Black holes: the story of the discovery of the most mysterious objects in the Universe that we will never see. What is a black hole in space
Mysterious and elusive black holes. The laws of physics confirm the possibility of their existence in the universe, but many questions still remain. Numerous observations show that holes exist in the universe and there are more than a million of these objects.
What are black holes?
Back in 1915, when solving Einstein’s equations, such a phenomenon as “black holes” was predicted. However, the scientific community became interested in them only in 1967. They were then called “collapsed stars”, “frozen stars”.
Nowadays, a black hole is a region of time and space that has such gravity that even a ray of light cannot escape from it.
How are black holes formed?
There are several theories for the appearance of black holes, which are divided into hypothetical and realistic. The simplest and most widespread realistic one is the theory of gravitational collapse of large stars.
When a sufficiently massive star, before “death,” grows in size and becomes unstable, using up its last fuel. At the same time, the mass of the star remains unchanged, but its size decreases as the so-called densification occurs. In other words, when compacted, the heavy core “falls” into itself. In parallel with this, compaction leads to a sharp increase in the temperature inside the star and the outer layers of the celestial body tear off, from which new stars are formed. At the same time, in the center of the star, the core falls into its own “center”. As a result of the action of gravitational forces, the center collapses to a point - that is, the gravitational forces are so strong that they absorb the compacted core. This is how a black hole is born, which begins to distort space and time so that even light cannot escape from it.
At the center of all galaxies is a supermassive black hole. According to Einstein's theory of relativity:
“Any mass distorts space and time.”
Now imagine how much a black hole distorts time and space, because its mass is enormous and at the same time squeezed into an ultra-small volume. This ability causes the following oddity:
“Black holes have the ability to practically stop time and compress space. Because of this extreme distortion, the holes become invisible to us.”
If black holes are not visible, how do we know they exist?
Yes, even though a black hole is invisible, it should be noticeable due to the matter that falls into it. As well as stellar gas, which is attracted by a black hole; when approaching the event horizon, the temperature of the gas begins to rise to ultra-high values, which leads to a glow. This is why black holes glow. Thanks to this, albeit weak, glow, astronomers and astrophysicists explain the presence in the center of the galaxy of an object with a small volume but a huge mass. Currently, as a result of observations, about 1000 objects have been discovered that are similar in behavior to black holes.
Black holes and galaxies
How can black holes affect galaxies? This question plagues scientists all over the world. There is a hypothesis according to which it is the black holes located in the center of the galaxy that influence its shape and evolution. And that when two galaxies collide, black holes merge and during this process such a huge amount of energy and matter is released that new stars are formed.
Types of black holes
- According to existing theory, there are three types of black holes: stellar, supermassive, and miniature. And each of them was formed in a special way.
- - Black holes of stellar masses, it grows to enormous sizes and collapses.
- Supermassive black holes, which can have a mass equivalent to millions of Suns, are likely to exist at the centers of almost all galaxies, including our Milky Way. Scientists still have different hypotheses for the formation of supermassive black holes. So far, only one thing is known - supermassive black holes are a by-product of the formation of galaxies. Supermassive black holes - they differ from ordinary ones in that they have a very large size, but paradoxically low density. - - No one has yet been able to detect a miniature black hole that would have a mass less than the Sun. It is possible that miniature holes could have formed shortly after the "Big Bang", which is the exact beginning of the existence of our universe (about 13.7 billion years ago).
- - Quite recently, a new concept was introduced as “white black holes”. This is still a hypothetical black hole, which is the opposite of a black hole. Stephen Hawking actively studied the possibility of the existence of white holes.
- - Quantum black holes - they exist only in theory so far. Quantum black holes can be formed when ultra-small particles collide as a result of a nuclear reaction.
- - Primary black holes are also a theory. They were formed immediately after their origin.
At the moment, there are a large number of open questions that have yet to be answered by future generations. For example, can so-called “wormholes” really exist, with the help of which one can travel through space and time. What exactly happens inside a black hole and what laws these phenomena obey. And what about the disappearance of information in a black hole?
Both for scientists of past centuries and for researchers of our time, the greatest mystery of the cosmos is the black hole. What's inside this completely unfamiliar system to physics? What laws apply there? How does time pass in a black hole, and why can’t even light quanta escape from there? Now we will try, of course, from the point of view of theory and not practice, to understand what is inside a black hole, why it, in principle, was formed and exists, how it attracts the objects that surround it.
