Betelgeuse: Now A Giant Red Star, It Was Once Yellow, Study Finds

The new study reports that Betelgeuse, the bright red giant star in the upper left of the constellation Orion, was yellow-orange about 2,000 years ago. 

The brightness, size, and colour of a star can change with progressing nuclear fusion in the centre of the celestial body. These properties unveil important information about the age and mass of a star. 

Stars which are significantly more massive than the Sun are blue-white or red. In astronomical time scales, the transition of a star from yellow to orange, and from orange to red is considered to be a rapid phenomenon. 

A team of astrophysicists have successfully detected and dated the colour change in a bright star, called Betelgeuse. The study describing their findings was recently published in the Monthly Notices of the Royal Astronomical Society.

The new study reports that Betelgeuse, the bright red giant star in the upper left of the constellation Orion, was yellow-orange about 2,000 years ago. 

Colour Evolution Of Stars

Massive stars evolve from blue-white dwarfs to red supergiants, after core hydrogen burning. These stars evolve by expanding, brightening, and cooling within a few millennia. 

In the new paper, the researchers discuss a previously neglected constraint on mass, age, and evolutionary state of Betelgeuse and Antares. This constraint is the observed colour evolution of the stars over historical times. 

The researchers studied pre-telescopic records on star colour with historically critical methods to find stars that have evolved noticeably in colour within the last millennia. 

How Were Star Colours Described In History?

Around 100 BC, the Chinese court astronomer Sima Qian wrote about star colours. He used the colours red, yellow, white, and blue to describe Antares, Betelgeuse, Sirius, and Bellatrix, respectively. 

In a statement released by University of Jena, Professor Ralph Neuhäuser, the lead author of the study, said from these specifications, one can conclude that Betelgeuse at that time was in colour between the blue-white Sirius and Bellatrix and the red Antares. 

Some 100 years after Sima Qian, Roman scholar Hyginus described that Betelgeuse was in colour like the yellow-orange Saturn. Therefore, one can quantify the former colour of Betelgeuse with even more precision. 

Hygnius and Sima Qian independently reported Betelgeuse as appearing like Saturn in colour, and 'yellow', respectively, two millennia ago. 

According to the University of Jena, the writings of Ancient astronomer Claudius Ptolemy indicate that Betelgeuse, at his time, did not belong to the group of bright red stars like Antares, located in the constellation Scorpion, and Aldebaran, located in the constellation Taurus. Ptolemy lived from 100 AD to 170 AD. 

Antares is a Greek name which means "like Mars". For centuries, Antares has been compared to Mars, and reported as red in colour. 

One can conclude from a statement by the Danish astronomer Tycho Brahe that, in the 16th century, Betelgeuse was more red than Aldebaran, Neuhäuser noted. Today, the brightness and colour of Betelgeuse are comparable to that of Antares.

What Does The Colour Evolution Of Betelgeuse Tell Us About its Mass?

The main result of the study is that Betelgeuse was recorded with a colour significantly different from today. 

The authors note in the study that the colour change of Betelgeuse is a new, tight constraint for single-star theoretical evolutionary models, or merger models. They also state that the roughly constant colour of Antares for the last three millennia also constrains its mass and age. 

Delta Canis Majoris, officially named Wezen, is a star that was reported white historically, but is now yellow. Wezen is located in the constellation Canis Major.

Neuhäuser has been including historical celestial observations in his astrophysical research for about 10 years, according to the University of Jena. The field, in which historical celestial observations are included in astrophysical research, is known as "Terra-Astronomy". 

Neuhäuser closely collaborates with colleagues from languages, history, and natural philosophy. 

Neuhäuser said in the statement that the view back in time delivers strong impulses and important results. He added that there are quite a number of astrophysical problems which can hardly be solved without historical observations. 

These historical transmissions tell us a lot about Betelgeuse. Neuhäuser explained that the very fact that Betelgeuse changed colour within two millennia from yellow-orange to red tells us, together with theoretical calculations, that the star has 14 times the mass of our Sun. The mass is the main parameter defining the evolution of stars. 

When Will Betelgeuse Explode As A Supernova?

Neuhäuser further said that Betelgeuse is now 14 million years old and in its late evolutionary phases. He said that in about 1.5 million years, Betelgeuse will finally explode as a supernova.