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Ancient astronomers saw Betelgeuse shining in a different color

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When Sima Tian, ​​the great scribe-astrologer prefect of China’s early Han dynasty, gazed at the constellation Orion just over 2,000 years ago, he failed to see the brilliant crimson star on the hunter’s right shoulder that we know today as Betelgeuse. According to an astronomical treatise he compiled, at that time Orion’s shoulder was marked with a yellow star. If Sima Tian’s observations were correct, they suggest that ancient astronomers happened to observe Betelgeuse during a profound astrophysical transformation that has important implications for the star’s evolution and eventual demise.

Sima Tian’s record is one of a series of ancient observations that could place Betelgeuse at about 10 million years old and a total mass of about 14 times that of our sun. Both mass and color are crucial in determining how a star’s future evolution will unfold. Color changes over the past few millennia indicate that the star had another million years or so before it collapses and then explodes as a supernova, according to a study published in July at Monthly Notices of the Royal Astronomical Society.

Whatever color Betelgeuse once was, it is now known for certain that the star is distinctly red. That’s because the nuclear fusion reactions that make Betelgeuse glow have burned up almost all of the hydrogen in its core. Betelgeuse is now mostly burning more energetic helium for fuel, and its outer layers have become red and puffy from the extra heat. It became a red giant star, so huge that if it replaced our sun, its swollen atmosphere would envelop all the planets up to Jupiter.

To determine the color of Betelgeuse over the centuries, the study authors consulted the records of Northern Hemisphere historians, astronomers and astrologers from antiquity to the development of the telescope. The result is a unique 2,000-year evolutionary line of the star that traces its development from a yellow hydrogen-burning star somewhat similar to our sun to the red giant we know today.

It is difficult to catch a star in the midst of this metamorphosis, which, in the language of astronomers, is known as the “crossing of the Hertzsprung fissure.” From the point of view of star life, such episodes are fleeting. A star can shine for tens of billions of years and cross the Hertzsprung gap in a few thousand. This is practically instantaneous on a cosmic scale, but for a human it remains an enormous amount of time. Stars can oscillate back and forth across the gap on millennial timescales, offering more chances for observers to witness the transition. But each crossing still far exceeds the lifespan of any individual astronomer—and most societies, for that matter.

“Modern Instrumental Measurements [have been] taken from about 100 years ago or so; telescopic observations [have been] about 400 years,” says the study’s lead author and astrophysicist Ralf Neuhauser of the Institute for Astrophysics and University Observatory of the Friedrich Schiller University of Jena in Germany. “For anything [lasts] longer or develops more slowly, etc [kinds] data, i.e. historical data, important.”

Tracking historical observations and correctly interpreting the meanings of words across cultures and centuries are just some of the challenges Neuhauser and his colleagues faced. They also looked at the effects of dust in Earth’s atmosphere and in interstellar space, which can change the color of the starlight that passes through. They even thought about how the anatomical structure of the human eye affects the perception of color. Still, there remains a significant amount of uncertainty in the analysis, says astronomer Stella Kafka of the American Meteorological Society, who was not involved in the study.

“The assumptions they make have to do with how we define different colors,” says Kafka. In previous eras, observers did not have standards for measuring the color of stars, such as the highly accurate calibration methods used today. “[In the new study] they associate the words and descriptions of the colors red, orange and yellow with the distribution of colors within them [current astronomical catalogs].”

For the study, some of the most valuable ancient observations were those that compared Betelgeuse to other objects in the night sky that display colors believed to be more stable over time. For example, around the beginning of the first millennium, Gaius Julius Hyginus, the librarian of the Roman emperor Augustus, noted that Betelgeuse had a yellow color comparable to that of Saturn.

More circumstantial evidence comes from Betelgeuse’s conspicuous absence from ancient red star lists. The Greek scientist Ptolemy, for example, noted red stars visible to the naked eye in his second-century treatise on astrology, but Betelgeuse was not included. Given that it is one of the brightest and reddest stars in the sky now, the omission suggests that it was not red in Ptolemy’s time. In another writing, he described the star as “hypokirosis,” a Greek word that the authors of the new research note have interpreted by various scholars to mean pale yellow to reddish.

The researchers also found ancient records for 235 other stars, including another red giant that looks a lot like Betelgeuse today – the southern sky star Antares. Chinese inscriptions dating back to 1300 BC suggest that Antares was red like Mars – a comparison that is repeated again and again in the Northern Hemisphere and throughout the following centuries.

“For Antares from not changing color since the second millennium [B.C.E.]we conclude that it is either on the second or the third [Hertzsprung gap] intersection,” Neuhauser says. When it’s on its second pass, he says, Antares has about 13 times the mass of our sun. Otherwise, it is likely 15-16 times the mass of the Sun. In any case, the historical consistency in the descriptions of red Antares supports the argument that Betelgeuse looked different – and almost certainly less red – thousands of years ago.

Despite the complexities of collecting and interpreting ancient observations, for astronomers such as Frederick Walter of Stony Brook University, who was not involved in the study, the historical timelines offer a valuable contribution to modern research and understanding of stars in transition.

“As astronomers, we’re only observing a snapshot in time,” Walter says. “Scientific data is several hundreds of years old, and historical data can be carried back several millennia, albeit with enormous uncertainty. Whenever we can get astronomical information from the historical record, we should take it seriously.”

For Neuheuser and his co-authors, recognizing the star’s ancient color change was just the first step. They then fed their hard-earned historical data into state-of-the-art models that simulate the evolution of stars crossing the Hertzsprung Gap.

Looking at Betelgeuse’s state today, Neuhauser says, “Its historical color change from yellow-orange around two millennia ago to red now would limit its mass to about 14 [times the sun’s mass].” There is no doubt among astrophysicists that the star will eventually explode as a supernova. But the estimated stellar mass derived from Neuhauser’s study suggests that Betelgeuse is still more than a million years away from this explosive end. That’s a prediction that Neuheuser said is slightly longer than the roughly 100,000-year supernova lifetime predicted by other studies for the star.

For Kafka, in addition to the difficulty of assigning quantitative values ​​to qualitative descriptions of color, other major uncertainties in the study arise from the mercurial nature of Betelgeuse, Antares, and other aged massive stars.

“Betelgeuse is a variable star. It changes with time…, and Antares is also changeable, but we also do not know the cycles,” says Kafka. “If Betelgeuse and Antares are in two different stages of their cycles… then it’s a bit difficult to compare them” to infer the long-term changes that have occurred in each.

Kafka would also like to see more precise dates for ancient records – a task more suited to archaeologists and historians than astronomers. But overall, she finds the scope of the study impressive and praises the understanding of Betelgeuse that Neuhauser and his group have achieved.

“It’s a beautiful paper,” she says. “The identification of Betelgeuse or even Antares as members of this very unique class would be exciting.”

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