One of the largest stars in the known universe is undergoing a strangely rapid transformation and may soon explode as a supernova.
First catalogued in 1981, WOH G64 sits some 160,000 light years from Earth in the Large Magellanic Cloud, a small satellite galaxy of the Milky Way. It is one of the biggest red supergiants, the largest stars we know of. These are massive, cool stars that have run out of hydrogen fuel in their core and instead burn an envelope of hydrogen gas that surrounds them.
WOH G64 was thought to be about 1500 times the size of the sun, but is pretty unstable, losing mass faster than any other known red supergiant. Now, using data from the Very Large Telescope (VLT) and Magellan Telescopes in Chile, at the National Observatory of Athens, Greece, and her colleagues have spotted an even more dramatic transformation.
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By analysing the light given off by the star, they found that it had gone from being about 3000掳C with a strong signature of titanium oxide and a reddish colour, which is characteristic of a red supergiant, to heating up to about 4500掳C and having strong emission lines of elements such as iron and nickel and a bluer colour.
The star had changed so much, says Bonanos, that 鈥渙ne of our co-authors said, 鈥榃ait, did I observe the wrong star?鈥欌�� But it was the right one, she says. 鈥淭hat was the first clue that something was going on.鈥�
Exactly when this transition occurred is unclear, given a lack of continuous observations of the star, but it seems to have happened over the course of just a few years 鈥� the astronomical blink of an eye. It is now about half its earlier size and its red light is about a hundredth as bright as it was.
The researchers suspect that a few things are going on. First, that WOH G64 turned from a red supergiant into a quieter yellow hypergiant between 2009 and 2016, but probably in about 2014. Such a transformation has previously been hypothesised, but we have never actually seen it happen.
Second, the team thinks that the outer layers of the star have been stripped away. This may have happened, they say, because its stellar wind has ramped up, releasing huge amounts of gas and hinting it might soon explode as a supernova. Alternatively, the stripping may have been caused by interactions with another star, suggesting that WOH G64 is actually part of a binary system. The presence of a second star is strongly hinted at by details in the light signature that can鈥檛 be explained by the characteristics of a yellow hypergiant.
鈥淲e knew something was brewing, and that what WOH G64 had been doing was unsustainable 鈥� it has been losing mass at such a rate,鈥� says at Keele University, UK, whose team revealed an image of the star last year, the first picture of a star in another galaxy. 鈥淏ut we didn鈥檛 know it was going to happen in our lifetime. To have caught that in the act is really great.鈥�
at the University of Minnesota thinks something different has been happening. She suspects that WOH G64 might have been a yellow hypergiant all along that went through a period of intense activity during which it looked like a red supergiant and it has now reverted. Such behaviour has been seen in other stars, but it is unusual, she says.
鈥淲e have to ask ourselves, what exactly is the evolutionary state of this star, is it soon to go supernova?鈥� says Humphreys.
Bonanos says WOH G64 is letting us watch stellar evolution in real time, and her team will be using the VLT telescope to observe it regularly over the next year to see what happens next.
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