First, let's describe this object
So, a black hole is a certain region of space in the Universe. It is impossible to single it out as a separate star or planet, since it is neither a solid nor a gaseous body. Without a basic understanding of what spacetime is and how these dimensions can change, it is impossible to comprehend what is inside a black hole. The point is that this area is not just a spatial unit. which distorts both the three dimensions we know (length, width and height) and the timeline. Scientists are confident that in the horizon region (the so-called area surrounding the hole), time takes on a spatial meaning and can move both forward and backward.
Let's learn the secrets of gravity
If we want to understand what's inside a black hole, let's take a closer look at what gravity is. It is this phenomenon that is key in understanding the nature of the so-called “wormholes”, from which even light cannot escape. Gravity is the interaction between all bodies that have a material basis. The strength of such gravity depends on the molecular composition of bodies, on the concentration of atoms, as well as on their composition. The more particles collapse in a certain area of space, the greater the gravitational force. This is inextricably linked to the Big Bang Theory, when our Universe was the size of a pea. This was a state of maximum singularity, and as a result of a flash of light quanta, space began to expand due to the fact that particles repelled each other. Scientists describe a black hole exactly the opposite. What is inside such a thing in accordance with the TBZ? A singularity that is equal to the indicators inherent in our Universe at the moment of its birth.
How does matter get into a wormhole?
There is an opinion that a person will never be able to understand what is happening inside a black hole. Because once there, he will be literally crushed by gravity and the force of gravity. Actually this is not true. Yes, indeed, a black hole is a region of singularity where everything is compressed to the maximum. But this is not at all a “space vacuum cleaner” that can suck in all the planets and stars. Any material object that finds itself on the event horizon will observe a strong distortion of space and time (for now, these units stand separately). The Euclidean system of geometry will begin to malfunction, in other words, they will intersect, and the outlines of stereometric figures will no longer be familiar. As for time, it will gradually slow down. The closer you get to the hole, the slower the clock will go relative to Earth time, but you won't notice it. When falling into a wormhole, the body will fall at zero speed, but this unit will be equal to infinity. curvature, which equates the infinite to zero, which finally stops time in the region of singularity.
Reaction to emitted light
The only object in space that attracts light is a black hole. What is inside it and in what form it is there is unknown, but it is believed that it is pitch darkness, which is impossible to imagine. Light quanta, getting there, do not simply disappear. Their mass is multiplied by the mass of the singularity, which makes it even larger and enlarges it. Thus, if inside the wormhole you turn on a flashlight to look around, it will not glow. The emitted quanta will constantly multiply by the mass of the hole, and you, roughly speaking, will only make your situation worse.
Black holes at every step
As we have already figured out, the basis of formation is gravity, the magnitude of which there is millions of times greater than on Earth. An accurate idea of what a black hole is was given to the world by Karl Schwarzschild, who, in fact, discovered the very event horizon and the point of no return, and also established that zero in a state of singularity is equal to infinity. In his opinion, a black hole can form at any point in space. In this case, a certain material object having a spherical shape must reach the gravitational radius. For example, the mass of our planet must fit into the volume of one pea in order to become a black hole. And the Sun should have a diameter of 5 kilometers with its mass - then its state will become singular.
The horizon for the formation of a new world
The laws of physics and geometry work perfectly on earth and in outer space, where space is close to a vacuum. But they completely lose their significance on the event horizon. This is why, from a mathematical point of view, it is impossible to calculate what is inside a black hole. The pictures that you can come up with if you bend space in accordance with our ideas about the world are probably far from the truth. It has only been established that time here turns into a spatial unit and, most likely, some more are added to the existing dimensions. This makes it possible to believe that inside a black hole (a photo, as you know, will not show this, since the light there eats itself) completely different worlds are formed. These Universes may be composed of antimatter, which is currently unknown to scientists. There are also versions that the sphere of no return is just a portal that leads either to another world or to other points in our Universe.
