Whenever something happens to Betelgeuse, speculation about its supernova explosion immediately begins. It would certainly be an incredible sight to behold, and we are far enough away that it does not affect us. Now Betelgeuse has increased its brightness again by almost 50% and this has reinstated the rumors of a possible imminent explosion. Betelgeuse will explode as a supernova, everyone agrees on that. But knowing exactly when it will happen is complex because Betelgeuse is a star with highly variable behavior.
Betelgeuse is not just a red supergiant, it is also a pulsating semiregular variable star. This means that there is some periodicity in their brightness changes, although the amplitudes can vary. It has a cycle of about 400 days in which its brightness changes. It also has a shorter 125-day cycle, another 230-day cycle, and a whopping 2,200-day cycle, all of which are determined by pulse, which can make its behavior very difficult to understand. A couple of years ago, Betelgeuse had gone dark and people were wondering what that meant. It had been discovered that the brightness of the star had not actually changed. Instead, the star ejected material from its surface that cooled in a cloud of dust that passed right in front of our vantage point, causing the star to be dipped in light. Now that its brightness is increasing, it has once again attracted the attention of scientists who want to know what evolutionary stage it is in and what all this activity means.
New research has indicated that it could explode as a supernova sooner than expected. As a red supergiant, Betelgeuse has left the main sequence. During its long history of 8 to 8.5 million years, it used large amounts of hydrogen by fusing it into helium and releasing the mass lost from that fusion as energy. That means it’s no longer fusing hydrogen into helium like the Sun does. When stars like Betelgeuse lose mass, their gravity can no longer contain their outer pressure and they expand into a bulkier envelope. Then, despite the loss of mass, they grow in size. After stars like Betelgeuse leave the main sequence and no longer fuse hydrogen with helium in their cores, things change dramatically. During the later phase of helium fusion, carbon accumulates in their nuclei. They then begin a period of burning the central coal which produces other elements. The authors of the new paper say Betelgeuse is in the late stages of that period.
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But how much time is left? There is still no exact answer to this question. Despite, and to some extent because of, the relatively small distance from Earth, it has been difficult to obtain strict constraints on the distance, luminosity, radius, current, and masses of the “zero-age main sequence” ( ZAMS), i.e. the phase the star is in now, and information about the state of internal rotation and associated mixing, and thus about the evolutionary state of Betelgeuse and when it might explode. ZAMS is particularly important for understanding the evolutionary stage of particular stars. It’s key, though not solely responsible, and the studio has some solid possibilities. In fact, the work was a combination of observations and models, each of which fitted the observations in different ways. It’s a tricky business, so those who say it could explode in a few decades are partly speculating. The core period of carbon burning has several phases. The difficulty in determining when Betelgeuse will go supernova stems in part from determining which of these stages it is in. Betelgeuse pulsates, ejects material, spins, and what’s more, it’s a runaway star accelerating through space. Its distance from us is also up for debate, but in the end astronomers can only see the surface and it’s what happens inside the star that holds the answers.
Not everyone agrees that Betelgeuse is even in the middle stage of carbon burning. The authors of a review of Betelgeuse claim that the star is still in the helium phase. Since the burning of the core helium takes much longer than subsequent burn phases, it is very likely that Betelgeuse is burning the core helium. The pulsation period likely constrains the radius, distance, and evolutionary status of helium burning, while acknowledging that there are “arguments to the contrary.” When it finally does explode, and no one disagrees with its eventual explosion as a supernova, it will be a light show visible to all of humanity, and one that will change the Orion constellation forever. Scientists say it will probably leave behind a neutron star, perhaps a pulsar that will be visible for millions of years. The entire event, from start to finish, will be an unprecedented opportunity to study stellar evolution, supernovae and stellar remnants. At that time the scientists will be able to observe thanks to the data obtained from the explosion and reconstruct what happened to compare them with all the investigations and observations carried out to identify where they were right and where they were not. The shock wave from the supernova will arrive in about 100,000 years and will be easily deflected by our Sun’s magnetosphere.
source: ArXiv, Oxford Academy