Discovery suggests red supergiant Betelgeuse was actually yellow 2,000 years ago

Using historical images and descriptions of Betelgeuse over the centuries, scientists have been able to roughly determine when the supergiant star turned red.

The team found that a star located in the constellation of Orion in the Milky Way about 640 light-years from Earth changed color from yellow-orange to red about 2,000 years ago. Betelgeuse is a red giant star, the stage that stellar bodies go through when they finish burning hydrogen in their cores, which causes the core to collapse and blow out the outer layers of the star.

When our Sun passes this stage of its evolution in about 5 billion years, it will swell to a radius around the orbit of Mars and engulf the inner rocky worlds of the solar system, including Earth.

Related: Betelgeuse comes to life after a strange blackout episode

Astronomers have long known that stars change color during their lives as nuclear fusion consumes the hydrogen in their cores. These color changes are accompanied by changes in brightness and size, and can provide important information about a star’s age and mass.

Stars more massive than the Sun, such as Betelgeuse, which is 11 times the mass of our Sun but at least 764 times its size, tend to be blue-white or red. But as they transition from hot, young blue stars to cooler, older red giants, they go through a brief yellow-orange phase.

By studying historical documents, the researchers found that Betelgeuse went through this phase two millennia ago. The findings could help researchers better understand the life cycles of stars.

One of the sources used by the team was the Chinese court astronomer Sima Qian, who wrote about the colors of the stars in 100 BC, commenting: “white is like Sirius, red is like Antares, yellow is like Betelgeuse, blue is like Bellatrix.”

“From these specifications, we can conclude that the color of Betelgeuse at that time was between the blue-white Sirius and Bellatrix and the red Antares,” said Ralf Neuhäuser, an astrophysicist at the University of Jena, who is part of the group behind the discovery. (will open in a new tab).

Fast forward 100 years in history, the Roman scientist Hyginus wrote that Betelgeuse was similar in color to Saturn, suggesting that at that time the stars had a yellow-orange hue.

The 14th-century astronomer Ptolemy compared Betelgeuse to other stars, distinguishing it from bright red stars such as Antares, a red supergiant about 700 times the size of the Sun, whose name itself means “like Mars” or Aldebaran in Greek.

Image of the astronomer Ptolemy, who distinguished Betelgeuse from other red stars. (Image credit: public domain) (will open in a new tab)

“From the statement of the Danish astronomer Tycho Brahe, we can conclude that in the 16th century Betelgeuse was redder than Aldebaran,” Neuhäuser added.

Today, astronomers consider Betelgeuse to be similar in brightness and color to Antares, found in the constellation Taurus and located about 604 light-years from Earth.

The process used by Neuhäuser and his team is described as “terra astronomy”. blending astrophysical research with work being studied by researchers in fields as diverse as languages, history, and natural philosophy.

“Looking back provides strong impulses and important results,” adds Neuhäuser. “There are quite a few astrophysical problems that can hardly be solved without historical observations.”

Color shifts aren’t the only changes Betelgeuse has gone through recently that have caught the attention of astronomers.

In 2019 and 2020, the star underwent what astronomers have called a “great dimming” when its brightness dropped at an unprecedented rate and dropped to about 35 percent of its typical brightness. The dimming began in December 2019 and Betelgeuse regained its brightness over the next few months.

This mysterious dimming of the red star’s brightness has been met with a flurry of explanations, with some astronomers even suggesting that it could be the result of the star collapsing before going supernova.

Scientists have finally been able to unravel the mystery of the Great Blackout using data collected by Japan’s Himawari-8 weather satellite. Infrared and optical observations from the satellite showed that the dimming was caused by a combination of the cooling of the star and the condensation of the dust cloud around it.

With that mystery solved, Betelgeuse is now expected to go supernova again in about 1.5 million years, which Neuhäuser said helped confirm this historical study.

“The very fact that it changed color from yellow-orange to red over the course of two millennia tells us, along with theoretical calculations, that its mass is 14 times the mass of our Sun – and mass is the main parameter that determines the evolution of stars, Neuhäuser concluded. “Betelgeuse is now 14 million years old and in the late stages of evolution.

“In about 1.5 million years, it will finally explode like [a] supernova.”

The team’s research is published in the latest issue of Monthly Notices of the Royal Astronomical Society. (will open in a new tab)

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