The demise of large stars is dramatic: when they run out of nuclear fuel, their core collapses into a neutron star or black hole, while the outer layers are ejected into space with tremendous force. The star lights up brightly as a so-called supernova - it can then shine brighter than all the stars in a galaxy together. While astronomers observe hundreds of supernovae in distant galaxies every year, the last supernova in our Milky Way appeared a long time ago, in 1604.
Sky explorers are eagerly awaiting another event right on our cosmic doorstep to study with modern instruments. And perhaps such a supernova is now imminent: Betelgeuse, the bright left shoulder star of the constellation Orion, is showing strange fluctuations in brightness that indicate the end is near. However, scientists are arguing about how close the spectacular end actually is.
Some even assume that it could take up to 1.5 million years. Others assume that it will be visible in the sky within a few decades. Betelgeuse could therefore have exploded long ago: the star is 550 light-years away from us, so its light needs 550 years to reach Earth. So when we see it flash up in the sky as a supernova, more than half a millennium has passed since the explosion.
760 times larger than the sun
Betelgeuse is a so-called red giant star that contains up to 19 times more mass than our sun and is about 760 times larger than it. The orbits of our solar system's inner planets—Mercury, Venus, Earth, and Mars—would all disappear inside if our sun were swapped for Betelgeuse.
At its end, the star's core will collapse under its own gravity, forming a neutron star, an object only about 20 kilometers across, in which matter is packed as densely as in the nuclei of atoms. A teaspoon of neutron star matter would weigh about a billion tons on Earth. The collapse of the star's interior unleashes an outward shock wave that rips apart the outer portion in a violent explosion.
Betelgeuse's brightness fluctuates erratically and unpredictably, a sign of its advanced age. Nevertheless, it always remained one of the ten brightest stars in the entire sky. But in October 2019, Betelgeuse was unexpectedly fading and fading, causing astronomers to fret: could that be a harbinger of its explosion? The brightness dropped to about 40 percent, lower than ever observed before. Betelgeuse did not even count among the twenty brightest stars.
Shrouded in dust
But eventually the brightness increased again and reached normal values in April 2020. Observations with the "Hubble" space telescope provided an explanation for the spectacular "great eclipse": The star had apparently ejected a huge cloud of material into space, in which a large amount of dust had formed as a result of cooling - and this dust had come from Earth seen, shielded a large part of the starlight.
This gave the all-clear again. An explosion at Betelgeuse doesn't seem imminent after all. According to experts, it could still be 10,000 to 100,000 years before the big event. Perhaps even longer, assumes a team led by Ralph Neuhäuser from the University of Jena. The researchers had studied historical texts in which Betelgeuse is mentioned. A very specific piece of information was sought: Has Betelgeuse always been such a conspicuous red star as it is today?
The research revealed: no. Around the year 100 BC, the Chinese court astronomer Sima Qian describes Betelgeuse as yellow. And the Roman scholar Hyginus wrote a hundred years later that Betelgeuse was yellow-orange like the planet Saturn. "From a statement by the Danish astronomer Tycho Brahe, it can be concluded that Betelgeuse surpassed the star Aldebaran in redness in the 16th century," reports Neuhäuser - today, in turn, Betelgeuse is almost as red as Antares, the brightest star in the constellation Scorpio.
Decades or millions of years?
Accordingly, Betelgeuse has changed its color significantly over the past 2000 years - for the researchers this is a sign that it only developed into a red giant during this period. That would put him much further away from his brilliant end than previously assumed. According to Neuhäuser, it will only explode as a supernova in around 1.5 million years.
However, researchers from Japan and Switzerland recently came to a completely different conclusion. "Betelgeuse is in the final stages of carbon burning," writes Hideyuki Saio's team at Tohoku University in the Monthly Notices of the Royal Astronomical Society. The star is a good candidate for the next supernova in the Milky Way and could light up in just a few decades, the researchers concluded from telescope data and theoretical models of stellar evolution.
Either way, the experts will continue to keep an eye on Betelgeuse. Its explosion as a supernova would be spectacular even for non-experts: the star may then shine as brightly as the full moon and can even be clearly seen in the daytime sky.