About neutron stars. Pulsars and neutron stars

Neutron stars, often called “dead” stars, are amazing objects. Their study in last 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 quanta 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 high temperature surface, and the rotating magnetic field creates an electric field of enormous intensity, 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 with 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", very similar to the substance from atomic nuclei, 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. A super-dense nuclear liquid, if brought to Earth, would explode like 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 in old star the fuel runs out and the thermonuclear reaction can no longer heat its depths to the required temperature; a collapse of the gas cloud occurs, as it were, 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 border inner core At the 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 smallest size have the most massive neutron stars.

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 peculiar shock wave, 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 flow 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, and about the speed of rotation, and about future fate 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 once every 3 years, and this makes it very interesting object to study 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-ray 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 conductive ball can be unstable, and a sharp restructuring of its structure can be accompanied by the release of colossal amounts of energy ( clear example such instability 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 kinetic energy transform into something unusual, such as 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. Other interesting phenomenon, associated with neutron stars that have a low-mass companion, X-ray bursts (bursters). 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 reaction begins thermonuclear fusion and stands out great amount energy. In terms of power, this is equivalent to the explosion of everything nuclear arsenal 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 power lines to 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. Age bright giants does not exceed several tens of millions of years, while the age of faint dwarf stars can be billions of years, 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, while 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 are also associated with binary systems with the most short periods(less than 30 milliseconds) so-called millisecond pulsars. 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 former neighbors began an independent journey through the Universe.

Baby star eater
Just as stones fall to the ground, so big star, releasing its mass piece by piece, 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 round dance, the falling matter, before it reaches the surface, must lose most its angular momentum. 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 them hectic life gives scientists unique information necessary to understand the nature of Matter and the evolution of the Universe.

Universal standard
Send something outside solar system very difficult, therefore, together with the Pioneer-10 and -11 spaceships heading there 30 years ago, the earthlings also sent messages to their brothers in mind. To draw something that would 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 first flight spaceship, who left the solar system.

Nikolay Andreev

August 29th, 2013 , 10:33 pm

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 forced 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 (about 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 .

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 around 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 a strobe beacon - only momentarily cutting through the surrounding darkness.

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

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. The “nuclear liquid,” which closely resembles the substance of atomic nuclei, is 1014 times denser than ordinary water. This huge difference is understandable - after all, atoms consist mainly 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.

Origin of the neutron star
A supernova explosion is simply the conversion 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 occurs—the collapse of a gas cloud toward 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 20-40 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.

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 the neutron star. The red outline at the border of the black space around the neutron star and the red glowing clouds of plasma is where the stream of relativistic particles flying at almost 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 way 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 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 bursts of energy can be caused by starquakes—powerful versions of earthquakes that rupture the solid surface of neutron stars and release powerful streams of protons from their cores, which, stuck in a magnetic field, emit gamma and X-rays. 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-ray 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 model of a magnetar—a neutron star with a super-strong 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 are so-called anomalous X-ray pulsars - AXPs. 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.

Scientists believe that AXP and SGR are simply two stages in the life of the same giant magnet - a neutron star. For the first 10,000 years, the magnetar is an SGR - a pulsar, visible in ordinary light and producing repeated bursts of soft X-ray radiation, and for the next millions of years it, already 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 larger the field, the larger the currents, the larger the currents, the larger the field - and all this is due to banal convective flows, due to the fact that hot matter is lighter than cold matter, 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 in 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, while 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.

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.

Little 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 true 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 40 years ago, earthlings also sent messages to their brothers in mind. Drawing 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 - the 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 objects discussed in the article were discovered by accident, although scientists L. D. Landau and R. Oppenheimer predicted their existence back in 1930. We are talking about neutron stars. The characteristics and features of these cosmic luminaries will be discussed in the article.

