Neutron star mass. White dwarf, neutron star, black hole

More than ten billion years have passed since the birth of the Universe, during which stellar evolution occurs and the composition of outer space changes. Some space objects disappear, and others appear in their place. This process occurs constantly, however, due to the huge time intervals, we are able to observe only one single frame of a colossal and fascinating multi-session.

We see the Universe in all its glory, observing the life of stars, the stages of evolution and the moment of death agony. The death of a star is always a grandiose and bright event. The larger and more massive the star, the larger the cataclysm.

The neutron star is a striking example of such evolution, a living monument to past stellar power. This is the whole paradox. In place of a massive star, the size and mass of which are tens and hundreds of times greater than those of our Sun, a tiny celestial body with a diameter of a couple of tens of kilometers appears. This transformation does not happen overnight. The formation of neutron stars is the result of a long evolutionary path of development of a cosmic monster, extended in space and time.

Physics of Neutron Stars

Such objects are few in number in the Universe, as it might seem at first glance. Typically, a neutron star may be one in a thousand stars. The secret to such a small number lies in the unique evolutionary processes that precede the birth of neutron stars. All stars live their lives differently. The ending of the star drama also looks different. The scale of the action is determined by the mass of the star. The greater the mass of the cosmic body, the more massive the star, the higher the likelihood that its death will be quick and bright.

Constantly increasing gravitational forces lead to the transformation of stellar matter into thermal energy. This process is involuntarily accompanied by a colossal ejection - a Supernova explosion. The result of such a cataclysm is a new space object - a neutron star.

Simply put, stellar matter ceases to be fuel, thermonuclear reactions lose their intensity and are unable to maintain the required temperatures in the depths of a massive body. The way out of this state is collapse—the collapse of stellar gas onto the central part of the star.

All this leads to an instant release of energy, scattering the outer layers of stellar matter in all directions. An expanding nebula appears in place of the star. Such a transformation can happen to any star, but the results of the collapse may be different.

If the mass of the cosmic object is small, for example, we are dealing with a yellow dwarf like the Sun, a white dwarf remains at the site of the flare. In the event that the mass of the space monster exceeds the solar mass by tens of times, as a result of the collapse we observe a Supernova outbreak. In place of the former stellar greatness, a neutron star is formed. Supermassive stars, whose mass is hundreds of times greater than the mass of the Sun, complete their life cycle; a neutron star is an intermediate stage. Continued gravitational compression leads to the fact that the life of a neutron star ends with the appearance of a black hole.

As a result of the collapse, only the core remains of the star, which continues to shrink. In this regard, a characteristic feature of neutron stars is high density and enormous mass with tiny sizes. So the mass of a neutron star with a diameter of 20 km. 1.5-3 times the mass of our star. The compaction or neutronization of electrons and protons into neutrons occurs. Accordingly, with a decrease in volume and size, the density and mass of stellar matter rapidly increases.

Composition of neutron stars

There is no exact information about the composition of neutron stars. Today, astrophysicists, when studying such objects, use the working model proposed by nuclear physicists.

Presumably, stellar matter as a result of collapse is transformed into a neutron, superfluid liquid. This is facilitated by the enormous gravitational attraction that exerts constant pressure on the substance. This “nuclear liquid substance” is called a degenerate gas and is 1000 times denser than water. Atoms of a degenerate gas consist of a nucleus and electrons revolving around it. During neutronization, the internal space of atoms disappears under the influence of gravitational forces. Electrons fuse with the nucleus to form neutrons. Internal gravity gives stability to the superdense substance. Otherwise, a chain reaction would inevitably begin, accompanied by a nuclear explosion.

The closer to the outer edge of the star, the lower the temperature and pressure. As a result of complex processes, the neutron substance “cools”, from which iron nuclei are intensively released. The collapse and subsequent explosion is a factory of planetary iron, which spreads into outer space, becoming the building material for the formation of planets.

It is to supernova explosions that the Earth owes the fact that particles of cosmic iron are present in its structure and structure.

Conventionally, examining the structure of a neutron star through a microscope, we can distinguish five layers in the structure of the object:

• the atmosphere of the object;
• outer cortex;
• inner layers;
• outer core;
• the inner core of a neutron star.

The atmosphere of a neutron star is only a few centimeters thick and is the thinnest layer. In terms of its composition, it is a layer of plasma responsible for the thermal irradiation of the star. Next comes the outer crust, which is several hundred meters thick. Between the outer crust and the inner layers is the realm of degenerate electron gas. The deeper into the center of the star, the faster this gas becomes relativistic. In other words, the processes occurring inside the star are associated with a decrease in the proportion of atomic nuclei. At the same time, the number of free neutrons increases. The inner regions of a neutron star represent the outer core, where neutrons continue to coexist with electrons and protons. The thickness of this layer of substance is several kilometers, while the density of matter is tens of times higher than the density of the atomic nucleus.

This entire atomic soup exists thanks to colossal temperatures. At the time of the Supernova explosion, the temperature of the neutron star is 1011K. During this period, the new celestial object has maximum luminosity. Immediately after the explosion, a rapid cooling stage begins, the temperature drops to 109K in a few minutes. Subsequently, the cooling process slows down. Even though the star's temperature is still high, the object's luminosity is decreasing. The star continues to glow only due to thermal and infrared radiation.

Classification of neutron stars

This specific composition of the stellar-nuclear substance determines the high nuclear density of a neutron star, 1014-1015 g/cm³, while the average size of the resulting object is no less than 10 and no more than 20 km. A further increase in density is stabilized by neutron interaction forces. In other words, the degenerate stellar gas is in a state of equilibrium, keeping the star from collapsing again.

The rather complex nature of such cosmic objects as neutron stars became the reason for the subsequent classification, which explains their behavior and existence in the vastness of the Universe. The main parameters on the basis of which the classification is carried out are the rotation period of the star and the scale of the magnetic field. During its existence, a neutron star loses rotational energy, and the object’s magnetic field also decreases. Accordingly, the celestial body passes from one state to another, among which the following types are the most characteristic:

• Radio pulsars (ejectors) are objects that have a short rotation period, but the strength of their magnetic field remains quite large. Charged particles, moving along force fields, leave the star shell at break points. A celestial body of this type ejects, periodically filling the Universe with radio pulses detected in the radio frequency range;
• A neutron star is a propeller. In this case, the object has an extremely low rotation speed, however, the magnetic field does not have sufficient strength to attract elements of matter from the surrounding space. The star does not emit impulses, and accretion (fall of cosmic matter) does not occur in this case;
• X-ray pulsar (accretor). Such objects have a low rotation speed, but due to the strong magnetic field, the star intensively absorbs material from outer space. As a result, in places where stellar matter falls, plasma heated to millions of degrees accumulates on the surface of a neutron star. These points on the surface of a celestial body become sources of pulsating thermal and X-ray radiation. With the advent of powerful radio telescopes capable of peering into the depths of space in the infrared and X-ray range, it has become possible to more quickly identify quite a few ordinary X-ray pulsars;
• A georotator is an object that has a low rotation speed, while stellar matter accumulates on the surface of the star as a result of accretion. A strong magnetic field prevents the formation of plasma in the surface layer, and the star gradually gains mass.

As can be seen from the existing classification, each of the neutron stars behaves differently. This leads to different ways of detecting them, and perhaps the fate of these celestial bodies in the future will be different.

