Just 2,000 years ago, Betelgeuse was yellow, not red

Compared to the lifespan of stars, human life is quite short. Stars like Betelgeuse (in Orion) live for millions of years. Others have existed for billions of years. We (if we’re lucky) might get 100 years (more or less). So, to us, stars don’t seem to change much over the course of our lives, unless they explode as supernovae. But what about over the course of 20 or 30 consecutive lifetimes?

Well, it turns out that Betelgeuse has seen obvious changes in that span of time – and very visibly. And these changes are contained in the historical record. In fact, Betelgeuse has been haunted for thousands of years (as we reported earlier this year). In the year 1800 BC In about 300 BC, an astronomer in China named Sima Qian noticed that Betelgeuse was a rich, yellowish color. It’s not at all what we look like these days. In our night sky it is more of a reddish orange.

Qian wasn’t the only sky observer to record the color of this star. Based on historical records, a hundred years after Qian, the Roman observer Hyginus described it as yellow-orange, like Saturn. But nearly two thousand years later, around AD 2, astronomer Claudius Ptolemy determined it was a “bright, reddish star.” That’s a lot of change in a few thousand years. And it went on. In the 16th century, the astronomer Tycho Brahe found that the star was even redder than Aldebaran (in Taurus). Many observers also compared it to the redness of Antares (another supergiant).

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Alternating colors indicate internal changes

Rapid color shifting is really about evolution, according to astronomer Ralph Neuhäuser of the University of Jena in Germany. “The mere fact that it changed color from yellow-orange to red in two millennia, together with theoretical calculations, tells us that it has 14 times the mass of our Sun – and mass is the main parameter that drives the evolution.” defined by stars,” he said. “Betelgeuse is now 14 million years old and in its late stages of evolution. In about 1.5 million years it will eventually explode as a supernova.”

Neuhauser and colleagues examined the historical record for star observations of a number of stars. They reported their conclusions in a paper published in MNRAS. For Betelgeuse, they wrote: “The color change of Betelgeuse is a new, severe constraint on single-star theoretical evolution (or merger) models. It is most likely less than a millennium behind the lower end of the red giant branch, before which rapid color development can be expected. Evolutionary traces of MIST, consistent with both its color evolution and position on the CMD, suggest a mass of ~14M_s at ~14 million.”

Color: An indication of the aging of Betelgeuse

So what happens to this massive old star that is changing color so rapidly that humans can visually track its change over historical time? As a star like Betelgeuse ages, it changes in brightness, size, and color. These properties give astronomers clues as to the ages and masses of stars. When Betelgeuse’s core ran out of hydrogen, it essentially evolved from a yellow-white star to a red supergiant. In astrophysical terms, it crossed the Hertzsprung gap, meaning it stopped hydrogen burning in the core.

As Betelgeuse aged it suffered a loss of mass and began to cool. It only took a few thousand years for it to change color. This means that this development was quite fast. They typically evolve from blue-white dwarfs to red supergiants over the course of a few millennia. Betelgeuse made it into two parts, indicating its mass, and from this the scientists in Jena were able to determine its age. So it turns out that the color change observed (from white to red) in the centuries between the observations of Sima Qian and Ptolemy is a feature of this evolution.

The story helps in examining the Hertzsprung gap

This idea of ​​using color evolution to study the Hertzsprung gap (the end of hydrogen burning in Betelgeuse and other similar stars) is a novel way to track their physical evolution. In general, such color changes should be too slow compared to human lifetimes. Researchers must also consider different color perceptions between observers and other issues that arise when using historical records. However, the records of Betelgeuse are clearly drawing attention. The speed of change is indicative of a process within Betelgeuse that has helped him “quickly jump the gap”. Does it happen to other stars? The scientists also studied other stars to see how their colors changed over human time. In particular, they compared Antares, which has remained red from ancient times to modern times. It appears to be a much slower evolving star.

The observable properties (brightness, color, temperature, chemical composition, etc.) of Betelgeuse and other stars whose colors have been observed throughout history could provide further insight into the physics at work in the evolution of these stars. Of course, astronomers must carefully calibrate historical observations with today’s data.

However, this information should help locate stellar masses with even greater precision. As the authors note in the conclusions of their paper: “This could provide further insight into the physics of stellar interiors and the late evolution of supergiants (and the time remaining before they become supernova). The historical color evolution is a new severe constraint for either the single star evolution models or the Betelgeuse fusion models.”

For more informations

Red Giant Betelgeuse was yellow about 2,000 years ago
Color evolution of Betelgeuse and Antares over two millennia, derived from historical record, as new constraint on mass and age