Neutron and the star of the same name

After the prediction in the 30s of the 20th century about the existence of neutron stars and after the discovery of the neutron (1932), Baade V., together with Zwicky F., in 1933, at a congress of physicists in America, announced the possibility of the formation of an object called neutron star. This is a cosmic body that appears during a supernova explosion.

However, all calculations were only theoretical, since it was not possible to prove such a theory in practice due to the lack of appropriate astronomical equipment and the too small size of the neutron star. But in 1960, X-ray astronomy began to develop. Then, quite unexpectedly, neutron stars were discovered thanks to radio observations.

Opening

The year 1967 was significant in this area. Bell D., as a graduate student of Huish E., was able to discover a cosmic object - a neutron star. This is a body emitting constant radiation of radio wave pulses. The phenomenon was compared to a cosmic radio beacon due to the narrow directionality of the radio beam, which came from a very fast rotating object. The fact is that any other standard star would not be able to maintain its integrity at such a high rotational speed. Only neutron stars are capable of this, among which the first discovered was the pulsar PSR B1919+21.

The fate of massive stars is very different from small ones. In such luminaries there comes a moment when the gas pressure no longer balances the gravitational forces. Such processes lead to the fact that the star begins to shrink (collapse) without limit. With a star mass 1.5-2 times greater than the Sun, collapse will be inevitable. During the compression process, the gas inside the stellar core heats up. At first everything happens very slowly.

Collapse

Reaching a certain temperature, a proton can turn into neutrinos, which immediately leave the star, taking energy with them. The collapse will intensify until all protons turn into neutrinos. This creates a pulsar, or neutron star. This is a collapsing core.

During the formation of a pulsar, the outer shell receives compression energy, which will then be at a speed of more than one thousand km/sec. thrown into space. This creates a shock wave that can lead to new star formation. This one will be billions of times larger than the original. After this process, over a period of one week to a month, the star emits light in quantities exceeding an entire galaxy. This celestial body called a supernova. Its explosion leads to the formation of a nebula. At the center of the nebula is a pulsar, or neutron star. This is the so-called descendant of a star that exploded.

Visualization

In the depths of all space, amazing events take place, among which is the collision of stars. Thanks to the most complex mathematical model NASA scientists managed to visualize the riot of enormous amounts of energy and the degeneration of matter involved in it. An incredibly powerful picture of a cosmic cataclysm plays out before the eyes of observers. The probability that a collision of neutron stars will occur is very high. The meeting of two such luminaries in space begins with their entanglement in gravitational fields. Possessing enormous mass, they exchange hugs, so to speak. Upon collision, a powerful explosion occurs, accompanied by an incredibly powerful release of gamma radiation.

If we consider a neutron star separately, then this is the remnant of a supernova explosion, in which life cycle ends. The mass of a dying star is 8-30 times greater than that of the sun. The universe is often illuminated by supernova explosions. The probability that neutron stars will be found in the universe is quite high.

Meeting

It is interesting that when two stars meet, the development of events cannot be foreseen unambiguously. One of the options is described by a mathematical model proposed by NASA scientists from the Space Flight Center. The process begins with two neutron stars located at a distance of approximately 18 km from each other in outer space. By cosmic standards, neutron stars with a mass of 1.5-1.7 times the Sun are considered tiny objects. Their diameter varies within 20 km. Due to this discrepancy between volume and mass, a neutron star has a strong gravitational and magnetic field. Just imagine: a teaspoon of matter from a neutron star weighs as much as the entire Mount Everest!

Degeneration

The incredibly high gravitational waves of a neutron star around it are the reason why matter cannot exist in the form of individual atoms, which begin to collapse. The matter itself transforms into degenerate neutron matter, in which the structure of the neutrons themselves will not allow the star to pass into a singularity and then into a black hole. If the mass of degenerate matter begins to increase due to addition to it, then gravitational forces will be able to overcome the resistance of neutrons. Then nothing will prevent the destruction of the structure formed as a result of the collision of neutron stellar objects.

