What are supernovae, stellar explosions

The increasingly advanced exploration of space has allowed us to observe a good amount of phenomena that, here on Earth, would be impossible to replicate. Phenomena now spectacular now destructive, among which we can include supernovae. The word supernova was first used by Walter Baade and Fritz Zwicky in 1931, and, in fact, is the largest explosion that man has ever seen.

One of the most catastrophic events in the universe, it involves a very massive star in its last moments of life, when it explodes, destroying itself and releasing enormous energy. At that moment, the celestial body becomes so bright that it shines even more than an entire galaxy. The light that is emitted by the star as a result of the explosion lasts a few months and is comparable to what our Sun is able to emit in a billion years.

Not only that, this somehow generates objects among the most exotic, even accompanying neutron stars, pulsars and black holes. All of course at very high temperatures, which can reach the impressive number of one hundred billion Kelvin.

What a supernova is and how it is created

Some would say that a supernova is the last hurrah of a dying massive star. It involves stars of large mass, greater than 8 solar masses, under certain conditions 10, and represents a stellar explosion more energetic than that of a nova. Supernovae are very bright and cause an emission of radiation that can, at least for short periods, exceed that of an entire galaxy.

In a time interval that can usually range from a few weeks to a few months, the explosion of a supernova, in addition to its big "bang," emits as much energy as the Sun is expected to emit during its entire existence. For about fifteen seconds, then, it reaches a temperature of one hundred billion Kelvin, but only if the star has a mass at least nine times greater than that of our Sun.

You will understand then that we are faced with an end much more tragic, and at the same time able to offer a real spectacle of nature, than that which is due to minor stars. Just think that this type of stellar explosion ejects most or all of the material that makes up the star, at a speed that can reach 30,000 kilometers per second, practically 10% of the speed of light.

While for small-mass stars the only possible nuclear reactions are those of hydrogen and helium, and only rarely carbon, higher-mass stars are able to reach temperatures sufficient to trigger further nuclear fusion during periods of compression related to the exhaustion of one of the forms of fuel.

Another difference with smaller stars is the possibility, for the largest stars, to better mix the internal elements allowing hydrogen to slip back to the core. Technicalities aside, this process still produces tons of energy, and the core becomes very hot. The heat is such as to generate a strong pressure that, in a long tug-of-war with gravity, sees the creation of various reactions that occur simultaneously at the various layers of the stellar structure.

While the helium is progressively fused into heavier and heavier elements, it is a sequence known as helium capture, the core continues to collapse with a temperature that rises up to 600 million degrees Kelvin: this is enough to trigger the inevitable reaction of carbon into heavier elements, such as oxygen, neon, sodium and magnesium.

The same fusion of carbon, finally, provides a whole new energy source, able to rebalance the "fight" between gravity and pressure that characterizes these extremely bright supergiants, of large radius and very low density. Once the nuclear fusions able to cope with gravity are finished, the star implodes and the mass is too big to allow the stellar nucleus to resist.

A supernova explosion is generated, which as we said is among the most violent events of the inner universe. To make it even simpler, we can say that when a massive star runs out of steam, it goes cold, causing the pressure to drop. Gravity thus wins and the star suddenly collapses. The collapse happens so quickly that it creates huge shock waves that explode the outer part of the star.

Types of supernova

The term supernova comes from the term "nova", with which in the past were indicated stars that appeared in the sky in places where previously there was no trace, which suggested the birth of a "new" star. Because of the brightness of these appearances, the word was emphasized with "supernova", although we know that in reality it is a star that is actually dying. To date, according to the observations of astronomers there are two types of supernovae, which differ in the mechanism of explosion and the type of stars from which the explosion itself originates.

Type I supernovae do not originate from single stars, but from so-called binary systems, which are those composed of two nearby stars rotating around a common center of gravity. Binary systems that can trigger a Type I supernova are generally those consisting of a white dwarf of oxygen and carbon and a star called its companion. The matter that composes the first, because of the extremely high pressure and density, is in a state that scientists call "degenerate"

A state, this, which is stable only if the mass of the star concerned is less than a limit value called "Chandrasekar mass", equal to 1.4 times the mass of the Sun. In case the white dwarf is in a binary system, its gravitational field can be so strong to push the nearby companion star to transfer its mass. As a consequence, the dwarf starts to grow exponentially until it exceeds the Chandrasekar limit, and it contracts.

The contraction triggers the nuclear reactions that we already know, and the energy released is enough to completely explode the star, which disintegrates leaving nothing but dust in space.

The type II supernovae, however, originate from particularly massive stars, usually about 10 times the mass of our Sun. These live a relatively short time, not exceeding 10 million years, and throughout their life the nuclear fuel at the center of the star tends to change cyclically element.

Until you get to iron. At each "transformation", the nucleus goes to contract for the action of gravity and is able to raise the temperature so much to trigger the burning of the new chemical element. Taking into account that iron for its own nature cannot undergo a further fusion to produce energy, once arrived its turn the contraction of the nucleus will be unstoppable and completely irreversible.

In only few tens of seconds the diameter of the nucleus contracts from about half the Earth radius to a little more than 10 kilometers, and the shock wave so produced propagates in about two hours through the external layers of the star. When it manages to reach the surface, the star explodes. All of the material that makes up the outer portion of the star is projected into surrounding space at a speed of about 15,000 kilometers per second, while the residue left behind can be, depending on its mass, a neutron star, also called a pulsar, or a black hole.

The importance of a stellar explosion

Despite being a destructive phenomenon, the supernova plays a key role in the evolution of the universe, with effects perceptible even here on Earth. First of all, this thunderous stellar explosion has proven to be the most efficient and complete mechanism of chemical enrichment of galaxies. In fact, not everyone knows that most of the elements found today in the Galaxy, on Earth and in humans themselves did not appear with the birth of the Universe following the Big Bang, but were synthesized within the stars, including the oxygen we breathe!

Following the explosion of supernovae, the stellar material, which is rich in chemical elements, is returned to space and enriches the clouds of gas and interstellar dust that will later give rise to new stars, planets and galaxies. Not only, the energy of the explosion allows the elements already present to transform, so as to complete all the elements of the periodic table that we have learned to know.

In addition, the explosion of a supernova promotes the birth of new stars, for a virtuous circle that translates into potential new forms of life. This is because the shock wave generated by the explosion propagates in the interstellar gas and dust clouds and causes density variations. The variations trigger the contraction of gas and the subsequent formation of a new star.

The cosmos, therefore, has its own peculiar life cycle: the death of a star creates the necessary conditions for others to live. It should come as no surprise then that supernovae are so much studied. Unfortunately it is not possible with our means to know when and where a supernova will explode, but astrophysicists can go and discover them by continuously monitoring a large number of galaxies. You also need to have a lot of patience, considering that the number of galactic supernova explosions is only one every 30-50 years on average.