One of the largest known stars in the universe underwent a dramatic transformation in 2014, new research shows, and may be preparing to explode.
A study led by Gonzalo Muñoz-Sanchez at the National Observatory of Athens, published in Nature Astronomy today, argues that the enormous star WOH G64 has transitioned from a red supergiant to a rare yellow hypergiant – in what may be evidence of impending supernova.
The evidence suggests we may be witnessing, in real time, a massive star shedding its outer layers, shrinking as it heats up, and moving closer to the end of its short life.
A very special star
WOH G64 was first discovered in the 1970s as as star of interest in the Large Magellanic Cloud, a dwarf galaxy orbiting the Milky Way.
It turned out the star was not only extremely luminous, but also one of the biggest ever discovered: more than 1,500 times the radius of the Sun.
In 2024, WOH G64 was the first star beyond our galaxy ever photographed in detail, thanks to the Very Large Telescope Interferometer. The image showed a clear dusty cocoon around the central giant star, which confirmed it was losing mass as it aged.
From supergiant to hypergiant, big is big
WOH G64 is a young star in the grand scheme of the cosmos, with an estimated age of less than 5 million years old. Unlike our Sun (currently about 4.6 billion years old), WOH G64 is destined to live fast and die young.
WOH G64 was born big, forming from a huge cloud of gas and dust collapsing until the pressure made it ignite. Like our Sun, it would have burned hydrogen in its core by nuclear fusion.
Later it would have expanded and burned helium, becoming what is called a red supergiant.
Not all supergiants become hypergiants. It’s been theorised that hypergiants form when very large stars quickly burn and evolve from burning hydrogen to burning helium.
During this transition, these stars start to shed their outer layers, while their cores begin to shrink inwards. Once a star becomes a hypergiant, it is destined for a quick death in the fiery explosion of a supernova.
What has caused this change seen in WOH G64?
So what happened to WOH G64 in 2014? The new study proposes that a large part of the original supergiant’s surface was ejected away from the star.
This may have been due to interactions with a companion star, which the authors have confirmed exists by looking at the spectrum of light from WOH G64.
Another theory: the star is getting ready to explode. We know stars this big will inevitably go kaboom, but exactly when it will happen can be hard to determine in advance.
One possible scenario is that the transition we’re seeing is due to a pre-supernova “superwind” phase. This is theorised to occur due to strong internal pulsations as the fuel in the core is spent quickly.
Only time will tell
Most stars live for tens of millions or even tens of billions of years. It was never a given we would witness and be able to document so much transformation in a star, let alone one outside our galaxy.
If we are lucky, we will see the death of WOH G64 in our lifetimes – not only providing an incredible intergalactic spectacle but also helping scientists complete the puzzle of this fascinating star.