Mathematical model

By studying these celestial objects, scientists came to the conclusion that the density of a neutron star is comparable to the density of matter in the nucleus of an atom. Its indicators range from 1015 kg/m³ to 1018 kg/m³. Thus, the independent existence of electrons and protons is impossible. The star's matter practically consists of only neutrons.

The created mathematical model demonstrates how powerful periodic gravitational interactions arising between two neutron stars break through thin shell two stars and emit a huge amount of radiation (energy and matter) into the space surrounding them. The process of rapprochement occurs very quickly, literally in a split second. As a result of the collision, a toroidal ring of matter is formed with a newborn black hole in the center.

Important

Modeling such events is important. Thanks to them, scientists were able to understand how a neutron star and a black hole are formed, what happens when stars collide, how supernovae are born and die, and many other processes outer space. All these events are the source of the most severe chemical elements in the Universe, even heavier than iron, incapable of being formed in any other way. This speaks volumes importance neutron stars throughout the Universe.

The rotation of a celestial object of enormous volume around its axis is amazing. This process causes collapse, but at the same time the mass of the neutron star remains practically the same. If we imagine that the star will continue to contract, then, according to the law of conservation of angular momentum, the angular velocity of rotation of the star will increase to incredible values. If a star needed about 10 days to complete a full revolution, then as a result it will complete the same revolution in 10 milliseconds! These are incredible processes!

Development of collapse

Scientists are studying such processes. Perhaps we will witness new discoveries that still seem fantastic to us! But what could happen if we imagine the development of the collapse further? To make it easier to imagine, let’s take for comparison the neutron star/Earth pair and their gravitational radii. So, with continuous compression, a star can reach a state where neutrons begin to turn into hyperons. The radius of the celestial body will become so small that we will see a lump of a superplanetary body with the mass and gravitational field of a star. This can be compared to how if the earth became the size of a ping-pong ball, and the gravitational radius of our luminary, the Sun, were equal to 1 km.

If we imagine that a small lump of stellar matter has the attraction of a huge star, then it is capable of holding an entire planetary system near it. But the density of such a celestial body is too high. Rays of light gradually stop breaking through it, the body seems to go out, it ceases to be visible to the eye. Only the gravitational field does not change, which warns that there is a gravitational hole here.

Discoveries and observations

The first time neutron star mergers were recorded was quite recently: August 17. Two years ago, a black hole merger was detected. This is such an important event in the field of astrophysics that observations were simultaneously carried out by 70 space observatories. Scientists were able to verify the correctness of the hypotheses about gamma-ray bursts; they were able to observe the synthesis of heavy elements previously described by theorists.

This widespread observation of the gamma-ray burst, gravitational waves and visible light made it possible to determine the region in the sky where it occurred. significant event, and the galaxy where these stars were. This is NGC 4993.

Of course, astronomers have been observing short ones for a long time, but until now they could not say for sure about their origin. Behind the main theory was a version of the merger of neutron stars. Now it has been confirmed.

To describe a neutron star using mathematics, scientists turn to the equation of state that relates density to pressure of matter. However, there are a lot of such options, and scientists simply do not know which of the existing ones will be correct. It is hoped that gravitational observations will help resolve this issue. At the moment, the signal has not given a clear answer, but it already helps to estimate the shape of the star, which depends on the gravitational attraction to the second body (star).

Illustration copyright Getty Images Image caption The phenomenon was observed using space observatories and ground-based telescopes

Scientists have been able to detect gravitational waves from the merger of two neutron stars for the first time.

The waves were recorded by LIGO detectors in the USA and the Italian Virgo Observatory.

According to researchers, as a result of such mergers, elements such as platinum and gold appear in the Universe.

The discovery was made on August 17th. Two detectors in the United States detected the gravitational signal GW170817.

Data from the third detector in Italy made it possible to clarify the localization of the cosmic event.

“This is what we've all been waiting for,” said LIGO Laboratory Executive Director David Reitze, commenting on the discovery.

