The red giant Betelgeuse was yellow only 2,000 years ago

Sep. 5 () –

Ancient observations show that Betelgeuse, the red giant star in the upper left of the Orion constellation, it was orange-yellow only 2,000 years ago.

As nuclear fusion proceeds in the center of a star, also change the brightness, size and color. Astrophysicists can derive important information about a star’s age and mass from such properties. Those stars with much more mass than our sun are blue-white or red; the transition from red to yellow and orange is relatively fast for astronomical time scales.

Astrophysicists at the Friedrich Schiller University of Jena, Germany, together with colleagues from the US and Italy, have successfully detected and dated such a color change in a bright star. Using various historical sources, they discovered that Betelgeuse was yellow-orange in color about 2,000 years ago. They report their results in the current issue of Monthly Notices of the Royal Astronomical Society.

The Chinese court astronomer Sima Qian wrote around 100 BC about the colors of the stars, referring to how white is like Sirius, red like Antares, yellow like Betelgeuse, blue like Bellatrix. “Based on these specifications, it can be concluded that Betelgeuse at that time had a color between the blue-white Sirius and Bellatrix and the red Antares“, says Professor Ralph Neuhäuser of the University of Jena.

Regardless of the above, the Roman scholar Hyginus described some 100 years later that Betelgeuse was colored like the yellow-orange of Saturn; Thus, the previous color of Betelgeuse can be quantified even more precisely.

Additional ancient authors such as Ptolemy provide further indications that Betelgeuse in his day did not belong to the group of bright red stars such as Antares (in the constellation Scorpio) and Aldebaran (in Taurus, the Bull).

The Greek name Antares means “like Mars” in color; in fact, it was reported to be red and compared to Mars for millennia in cultures around the world. “From a statement by the Danish astronomer Tycho Brahe, it can be concluded that, in the 16th century, Betelgeuse was redder than Aldebaran,” says Neuhäuser. Today, Betelgeuse is comparable in brightness and color to Antares.

Jena astronomer Ralph Neuhäuser has included historic celestial observations in his astrophysical research for the last ten years; this field is called “Terra-Astronomy”. He collaborates closely with colleagues in languages, history and natural philosophy, including his wife Dagmar. “Looking back in time provides strong impulses and important results”, adds Neuhäuser. “There are a lot of astrophysical problems that can hardly be solved without historical observations.”

What do those historical transmissions tell us about Betelgeuse? “The very fact that it changed color in two millennia from yellow-orange to red tells us, along with theoretical calculations, which has 14 times the mass of our sun, and mass is the main parameter that defines the evolution of stars“, explains Neuhäuser. “Betelgeuse is now 14 million years old and is in its last evolutionary stages. In about 1.5 million years, it will finally explode as a supernova.”