Paradoxes of the birth of neutron stars

The first version, that neutron stars are products of a Supernova explosion, is not a postulate today. There is a theory that another mechanism may be used here. In binary star systems, white dwarfs become food for new stars. Stellar matter gradually flows from one cosmic object to another, increasing its mass to a critical state. In other words, in the future, one of the white dwarf pair is a neutron star.

Often, a single neutron star, being in a close environment of star clusters, turns its attention to its nearest neighbor. Any stars can become companions of neutron stars. These pairs occur quite often. The consequences of such friendship depend on the mass of the companion. If the mass of the new companion is small, then the stolen stellar matter will accumulate around in the form of an accretion disk. This process, accompanied by a long rotation period, will cause the stellar gas to heat up to a temperature of a million degrees. The neutron star will erupt in a stream of X-rays, becoming an X-ray pulsar. This process has two paths:

• the star remains in space as a dim celestial body;
• the body begins to emit short X-ray bursts (bursters).

During X-ray flares, the star's brightness rapidly increases, making such an object 100 thousand times brighter than the Sun.

History of the study of neutron stars

Neutron stars became a discovery of the second half of the 20th century. Previously, it was technically impossible to detect such objects in our galaxy and in the Universe. The dim light and small size of such celestial bodies did not allow them to be detected using optical telescopes. Despite the lack of visual contact, the existence of such objects in space was theoretically predicted. The first version of the existence of stars with enormous density appeared at the suggestion of the Soviet scientist L. Landau in 1932.

A year later, in 1933, a serious statement was made overseas about the existence of stars with an unusual structure. Astronomers Fritz Zwicky and Walter Baade have put forward a reasonable theory that a neutron star will inevitably remain at the site of a Supernova explosion.

In the 60s of the 20th century there was a breakthrough in astronomical observations. This was facilitated by the emergence of X-ray telescopes capable of detecting sources of soft X-ray radiation in space. Using in their observations the theory of the existence of sources of strong thermal radiation in space, astronomers came to the conclusion that we are dealing with a new type of star. A significant addition to the theory of the existence of neutron stars was the discovery of pulsars in 1967. American Jocelyn Bell, using his radio equipment, discovered radio signals coming from space. The source of the radio waves was a rapidly rotating object, which acted like a radio beacon, sending signals in all directions.

Such an object certainly has a high rotation speed, which would be fatal for an ordinary star. The first pulsar to be discovered by astronomers is PSR B1919+21, located at a distance of 2283.12 light years. years from our planet. According to scientists, the closest neutron star to Earth is the space object RX J1856.5-3754, located in the constellation Corona South, which was discovered in 1992 at the Chandra Observatory. The distance from Earth to the nearest neutron star is 400 light years.

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German astrophysicists have clarified the maximum possible mass of a neutron star, based on the results of measurements of gravitational waves and electromagnetic radiation from. It turned out that the mass of a non-rotating neutron star cannot be more than 2.16 solar masses, according to an article published in Astrophysical Journal Letters.

Neutron stars are ultra-dense compact stars that form during supernova explosions. The radius of neutron stars does not exceed several tens of kilometers, and their mass can be comparable to the mass of the Sun, which leads to a huge density of star matter (about 10 17 kilograms per cubic meter). At the same time, the mass of a neutron star cannot exceed a certain limit - objects with large masses collapse into black holes under the influence of their own gravity.

According to various estimates, the upper limit for the mass of a neutron star lies in the range from two to three solar masses and depends on the equation of state of matter, as well as on the speed of rotation of the star. Depending on the density and mass of the star, scientists distinguish several different types of stars; a schematic diagram is shown in the figure. First, non-rotating stars cannot have a mass greater than M TOV (white region). Secondly, when a star rotates at a constant speed, its mass can be either less than M TOV (light green region) or more (bright green), but still must not exceed another limit, M max. Finally, a neutron star with a variable rotation rate could theoretically have an arbitrary mass (red regions of different brightness). However, you should always remember that the density of rotating stars cannot be greater than a certain value, otherwise the star will still collapse into a black hole (the vertical line in the diagram separates stable solutions from unstable ones).

Diagram of different types of neutron stars based on their mass and density. The cross marks the parameters of the object formed after the merger of the stars of the binary system, the dotted lines indicate one of two options for the evolution of the object

L. Rezzolla et al. / The Astrophysical Journal

A team of astrophysicists led by Luciano Rezzolla has set new, more precise limits on the maximum possible mass of a non-rotating neutron star, M TOV. In their work, scientists used data from previous studies on processes that occurred in a system of two merging neutron stars and led to the emission of gravitational (event GW170817) and electromagnetic (GRB 170817A) waves. The simultaneous registration of these waves turned out to be a very important event for science; you can read more about it in ours and in the material.

From previous works of astrophysicists, it follows that after the merger of neutron stars, a hypermassive neutron star was formed (that is, its mass is M > M max), which subsequently developed according to one of two possible scenarios and after a short period of time turned into a black hole (dashed lines in the diagram ). Observation of the electromagnetic component of the star's radiation points to the first scenario, in which the baryonic mass of the star remains essentially constant and the gravitational mass decreases relatively slowly due to the emission of gravitational waves. On the other hand, the gamma-ray burst from the system arrived almost simultaneously with the gravitational waves (only 1.7 seconds later), which means that the point of transformation into a black hole should lie close to M max.

Therefore, if we trace the evolution of a hypermassive neutron star back to the initial state, the parameters of which were calculated with good accuracy in previous works, we can find the value of M max that interests us. Knowing M max, it is not difficult to find M TOV, since these two masses are related by the relation M max ≈ 1.2 M TOV. In this article, astrophysicists performed such calculations using so-called “universal relations,” which relate the parameters of neutron stars of different masses and do not depend on the type of equation of state of their matter. The authors emphasize that their calculations use only simple assumptions and do not rely on numerical simulations. The final result for the maximum possible mass was between 2.01 and 2.16 solar masses. A lower bound for it was previously obtained from observations of massive pulsars in binary systems - simply put, the maximum mass cannot be less than 2.01 solar masses, since astronomers have actually observed neutron stars with such a large mass.

Previously, we wrote about how astrophysicists used computer simulations to estimate the mass and radius of neutron stars, the merger of which led to the events GW170817 and GRB 170817A.

Dmitry Trunin

The substance of such an object is several times higher than the density of the atomic nucleus (which for heavy nuclei is on average 2.8⋅10 17 kg/m³). Further gravitational compression of the neutron star is prevented by the pressure of nuclear matter arising due to the interaction of neutrons.

Many neutron stars have extremely high rotation speeds, up to several hundred revolutions per second. Neutron stars arise from supernova explosions.

General information

Among neutron stars with reliably measured masses, most fall in the range of 1.3 to 1.5 solar masses, which is close to the Chandrasekhar limit. Theoretically, neutron stars with masses from 0.1 to about 2.16 solar masses are acceptable. The most massive neutron stars known are Vela X-1 (has a mass of at least 1.88±0.13 solar masses at the 1σ level, which corresponds to a significance level of α≈34%), PSR J1614–2230 en (with a mass estimate of 1. 97±0.04 solar), and PSR J0348+0432 en (with a mass estimate of 2.01±0.04 solar). Gravity in neutron stars is balanced by the pressure of the degenerate neutron gas, the maximum value of the mass of a neutron star is set by the Oppenheimer-Volkoff limit, the numerical value of which depends on the (still poorly known) equation of state of matter in the star's core. There are theoretical premises that with an even greater increase in density, the degeneration of neutron stars into quark stars is possible.