The merger occurred in the galaxy NGC4993, which is located about 130 million light years from Earth in the constellation Hydra.

The star masses ranged from 1.1 to 1.6 solar masses, which falls within the mass region of neutron stars. Their radius is 10-20 km.

Stars are called neutron stars because, during the process of gravitational compression, protons and electrons inside the star merge, resulting in an object consisting almost exclusively of neutrons.

Such objects have incredible density - a teaspoon of matter would weigh about a billion tons.

Illustration copyright NSF/LIGO/SONOMA STATE UNIVERSITY Image caption The merger of neutron stars in the minds of scientists looks something like this (pictured is a computer model)

The LIGO laboratory in Livingston, Louisiana, is a small building from which two pipes extend at right angles - the arms of the interferometer. Inside each of them there is a laser beam, recording changes in the length of which gravitational waves can be detected.

The LIGO detector, set in the middle of vast forests, was designed to detect gravitational waves that generate large-scale cosmic cataclysms such as neutron star mergers.

The detector was upgraded four years ago, and since then it has detected black hole collisions four times.

Gravitational waves that result from large-scale events in space, lead to the emergence of temporal-spatial curvatures, somewhat similar to ripples on water.

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Discovery of the year: what does a neutron star collision sound like?

They stretch and compress all the matter they pass through to an almost insignificant degree - less than the width of one atom.

“I’m delighted with what we’ve done. I first started working on gravitational waves in Glasgow while I was still a student. Many years have passed since then, there have been ups and downs, but now everything has come together,” says LIGO worker, Professor Norna Robertson.

“Over the past few years, we have first detected the merger of black holes and then neutron stars, and I feel like we are opening up a new field for research,” she adds.

The existence of gravitational waves was predicted by Einstein's general theory of relativity
It took decades to develop the technology that made it possible to record the waves.
Gravitational waves are distortions in time and space that arise as a result of large-scale events in space
Rapidly accelerating matter generates gravitational waves that travel at the speed of light
Among the visible sources of waves are mergers of neutron stars and “black holes.”
Wave research opens up a fundamentally new field for research

Scientists believed that the release of energy on such a scale leads to the emergence rare elements- such as gold and platinum.

According to Dr Kate Maguire from Queen's University Belfast, who analyzed the first outbreaks that arose from the merger, this theory has now been proven.

"Using the world's most powerful telescopes, we discovered that this neutron star merger produced a high-velocity release of heavy chemical elements such as gold and platinum into space," says Maguire.

"These new results make significant progress toward resolving a long-standing dispute about where elements heavier than iron come from on the periodic table," she adds.

New frontiers

Observations of the neutron star collision also confirmed the theory that it is accompanied by short bursts of gamma rays.

By combining the information collected about the gravitational waves resulting from the collision with data on light radiation collected using telescopes, scientists used a previously unused method to measure the rate of expansion of the Universe.

One of the most influential theoretical physicists on the planet, Professor Stephen Hawking, speaking to the BBC, called it "the first rung on the ladder" to a new way of measuring distances in the Universe.

"New ways of observing the universe tend to lead to surprises, many of which cannot be foreseen. We are still rubbing our eyes, or rather, clearing our ears, after hearing the sound of gravitational waves for the first time," Hawking said.

Illustration copyright N.S.F. Image caption LIGO Observatory complex in Livingston. “Shoulders” extend from the building - pipes, inside of which laser beams pass in a vacuum.

Now the equipment of the LIGO complex is being modernized. In a year, it will become twice as sensitive, and will be able to scan a section of space that is eight times larger than it is now.

Scientists believe that in the future, observations of collisions between black holes and neutron stars will become commonplace. They also hope to learn to observe objects that they cannot even imagine today, and begin new era in astronomy.

A pulsar (pink) can be seen at the center of the M82 galaxy.

Explore pulsars and neutron stars The Universe: description and characteristics with photos and videos, structure, rotation, density, composition, mass, temperature, search.

