Throughout the year we can enjoy a multitude of shows in our night sky. From the moon in its different phases, to the occasional meteor shower, to the rare comet that comes close enough to our planet to be visible for a few days or weeks. Also, if we get far enough away from the big cities, we can enjoy the sky as it is, with its thousands of stars, the wandering planets and even the silhouette of the Milky Way. However, few phenomena can be as rare and spectacular as a supernova explosion. These explosions, if they occur close enough, can rival the full moon in brightness and even be visible during the day.
Before we go any further, let’s refresh what exactly a supernova is. The life of a star comes to an end when the fuel that made it shine is exhausted. This will take more or less time depending on the total mass of the star, the life of more massive stars being shorter, as they spend their fuel at a much higher rate. In truly massive stars, at least 8 to 10 times the mass of the Sun, this process begins when nuclear fusion of the hydrogen inside them stops . This moment will mark the beginning of the end for the star.
As the hydrogen in the core is depleted, the core will contract under its own weight, thereby increasing the temperature until more hydrogen can once again be fused. This increase in temperature will also cause the outer layers of the star to expand. After several repetitions of this process, in which heavier elements are fused and the envelope of these stars expands further, we will reach the point where no more atomic nuclei can fuse. This will happen specifically when we get to iron. Until then, the fusion of different nuclei to form others lighter than iron released energy, keeping the star hot and working. However, when trying to fuse iron nuclei to form heavier ones, energy will be absorbed . When this happens, there will be a sudden cooling of the core of the star that will cause the contraction of the inner layers. The much lighter outer layers will fall on top of them, bouncing catastrophically, giving rise to what we know as a supernova explosion . During the explosion, so much energy will be released that elements much heavier than iron, such as gold or uranium, can be synthesized. In fact, the origin of the heaviest elements in the periodic table is mainly due to these explosions.
Actually this process that I have just described is responsible for a type of supernovae, but the end result is the same, regardless of its origin: an explosion so intense that it rivals in luminosity the galaxy of which it is a part .
In recent decades, thousands of supernovae in neighboring and different galaxies have been detected thanks to the latest generation telescopes. However, before that several of these explosions were already observed with the naked eye. We have plenty of records of astronomers mentioning the appearance of new stars in the night sky centuries ago. In the year 185 AD , precisely this was observed, the appearance of a bright point, which twinkled like a star and did not move from night to night. A similar event was observed in the year 1006 AD . This was collected by astronomers from China, Egypt, Italy, Iraq, Japan and Switzerland. Shortly thereafter, in AD 1054 , another supernova was observed, which we now believe to be the one that created the Crab Nebula. This supernova became 4 times brighter than Venus (the brightest object in the sky after the Sun and the Moon) and was visible during the day for more than 3 weeks and at night for almost 2 years.
The last of these explosions observed with the naked eye took place in the year 1604. This supernova was studied by Johannes Kepler, thanks to whom we use the term “nova” to refer to this type of event. Since then no supernovae have been observed directly and all of them have required a telescope. The question therefore would be , could any of these observations from centuries ago be repeated? And the answer would be yes, without a doubt. It is estimated that in our galaxy about 2 supernovae explode per century. Bearing in mind that we haven’t seen one for more than 4 centuries, it seems that it is already touching.
One possible candidate, which is constantly being watched, is the star Betelgeuse, in the constellation Orion. This star is a red supergiant, which means that it is in the final stage of expansion and heating of the core. Since it is about 18 times the mass of the Sun and 1,000 times larger in diameter, we would expect it to explode relatively soon. However, “soon” does not mean the same on a human scale as it does on an astronomical scale . The most optimistic predictions predict that it will explode within about 100,000 years, although it could last several million years without exploding. When it does, whether or not we are alive to see it, it will shine almost as bright as a full moon and will be visible during the day. Over the months it will fade, until the Orion warrior’s shoulder disappears completely, becoming a white dwarf or perhaps a neutron star, too faint to be seen with the naked eye.
