A team of astronomers from the University of Jena and elsewhere has analyzed pre-telescopic records on star color from Europe, the Mediterranean, West Asia (Near East), and East Asia, as well as First Nations around the world, to find stars that have evolved noticeably in color within the last millennia. Their main result is that Betelgeuse — a red supergiant located approximately 650 light-years away from Earth — was recorded with a color significantly different than today.
“Historical observations provide valuable input for many fields of astrophysics,” said University of Jena’s Professor Ralph Neuhäuser and his colleagues.
“Examples include the reconstruction of past solar activity with sunspots and aurorae, the determination of cometary orbits, or the study of supernovae in our Milky Way Galaxy.”
“We consider pre-telescopic color records of stars with historical-critical methods as a new test of theoretical evolutionary models, and to better constrain masses, ages, and the evolutionary state of supergiants,” they explained.
“We aimed to show that, two millennia ago, Betelgeuse was reported with a significant different (non-red) color than Antares, which was always given as red, while today, both have almost the same red color.”
According to the team, the Chinese court astronomer Sima Qian wrote around 100 BCE about star colors: ‘white is like Sirius, red like Antares, yellow like Betelgeuse, blue like Bellatrix.’
“From these specifications, one can conclude that Betelgeuse at that time was in color between the blue-white Sirius and Bellatrix and the red Antares,” Professor Neuhäuser said.
“Independently, the Roman scholar Hyginus described some 100 years later that Betelgeuse was in color like the yellow-orange Saturn — thus, one can quantify the former color of Betelgeuse with even more precision.”
“Authors from the Antiquity like Ptolemy bring further indications that Betelgeuse at their time did not belong to the group of bright red stars like Antares in the constellation Scorpion and Aldebaran in Taurus,” he added.
“The Greek name Antares means ‘like Mars’ in color, it was indeed reported as red and compared to Mars since millennia from cultures around the world.”
“From a statement by the Danish astronomer Tycho Brahe, one can conclude that, in the 16th century, Betelgeuse was more red than Aldebaran. Today, Betelgeuse is comparable in brightness and color to Antares.”
“The view back in time delivers strong impulses and important results,” Professor Neuhäuser said.
“There are quite a number of astrophysical problems which can hardly be solved without historical observations.”
The very fact that Betelgeuse changed its color within two millennia from yellow-orange to red tells the scientists, together with theoretical calculations, that the star has a mass of around 14 solar masses.
“Betelgeuse is now 14 million years old and in its late evolutionary phases,” Professor Neuhäuser said.
“In about 1.5 million years, it will finally explode as supernova.”
The team’s paper appears in the Monthly Notices of the Royal Astronomical Society.
R. Neuhäuser et al. 2022. Colour evolution of Betelgeuse and Antares over two millennia, derived from historical records, as a new constraint on mass and age. MNRAS 516 (1): 693-719; doi: 10.1093/mnras/stac1969
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