Betelgeuse: Finally, the giant supernova's true color revealed after 2,000 years

Betelgeuse is one of the brightest stars observable in the night sky. Still, its color was not always the same. An interdisciplinary team led by a Jena astrophysicist discovered that Betelgeuse appeared orange-yellow to observers on Earth nearly 2,000 years ago.

A star changes its brightness, size, and color as the nuclear fusion inside its core progresses.

Measuring the duration of this color transition allows researchers to determine the age and mass of a star, which, in turn, gives information about its inevitable supernova.

Why does Betelgeuse appear red now?

Betelgeuse, 642.5 light years away from Earth, is the giant red star found in the upper left of the Orion constellation. Researchers used various historical sources to determine that Betelgeuse appeared as a bright yellow star come 2000 years ago.

The Chinese court astronomer Sima Qian wrote around 100 BC about star colors: white is like Sirius, red is like Antares, yellow is like Betelgeuse, and blue is like Bellatrix.

Some 100 years later, the Roman scholar Hyginus described that the star in the right shoulder of Orion has a color similar to Saturn — yellow.

Further evidence from ancient authors like Ptolemy suggests that Betelgeuse didn't fit into the group of bright red stars like Antares and Aldebaran.

"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," says lead author Prof. Ralph Neuhäuser from the University of Jena.

The color of a star is a sure sign of its evolutionary stage. When the luminaries burn hydrogen thermonuclear fuel in their cores, they inflate and emit gases into space. This expansion leads to a decrease in their surface temperature, causing a transition from yellow-orange to red. This color change happened over thousands of years, which is relatively short according to astronomical standards.

Supernova in 1.5 million years

Betelgeuse is far away, yet it's one of the brightest stars in Earth's sky. That's because it's 14 times bigger and about 100,000 times brighter than our sun.

"The very fact that it changed in color within two millennia from yellow-orange to red tells us, together with theoretical calculations, that it has 14 times the mass of our sun," according to professor Ralph Neuhäuser.

Such brilliance comes at a price. Betelgeuse's enormous energy requires that its fuel be expended quickly, which shortens its life.

The relatively recent color change indicates that Betelgeuse almost burnt through its hydrogen reserves, and it's now working its way through its helium. Once the star runs out of fuel, it will collapse under its own weight and rebound in a spectacular supernova explosion.

The end is foreseeable, but not for another million years or so. "It is 14 million years old and is in the final phase of its development. In about 1.5 million years, it will finally explode as a supernova," explains Ralph Neuhäuser.