Astronomy: Red giant star Betelgeuse used to be yellow 2000 years ago

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The bright red giant star Betelgeuse in the constellation Orion still shone yellow-orange just 2000 years ago. This is what a team of astrophysicists from Friedrich Schiller University Jena, with the participation of colleagues from the USA, has now determined using historical sources. It is the first time that such a colour change of a star could be classified so precisely in time. Ancient observations from ancient China, Rome and Greece, among others, were evaluated for this purpose. With the help of astrophysics, important information about the age and mass, for example, could be read from the chronological sequence of such changes, the group writes - and also about further development.

Colour indications independent of each other

As Ralph Neuhäuser's team from the University of Jena explains, the Chinese court astronomer Sima Qian wrote in 100 BC, "White is like Sirius, red like Antares, yellow like Betelgeuse, blue like Bellatrix". In terms of colour, Betelgeuse lay between the red Antares and the blue-white Bellatrix - the other shoulder star of the constellation Orion. 100 years later, the Roman scholar Hyginus stated that Betelgeuse resembled the yellow-orange Saturn in colour. Other ancient descriptions also suggest that Betelgeuse was not one of the brightest red stars like Antares and Aldebaran. Then, in the 16th century, the Danish astronomer Tycho Brahe noted that Betelgeuse was redder than Aldebaran. In the meantime, it almost corresponds to Antares, writes Neuhäuser.

The fact that its colour has changed from yellow-orange to red in the past 2000 years suggests that it has about 14 times more mass than the sun, explains Neuhäuser. It is therefore 14 million years old and at the end of its development, in 1.5 million years, it will end in a supernova. That is much later than many other current estimates. Betelgeuse, which is actually one of the brightest stars in the night sky, caused a stir at the end of 2019 when it became noticeably dimmer. As a result, there had been speculation that its end in such a supernova was already imminent. It is now clear that a gigantic explosion occurred on the surface of the star. This created a cooler spot that made the star darker. The red giant star has still not fully recovered from the powerful explosion.

With his research work, Neuhäuser is now contributing further to knowledge about the prominent star. The astrophysicist has been incorporating historical celestial observations into his work for about ten years and cooperates closely with researchers from the humanities: "There is a whole series of astrophysical questions that can be solved better or at all with the help of historical observations. Neuhäuser calls this "terra-astronomy". He and his team are now presenting their work in the Monthly Notices of the Royal Astronomical Society.

(mho)