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

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

Well, apparently Betelgeuse has undergone obvious changes over that time, and in a very visible way. And those changes are in the historical record. In fact, Betelgeuse has been tracked for thousands of years (as reported earlier this year). In the year 1800 BC, a Chinese astronomer named Sima Qian noticed that Betelgeuse was a rich yellowish color. It is not at all what it appears to us these days. It’s more of an orange-red in our night sky.

Qian was not the only observer of the sky to record the color of this star. Based on historical documents, one hundred years after Qian, the Roman observer Hyginus described it as yellow-orange, like Saturn. Yet, nearly two thousand years later, around 2 AD, the astronomer Claudius Ptolemy noted that it was a “bright and reddish star”. That’s a lot of changes in a couple of thousand years. And, he continued. In the 16th century, astronomer Tycho Brahe noted that the star was even redder than Aldebaran (in Taurus). Many observers have also compared it to the blush of Antares (another supergiant).

Changing colors indicates an internal change

According to astronomer Ralph Neuhäuser of the University of Jena in Germany, the rapid color change is really about evolution. “The very fact that it has changed color in the space of two millennia from yellow-orange to red tells us, together with the theoretical calculations, that it has 14 times the mass of our Sun – and mass is the main parameter that defines the evolution of stars, “He said.” Betelgeuse is now 14 million years old and in its last stages of evolution. In about 1.5 million years, it will finally explode as a supernova. “

Neuhäuser and colleagues studied historical records for stellar observations of a number of stars. They reported their findings in a paper published in MNRAS. For Betelgeuse, they wrote: “Betelgeuse’s color change is a new, tight constraint for single-star theoretical evolutionary models (or fusion models). Most likely it is located less than a millennium from the bottom of the red giant branch. , before which rapid color evolution is expected. Evolutionary traces of MIST consistent with both its color evolution and its position on the CMD suggest a mass of ~ 14 M_S. to ~ 14 million euros.

Color – a hint of Betelgeuse aging

So what is happening to this huge old star that makes it change color so fast that humans can follow its change by eye over historical time? As a star like Betelgeuse ages, its brightness, size and color change. These properties provide astronomers with clues to the age and mass of stars. Essentially, when Betelgeuse’s core ran out of hydrogen, it evolved from a yellow-white star to a red supergiant. In astrophysical terms, it crossed the Hertzsprung gap, which means it stopped burning hydrogen in the core.

With aging, Betelgeuse experienced a loss of mass and began to cool down. It only took a couple of thousand years for it to change color. This means that this evolution has been quite rapid. Usually, they evolve from blue-white dwarfs to red supergiants over the course of a few millennia. Betelgeuse did it in two, which indicates its mass, and from that the scientists of Jena could understand its age. So, now it turns out that the change in color seen over the centuries between Sima Qian’s and Ptolemy’s observations (from white to red) is a feature of that evolution.

The story helps investigate the Hertzsprung gap

This idea of ​​using color evolution to study the Hertzsprung gap (the end of hydrogen combustion at Betelgeuse and other similar stars) is a new way to track their physical evolution. In general, such color changes should be too slow compared to human lives. Researchers must also take into account the different color perceptions among observers and other problems that arise when using historical documentation. However, the records on Betelgeuse clearly draw attention. The speed of change is an indication of some processes within Betelgeuse that have helped it “jump the gap” quickly. Does it happen to other stars? Scientists also studied other stars to see how their colors changed over human time. In particular, they took Antares by comparison, which has remained red from antiquity 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 noted throughout history could provide further information on the physics at work as these stars evolve. Of course, astronomers will have to carefully calibrate historical observations with today’s data.

But this information should help pinpoint stellar masses with even greater accuracy. As the authors state in their paper’s conclusions, “This could provide further information on the physics of stellar interiors and the late evolution of supergiants (and the time left before they become supernovae). The historical evolution of color is a new tight constraint on evolutionary single-star models or Betelgeuse fusion models. “

For more information

The red giant Betelgeuse was yellow about 2,000 years ago
Evolution of the color of Betelgeuse and Antares over two millennia, derived from historical documents, as a new bond of mass and age