By 2015, more than 2,500 neutron stars had been discovered. About 90% of them are single. In total, 10 8 -10 9 neutron stars can exist in our Galaxy, that is, about one per thousand ordinary stars. Neutron stars are characterized by high speed (usually hundreds of km/s). As a result of the accretion of cloud matter, a neutron star in this situation can be visible from Earth in different spectral ranges, including optical, which accounts for about 0.003% of the emitted energy (corresponding to magnitude 10).

Structure

A neutron star has five layers: atmosphere, outer crust, inner crust, outer core and inner core.

The atmosphere of a neutron star is a very thin layer of plasma (from tens of centimeters for hot stars to millimeters for cold ones), in which the thermal radiation of a neutron star is formed.

The outer crust consists of ions and electrons, its thickness reaches several hundred meters. The thin (no more than a few meters) near-surface layer of a hot neutron star contains non-degenerate electron gas, deeper layers contain degenerate electron gas, and with increasing depth it becomes relativistic and ultra-relativistic.

The inner crust consists of electrons, free neutrons and neutron-rich atomic nuclei. With increasing depth, the proportion of free neutrons increases, and that of atomic nuclei decreases. The thickness of the inner crust can reach several kilometers.

The outer core consists of neutrons with a small admixture (several percent) of protons and electrons. In low-mass neutron stars, the outer core can extend to the center of the star.

Massive neutron stars also have an inner core. Its radius can reach several kilometers, the density in the center of the nucleus can exceed the density of atomic nuclei by 10-15 times. The composition and equation of state of the inner core are not reliably known: there are several hypotheses, the three most probable of which are 1) a quark core, in which neutrons fall apart into their constituent up and down quarks; 2) a hyperonic core of baryons including strange quarks; and 3) a kaonic core consisting of two-quark mesons, including strange (anti)quarks. However, it is currently impossible to confirm or refute any of these hypotheses.

A free neutron, under normal conditions, not being part of the atomic nucleus, usually has a lifetime of about 880 seconds, but the gravitational influence of a neutron star does not allow the neutron to decay, so neutron stars are among the most stable objects in the Universe. [ ]

Cooling of neutron stars

At the moment of birth of a neutron star (as a result of a supernova explosion), its temperature is very high - about 10 11 K (that is, 4 orders of magnitude higher than the temperature at the center of the Sun), but it drops very quickly due to neutrino cooling. In just a few minutes, the temperature drops from 10 11 to 10 9 K, in a month - to 10 8 K. Then the neutrino luminosity decreases sharply (it depends very much on temperature), and cooling occurs much more slowly due to photon (thermal) radiation from the surface. The surface temperature of known neutron stars for which it has been possible to measure it is on the order of 10 5 -10 6 K (although the core is apparently much hotter).

History of discovery

Neutron stars are one of the few classes of cosmic objects that were theoretically predicted before their discovery by observers.

For the first time, the idea of ​​the existence of stars with increased density, even before the discovery of the neutron made by Chadwick in early February 1932, was expressed by the famous Soviet scientist Lev Landau. Thus, in his article “On the Theory of Stars,” written in February 1931 and for unknown reasons belatedly published on February 29, 1932 (more than a year later), he writes: “We expect that all this [violation of the laws of quantum mechanics] should manifest itself when the density of matter becomes so great that atomic nuclei come into close contact, forming one giant nucleus.”

"Propeller"

The rotation speed is no longer sufficient for the ejection of particles, so such a star cannot be a radio pulsar. However, the rotation speed is still high, and the matter surrounding the neutron star captured by the magnetic field cannot fall, that is, accretion of matter does not occur. Neutron stars of this type have virtually no observable manifestations and are poorly studied.

Accrector (X-ray pulsar)

The rotation speed decreases so much that nothing now prevents matter from falling onto such a neutron star. Falling, the matter, already in a plasma state, moves along the magnetic field lines and hits the solid surface of the neutron star’s body in the region of its poles, heating up to tens of millions of degrees. Matter heated to such high temperatures glows brightly in the X-ray range. The region in which the collision of falling matter with the surface of the neutron star body occurs is very small - only about 100 meters. Due to the rotation of the star, this hot spot periodically disappears from view, so regular pulsations of X-ray radiation are observed. Such objects are called X-ray pulsars.

Georotator

The rotation speed of such neutron stars is low and does not prevent accretion. But the size of the magnetosphere is such that the plasma is stopped by the magnetic field before it is captured by gravity. A similar mechanism operates in the Earth’s magnetosphere, which is why this type of neutron star got its name.

Notes

1. Dmitry Trunin. Astrophysicists have clarified the maximum mass of neutron stars (undefined) . nplus1.ru. Retrieved January 18, 2018.
2. H. Quaintrell et al. The mass of the neutron star in Vela X-1 and tidally induced non-radial oscillations in GP Vel // Astronomy and Astrophysics. - April 2003. - No. 401. - pp. 313-323. - arXiv:astro-ph/0301243.
3. P. B. Demorest, T. Pennucci, S. M. Ransom, M. S. E. Roberts & J. W. T. Hessels. A two-solar-mass neutron star measured using Shapiro delay (English) // Nature. - 2010. - Vol. 467. - P. 1081-1083.

In astrophysics, as indeed in any other branch of science, the most interesting are evolutionary problems associated with the eternal questions “what happened?” and that will be?". We already know what will happen to a stellar mass approximately equal to the mass of our Sun. Such a star, having gone through a stage red giant, will become white dwarf. White dwarfs on the Hertzsprung-Russell diagram lie off the main sequence.

White dwarfs are the end of the evolution of solar mass stars. They are a kind of evolutionary dead end. Slow and quiet extinction is the end of the road for all stars with a mass less than the Sun. What about more massive stars? We saw that their lives were full of stormy events. But a natural question arises: how do the monstrous cataclysms observed in the form of supernova explosions end?

In 1054, a guest star flashed in the sky. It was visible in the sky even during the day and went out only a few months later. Today we see the remnants of this stellar catastrophe in the form of a bright optical object designated M1 in the Messier Nebula Catalog. This is famous Crab Nebula- remnant of a supernova explosion.

In the 40s of our century, the American astronomer V. Baade began to study the central part of the “Crab” in order to try to find a stellar remnant from a supernova explosion in the center of the nebula. By the way, the name “crab” was given to this object in the 19th century by the English astronomer Lord Ross. Baade found a candidate for a stellar remnant in the form of an asterisk 17t.

But the astronomer was unlucky; he did not have the appropriate equipment for a detailed study, and therefore he could not notice that this star was twinkling and pulsating. If the period of these brightness pulsations were not 0.033 seconds, but, say, several seconds, Baade would undoubtedly have noticed this, and then the honor of discovering the first pulsar would not have belonged to A. Hewish and D. Bell.

About ten years before Baade pointed his telescope at the center Crab Nebula, theoretical physicists began to study the state of matter at densities exceeding the density of white dwarfs (106 - 107 g/cm3). Interest in this issue arose in connection with the problem of the final stages of stellar evolution. It is interesting that one of the co-authors of this idea was the same Baade, who connected the very fact of the existence of a neutron star with a supernova explosion.

