Our Universe is home to an incredible variety of celestial bodies. However, even the stars are no different and, in comparison to the "simple" worlds, their characteristics can be even stranger, particular and extreme .. especially when they "die".
There are worlds where diamonds most likely rain, so imagine what stars can exist. Short excursus: white dwarfs, neutron stars and other celestial bodies made up of exotic matter are called "degenerate stars", a term that describes them as small bodies despite their large mass.
The white dwarfs
White dwarfs are among the most common stars but still remain fascinating. A white dwarf is a small star with one size comparable to that of the Earth, but with a mass similar or slightly higher than that of the Sun. It is therefore a very compact object, with a very high density and gravity (a teaspoon of matter from a white dwarf would weigh 5 tons). Before becoming a white dwarf, the star goes through a red giant phase. If the mass is not sufficient to melt carbon then this element and oxygen accumulate in the center. Subsequently the outer layers are expelled and only a central body is left.
The death of a star depends on its mass: the most massive stars, with eight times the mass of the sun or more, do not become white dwarfs, but at the end of their life they explode in a violent supernova, leaving behind a neutron star or a black hole. The smaller stars take a slightly quieter path, those of medium-low mass, such as the Sun, which eventually swell into red giants.
Subsequently, the stars spread their outer layers in a ring known as the planetary nebula, and the remaining nucleus becomes a white dwarf, a star in which no hydrogen fusion occurs. Even smaller stars, such as red dwarfs, do not reach the state of a giant red, they simply burn off all their hydrogen, immediately becoming a weak white dwarf. However, we have no proof of what has been said, since the red dwarfs they take billions of years to consume their fuel, much longer than the universe age of 13.8 billion years. According to NASA, gravity on the surface of a white dwarf is 350,000 times that of Earth: This means that a 68 kilogram person would weigh 22.7 million kilos on the surface.
At the end of its life a white dwarf does not disappear completely, but simply becomes a "black dwarf", a celestial body that no longer emits any type of light. To reach this stage, a star is thought to need 100 billion years. Given the age of the universe (13.8 billion years) it can be concluded that a black dwarf has not yet formed.
Neutron star
Compared to the white dwarf, protagonist of the end of life of medium-small stars, a neutron star is instead the result of gravitational collapse of the core of a massive star, which follows the cessation of nuclear fusion reactions due to the exhaustion of the light elements inside, and therefore represents the last stage of life of stars with masses greater than 10 times that of the Sun.
Neutron stars are the remains of giant stars that ended their life in the most spectacular way possible: through a supernova (the event that gives rise to a neutron star imparts a lot of energy to the object, making it rotate on its axis from 60 up to 700 times per second). After the explosion, the nuclei of these former stars are enclosed in an ultra-dense object having the mass of the Sun, but small in size, of an order not exceeding ten kilometers.
The immense gravitational force crushes the atomic nuclei together bringing the subatomic particles into contact, thus merging the electrons with the protons, transforming them into neutrons (the matter of these neutron stars is different from the ordinary one and not yet fully understood). Its physical density characteristics are more close to those of atomic nuclei rather than the common matter composed of atoms. A teaspoon of matter from a neutron star would weigh about a thousand tons.
Gravity is approx 2 billion times stronger than that of Earth. As you can imagine, a medium neutron star boasts a powerful magnetic field. Just think that the Earth's magnetic field is about 1 gauss, that of the Sun of a few hundred gauss while a neutron star, on the other hand, has a magnetic field of trillion gauss. These degenerate stars are not all the same and are mainly divided into two categories: pulsar and magnetar.
The pulsar
Pulsars are rotating neutron stars, characterized by radiation pulses at very regular intervals that generally range from milliseconds to seconds. Pulsars have very powerful magnetic fields which they channel jets of particles along the two magnetic poles. These accelerated particles produce very intense light beams.
Often, the magnetic field is not aligned with the rotation axis, so those rays of particles and light are dragged as the star rotates. When the ray crosses our field of vision from the perspective of the Earth we see an impulse.
To understand this concept just compare a pulsar to a lighthouse. At night, a lighthouse emits a ray of light that crosses the sky. Even if the light constantly shines, it is only possible to see the beam when it points directly in our direction.
There are also the so-called "millisecond pulsar", with a rotational period of between 1 and 10 milliseconds. Current theories on the evolution of the structure of neutron stars predict that pulsars could literally" break "if their rotation speed exceeds 1500 rpm.
The magnetar
Another type of neutron star (in some ways the most extreme of all!) Is called magnetar. In a typical neutron star the magnetic field is trillions of times stronger than the earth's; however, in a magnetar, the magnetic field is 1000 times stronger than a simple neutron star. It is believed that about 1 in 10 supernova degenerates into a magnetar instead of a more common neutron star or a pulsar: this happens when the star already has a fast rotation and a strong magnetism. The life of such an extreme celestial body is limited and due to the strong magnetic fields they decay after about 10,000 years.
In a magnetar, with its huge magnetic field, the movements in the crust cause the neutron star to release a large amount of energy in the form of electromagnetic radiation. To understand the power of these incredible celestial bodies, imagine that a magnetar – called SGR 1806-20 – on 27 December 2004 emitted an X-ray pulse which in a tenth of a second released more energy than that emitted by the sun in the last 100,000 years.
