An interdisciplinary team centered on Jena astrophysicists has used ancient observations to prove that Betelgeuse — the bright red giant star in the upper left corner of the constellation Orion — was orange-yellow about 2,000 years ago.
As nuclear fusion takes place in the center of the star, the brightness, size and color also change. Astrophysicists can obtain important information about the age and mass of stars from these properties. Those stars that are much more massive than our sun are blue-white or red — the transition from red to yellow and orange is relatively rapid on astronomical timescales.
Astrophysicists from Friedrich Schiller University in Jena, Germany, and colleagues from other disciplines in the US and Italy have now successfully detected and dated this color change in a bright star. Using several historical sources, they found that Betelgeuse — the bright red giant star in the upper left corner of the constellation Orion — was orange-yellow about 2,000 years ago.They are in the latest issue Monthly Notices of the Royal Astronomical Society.
Ancient sources from around the world
Chinese court astronomer Sima Qian wrote about the colors of stars around 100 BC: white like Sirius, red like Antares, yellow like Betelgeuse, and blue like Bellatrix. Professor Ralph Neuhäuser from the University of Jena said: “From these specifications, it can be concluded that the colour of Betelgeuse at the time was between the blue-white Sirius and Bellatrix and the red Antares.”
Independent of the above, about 100 years later, the Roman scholar Hyginus described Betelgeuse’s color as similar to the yellow-orange Saturn — so one could quantify Betelgeuse’s previous color more precisely.
Other authors from antiquity, such as Ptolemy, further suggest that Betelgeuse at the time did not belong to bright red star groups like Antares (in the constellation Scorpion) and Aldebaran (in the constellation Taurus, the Bull).
The Greek name Antares means “like Mars” in color; it has indeed been reported as red for thousands of years and has been compared to Mars 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,” Neuhäuser noted. Today, Betelgeuse rivals Antares in brightness and color.
1.5 million years before Betelgeuse explodes into supernova
Jena-based astronomer Ralph Neuhäuser has been incorporating historical astronomical observations into his astrophysics research over the past decade – a field known as “Earth astronomy”. He works closely with colleagues in the fields of language, history and natural philosophy, including his wife Dagmar. “The idea of going back in time brought strong impulses and important results,” Neuhäuser added. “There are quite a few astrophysical questions that are difficult to solve without historical observations.”
What do these historical transmissions tell us about Betelgeuse? “The fact that it turned from yellow-orange to red in two thousand years tells us, together with theoretical calculations, that it 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, in a late stage of evolution. In about 1.5 million years, it will eventually explode as a supernova.”
Classifying stars using ancient observations
R Neuhäuser et al. Color evolution of Betelgeuse and Antares over two thousand years, derived from historical records, as new constraints on mass and age, Monthly Notices of the Royal Astronomical Society (2022). DOI: 10.1093/mnras/stac1969
Courtesy of Friedrich-Schiller-Jena University
Citation: About 2000 years ago (September 5, 2022) red giant Betelgeuse was yellow, September 6, 2022 from https://phys.org/news/2022-09-red-giant-betelgeuse-yellow -years.html retrieved
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