If matter is compressed to densities greater than those of white dwarfs, so-called neutronization processes begin. The monstrous pressure inside the star “drives” electrons into atomic nuclei. Under normal conditions, a nucleus that has absorbed electrons will be unstable because it contains an excess number of neutrons. However, this is not the case in compact stars. As the density of the star increases, the electrons of the degenerate gas are gradually absorbed by the nuclei, and little by little the star turns into a giant neutron star- a drop. The degenerate electron gas is replaced by a degenerate neutron gas with a density of 1014-1015 g/cm3. In other words, the density of a neutron star is billions of times greater than that of a white dwarf.

For a long time, this monstrous configuration of the star was considered a game of theorists' minds. It took more than thirty years for nature to confirm this outstanding prediction. In the same 30s, another important discovery was made, which had a decisive influence on the entire theory of stellar evolution. Chandrasekhar and L. Landau established that for a star that has exhausted its sources of nuclear energy, there is a certain limiting mass when the star still remains stable. At this mass, the pressure of the degenerate gas is still able to resist the forces of gravity. As a consequence, the mass of degenerate stars (white dwarfs, neutron stars) has a finite limit (Chandrasekhar limit), exceeding which causes catastrophic compression of the star, its collapse.

Note that if the core mass of a star is between 1.2 M and 2.4 M, the final “product” of the evolution of such a star should be a neutron star. With a core mass of less than 1.2 M, evolution will ultimately lead to the birth of a white dwarf.

What is a neutron star? We know its mass, we also know that it consists mainly of neutrons, the sizes of which are also known. From here it is easy to determine the radius of the star. It turns out to be close to... 10 kilometers! Determining the radius of such an object is indeed not difficult, but it is very difficult to visualize that a mass close to the mass of the Sun can be placed in an object whose diameter is slightly larger than the length of Profsoyuznaya Street in Moscow. This is a giant nuclear drop, a supernucleus of an element that does not fit into any periodic systems and has an unexpected, peculiar structure.

The matter of a neutron star has the properties of a superfluid liquid! This fact is hard to believe at first glance, but it is true. The substance, compressed to monstrous densities, resembles to some extent liquid helium. In addition, we should not forget that the temperature of a neutron star is about a billion degrees, and, as we know, superfluidity in terrestrial conditions manifests itself only at ultra-low temperatures.

True, temperature does not play a special role in the behavior of the neutron star itself, since its stability is determined by the pressure of the degenerate neutron gas - liquid. The structure of a neutron star is in many ways similar to the structure of a planet. In addition to the “mantle”, consisting of a substance with the amazing properties of a superconducting liquid, such a star has a thin, hard crust about a kilometer thick. It is assumed that the bark has a peculiar crystalline structure. It is peculiar because, unlike the crystals known to us, where the structure of the crystal depends on the configuration of the electron shells of the atom, in the crust of a neutron star the atomic nuclei are devoid of electrons. Therefore, they form a lattice reminiscent of the cubic lattices of iron, copper, zinc, but, accordingly, at immeasurably higher densities. Next comes the mantle, the properties of which we have already talked about. At the center of a neutron star, densities reach 1015 grams per cubic centimeter. In other words, a teaspoon of the material from such a star weighs billions of tons. It is assumed that in the center of a neutron star there is a continuous formation of all known in nuclear physics, as well as not yet discovered exotic elementary particles.

Neutron stars cool quite quickly. Estimates show that over the first ten to one hundred thousand years the temperature drops from several billion to hundreds of millions of degrees. Neutron stars rotate rapidly, and this leads to a number of very interesting consequences. By the way, it is the small size of the star that allows it to remain intact during rapid rotation. If its diameter were not 10, but, say, 100 kilometers, it would simply be torn apart by centrifugal forces.

We have already talked about the intriguing history of the discovery of pulsars. The idea was immediately put forward that the pulsar was a rapidly rotating neutron star, since of all the known stellar configurations, only it could remain stable, rotating at high speed. It was the study of pulsars that made it possible to come to the remarkable conclusion that neutron stars, discovered “at the tip of the pen” by theorists, actually exist in nature and they arise as a result of supernova explosions. The difficulties of detecting them in the optical range are obvious, since due to their small diameter, most neutron stars cannot be seen in the most powerful telescopes, although, as we have seen, there are exceptions - a pulsar in Crab Nebula.

So, astronomers have discovered a new class of objects - pulsars, rapidly rotating neutron stars. A natural question arises: what is the reason for such a rapid rotation of a neutron star, why, in fact, should it spin around its axis at enormous speed?

The reason for this phenomenon is simple. We know well how a skater can increase the speed of rotation when he presses his arms closer to his body. In doing so, he uses the law of conservation of angular momentum. This law is never violated, and it is precisely this law that, during a supernova explosion, increases the rotation speed of its remnant, the pulsar, many times over.

Indeed, during the collapse of a star, its mass (what is left after the explosion) does not change, but the radius decreases by about a hundred thousand times. But the angular momentum, equal to the product of the equatorial rotation speed by the mass and the radius, remains the same. The mass does not change, therefore, the speed must increase by the same hundred thousand times.

Let's look at a simple example. Our Sun rotates quite slowly around its own axis. The period of this rotation is approximately 25 days. So, if the Sun suddenly became a neutron star, its rotation period would decrease to one ten-thousandth of a second.

The second important consequence of conservation laws is that neutron stars must be very strongly magnetized. In fact, in any natural process we cannot simply destroy the magnetic field (if it already exists). Magnetic field lines are forever associated with the stellar matter, which has excellent electrical conductivity. The magnitude of the magnetic flux on the surface of the star is equal to the product of the magnetic field strength by the square of the radius of the star. This value is strictly constant. That is why, when a star contracts, the magnetic field should increase very strongly. Let us dwell on this phenomenon in some detail, since it is this phenomenon that determines many of the amazing properties of pulsars.

The magnetic field strength can be measured on the surface of our Earth. We will get a small value of about one gauss. In a good physics laboratory, magnetic fields of a million gauss can be obtained. On the surface of white dwarfs, the magnetic field strength reaches one hundred million gauss. Nearby the field is even stronger - up to ten billion gauss. But on the surface of a neutron star, nature reaches an absolute record. Here the field strength can be hundreds of thousands of billions of gauss. The void in a liter jar containing such a field would weigh about a thousand tons.

Such strong magnetic fields cannot but affect (of course, in combination with the gravitational field) the nature of the interaction of the neutron star with the surrounding matter. After all, we have not yet talked about why pulsars have enormous activity, why they emit radio waves. And not only radio waves. Today, astrophysicists are well aware of X-ray pulsars observed only in binary systems, gamma-ray sources with unusual properties, the so-called X-ray bursters.

To imagine the various mechanisms of interaction of a neutron star with matter, let us turn to the general theory of slow changes in the modes of interaction of neutron stars with the environment. Let us briefly consider the main stages of such evolution. Neutron stars - remnants of supernova explosions - initially rotate very quickly with a period of 10 -2 - 10 -3 seconds. With such rapid rotation, the star emits radio waves, electromagnetic radiation, and particles.

One of the most amazing properties of pulsars is the monstrous power of their radiation, billions of times greater than the power of radiation from the stellar interior. For example, the radio emission power of the pulsar in the “Crab” reaches 1031 erg/sec, in optics it is 1034 erg/sec, which is much more than the emission power of the Sun. This pulsar emits even more in the X-ray and gamma-ray ranges.