Birth and death
Much more than the existence of a black hole is its creation or disappearance. A sphere that distorts space-time, as we have already found out, is formed as a result of collapse. This could be the explosion of a large star, a collision of two or more bodies in space, and so on. But how did matter that could theoretically be touched become a domain of time distortion? The puzzle is a work in progress. But it is followed by a second question - why do such spheres of no return disappear? And if black holes evaporate, then why doesn’t that light and all the cosmic matter that they sucked in come out of them? When matter in the singularity zone begins to expand, gravity gradually decreases. As a result, the black hole simply dissolves, and ordinary vacuum outer space remains in its place. Another mystery follows from this - where did everything that got into it go?
Is gravity our key to a happy future?
Researchers are confident that the energy future of humanity can be shaped by a black hole. What is inside this system is still unknown, but it has been established that at the event horizon any matter is transformed into energy, but, of course, partially. For example, a person, finding himself near the point of no return, will give up 10 percent of his matter for processing into energy. This figure is simply colossal; it became a sensation among astronomers. The fact is that on Earth, only 0.7 percent of matter is converted into energy.
Every person who gets acquainted with astronomy sooner or later experiences a strong curiosity about the most mysterious objects of the Universe - black holes. These are real lords of darkness, capable of “swallowing” any atom passing nearby and not allowing even light to escape - their attraction is so powerful. These objects pose a real challenge for physicists and astronomers. The former cannot yet understand what happens to the matter that falls inside the black hole, and the latter, although they explain the most energy-consuming phenomena in space by the existence of black holes, have never had the opportunity to observe any of them directly. We will tell you about these interesting celestial objects, find out what has already been discovered and what remains to be learned in order to lift the veil of secrecy.
What is a black hole?
The name “black hole” (in English - black hole) was proposed in 1967 by the American theoretical physicist John Archibald Wheeler (see photo on the left). It served to designate a celestial body, the attraction of which is so strong that even light does not let go of itself. That is why it is “black” because it does not emit light.
Indirect observations
This is the reason for such mystery: since black holes do not glow, we cannot see them directly and are forced to look for and study them using only indirect evidence that their existence leaves in the surrounding space. In other words, if a black hole engulfs a star, we cannot see the black hole, but we can observe the devastating effects of its powerful gravitational field.
Laplace's intuition
Although the expression “black hole” to denote the hypothetical final stage of the evolution of a star that has collapsed into itself under the influence of gravity is relatively recent, the idea of the possibility of the existence of such bodies arose more than two centuries ago. The Englishman John Michell and the Frenchman Pierre-Simon de Laplace independently hypothesized the existence of “invisible stars”; at the same time, they were based on the usual laws of dynamics and Newton’s law of universal gravitation. Today, black holes have received their correct description based on Einstein's general theory of relativity.
In his work “Exposition of the System of the World” (1796), Laplace wrote: “A bright star of the same density as the Earth, with a diameter 250 times greater than the diameter of the Sun, would, thanks to its gravitational attraction, prevent light rays from reaching us. Therefore, it is possible that the largest and brightest celestial bodies are invisible for this reason.”
Invincible gravity
Laplace's idea was based on the concept of escape velocity (second cosmic velocity). A black hole is such a dense object that its gravity can hold back even light, which develops the highest speed in nature (almost 300,000 km/s). In practice, escaping from a black hole requires speeds greater than the speed of light, but this is impossible!
This means that a star of this kind will be invisible, since even light will not be able to overcome its powerful gravity. Einstein explained this fact through the phenomenon of light bending under the influence of a gravitational field. In reality, near a black hole, space-time is so curved that the trajectories of light rays also close on themselves. In order to turn the Sun into a black hole, we will have to concentrate all of its mass in a ball with a radius of 3 km, and the Earth will have to turn into a ball with a radius of 9 mm!
Types of black holes
Just about ten years ago, observations suggested the existence of two types of black holes: stellar, whose mass is comparable to the mass of the Sun or slightly exceeds it, and supermassive, whose mass ranges from several hundred thousand to many millions of solar masses. However, relatively recently, X-ray images and high-resolution spectra obtained from artificial satellites such as Chandra and XMM-Newton brought to the fore a third type of black hole - with an average mass exceeding the mass of the Sun by thousands of times.
Stellar black holes
Stellar black holes became known earlier than others. They are formed when a large-mass star, at the end of its evolutionary path, exhausts its reserves of nuclear fuel and collapses into itself due to its own gravity. An explosion that shakes a star (a phenomenon known as a “supernova explosion”) has catastrophic consequences: if the star’s core is more than 10 times the mass of the Sun, no nuclear force can resist the gravitational collapse that will result in the creation of a black hole.
