Astronomers were taken aback by the disappearance of a massive star they were observing.
They are now wondering if the distant object collapsed to form a black hole without exploding in a supernova.
If correct, this would be the first example of such a huge stellar object reaching the end of its life.
But there is another possibility, according to the study published in the monthly notices of the Royal Astronomical Society.
The brightness of the object may have decreased because it is partially obscured by dust.
It is located about 75 million light-years away in the Kinman dwarf galaxy, in the constellation Aquarius.
The giant star belongs to – or belonged to – a type known as the bright blue variable; it is about 2.5 million times brighter than the sun.
Stars like this are unstable, showing occasional dramatic changes in their spectra – the amount of light emitted at different wavelengths – and the brightness.
Between 2001 and 2011, various teams of astronomers studied the massive star, concluding that it was in an advanced stage of its evolution. 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.
In 2019, a team led by doctoral student Andrew Allan of Trinity College Dublin, Ireland, targeted the galaxy, with the aim of learning more about how very massive stars end their lives.
But when they pointed to the Very Large Telescope (VLT) of the European Southern Observatory (Eso), they could no longer find the revealing signatures of the star.
Mr. Allan commented, “We were surprised to find that the star was gone! ”
He added: “It would be very unusual for such a massive star to disappear without producing a luminous supernova explosion. ”
Earlier observations seem to indicate that the star was experiencing giant eruptions, in which material is lost from the star. These would have stopped some time after 2011.
Bright blue variable stars like this one are prone to such explosions during their lifetime. They cause the star to lose mass and lead to a dramatic peak in brightness.
Based on their observations and models, astronomers suggest two explanations for the disappearance of the star and the absence of a supernova.
The explosions may have resulted in the transformation of the bright blue variable into a less bright star, which could also be partially hidden by dust.
Alternatively, the team says the star may have collapsed in a black hole, without producing a supernova explosion.
It would be a rare occurrence: our current understanding of how massive stars die suggests that most of them end in a violent nova.
If the explanation for the black hole is correct, says Allan, “it would be the first direct detection of such a monster star ending its life in this way.”
Co-author Jose Groh, also of Trinity College Dublin, said: “We may have detected one of the most massive stars in the local universe slowly penetrating into the night. ”
Future studies are needed to confirm what happened to the star.
Eso’s extremely large telescope (ELT) – slated to begin operations in 2025 – will be able to resolve stars in distant galaxies such as the Kinman dwarf, helping to unravel cosmic mysteries like this one.
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