Today, he stands on the brink of death: a huge ball of red fire, dazzling like an evil bloodshot eye before transforming into a tiny pinprick of degenerated matter.
But the red supergiant Betelgeuse was not always like this. Once upon a time, the star was a main-sequence monster – a blue-white O-type star, the most massive stellar weight class, fusing hydrogen as if it were going out of fashion. As he reached the end of this fast and youthful lifestyle, he would have taken on a more golden hue. And now astronomers have realized how recent it was.
According to a report of observations of the star dating from Antiquity, it would have been yellow-orange in color around 2,000 years ago. The transition to its current reddish hue happened in the cosmic blink of an eye, even for a star as short-lived as Betelgeuse.
But measuring the duration of this transition is not just to satisfy idle curiosity. This allowed an international team of scientists led by astronomer Ralph Neuhäuser of the Friedrich Schiller University of Jena in Germany to make a new estimate of the star’s age – which, in turn, gave us a new timeline for its inevitable supernova.
Betelgeuse, only about 700 light-years away, is one of the largest and brightest stars in our sky – a giant star nearing the end of its life.
It is almost burned thanks to its reserves of hydrogen, the fusion of which in the core of a star feeds this star throughout its life. Instead, it pushes its way through its helium, the fusion of which into carbon and oxygen caused the star to expand outward to gargantuan size.
Once there is no more material to fuse, the star will become a supernova; its core will probably collapse into a neutron star.
And in recent years, after a series of dimming events, the star has been the subject of tabloid alarmism, stating that Betelgeuse could explode any day, and by God, we’re all going with it. (Spoiler: It won’t, and we won’t.)
However, while we know Betelgeuse’s demise isn’t imminent – at least on a human scale – we don’t know exactly when it will. Or even how long ago it swelled into a red supergiant. The current best estimate is around 40,000 years old.
But we don’t have to rely solely on our observations of the star now. Humans have been recording the skies for millennia…and these ancient texts, Neuhäuser and his colleagues speculated, might hold the answer.
They scoured historical records for references to the star. And they found them. Two millennia ago, ancient astronomers called Betelgeuse yellow.
In 100 BCE, in its Treaty of celestial offices, the Han dynasty court astrologer Sima Qian described the star as having a yellow hue. Sirius, on the other hand, has been described as white, Bellatrix as blue, and the red supergiant star Antares has been described as…well, red.
If Betelgeuse were the same color, surely it would not be described as yellow.
“From these specifications,” says Neuhäuser, “one can conclude that Betelgeuse at that time was colored between Sirius and Bellatrix blue-white and Antares red.”
Then, about 100 years later, the Roman scholar Hyginius, author of a work entitled astronomy.
“The star of the sun…has a large body, and a fiery color, like that star which is on the right shoulder of Orion;…Some have said that this star was Saturn…“Hyginius wrote. “The star of the Sun… the body is large (i.e. brilliant), and of fiery/burning color/coloration; similar to that star which is in the right shoulder of Orion (i.e. Betelgeuse)… Many have said that this star is (the star) of Saturn…”
This is an even stronger example – the color of Betelgeuse compared to the color of the Sun, and Saturn, which appears more tawny than red. Other stars described by these observers are given accurate hues, the researchers say, paying particular attention to descriptions of Antares, the red giant Aldebaran and the red giant Arcturus, all described as red.
The changing color of Betelgeuse can be traced. In the 16th century, according to the observations of the Danish astronomer Tycho Brahe, Betelgeuse was redder than Aldebaran. Today it is even redder, closer to Antares, a star whose name means “like Mars”.
This transition, and the time it took, gave the researchers a parameter to estimate Betelgeuse’s current age and how long it will have before it becomes kaboom.
“The very fact that it changed color over two millennia from yellow-orange to red tells us, with theoretical calculations, that it has 14 times the mass of our sun – and mass is the main parameter defining the evolution of stars,” explains Neuhäuser.
“Betelgeuse is now 14 million years old and in its final stages of evolution. In about 1.5 million years it will finally explode as a supernova.”
We will therefore enjoy our fascinating red friend for a long time to come.
The research was published in the Royal Astronomical Society Monthly Notices.
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