Soon a supernova to see live? “I think we’ll see some”

The most massive stars go supernova at the end of their “life”, but this event has never been observed live. However, some warning signs can be seen and should be monitored to finally see such an event soon.

In 1604, the sky shone strangely, and astronomers of the time observed, amazed, a star exploding live. Several testimonies, notably signed by Johannes Kepler, describe this phenomenon, which remains visible for a whole year and which many centuries later will be called “supernova”.

Since then, humanity has not witnessed a similar phenomenon, although our knowledge of these events is much greater. We now know that it is an explosion of a star, a star much more massive than our Sun, which suddenly gets rid of all its matter. The remnants of this huge explosion remain visible to the naked eye for months, even in the case of very distant events.

Such a spectacle would be incredible, and there are permanents throughout the universe. But only a handful of supernovae occur every century in a galaxy like ours, no more than three according to the most optimistic studies. All the ones we can observe take place in other galaxies, and each time they are detected after the explosion, which lasts only a second. Fortunately, it may be possible to predict these events in order to arrive on time for the session. This is confirmed by a new study published in Monthly Notices of the Royal Astronomical Society13 October 2022.

Remnants of a massive star that exploded in a supernova. // Source: NASA and Hubble Heritage Team (STScI/AURA) (cropped image)

First we looked at red supergiantstells Numerama one of the authors, Bertrand Plez, CNRS researcher at the University of Montpellier. These are the stars that later go supernova and experience strong winds that cause them to lose mass rapidly. »

For the Betelgeuse star, it didn’t fit

These winds were already known, they are phenomena that eject matter from the star. A dust that is then present after a supernova. It was therefore more or less accepted that the star, by a still ill-defined mechanism, lost large amounts of material before exploding.

But how long can this giant flossing last? Some theories speak of decades, even a century. The interesting thing is that this ejected material is very easily visible on the brightest red giants, for example Betelgeuse. And when the star experienced sharp dips in brightness in 2019 and 2020, many observers expected it to go supernova. Hopes were high among the scientific community and the general public (as well as the editors at Numerama). But nothing happened, and Betelgeuse ended up regaining its usual brightness.

In the case of Betelgeuse, it did not quite fit, the decrease in brightness should have been much strongerspecifies Bertrand Plez. But we have designed a model that specifies the behavior of stars before they explode. »

This model, which takes into account these violent winds and the decrease in brightness, allows to identify two possibilities to explain the mechanism before the supernova.

  • In the first case, the superwinds eject matter for several decades. This dust surrounds the star and covers it more and more until it gradually becomes invisible to our eyes. Then the supernova occurs.
  • In the second case, a large puff of dust obscures the star in less than a year. A large cloud representing 5 to 10% of the star’s total mass suddenly appears and the instability causes the supernova.

Intact stars just before they explode

Both models seemed valid, but the authors went further by digging into the records of telescopes that have observed supernovae in other galaxies. Each time the phenomenon had not been seen live, but observers had encountered already formed supernovae. ” What is interestingsays Bertrand Plez, it’s that in the images taken by the same telescopes less than a year before the supernova, the stars were quite ordinary! In other words, nothing had begun to hide them. A proof that of the two models described above, only the breath of the fabric can be retained.

This finding has two implications. First, Betelgeuse’s behavior could have been better predicted. The amount of matter that darkened it was too small and did not stick together enough to suggest a supernova. It was indeed a loss of material, but much more modest and localized. Hope could have been set aside much sooner!

And most importantly, this model can help better predict future supernovae in our galaxy. Instead of wondering if a decrease in brightness must be seen for a century to hope to see anything, we now know that if the star suddenly dims, the explosion will happen within a year. ” I think we will see some soonassures Bertrand Plez. We now have telescopes that look at entire skies and even stars in other galaxies, increasing the chances of encountering such a phenomenon. »

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