February 14, 2020

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They discover what may be happening to Betelgeuse, the star that will explode in the sky

Gonzalo López SánchezGonzalo López Sánchez

Madrid

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Betelgeuse is such a huge and luminous star that, although it is 700 light years away, it is one of the brightest in the sky. Actually, it is very easy to find with the naked eye: it is an orange and thick point that is suspended on the shoulder of the constellation Orion, the hunter who points his bow to the west in the sky. But since October 2019 the brightness of Betelgeuse has been reduced three times. This is a semi-variable star, which means that it changes its size and brightness every so often, but never in the last 150 years has it experienced such a change. Therefore, as its light died out, astronomers have launched themselves to observe it and many have remembered that Betelgeuse is a star that is condemned to explode in the form of a supernova sometime between today and the next 100,000 years. The explosion will be so violent that it will illuminate the nights as much as a full moon.

But what is happening to Betelgeuse now? Does it have to see the drop in brightness with which it will explode? In December and January, the astronomer Miguel Montargès, astronomer of the Observatory of the Catholic University of Leuven (Belgium), made some urgent observations, with the "Very Large Telescope" (VLT), to see the surface of the star. Their results have been published today and show that the surface has experienced a noticeable darkening, which places its brightness at 36% of normal, especially compared to January last year. The astronomer shuffles two possible causes and joins a majority of scientists who do not believe their explosion is imminent.

Betelgeuse area in January 2019 (left) and December (right). You can clearly see its darkening
Betelgeuse area in January 2019 (left) and December (right). You can clearly see its darkening – ESO / Miguel Montargès

“The two scenarios we are working on – Montargès explained in a statement from the European Southern Observatory (ESO) – are a surface cooling because of exceptional star activity, or a dust ejection in our direction».

A swollen star that dies

To understand it, you have to take a look at the star. Our Sun is today a relatively calm and stable sphere, what is known as a yellow dwarf. But Betelgeuse is a star at the end of his life: a red supergiant. It is a swollen object and a thousand times wider than the Sun, with limits much more difficult to see because it is expelling a huge amount of its gas to the surrounding space. In its day it was a blue giant, but after having consumed a good part of its fuel, it has been falling apart for hundreds of thousands of years.

"Red supergiants are known for losing their material," Montargès explained to this newspaper. «That is, by the way, the famous stardust we are made of. Well, a star like Betelgeuse can lose up to 60% of its mass through the stellar wind. Perhaps now, by chance, we are seeing dust ejected in our direction for the first time ».

Betelgeuse's beats

Apart from dust covering its light, another option is that the luminosity and temperature of the star have decreased since October. Why would that happen? Betelgeuse is what is known as a semi-variable star, an object that contracts and expands more or less regularly: specifically, a main period of about 400 days has been observed, and a secondary period of about 2,100.

It is believed that this occurs because of changes in the "speed" of nuclear fusion reactions that occur in its nucleus and as a consequence of the presence of what is known as "convective cells." These cells are regions almost the size of the Sun, which are formed due to changes in density and temperature (as in a boiling cauldron) and that they move, shrink and swell, influencing changes in size and brightness. By the way, the Sun also has its own convective cells, but much smaller.

Crown of flame

The observations made by Miguel Montargès have been made in the visible part of the light, thanks to the SPHERE instrument of the VLT, located in the Cerro Paranal Observatory, in Chile. In addition, these facilities have allowed observing another part of the star. Thanks to the VISIR instrument, astronomers led by Pierre Kervella, a scientist at the Paris Observatory in France, have been able to observe the infrared light emitted by the dust surrounding the star. The image shows a black sphere (of which there is no information with that instrument) that seems to be surrounded by a crown of flames.

Infrared light emitted by dust ejected by the star
Infrared light emitted by dust ejected by the star – ESO / Pierre Kervella

These observations have not been simple. Betelgeuse is so large (it is the second largest in the sky), that the amount of radiation flow with which telescopes work must be reduced. In this case an adaptive optics instrument (SPHERE) has been used that has allowed to obtain a direct image of the star.

So when will it explode?

So when is Betelgeuse going to explode? As he explained to ABC Sylvia Ekström, an astronomer from the University of Geneva (Switzerland) who works on models to explain the life of this type of stars, it is unknown what is happening in the nucleus of Betelgeuse.

"Really, We don't know what stage it is. It is possible that he is now "burning" helium – joining these atoms through nuclear fusion reactions – but it could be burning other things, such as carbon, neon or even oxygen.

The fact that he merges some things or others indicates how much time he has left. Gradually, this star will be merging increasingly heavy atoms but, upon reaching silicon, there will be nothing else to keep it alive. Iron will form in its core and it will explode in a violent supernova, after Betelgeuse collapses on itself. Nobody knows when it will happen, but when we see it, it will have been a long time ago: the light of its explosion will take about 700 years to reach us because of the enormous distance.

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