When Sima Qian, prefect grand scribe astrologer of China’s early Han dynasty, gazed up at the constellation of Orion a little more than 2,000 years ago, he didn’t 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 the time, Orion’s shoulder was instead marked by a yellow star. If Sima Qian’s observations were correct, they suggest astronomers from antiquity had serendipitously witnessed Betelgeuse during a profound astrophysical transformation—one that has important implications for the star’s evolution and eventual demise.
Sima Qian’s record is one of a series of ancient observations that may pin down the age of Betelgeuse to about 10 million years and its total mass to some 14 times that of our sun. Both mass and color are crucial for determining how a star’s future evolution will unfold. Color changes through the past few millennia suggest that the star has another million years or so before it collapses and then explodes as a supernova, according to a study published in July in the Monthly Notices of the Royal Astronomical Society.
Whatever color Betelgeuse once was, what’s certain now is the star is clearly red. That’s because the nuclear fusion reactions that make Betelgeuse shine have burned through almost all the hydrogen at its core. Betelgeuse now mostly burns more energy-rich helium as fuel, and its outer layers have grown ruddy and puffy from the extra heat. It has become a red giant star so enormous that, were it to replace our sun, its swollen atmosphere would envelop all the planets out to Jupiter.
To deduce the color of Betelgeuse over the ages, the study’s authors turned to records from historians, astronomers and astrologers across the Northern Hemisphere from antiquity up until the development of the telescope. The result is a unique, 2,000-year time line of the star’s evolution that tracks its progression from a yellow hydrogen-burning star somewhat similar to our sun to the red giant we know today.
It’s tricky to catch a star amid this metamorphosis, which, in astronomers’ parlance, is known as “crossing the Hertzsprung gap.” In terms of stellar lifetimes, such episodes are fleeting. A star may shine for tens of billions of years and cross the Hertzsprung gap in a few thousand. That’s practically instantaneous on cosmic scales, but for human beings, it remains an enormous span 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 surpasses the longevity of any single astronomer—and of most societies, for that matter.
“Modern instrumental measurements [have been] taken since about 100 years or so; telescopic observations [have been] since some 400 years,” says lead study author and astrophysicist Ralph Neuhäuser of the Astrophysical Institute and University Observatory at the Friedrich Schiller University of Jena in Germany. “For anything that [lasts] longer or evolves slower, other [kinds] of data—i.e., historical data—are important.”
Tracking down historical observations and correctly interpreting the meanings of words across cultures and centuries are just some of the challenges Neuhäuser and his colleagues faced. They also considered the impact of dust in Earth’s atmosphere and in interstellar space that can change the color of starlight passing through. They even pondered how the anatomical structure of the human eye affects color perception. Still, there’s significant uncertainty left in the analysis, says astronomer Stella Kafka of the American Meteorological Society, who was not affiliated with the study.
“The assumptions they are making have to do with how we define different colors,” Kafka says. In prior eras, observers had no standards for measuring star color like the high-precision calibration techniques currently in use. “[In the new study] they are associating words and descriptions of the colors red, orange and yellow with the distribution of the colors in [current astronomical catalogs].”
For the study, among the most valuable ancient observations were those comparing Betelgeuse to other objects in the night sky that display colors thought to be more constant through time. For instance, around the onset of the first millennium, Gaius Julius Hyginus, librarian for Roman emperor Augustus, noted that Betelgeuse was a yellow color comparable to Saturn.
More circumstantial evidence comes from Betelgeuse’s conspicuous absence from lists of red stars in antiquity. The Greek polymath Ptolemy, for example, noted red stars visible with the naked eye in his second-century treatise on astrology, but Betelgeuse was not included. Considering that it’s one of the brightest and reddest stars in the sky now, the omission suggests that it was not red in Ptolemy’s time. In other writing, he described the star as “hypokirros,” a Greek word that the authors of the new study note has been interpreted by various scholars to mean anything from pale yellow to reddish.
The researchers also dug up ancient records for 235 other stars, including another red giant that looks much like Betelgeuse today—the southern-sky star Antares. Chinese inscriptions dating to 1300 B.C.E. record Antares as being red like Mars—a comparison that turns up time and time again across the Northern Hemisphere and through subsequent ages.
“For Antares, from the fact that it did not change color since the second millennium [B.C.E.], we conclude that it is either on the second or third [Hertzsprung gap] crossing,” Neuhäuser says. If it’s on its second pass, he says, Antares has a mass about 13 times that of our sun. Otherwise, it’s likely 15 or 16 times the mass of the sun. In either case, the historical consistency in descriptions of a red Antares bolsters the case for Betelgeuse looking different—and almost certainly less red—thousands of years ago.
Despite the challenges inherent in gathering and interpreting ancient observations, to astronomers such as Frederick Walter of Stony Brook University, who was not affiliated with the study, historical time lines offer a valuable contribution to modern research and the understanding of stars in transition.
“As astronomers, we observe only a snapshot in time,” Walter says. “The scientific record goes back a few hundred years, and the historical record can be pushed to a few millennia, though with enormous uncertainties. Any time we can glean astronomical information from the historical record, we need to consider it seriously.”
For Neuhäuser and his co-authors, discerning an ancient shift in a star’s color was only the first step. Next, they fed their hard-won historical data into state-of-the-art models simulating the evolution of stars crossing the Hertzsprung gap.
Considering the state of Betelgeuse today, Neuhäuser says, “its historical color change from yellow-orange some two millennia ago to red now would then constrain its mass to about 14 [times the sun’s mass].” There’s no question among astrophysicists that the star will eventually detonate as a supernova. But the estimated stellar mass that has emerged from Neuhäuser's study suggests Betelgeuse is still more than a million years from that explosive end. It’s a projection that Neuhäuser says is a bit longer than the circa 100,000-year supernova deadline other studies have forecast for the star.
For Kafka, besides the difficulty of assigning quantitative values to qualitative descriptions of color, the study’s other great uncertainties 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 variable as well, but we don’t know the cycles as well,” Kafka says. “If Betelgeuse and Antares are in two different stages of their cycles..., then it’s a little bit of a stretch to compare them” to deduce the long-term changes each has experienced.
Kafka also would like to see more precise dates for the ancient records—a task more suited for archeologists and historians than astronomers. But overall, she finds the scope of the study impressive and lauds the insight into Betelgeuse that Neuhäuser and his group have achieved.
“It’s a beautiful paper,” she says. “Identifying Betelgeuse or even Antares as members of this very unique class would be spectacular.”