Today, it’s on the brink of death: a giant red ball of flame that dazzles like a bloodshot evil eye before blinking into a tiny pinhole of degenerate matter.
But that wasn’t always the case with the red supergiant Betelgeuse. Once upon a time, this star was a main-sequence monster—a blue-white O-type star, the most massive stellar heavyweight, fusing hydrogen as if it had become obsolete. As it reaches the end of this dying young lifestyle, it takes on a more golden hue. Now astronomers have figured out that this is very recent.
Based on a review of stellar observations dating back to ancient times, it should have been yellow-orange 2,000 years ago. The transition to its present rosy color took place in the blink of an eye, even for short-lived stars like Betelgeuse.
But measuring the duration of this shift isn’t just for idle curiosity. It has allowed an international team of scientists led by astronomer Ralph Neuhauser of Friedrich Schiller University in Jena, Germany to produce a new estimate of the star’s age – which in turn provides us with its inexorable A new timeline for avoided supernovae.
Just about 700 light-years away, Betelgeuse is one of the largest and brightest stars in our sky—a giant star that is nearing the end of its life.
It has nearly exhausted its hydrogen reserves, and fusion in the star’s core powers the star for a lifetime. Instead, it’s now passing through its helium, and its fusion with carbon and oxygen causes the star to expand outward to enormous dimensions.
Once no material has fused, the star will go supernova; its core will likely collapse into a neutron star.
The star has been the subject of tabloid scare in recent years following a series of dimming events that Betelgeuse could explode any day now, and God we’re all accepting it. (Spoiler: it won’t, and neither will we.)
However, while we know Betelgeuse’s demise isn’t imminent — at least on a human timescale — we don’t know exactly when it will. It hasn’t even swelled into a red supergiant in a while. The current best estimate is about 40,000 years ago.
But we don’t have to rely solely on our observations of stars right now. Humans have been documenting the sky for thousands of years…and those ancient texts, Neuhäuser and his colleagues speculate, may contain the answer.
They scoured historical records for references to the star. They found them. Two thousand years ago, ancient astronomers called Betelgeuse yellow.
100 BC, in his vocation, the Han Dynasty court astrologer Sima Qian described the star as yellow. By contrast, Sirius is described as white, Bellatrix as blue, and the red supergiant Antares as…well, red.
If Betelgeuse were the same color, it would certainly not be described as yellow.
“From these specifications,” Neuhäuser said, “it can be concluded that the color of Betelgeuse at the time was between the blue-white Sirius and Bellatrix and the red Antares.”
Then, about 100 years later, came the Roman scholar Hyginius, whose work was titled astronomy.
“Solis stella… corpore est magno, coloure autem igneo, similis eius stellae quae est in humero dextro Orionis;… Hanc stellam nonnulli Saturni esse dixerunt…“Hyginius wrote. “The sun’s star… large in body (i.e. bright) and fiery/burning in color/color; similar to the star on the right shoulder of Orion (i.e. Betelgeuse)…many say this star is Saturn’s (star)…”
Here’s a more reliable example – Betelgeuse’s color is compared to that of the Sun, and Saturn’s color looks more yellow than red. The other stars described by these observers were given accurate hues, the researchers said, paying particular attention to the descriptions of Antares, the red giant Aldebaran, and the red giant Arcturus, all of which are described as red.
Betelgeuse’s color changes can be tracked. By the 16th century, according to the observations of the Danish astronomer Tycho Brahe, Betelgeuse was redder than Aldebaran. Today, it’s redder and closer in color to Antares — a star whose name means “like Mars.”
This transition, and how long it took, gave researchers a parameter to estimate Betelgeuse’s current age and how long it was before it exploded.
“The fact that it turned from yellow-orange to red in two thousand years tells us, together with theoretical calculations, that it has 14 times the mass of our sun — and mass is the main parameter that defines the evolution of stars,” explains Neuhäuser.
“Betelgeuse is now 14 million years old and in a late stage of evolution. In about 1.5 million years, it will eventually explode as a supernova.”
So we’ll be enjoying our charming red friend for some time to come.
The research has been published in Monthly Notices of the Royal Astronomical Society.