Neutron stars . For sure the most exotic objects in the universe. These are “excessive” in almost all aspects: gravity, magnetic field strength, density and temperature. You could say that the black holes are more dense and in a sense it would be true, but in fact we can not determine the inner structure of a black hole that is permanently hidden beyond the horizon of events.



The neutron star with a solid outer surface is the densest solid object we can see, having a core density of several times the density of the atomic nucleus. A piece of a neutron star of grain size would have a mass of more than 500,000 tons. Neutron stars offer a whole range of features that make them a favorite target for astrophysicists. For the general public, however, they have an image problem because they can not provide us with direct observation and do not have the strange attraction of the black holes. 

The origin of neutron stars

Neutron stars are formed by explosions of supernovae, which represent the end of the “life” of a medium-sized star (with a mass greater than 8-20 times the Sun). Once they consume their nuclear “fuel”, these stars explode, losing most of the matter in space. What remains in small objects (according to astronomical standards), with a diameter of about 22 km, but with a mass greater than the Sun (1.5 times). The bark of the neutron star is mainly made of crystallized iron. But this kind of atom can not withstand the inside of the star, the material having a transition, the bizarre “nuclear paste” phase, to the neutron fluid in the star core. The kernel environment can not be reproduced on Earth, but the uncertainties surrounding this region (may contain matter consisting of “strange quarks” or exotic hypertones) are a motivating factor for studying these objects. Neutron stars emit little visible light, making it virtually impossible to detect in an unfamiliar search. Most of the few thousands of neutron stars have been discovered based on radio pulses. As cosmic headlamps, the pair of radio waves that emerge from these pulsating objects “sweep” the Universe. If a beam reaches Terra, it can be detected with the telescopes on the ground. The nearest pulsar is PSR J0437-4715, about 500 light years away. Of course, many of the beams do not reach the Earth, so the discovered ones represent only a small part of those existing in the galaxy.In addition to these “ordinary” pulsars, there are other “species” with interesting names:

Rotating radio transients (RRAT) – which are pulsers transmitting short and rare radio pulses.

Magnetars – pulsars with an incredibly powerful magnetic field.




Microquasations – pulsars with jets reaching relativistic speeds.



Dazzling rotation

typical neutron star rotates once per second, which is remarkable as it is a massive object. But if it happens that a neutron star also has a companion star, then the rotation speed is much higher. The process that generates this rotation is called accretion. In millions of years companion stars evolve (and increase their volume) so that the outer layer strongly senses the gravitational attraction of the neutron star. Then the companion star’s gas gets to “blow” into the neutron star, making it spin, just like you could turn a bicycle wheel with a stronger jet of water. This process has remarkable effects. The gas that falls into the neutron star is heated to millions of degrees, and it begins to shine strongly, especially in the X-ray area. This radiation is blocked by the terrestrial atmosphere, but it can be detected by the telescopes on the satellites. In fact, the brightest object detected on Terra (but in the X-ray area) outside the Sun is a neutron star (at least it is believed to be), Scorpius X-1 (the first X-ray source in the Scorpio constellation) , which orbits companion star and donor every 19 hours.



The gas that accumulates on the surface of a neutron star in the acrelation process is probably similar in composition to our Sun – essentially hydrogen and helium. 


The enormous gravity of the neutron star – a few million times more powerful than Earth’s – will compress and heat the gas, after several hours or days reaching the point where the nuclear fusion takes place. But this process is unstable, in a matter of seconds, the entire accumulated “fuel” is exhausted; this is the result of an X-ray explosion that spreads into the galaxy. These energy outbursts were observed in about 100 systems from the first launch of X-ray telescopes in the 1960s. Taking place at intervals of several hours or a few days (depending on the rate of accretion) – they are by far the most frequent thermonuclear explosions in the Universe. Of course, the supply of gas from the companion star will be exhausted at some point. And when that happens, the neutron star “gives up” its role as a pulsar radio, although it has reached stunning rotation speeds (hundreds of revolutions per second). The absolute holder of the record per second is PSRJ1748-2446ad (716 revolutions per second!). But the neutron stars do not remain active forever. In the end, the energy of rotation will dissipate, and in the absence of a companion star that will “recycle” it, the pulsar will become undetectable. Then lightly it will cool down. But until then the neutron stars will continue to be an extraordinary laboratory to study matter under extreme density and temperature.








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