How are supernovae

"No signs of an impending supernova"

Betelgeuse - a red supergiant in the constellation Orion about 600 light years away from us - has made headlines in recent months. Because since October 2019 the star has been losing its luminosity significantly. This dark phase may herald an explosion of Betelgeuse, was read in some places. But Hans-Thomas Janka from the Max Planck Institute for Astrophysics in Garching does not see any signs of a supernova, as he reports in an interview with Welt der Physik.

Hans-Thomas Janka

World of physics: There is currently a lot of speculation in the media about a possible star explosion by Betelgeuse. What happens in such a supernova?

Hans-Thomas Janka: A supernova occurs when a star with at least nine times the mass of the sun has reached the end of its evolution. If its fuel is no longer sufficient, the core of the star collapses. The star releases a huge amount of energy and shines brightly. Up until a few years ago it was thought that such a process was not in the offing at all. However, new observations showed that stars - which are about to explode - are already glowing in abrupt bursts of light a few days or even months earlier. The brightness of Betelgeuse has also changed in the last few months - however, the red supergiant did not become brighter, but rather lost its luminosity. So this observation is not a sign of an impending supernova.

How can the darkening of Betelgeuse then be explained?

Betelgeuse is a so-called variable star - its luminosity is subject to constant fluctuations. These fluctuations in brightness are not arbitrary, but occur almost regularly in two cycles. While one cycle lasts a little longer than a year, the second cycle is about six years long. At the moment Betelgeuse is in a very dark phase of the cycle. However, such larger fluctuations are also to be expected, albeit very rarely. There were times in the past when Betelgeuse was similarly dark.

How do these fluctuations in brightness arise?

Observations by Betelgeuse - for example through the Hubble space telescope or the ALMA telescope system - show that the star itself is not evenly bright. This indicates violent movements of matter in its shell, presumably associated with loss of mass in strong stellar winds. This blown off matter will then likely collect in a cloud around the star. This expanding and cooling gas cloud shields the star's radiation in such a way that it sometimes appears darker from Earth. As the stellar winds vary over time, the brightness of Betelgeuse we observed also changes. It is still unclear why the gas clouds occur in different cycles. The aim of our research is to better understand the processes behind the instabilities of such variable stars.

Can you explain the sudden media interest in Betelgeuse?

Size comparison between Betelgeuse and our solar system

Betelgeuse has always been a popular celestial object among amateur astronomers due to its short distance to the earth. The fluctuations in brightness that can be observed from Earth also make it interesting. In addition, Betelgeuse as a red supergiant - with a larger extent than the orbit of Jupiter - is definitely a possible candidate for a supernova. However, it cannot be said whether the explosion will happen in a hundred or ten thousand years. It is also possible that the star will not end up as a supernova at all, but rather collapse into a black hole. However, should Betelgeuse actually explode at some point, the supernova would also be a spectacular event for researchers.

To what extent is such a supernova interesting for research?

At the moment we expect that the next supernova in our Milky Way will most likely occur at a distance of about 10,000 to 30,000 light years. Because in the direction of our galactic center, many stars come into question as candidates for a supernova. At 600 light years, the distance to Betelgeuse is significantly shorter. An explosion of Betelgeuse would release large amounts of high-energy particles - such as neutrinos - that could be detected on Earth. In addition, a supernova would also generate gravitational waves that we could detect with our detectors. The possibility of investigating such signals would mean winning the lottery for physicists.

Would the explosion also have a direct impact on us on earth?

No, Betelgeuse is too far away from us for that. The critical distance of a supernova that could have a very negative impact on our life on earth is around 200 light years. The Betelgeuse explosion would expose the earth to strong radioactive radiation, but the atmosphere and the ozone layer would remain largely undamaged. We therefore do not have to worry about the impact of a possible explosion on our biosphere. In addition, there are more threatening and avertable dangers to humanity and earthly life at the moment, which should concern us more than a Betelgeuse supernova explosion.