Pulsars

Pulsars They are spherical compact objects, the dimensions of which do not extend beyond the boundaries of a large city. The surprising thing is that with such a volume they exceed the solar mass in terms of mass. They are used to study extreme states of matter, detect planets beyond our system, and measure cosmic distances. In addition, they helped find gravitational waves that indicate energetic events, such as supermassive collisions. First discovered in 1967.

What is a pulsar?

If you look for a pulsar in the sky, it appears to be an ordinary twinkling star following a certain rhythm. In fact, their light does not flicker or pulsate, and they do not appear as stars.

The pulsar produces two persistent, narrow beams of light in opposite directions. The flickering effect is created because they rotate (beacon principle). At this moment, the beam hits the Earth and then turns again. Why is this happening? The fact is that the light beam of a pulsar is usually not aligned with its rotation axis.

If the blinking is generated by rotation, then the speed of the pulses reflects the speed at which the pulsar is spinning. A total of 2,000 pulsars were found, most of which rotate once per second. But there are approximately 200 objects that manage to make a hundred revolutions in the same time. The fastest ones are called millisecond ones, because their number of revolutions per second is equal to 700.

Pulsars cannot be considered stars, at least “living”. These are more likely neutron stars that form after massive star the fuel runs out and it collapses. As a result, a strong explosion is created - a supernova, and the remaining dense material is transformed into a neutron star.

The diameter of pulsars in the Universe reaches 20-24 km, and their mass is twice that of the Sun. To give you an idea, a piece of such an object the size of a sugar cube will weigh 1 billion tons. That is, something as heavy as Everest fits in your hand! True, there is an even denser object - a black hole. The most massive reaches 2.04 solar masses.

Pulsars have a strong magnetic field, which is 100 million to 1 quadrillion times stronger than Earth's. For a neutron star to start emitting light like a pulsar, it must have the right ratio of magnetic field strength and rotation speed. It happens that a beam of radio waves may not pass through the field of view of a ground-based telescope and remain invisible.

Radio pulsars

Astrophysicist Anton Biryukov on the physics of neutron stars, slowing down rotation and the discovery of gravitational waves:

Why do pulsars rotate?

The slowness of a pulsar is one rotation per second. The fastest ones accelerate to hundreds of revolutions per second and are called millisecond. The rotation process occurs because the stars from which they were formed also rotated. But to get to that speed, you need an additional source.

Researchers believe that millisecond pulsars were formed by stealing energy from a neighbor. You may notice the presence of a foreign substance that increases the rotation speed. And that's not a good thing for the injured companion, which could one day be completely consumed by the pulsar. Such systems are called black widows (after a dangerous type of spider).

Pulsars are capable of emitting light in several wavelengths (from radio to gamma rays). But how do they do it? Scientists cannot yet find an exact answer. It is believed that a separate mechanism is responsible for each wavelength. Beacon-like beams are made of radio waves. They are bright and narrow and resemble coherent light, where the particles form a focused beam.

The faster the rotation, the weaker the magnetic field. But the rotation speed is enough for them to emit rays as bright as slow ones.

During rotation, the magnetic field creates an electric field, which can bring charged particles into a mobile state (electric current). The area above the surface where the magnetic field dominates is called the magnetosphere. Here, charged particles are accelerated to incredibly high speeds due to a strong electric field. Each time they accelerate, they emit light. It is displayed in optical and x-ray ranges.

What about gamma rays? Research suggests that their source should be sought elsewhere near the pulsar. And they will resemble a fan.

Search for pulsars

Radio telescopes remain the main method for searching for pulsars in space. They are small and faint compared to other objects, so you have to scan the entire sky and gradually these objects get into the lens. Most were found using the Parkes Observatory in Australia. Much new data will be available from the Square Kilometer Array Antenna (SKA) starting in 2018.

