THEMES

Neutron stars: size, collision and fusion

Neutron stars: size, collision and fusion

Neutron stars, stars that come from a collapse of large stars and that have very special characteristics. Let's not imagine the stars we draw as children, in these stars a collapse takes place, waves are emitted from these stars. The neutron stars they are "curious objects" to be known in all their characteristics, even if they are not at hand nor do they tell us, for those who believe in horoscopes, our future and our destiny. Perhaps they have much better to say about the universe.

Neutron stars

It is about special stars, not the ones we are used to seeing in the sky as they form constellations that we learn to recognize. See or imagine, in truth, given the levels of light pollution recorded in many areas of Italy.

Neutron stars are very particular stars, they are the result of the evolution of stars with a mass sometimes equal to that of the Sun. In this process of transforming the outer layers of the stars of this size are blown away and the iron core collapses on itself rather quickly and abruptly.

If it were a small star, the nucleus, gradually contracting more and more, would arrive at a situation in which the pressure of the electrons inside it becomes so strong that it counterbalances the pressure of the outer layers.So no collapse in sight.

Considering larger stars, however, such as those that have a mass two or three or more times that of the Sun, it cannot be assumed that the same thing happens. In fact, the pressure of the internal electrons in this case is too great and a collapse phenomenon which comes to modify the structure of the atoms inside it. In this way we arrive at the neutron stars.

Neutron stars: collision

There collision that occurs in these stars has important consequences. If we were in a normal atom, we would have electrons quite distant from the nucleus and with a lot of space available, therefore with an atom that could be defined as "empty".

Taking the neutron stars instead, we see that the pressure is so great that it breaks the nuclei into protons and neutrons, this involves the possibility that electrons come to be very close to protons ending up colliding and merging. The result of the fusion are neutrons, the atom, as anticipated, thus changes its internal structure as rarely happens.

Neutron stars: merger

When these happen mergers, gradually there is a star composed only of neutrons and positioned so that they are at minimum distances between them if not practically one against the other. Consider that a neutron star has a density equal to 100 thousand billion times that of the rock. You know the spoonful of sugar we add to coffee?

If instead of sugar it were the stuff the neutron star is made of, this teaspoon would be impossible to lift because it would weigh as much as the entire population of the Earth, if it were on Earth itself. The merger, however, allows us to stop the collapse that previously seemed to promise disasters and to reach a situation of equilibrium, our star.

Neutron stars: dimensions

Despite having a mass greater than that of the Sun, a neutron star is very small in size. Obviously, reasoning on a spatial, universal scale. Small means diameters of about thirty kilometers, therefore comparable to large asteroids. Also thanks to this characteristic, the neutron stars are able to rotate with particularly high speeds compared to their colleagues, with speed peaks of 100 "revolutions" per second.

Another characteristic of these stars is the temperature, very high, therefore even 10 million degrees, in fact the radiation emitted by them is not comparable with that which reaches us from normal stars. This is also important from a practical point of view, because this vast temperature gap implies the need to use ad hoc instruments to observe stars like this: we cannot use the ones we already have for classical stars. A radio telescope, because this instrument detects the radio waves emitted by a neutron star by locating it, thus "seeing" it.

It dates back to 1967 the discovery of the first neutron star and it is thanks to the radio pulses emitted by it that it was possible to identify its presence: the astronomers were in fact aware of the star by perceiving radio pulses regular over time and going back to the source, located in a precise point that at the time they had called “Pulsating radio star”.

Neutron stars and gravitational waves

Neutron stars have on the one hand a very strong gravitational field, on the other also a very intense magnetic field. The result is that the radiation produced is not emitted by the entire star but is channeled into two very narrow cones, around the axis of the star's magnetic field. If, or rather, when one of the cones is facing us, the radiation emitted in the form of radio waves reaches us, certainly not the only ones emitted by a neutron star: there are also ultraviolet, X and gamma rays, and more faintly inde in the optical band.

Neutron stars and black holes

We often hear about neutron stars and black holes at the same time, let's see how these two realities are linked. The stars are what remain after the gravitational collapse of stars with masses between 8-10 and 20-30 times that of the Sun. When we have stars with masses greater than 20-30 solar masses, the same collapse mechanism gives life to black holes. Finally, going to consider much more "light" stars, with masses less than 8-10 times that of the Sun, then the gravitational collapse results in white dwarfs.

If you are interested in the starry sky, read the article on brightness of the stars

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Video: The Ultimate Collision: Neutron Stars Rattle, Shine, and Sparkle, Sanjay Reddy, Univ of Washington (September 2021).