How do these natural energy generators work? All radio pulsars have one common property, which served as the key to unraveling the mechanism of their action. This property lies in the fact that the period of pulse emission does not remain constant, it slowly increases. It is worth noting that this property of rotating neutron stars was first predicted by theorists, and then very quickly confirmed experimentally. Thus, in 1969 it was found that the period of emission of pulsar pulses in the “Crab” is growing by 36 billionths of a second per day.

We will not talk now about how such short periods of time are measured. What is important for us is the very fact of increasing the period between pulses, which, by the way, makes it possible to estimate the age of pulsars. But still, why does a pulsar emit pulses of radio emission? This phenomenon has not been fully explained within the framework of any complete theory. But a qualitative picture of the phenomenon can nevertheless be drawn.

The thing is that the neutron star's rotation axis does not coincide with its magnetic axis. It is well known from electrodynamics that if a magnet is rotated in a vacuum around an axis that does not coincide with the magnetic one, then electromagnetic radiation will arise exactly at the frequency of rotation of the magnet. At the same time, the rotation speed of the magnet will slow down. This is understandable from general considerations, since if braking did not occur, we would simply have a perpetual motion machine.

Thus, our transmitter draws the energy of radio pulses from the rotation of the star, and its magnetic field is like a driving belt of a machine. The real process is much more complicated, since a magnet rotating in a vacuum is only partially an analogue of a pulsar. After all, a neutron star does not rotate in a vacuum; it is surrounded by a powerful magnetosphere, a plasma cloud, and this is a good conductor that makes its own adjustments to the simple and rather schematic picture we have drawn. As a result of the interaction of the pulsar’s magnetic field with the surrounding magnetosphere, narrow beams of directed radiation are formed, which, with a favorable “location of the stars,” can be observed in various parts of the galaxy, in particular on Earth.

The rapid rotation of a radio pulsar at the beginning of its life causes not only radio emission. A significant portion of the energy is also carried away by relativistic particles. As the pulsar's rotation speed decreases, the radiation pressure drops. Previously, the radiation had pushed the plasma away from the pulsar. Now the surrounding matter begins to fall on the star and extinguishes its radiation. This process can be especially effective if the pulsar is part of a binary system. In such a system, especially if it is close enough, the pulsar pulls the matter of the “normal” companion onto itself.

If the pulsar is young and full of energy, its radio emission is still able to “break through” to the observer. But the old pulsar is no longer able to fight the accretion, and it “extinguishes” the star. As the pulsar's rotation slows, other remarkable processes begin to appear. Since the gravitational field of a neutron star is very powerful, the accretion of matter releases a significant amount of energy in the form of X-rays. If in a binary system the normal companion contributes a noticeable amount of matter to the pulsar, approximately 10 -5 - 10 -6 M per year, the neutron star will be observed not as a radio pulsar, but as an X-ray pulsar.

But that is not all. In some cases, when the magnetosphere of a neutron star is close to its surface, matter begins to accumulate there, forming a kind of shell of the star. In this shell, favorable conditions can be created for the passage of thermonuclear reactions, and then we can see an X-ray burster in the sky (from the English word burst - “flash”).

As a matter of fact, this process should not look unexpected to us; we have already talked about it in relation to white dwarfs. However, the conditions on the surface of a white dwarf and a neutron star are very different, and therefore X-ray bursters are clearly associated with neutron stars. Thermo-nuclear explosions are observed by us in the form of X-ray flares and, perhaps, gamma-ray bursts. Indeed, some gamma-ray bursts may appear to be caused by thermonuclear explosions on the surface of neutron stars.

But let's return to X-ray pulsars. The mechanism of their radiation, naturally, is completely different from that of bursters. Nuclear energy sources no longer play any role here. The kinetic energy of the neutron star itself also cannot be reconciled with observational data.

Let's take the X-ray source Centaurus X-1 as an example. Its power is 10 erg/sec. Therefore, the reserve of this energy could only be enough for one year. In addition, it is quite obvious that the rotation period of the star in this case would have to increase. However, for many X-ray pulsars, unlike radio pulsars, the period between pulses decreases over time. This means that the issue here is not the kinetic energy of rotation. How do X-ray pulsars work?

We remember that they manifest themselves in double systems. It is there that accretion processes are especially effective. The speed at which matter falls onto a neutron star can reach one third the speed of light (100 thousand kilometers per second). Then one gram of the substance will release the energy of 1020 erg. And to ensure an energy release of 1037 erg/sec, it is necessary that the flow of matter onto the neutron star be 1017 grams per second. This, in general, is not very much, about one thousandth of the Earth’s mass per year.

The material supplier may be an optical companion. A stream of gas will continuously flow from part of its surface towards the neutron star. It will supply both energy and matter to the accretion disk formed around the neutron star.

Because a neutron star has a huge magnetic field, gas will “flow” along magnetic field lines towards the poles. It is there, in relatively small “spots” of the order of only one kilometer in size, that grandiose-scale processes of the creation of powerful X-ray radiation take place. X-rays are emitted by relativistic and ordinary electrons moving in the magnetic field of the pulsar. The gas falling on it can also “feed” its rotation. That is why it is precisely in X-ray pulsars that a decrease in the rotation period is observed in a number of cases.

X-ray sources included in binary systems are one of the most remarkable phenomena in space. There are few of them, probably no more than a hundred in our Galaxy, but their significance is enormous not only from the point of view, in particular for understanding type I. Binary systems provide the most natural and efficient way for matter to flow from star to star, and it is here (due to the relatively rapid change in the mass of stars) that we may encounter various options for “accelerated” evolution.

Another interesting consideration. We know how difficult, almost impossible, it is to estimate the mass of a single star. But since neutron stars are part of binary systems, it may turn out that sooner or later it will be possible to empirically (and this is extremely important!) determine the maximum mass of a neutron star, as well as obtain direct information about its origin.

Neutron stars, often called “dead” stars, are amazing objects. Their study in recent decades has become one of the most fascinating and discovery-rich areas of astrophysics. Interest in neutron stars is due not only to the mystery of their structure, but also to their colossal density and strong magnetic and gravitational fields. The matter there is in a special state, reminiscent of a huge atomic nucleus, and these conditions cannot be reproduced in earthly laboratories.

Birth at the tip of a pen

The discovery of a new elementary particle, the neutron, in 1932 led astrophysicists to wonder what role it might play in the evolution of stars. Two years later, it was suggested that supernova explosions are associated with the transformation of ordinary stars into neutron stars. Then calculations were made of the structure and parameters of the latter, and it became clear that if small stars (like our Sun) at the end of their evolution turn into white dwarfs, then heavier ones become neutron ones. In August 1967, radio astronomers, while studying the flickering of cosmic radio sources, discovered strange signals: very short, lasting about 50 milliseconds, pulses of radio emission were recorded, repeated at a strictly defined time interval (of the order of one second). This was completely different from the usual chaotic picture of random irregular fluctuations in radio emission. After a thorough check of all the equipment, we became confident that the pulses were of extraterrestrial origin. It is difficult for astronomers to be surprised by objects emitting with variable intensity, but in this case the period was so short and the signals were so regular that scientists seriously suggested that they could be news from extraterrestrial civilizations.

