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

Compared to the lifespan of stars, human life is very short. Stars like Betelgeuse (in Orion) live for millions of years. Others have been around 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 20 or 30 consecutive lifetimes?

Well, it turns out that Betelgeuse has undergone distinct changes in that time period – and that was very evident. These changes are in the historical record. In fact, Betelgeuse has been tracked for thousands of years (As we reported earlier this year). In 1800 BC, an astronomer in China named Sima Qian noticed that Betelgeuse was rich and yellow in color. Nothing beats what it looks like to us these days. It’s more reddish-orange in the night sky.

Familiar constellation Orion. The belt of Orion can be clearly seen, as well as Betelgeuse (red star in the upper left corner). Astronomers of early antiquity saw this star as white-yellow. It has changed since then. Credit: NASA Astronomy Picture of the Day Collection NASA

Qian was not the only skywatcher who recorded 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. However, nearly two millennia later, around AD 2, the astronomer Claudius Ptolemy remarked that it was a “bright reddish star.” That’s a lot of change in a few thousand years. and continued. By the 16th century, astronomer Tycho Brahe noticed that the star was much redder than Aldebaran (in Taurus). Many observers have also compared it to the redness of the heart of the scorpion (another super-giant).

Changing colors indicates internal change

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

Neuhäuser and his colleagues studied historical records of stellar observations for 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 and tight constraint of theoretical single-star evolutionary models (or fusion models). It probably occurs less than a thousand years after the lower part of the red giant branch, before which rapid evolution is expected. Evolutionary trajectories from fog consistent with both color evolution and its location on the CMD indicate a mass of about 14 ms At ~14 Myr. “

Color: Evidence of aging Betelgeuse

So, what is happening to this massive ancient star that causes it to change color so quickly that humans can track its change by eye through historical time? As a star like Betelgeuse ages, its brightness, size and color change. These properties give astronomers clues about the age and mass of stars. Essentially, when the core of Betelgeuse ran out of hydrogen, it evolved from a yellow-white star into a red giant. In astrophysical terms, it crossed the Hertzsprung gap, which means it stopped burning basic hydrogen.

With age, Betelgeuse suffered a loss of mass, and began to cool off. It only took a few thousand years for their color to change. This means that this development was rather rapid. Usually, they evolve from blue and white dwarfs to red giant stars over a period of a few thousand years. Betelgeuse did this in two parts, which indicates its mass, and from this, scientists at Jena can find out its age. Therefore, it now turns out that the color change seen in the centuries between the observations of Sima Qian and Ptolemy (from white to red) is characteristic of this development.

History helps investigate the Hertzsprung hiatus

The idea of ​​using color evolution to explore the Hertzsprung gap (the end of hydrogen burning in Betelgeuse and other similar stars) is a new way to trace its physical evolution. In general, these color changes should be very slow compared to human age. Researchers also have to take into account different color perceptions among observers, and other issues that arise when using the historical record. However, the records on Betelgeuse are clearly a source of attention. The speed of change is evidence of some process within Betelgeuse that helped it “get through the gap” quickly. Does it happen to other stars? Scientists have also studied other stars to see how their colors have changed over time. In particular, they looked at Antares by comparison, which has remained red from antiquity to modern times. It appears to be a star that evolves much slower.

The observable properties (brightness, colour, temperature, chemical composition, etc.) of Betelgeuse and other stars whose colors have been observed throughout history can provide insight into the physics at work during the evolution of these stars. Of course, astronomers will need to carefully calibrate historical observations with current data.

But, this information should help determine stellar masses with higher accuracy. As the authors state in their paper’s conclusions, “This could provide further insight into the physics of stellar interiors and the late evolution of giant giants (and the time remaining until they transform into a supernova). Historical color evolution is a strict new limitation on single-star evolutionary models or models of Merger of Betelgeuse”.

for more information

Red Giant Betelgeuse was yellow about 2000 years ago
Color evolution from Betelgeuse and Antares over two millennia, derived from historical records, as a new constraint on mass and age