“For the first time we have witnessed the creation of atoms. We can measure the temperature of matter and observe the microphysics in this remote explosion.” Rasmus Damgaard, astrophysicist and researcher at the DAWN Cosmic Center at the Niels Bohr Institute (Denmark), has chosen these words to describe the magnitude of one of the most spectacular cosmic phenomena of how many the Hubble telescope has collected in recent years.
Damgaard and his colleagues at the DAWN Center have studied the collision of two neutron stars that triggered the formation of the smallest black hole observed to date. The Hubble Space Telescope and other instruments picked up this event even though it took place no less than 130 million light years from Earth. The most interesting thing is that the analysis of this collision can help scientists better understand the formation process of elements that are heavier than iron, and which, therefore, cannot condense inside stars through reactions. of nuclear fusion.
The collision and subsequent merger of two neutron stars is known as a kilonova, and is an extraordinarily energetic event capable of emitting as much light as several hundred million stars simultaneously. It’s something difficult to imagine. In any case, the most amazing thing is that the astrophysicists who are studying this kilonova have witnessed for the first time the processes that give rise to the creation of atoms, as they explain in the very interesting article they have published in Astronomy & Astrophysics.
Neutron stars are one of the most exciting objects in the cosmos
Neutron stars are not always solitary. Sometimes one of them is part of a binary system along with a “living” star, and if the appropriate conditions are met, the latter can also end up becoming a neutron star. In this scenario, the binary system ends up being made up of two neutron stars that rotate around each other. As time goes by, they lose angular momentum, which causes their orbits to narrow and get closer and closer. And when they are close enough, gravity takes over and the two neutron stars are doomed to collide.
This highly energetic cosmic event triggers the emission of electrons and neutrons that end up rotating around the massive object that remains after the collision of the two neutron stars. And finally this body collapses at a great speed and gives rise to the formation of a black hole. Very broadly this is what has happened in the galaxy NGC 4993. And the starting point of everything, as we have just seen, are neutron stars. We have explained how they are formed in other articles, but it is worth reviewing it before concluding this topic because it is an exciting process.
Neutron stars are not always solitary. Sometimes one of them is part of a binary system with a “living” star.
In the report we dedicate to the life of stars you will find very interesting information about brown dwarfs, white dwarfs or red giants. But in this article we are interested in sticking to neutron stars. If the object that remains after the star has expelled its outer layers into the stellar medium in the form of a supernova has more than 1.44 solar masses, a value known as the Chandrasekhar limit in honor of the Indian astrophysicist who calculated it, the stellar remnant will collapse once again to give rise to a neutron star.
A few moments before the supernova occurs, the iron core of our massive star is subjected to the enormous pressure of the upper layers of material, and also to the incessant action of gravitational contraction. These processes trigger a mechanism of quantum nature that entails very important changes in the structure of mattercausing the iron in the stellar core, which is subjected to a very high temperature, to photodisintegrate under the action of high-energy photons, which constitute a form of energy transfer known as gamma radiation.
The very high energy photons manage to disintegrate the iron and helium accumulated in the core of the star, giving rise to the production of alpha particles, which are helium nuclei that lack their electron envelope, and which, therefore, have a charge. positive electric, and neutrons. In addition, a mechanism known as beta capture takes place, which we are not going to delve into so as not to overly complicate the article. The important thing is that we know that it causes the electrons in the iron atoms to interact with the protons in the nucleus, neutralizing their positive charge and leading to the production of more neutrons.
During this process, the initial matter, which was made up of protons, neutrons and electrons, becomes made up only of neutrons because, as we have just seen, the electrons and protons have interacted through electronic capture. to give rise to more neutrons. From that moment on the star is no longer made up of ordinary matter; It has transformed into a kind of enormous crystal made up only of neutrons.
A one-cubic centimeter fragment of a neutron star weighs approximately one billion tons.
However, once the star has reached this state we can ask ourselves what mechanism allows this ball of neutrons to manage to withstand and counteract the pressure exerted by the tireless gravitational contraction. The phenomenon responsible for keeping the neutron star in balance is the Pauli exclusion principle, an effect of quantum nature in which we do not need to dive deeply to avoid complicating the article much more.
Very broadly this principle, which was stated by the Austrian physicist Wolfgang Ernst Pauli in 1925, establishes that two fermions of the same quantum system cannot remain in the same quantum state. Quarks, which are the elementary particles that make up the protons and neutrons of the atomic nucleus, are fermions. And the electrons, too. To approximate in a simple way what it means that two fermions cannot acquire the same quantum state and understand where the balance of neutron stars comes from, we can intuit that the impossibility of two neutrons occupying the same place generates the pressure necessary to maintain the star in balance.
And this brings us to what is undoubtedly the most surprising characteristic of neutron stars: their density. The average radius of one of these objects is approximately ten kilometers, but its mass is enormous. Compared, for example, to stars found on the main sequence, or even to white dwarfs, neutron stars are very small, and accumulating so much mass in such a small space causes a fragment of a cubic centimeter of A neutron star weighs approximately, no more and no less, a billion tons. It is amazing that a little piece of matter similar to a sugar cube can have such a monstrous weight.
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More information | Astronomy & Astrophysics
In WorldOfSoftware | From clouds of dust and gas to black holes: this is how stars are born, grow, die and reproduce