Therefore, the first pulsar was named LGM-1 (from the English Little Green Men “Little Green Men”), although attempts to find any meaning in the received pulses ended in vain. Soon, 3 more pulsating radio sources were discovered. Their period again turned out to be much less than the characteristic times of vibration and rotation of all known astronomical objects. Due to the pulsed nature of the radiation, new objects began to be called pulsars. This discovery literally shook up astronomy, and reports of pulsar detections began to arrive from many radio observatories. After the discovery of a pulsar in the Crab Nebula, which arose due to a supernova explosion in 1054 (this star was visible during the day, as the Chinese, Arabs and North Americans mention in their annals), it became clear that pulsars are somehow related to supernova explosions .

Most likely, the signals came from an object left after the explosion. It took a long time before astrophysicists realized that pulsars were the rapidly rotating neutron stars they had been looking for for so long.

Crab Nebula
The outbreak of this supernova (photo above), sparkling in the earth's sky brighter than Venus and visible even during the day, occurred in 1054 according to earth clocks. Almost 1,000 years is a very short period of time by cosmic standards, and yet during this time the beautiful Crab Nebula managed to form from the remains of the exploding star. This image is a composition of two pictures: one of them was obtained by the Hubble Space Optical Telescope (shades of red), the other by the Chandra X-ray telescope (blue). It is clearly seen that high-energy electrons emitting in the X-ray range very quickly lose their energy, so blue colors prevail only in the central part of the nebula.
Combining two images helps to more accurately understand the mechanism of operation of this amazing cosmic generator, emitting electromagnetic oscillations of the widest frequency range - from gamma rays to radio waves. Although most neutron stars have been detected by radio emission, they emit the bulk of their energy in the gamma-ray and x-ray ranges. Neutron stars are born very hot, but cool quickly enough, and already at a thousand years of age they have a surface temperature of about 1,000,000 K. Therefore, only young neutron stars shine in the X-ray range due to purely thermal radiation.

Pulsar physics
A pulsar is simply a huge magnetized top spinning around an axis that does not coincide with the axis of the magnet. If nothing fell on it and it did not emit anything, then its radio emission would have a rotational frequency and we would never hear it on Earth. But the fact is that this top has a colossal mass and a high surface temperature, and the rotating magnetic field creates a huge electric field, capable of accelerating protons and electrons almost to the speed of light. Moreover, all these charged particles rushing around the pulsar are trapped in its colossal magnetic field. And only within a small solid angle about the magnetic axis they can break free (neutron stars have the strongest magnetic fields in the Universe, reaching 10 10 10 14 gauss, for comparison: the earth’s field is 1 gauss, the solar one 10 50 gauss) . It is these streams of charged particles that are the source of the radio emission from which pulsars were discovered, which later turned out to be neutron stars. Since the magnetic axis of a neutron star does not necessarily coincide with the axis of its rotation, when the star rotates, a stream of radio waves propagates through space like the beam of a flashing beacon, only momentarily cutting through the surrounding darkness.

X-ray images of the Crab Nebula pulsar in its active (left) and normal (right) states

nearest neighbor
This pulsar is located only 450 light years from Earth and is a binary system of a neutron star and a white dwarf with an orbital period of 5.5 days. The soft X-ray radiation received by the ROSAT satellite is emitted by the polar ice caps PSR J0437-4715, which are heated to two million degrees. During its rapid rotation (the period of this pulsar is 5.75 milliseconds), it turns toward the Earth with one or the other magnetic pole, as a result, the intensity of the gamma ray flux changes by 33%. The bright object next to the small pulsar is a distant galaxy that, for some reason, actively glows in the X-ray region of the spectrum.

Almighty Gravity

According to modern evolutionary theory, massive stars end their lives in a colossal explosion, turning most of them into an expanding nebula of gas. As a result, what remains from a giant many times larger than our Sun in size and mass is a dense hot object about 20 km in size, with a thin atmosphere (of hydrogen and heavier ions) and a gravitational field 100 billion times greater than that of the Earth. It was called a neutron star, believing that it consists mainly of neutrons. Neutron star matter is the densest form of matter (a teaspoon of such a supernucleus weighs about a billion tons). The very short period of signals emitted by pulsars was the first and most important argument in favor of the fact that these are neutron stars, possessing a huge magnetic field and rotating at breakneck speed. Only dense and compact objects (only a few tens of kilometers in size) with a powerful gravitational field can withstand such a rotation speed without falling into pieces due to centrifugal inertial forces.

A neutron star consists of a neutron liquid mixed with protons and electrons. “Nuclear liquid,” which closely resembles the substance of atomic nuclei, is 1014 times denser than ordinary water. This huge difference is understandable, since atoms consist mostly of empty space, in which light electrons flit around a tiny, heavy nucleus. The nucleus contains almost all the mass, since protons and neutrons are 2,000 times heavier than electrons. The extreme forces generated by the formation of a neutron star compress the atoms so much that the electrons squeezed into the nuclei combine with protons to form neutrons. In this way, a star is born, consisting almost entirely of neutrons. The super-dense nuclear liquid, if brought to Earth, would explode like a nuclear bomb, but in a neutron star it is stable due to the enormous gravitational pressure. However, in the outer layers of a neutron star (as, indeed, of all stars), pressure and temperature drop, forming a solid crust about a kilometer thick. It is believed to consist mainly of iron nuclei.

Flash
The colossal X-ray flare of March 5, 1979, it turns out, occurred far beyond our Galaxy, in the Large Magellanic Cloud, a satellite of our Milky Way, located at a distance of 180 thousand light years from Earth. Joint processing of the gamma-ray burst on March 5, recorded by seven spacecraft, made it possible to quite accurately determine the position of this object, and the fact that it is located precisely in the Magellanic Cloud is today practically beyond doubt.

The event that happened on this distant star 180 thousand years ago is difficult to imagine, but it flashed then like 10 supernovae, more than 10 times the luminosity of all the stars in our Galaxy. The bright dot at the top of the figure is a long-known and well-known SGR pulsar, and the irregular outline is the most likely position of the object that flared up on March 5, 1979.

Origin of the neutron star
A supernova explosion is simply the transition of part of the gravitational energy into heat. When an old star runs out of fuel and the thermonuclear reaction can no longer heat its interior to the required temperature, a collapse of the gas cloud occurs at its center of gravity. The energy released in this process scatters the outer layers of the star in all directions, forming an expanding nebula. If the star is small, like our Sun, then an outburst occurs and a white dwarf is formed. If the mass of the star is more than 10 times that of the Sun, then such a collapse leads to a supernova explosion and an ordinary neutron star is formed. If a supernova erupts in the place of a very large star, with a mass of 20 x 40 solar, and a neutron star with a mass of more than three solar is formed, then the process of gravitational compression becomes irreversible and a black hole is formed.

Internal structure
The solid crust of the outer layers of a neutron star consists of heavy atomic nuclei arranged in a cubic lattice, with electrons flying freely between them, which is reminiscent of terrestrial metals, but only much denser.

Open question

Although neutron stars have been intensively studied for about three decades, their internal structure is not known for certain. Moreover, there is no firm certainty that they really consist mainly of neutrons. As you move deeper into the star, pressure and density increase and matter can be so compressed that it breaks down into quarks - the building blocks of protons and neutrons. According to modern quantum chromodynamics, quarks cannot exist in a free state, but are combined into inseparable “threes” and “twos”. But perhaps, at the boundary of the inner core of a neutron star, the situation changes and the quarks break out of their confinement. To further understand the nature of a neutron star and exotic quark matter, astronomers need to determine the relationship between the star's mass and its radius (average density). By studying neutron stars with satellites, it is possible to measure their mass quite accurately, but determining their diameter is much more difficult. More recently, scientists using the XMM-Newton X-ray satellite have found a way to estimate the density of neutron stars based on gravitational redshift. Another unusual thing about neutron stars is that as the mass of the star decreases, its radius increases; as a result, the most massive neutron stars have the smallest size.