In 2008, the GLAST telescope was launched, which found 2050 gamma-ray emitting pulsars, of which 93 were millisecond. This telescope is incredibly useful because it scans the entire sky, while others highlight only small areas along the plane.

Finding different wavelengths can be challenging. The fact is that radio waves are incredibly powerful, but they may simply not fall into the telescope lens. But gamma radiation spreads across more of the sky, but is inferior in brightness.

Scientists now know of the existence of 2,300 pulsars, found through radio waves and 160 through gamma rays. There are also 240 millisecond pulsars, of which 60 produce gamma rays.

Using pulsars

Pulsars are not just amazing space objects, but also useful tools. The emitted light can tell a lot about internal processes. That is, researchers are able to understand the physics of neutron stars. These objects have such high pressure that the behavior of matter differs from the usual. The strange content of neutron stars is called “nuclear paste.”

Pulsars bring many benefits due to the precision of their pulses. Scientists know specific objects and perceive them as cosmic clocks. This is how speculation about the presence of other planets began to appear. In fact, the first exoplanet found was orbiting a pulsar.

Don’t forget that pulsars continue to move while they “blink”, which means they can be used to measure cosmic distances. They were also involved in testing Einstein's theory of relativity, like moments with gravity. But the regularity of the pulsation can be disrupted by gravitational waves. This was noticed in February 2016.

Pulsar Cemeteries

Gradually, all pulsars slow down. The radiation is powered by the magnetic field created by the rotation. As a result, it also loses its power and stops sending beams. Scientists have drawn a special line where gamma rays can still be detected in front of radio waves. As soon as the pulsar falls below, it is written off in the pulsar graveyard.

If a pulsar was formed from supernova remnants, then it has a huge energy reserve and fast speed rotation. Examples include the young object PSR B0531+21. It can remain in this phase for several hundred thousand years, after which it will begin to lose speed. Middle-aged pulsars make up the majority of the population and produce only radio waves.

However, a pulsar can extend its life if there is a satellite nearby. Then it will pull out its material and increase the rotation speed. Such changes can occur at any time, which is why the pulsar is capable of rebirth. Such a contact is called a low-mass X-ray binary system. The oldest pulsars are millisecond ones. Some reach billions of years of age.

Neutron stars

Neutron stars- rather mysterious objects, exceeding the solar mass by 1.4 times. They are born after the explosion of larger stars. Let's get to know these formations better.

When a star 4-8 times more massive than the Sun explodes, a high-density core remains and continues to collapse. Gravity pushes so hard on a material that it causes protons and electrons to fuse together to become neutrons. This is how a high-density neutron star is born.

These massive objects can reach a diameter of only 20 km. To give you an idea of density, just one scoop of neutron star material would weigh a billion tons. The gravity on such an object is 2 billion times stronger than Earth's, and the power is enough for gravitational lensing, allowing scientists to view the back of the star.

The shock from the explosion leaves a pulse that causes the neutron star to spin, reaching several revolutions per second. Although they can accelerate up to 43,000 times per minute.

Boundary layers near compact objects

Astrophysicist Valery Suleymanov on the emergence of accretion disks, stellar wind and matter around neutron stars:

The interior of neutron stars

Astrophysicist Sergei Popov on extreme states of matter, the composition of neutron stars and methods for studying the interior:

When a neutron star is part of a binary system where a supernova has exploded, the picture is even more impressive. If the second star is inferior in mass to the Sun, then it pulls the mass of the companion into the “Roche lobe”. This is a spherical cloud of material orbiting a neutron star. If the satellite was 10 times larger than the solar mass, then the mass transfer is also adjusted, but not so stable. The material flows along the magnetic poles, heats up and creates X-ray pulsations.

By 2010, 1,800 pulsars had been found using radio detection and 70 using gamma rays. Some specimens even had planets.

Types of Neutron Stars

Some representatives of neutron stars have jets of material flowing almost at the speed of light. When they fly past us, they flash like the light of a beacon. Because of this, they are called pulsars.

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