility enabled us to find and use data of the same object obtained in 2002 and 2009,’ says Andrea Mehner from ESO.
The old data indicated that the star in the Kinman Dwarf could have been undergoing a strong outburst period that likely ended sometime after 2011.
Luminous blue variable stars such as this one are prone to experiencing giant outbursts over the course of their life, causing the stars’ rate of mass loss to spike and their luminosity to increase dramatically.
Based on their observations and models, the astronomers have suggested two explanations for the star’s disappearance and lack of a supernova, related to this possible outburst that ended in 2011.
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.
Alternatively, the team says the star may have collapsed into a black hole, without producing a supernova explosion.
This would be a rare event: our current understanding of how massive stars die points to most of them ending their lives in a supernova.
This question may not be answered until the ESO Extremely Large Telescope comes online in 2025 as it will be capable of resolving stars in distant galaxies such as this one in a way the VLT can’t.
The findings have been published in the journal Monthly Notices of the Royal Astronomical Society.