Black Widow
The explosion of a supernova quite often imparts considerable speed to a newborn pulsar. Such a flying star with a decent magnetic field of its own greatly disturbs the ionized gas filling interstellar space. A kind of shock wave is formed, running in front of the star and diverging into a wide cone after it. The combined optical (blue-green part) and X-ray (shades of red) image shows that here we are dealing not just with a luminous gas cloud, but with a huge stream of elementary particles emitted by this millisecond pulsar. The linear speed of the Black Widow is 1 million km/h, it rotates around its axis in 1.6 ms, it is already about a billion years old, and it has a companion star circling around the Widow with a period of 9.2 hours. The pulsar B1957+20 received its name for the simple reason that its powerful radiation simply burns its neighbor, causing the gas that forms it to “boil” and evaporate. The red cigar-shaped cocoon behind the pulsar is the part of space where the electrons and protons emitted by the neutron star emit soft gamma rays.

The result of computer modeling makes it possible to very clearly, in cross-section, present the processes occurring near a fast-flying pulsar. The rays diverging from a bright point are a conventional image of the flow of radiant energy, as well as the flow of particles and antiparticles that emanates from a neutron star. The red outline at the border of the black space around the neutron star and the red glowing clouds of plasma is the place where the stream of relativistic particles flying almost at the speed of light meets the interstellar gas compacted by the shock wave. By braking sharply, the particles emit X-rays and, having lost most of their energy, no longer heat up the incident gas so much.

Cramp of the Giants

Pulsars are considered one of the early stages of the life of a neutron star. Thanks to their study, scientists learned about magnetic fields, the speed of rotation, and the future fate of neutron stars. By constantly monitoring the behavior of a pulsar, one can determine exactly how much energy it loses, how much it slows down, and even when it will cease to exist, having slowed down so much that it cannot emit powerful radio waves. These studies confirmed many theoretical predictions about neutron stars.

Already by 1968, pulsars with a rotation period from 0.033 seconds to 2 seconds were discovered. The periodicity of the radio pulsar pulses is maintained with amazing accuracy, and at first the stability of these signals was higher than the earth's atomic clocks. And yet, with progress in the field of time measurement, it was possible to register regular changes in their periods for many pulsars. Of course, these are extremely small changes, and only over millions of years can we expect the period to double. The ratio of the current rotation speed to the rotation deceleration is one of the ways to estimate the age of the pulsar. Despite the remarkable stability of the radio signal, some pulsars sometimes experience so-called "disturbances." In a very short time interval (less than 2 minutes), the rotation speed of the pulsar increases by a significant amount, and then after some time returns to the value that was before the “disturbance.” It is believed that the “disturbances” may be caused by a rearrangement of mass within the neutron star. But in any case, the exact mechanism is still unknown.

Thus, the Vela pulsar undergoes large “disturbances” approximately every 3 years, and this makes it a very interesting object for studying such phenomena.

Magnetars

Some neutron stars, called repeating soft gamma ray burst sources (SGRs), emit powerful bursts of "soft" gamma rays at irregular intervals. The amount of energy emitted by an SGR in a typical flare lasting a few tenths of a second can only be emitted by the Sun in a whole year. Four known SGRs are located within our Galaxy and only one is outside it. These incredible explosions of energy can be caused by starquakes - powerful versions of earthquakes when the solid surface of neutron stars is torn apart and powerful streams of protons burst from their depths, which, stuck in a magnetic field, emit gamma and X-ray radiation. Neutron stars were identified as sources of powerful gamma-ray bursts after the huge gamma-ray burst on March 5, 1979, released as much energy in the first second as the Sun emits in 1,000 years. Recent observations of one of the most active neutron stars currently appear to support the theory that irregular, powerful bursts of gamma and X-ray radiation are caused by starquakes.

In 1998, the famous SGR suddenly woke up from its “slumber,” which had shown no signs of activity for 20 years and splashed out almost as much energy as the gamma-ray flare of March 5, 1979. What struck the researchers most when observing this event was the sharp slowdown in the speed of rotation of the star, indicating its destruction. To explain powerful gamma-ray and X-ray flares, a magnetar-neutron star model with a superstrong magnetic field was proposed. If a neutron star is born spinning very quickly, then the combined influence of rotation and convection, which plays an important role in the first few seconds of the neutron star's life, can create a huge magnetic field through a complex process known as an "active dynamo" (the same way the field is created inside the Earth and the Sun). Theorists were amazed to discover that such a dynamo, operating in a hot, newborn neutron star, could create a magnetic field 10,000 times stronger than the normal field of pulsars. When the star cools (after 10 or 20 seconds), convection and the action of the dynamo stop, but this time is enough for the necessary field to arise.

The magnetic field of a rotating electrically conducting ball can be unstable, and a sharp restructuring of its structure can be accompanied by the release of colossal amounts of energy (a clear example of such instability is the periodic transfer of the Earth’s magnetic poles). Similar things happen on the Sun, in explosive events called "solar flares." In a magnetar, the available magnetic energy is enormous, and this energy is quite enough to power such giant flares as March 5, 1979 and August 27, 1998. Such events inevitably cause deep disruption and changes in the structure of not only electrical currents in the volume of the neutron star, but also its solid crust. Another mysterious type of object that emits powerful X-ray radiation during periodic explosions is the so-called anomalous X-ray pulsarsAXP. They differ from ordinary X-ray pulsars in that they emit only in the X-ray range. Scientists believe that SGR and AXP are phases of the life of the same class of objects, namely magnetars, or neutron stars, which emit soft gamma rays by drawing energy from a magnetic field. And although magnetars today remain the brainchild of theorists and there is not enough data confirming their existence, astronomers are persistently searching for the necessary evidence.

Magnetar candidates
Astronomers have already studied our home galaxy, the Milky Way, so thoroughly that it costs them nothing to depict its side view, indicating the position of the most remarkable of the neutron stars.

Scientists believe that AXP and SGR are simply two stages in the life of the same giant magnet neutron star. For the first 10,000 years, the magnetar is an SGR pulsar, visible in ordinary light and producing repeated bursts of soft X-ray radiation, and for the next millions of years it, like an anomalous AXP pulsar, disappears from the visible range and puffs only in the X-ray.

The strongest magnet
Analysis of data obtained by the RXTE satellite (Rossi X-ray Timing Explorer, NASA) during observations of the unusual pulsar SGR 1806-20 showed that this source is the most powerful magnet known to date in the Universe. The magnitude of its field was determined not only on the basis of indirect data (from the slowing down of the pulsar), but also almost directly from measuring the rotation frequency of protons in the magnetic field of the neutron star. The magnetic field near the surface of this magnetar reaches 10 15 gauss. If it were, for example, in the orbit of the Moon, all magnetic storage media on our Earth would be demagnetized. True, taking into account the fact that its mass is approximately equal to that of the Sun, this would no longer matter, since even if the Earth had not fallen on this neutron star, it would have been spinning around it like crazy, making a full revolution in just an hour.

Active dynamo
We all know that energy loves to change from one form to another. Electricity easily turns into heat, and kinetic energy into potential energy. Huge convective flows of electrically conductive magma, plasma or nuclear matter, it turns out, can also convert their kinetic energy into something unusual, for example, into a magnetic field. The movement of large masses on a rotating star in the presence of a small initial magnetic field can lead to electric currents that create a field in the same direction as the original one. As a result, an avalanche-like increase in the own magnetic field of a rotating current-conducting object begins. The greater the field, the greater the currents, the greater the currents, the greater the field and all this is due to banal convective flows, due to the fact that a hot substance is lighter than a cold one, and therefore floats up

Troubled neighborhood

The famous Chandra space observatory has discovered hundreds of objects (including in other galaxies), indicating that not all neutron stars are destined to lead a solitary life. Such objects are born in binary systems that survived the supernova explosion that created the neutron star. And sometimes it happens that single neutron stars in dense stellar regions such as globular clusters capture a companion. In this case, the neutron star will “steal” matter from its neighbor. And depending on how massive the star is to accompany it, this “theft” will cause different consequences. Gas flowing from a companion with a mass less than that of our Sun onto such a “crumb” as a neutron star cannot immediately fall due to its own angular momentum being too large, so it creates a so-called accretion disk around it from the “stolen » matter. Friction as it wraps around the neutron star and compression in the gravitational field heats the gas to millions of degrees, and it begins to emit X-rays. Another interesting phenomenon associated with neutron stars that have a low-mass companion is X-ray bursts. They usually last from several seconds to several minutes and at maximum give the star a luminosity almost 100 thousand times greater than the luminosity of the Sun.

These flares are explained by the fact that when hydrogen and helium are transferred to the neutron star from the companion, they form a dense layer. Gradually, this layer becomes so dense and hot that a thermonuclear fusion reaction begins and a huge amount of energy is released. In terms of power, this is equivalent to the explosion of the entire nuclear arsenal of earthlings on every square centimeter of the surface of a neutron star within a minute. A completely different picture is observed if the neutron star has a massive companion. The giant star loses matter in the form of stellar wind (a stream of ionized gas emanating from its surface), and the enormous gravity of the neutron star captures some of this matter. But here the magnetic field comes into its own, causing the falling matter to flow along the lines of force towards the magnetic poles.

This means that X-ray radiation is primarily generated at hot spots at the poles, and if the magnetic axis and the rotation axis of the star do not coincide, then the brightness of the star turns out to be variable - it is also a pulsar, but only an X-ray one. Neutron stars in X-ray pulsars have bright giant stars as companions. In bursters, the companions of neutron stars are faint, low-mass stars. The age of bright giants does not exceed several tens of millions of years, while the age of faint dwarf stars can be billions of years old, since the former consume their nuclear fuel much faster than the latter. It follows that bursters are old systems in which the magnetic field has weakened over time, and pulsars are relatively young, and therefore the magnetic fields in them are stronger. Perhaps bursters pulsated at some point in the past, but pulsars are yet to burst in the future.

Pulsars with the shortest periods (less than 30 milliseconds)—the so-called millisecond pulsars—are also associated with binary systems. Despite their rapid rotation, they turn out to be not the youngest, as one would expect, but the oldest.

They arise from binary systems where an old, slowly rotating neutron star begins to absorb matter from its also aged companion (usually a red giant). As matter falls onto the surface of a neutron star, it transfers rotational energy to it, causing it to spin faster and faster. This happens until the neutron star's companion, almost freed of excess mass, becomes a white dwarf, and the pulsar comes to life and begins to rotate at a speed of hundreds of revolutions per second. However, recently astronomers discovered a very unusual system, where the companion of a millisecond pulsar is not a white dwarf, but a giant bloated red star. Scientists believe that they are observing this binary system just at the stage of “liberating” the red star from excess weight and turning into a white dwarf. If this hypothesis is incorrect, then the companion star could be an ordinary globular cluster star accidentally captured by a pulsar. Almost all neutron stars that are currently known are found either in X-ray binaries or as single pulsars.

And recently, Hubble noticed in visible light a neutron star, which is not a component of a binary system and does not pulsate in the X-ray and radio range. This provides a unique opportunity to accurately determine its size and make adjustments to ideas about the composition and structure of this bizarre class of burnt-out, gravitationally compressed stars. This star was first discovered as an X-ray source and emits in this range not because it collects hydrogen gas as it moves through space, but because it is still young. It may be the remnant of one of the stars in the binary system. As a result of a supernova explosion, this binary system collapsed and the former neighbors began an independent journey through the Universe.

Baby star eater
Just as stones fall to the ground, so a large star, releasing bits of its mass, gradually moves to a small and distant neighbor, which has a huge gravitational field near its surface. If the stars did not revolve around a common center of gravity, then the gas stream could simply flow, like a stream of water from a mug, onto a small neutron star. But since the stars swirl in a circle, the falling matter must lose most of its angular momentum before it reaches the surface. And here, the mutual friction of particles moving along different trajectories and the interaction of the ionized plasma forming the accretion disk with the magnetic field of the pulsar help the process of matter fall to successfully end with an impact on the surface of the neutron star in the region of its magnetic poles.

Riddle 4U2127 solved
This star has been fooling astronomers for more than 10 years, showing strange slow variability in its parameters and flaring up differently each time. Only the latest research from the Chandra space observatory has made it possible to unravel the mysterious behavior of this object. It turned out that these were not one, but two neutron stars. Moreover, both of them have companions: one star is similar to our Sun, the other is like a small blue neighbor. Spatially, these pairs of stars are separated by a fairly large distance and live an independent life. But on the stellar sphere they are projected to almost the same point, which is why they were considered one object for so long. These four stars are located in the globular cluster M15 at a distance of 34 thousand light years.

Open question

In total, astronomers have discovered about 1,200 neutron stars to date. Of these, more than 1,000 are radio pulsars, and the rest are simply X-ray sources. Over the years of research, scientists have come to the conclusion that neutron stars are real originals. Some are very bright and calm, others periodically flare up and change with starquakes, and others exist in binary systems. These stars are among the most mysterious and elusive astronomical objects, combining the strongest gravitational and magnetic fields and extreme densities and energies. And every new discovery from their turbulent life gives scientists unique information necessary to understand the nature of Matter and the evolution of the Universe.

Universal standard
It is very difficult to send something outside the solar system, so together with the Pioneer 10 and 11 spacecraft that headed there 30 years ago, earthlings also sent messages to their brothers in mind. To draw something that will be understandable to the Extraterrestrial Mind is not an easy task; moreover, it was also necessary to indicate the return address and the date of sending the letter... How clearly the artists were able to do all this is difficult for a person to understand, but the very idea of ​​​​using radio pulsars for indicating the place and time of sending the message is brilliant. Intermittent rays of various lengths emanating from a point symbolizing the Sun indicate the direction and distance to the pulsars closest to the Earth, and the intermittency of the line is nothing more than a binary designation of their period of revolution. The longest beam points to the center of our Galaxy Milky Way. The frequency of the radio signal emitted by a hydrogen atom when the mutual orientation of the spins (direction of rotation) of the proton and electron changes is taken as the unit of time in the message.

The famous 21 cm or 1420 MHz should be known to all intelligent beings in the Universe. Using these landmarks, pointing to the “radio beacons” of the Universe, it will be possible to find earthlings even after many millions of years, and by comparing the recorded frequency of pulsars with the current one, it will be possible to estimate when these man and woman blessed the flight of the first spaceship that left the solar system.

Nikolay Andreev