Supermassive black holes
Supermassive black holes, first noted in the nuclei of some active galaxies, have a different origin. There are several hypotheses regarding their birth: a stellar black hole, which over the course of millions of years devours all the stars around it; a cluster of black holes merging together; a colossal gas cloud collapsing directly into a black hole. These black holes are among the most energetic objects in space. They are located at the centers of many, if not all, galaxies. Our Galaxy also has such a black hole. Sometimes, due to the presence of such a black hole, the cores of these galaxies become very bright. Galaxies with black holes at the center, surrounded by large amounts of falling matter and therefore capable of producing colossal amounts of energy, are called "active" and their cores are called "active galactic nuclei" (AGN). For example, quasars (the most distant cosmic objects from us that are accessible to our observation) are active galaxies in which we see only a very bright core.
Medium and mini
Another mystery remains the medium-mass black holes, which, according to recent research, may be at the center of some globular clusters, such as M13 and NCC 6388. Many astronomers are skeptical about these objects, but some new research suggests the presence of black holes medium-sized even near the center of our Galaxy. English physicist Stephen Hawking also put forward a theoretical assumption about the existence of a fourth type of black hole - a “mini-hole” with a mass of only a billion tons (which is approximately equal to the mass of a large mountain). We are talking about primary objects, that is, those that appeared in the first moments of the life of the Universe, when the pressure was still very high. However, not a single trace of their existence has yet been discovered.
How to find a black hole
Just a few years ago, a light came on over black holes. Thanks to constantly improving instruments and technologies (both ground-based and space-based), these objects are becoming less and less mysterious; more precisely, the space surrounding them becomes less mysterious. In fact, since the black hole itself is invisible, we can only recognize it if it is surrounded by enough matter (stars and hot gas) orbiting around it at a short distance.
Watching binary systems
Some stellar black holes have been discovered by observing the orbital motion of a star around an unseen companion in a binary system. Close binary systems (that is, consisting of two stars very close to each other), in which one of the companions is invisible, are a favorite object of observation for astrophysicists searching for black holes.
An indication of the presence of a black hole (or neutron star) is the strong emission of X-rays caused by a complex mechanism that can be schematically described as follows. Thanks to its powerful gravity, a black hole can rip matter out of its companion star; this gas spreads out into a flat disk and spirals down into the black hole. Friction resulting from collisions between particles of falling gas heats the inner layers of the disk to several million degrees, which causes powerful X-ray radiation.
X-ray observations
X-ray observations of objects in our Galaxy and neighboring galaxies, carried out for several decades, have made it possible to detect compact binary sources, about a dozen of which are systems containing black hole candidates. The main problem is determining the mass of an invisible celestial body. The mass (although not very precise) can be found by studying the motion of the companion or, much more difficult, by measuring the intensity of the X-ray radiation of the falling material. This intensity is related by an equation to the mass of the body on which this substance falls.
Nobel laureate
Something similar can be said for supermassive black holes observed in the cores of many galaxies, the masses of which are estimated by measuring the orbital velocities of the gas falling into the black hole. In this case, caused by the powerful gravitational field of a very large object, a rapid increase in the speed of gas clouds orbiting in the center of galaxies is detected by observations in the radio range, as well as in optical rays. Observations in the X-ray range can confirm the increased release of energy caused by matter falling into the black hole. Research in X-rays was started in the early 1960s by the Italian Riccardo Giacconi, who worked in the USA. His Nobel Prize in 2002 recognized his "pioneering contributions to astrophysics leading to the discovery of X-ray sources in space."
Cygnus X-1: first candidate
Our Galaxy is not immune to the presence of candidate black hole objects. Fortunately, none of these objects are close enough to us to pose a threat to the existence of Earth or the solar system. Despite the large number of compact X-ray sources that have been identified (and these are the most likely candidates for black holes), we have no confidence that they actually contain black holes. The only one among these sources that does not have an alternative version is the close binary system Cygnus X-1, that is, the brightest source of X-ray radiation in the constellation Cygnus.
Massive stars
This system, whose orbital period is 5.6 days, consists of a very bright blue star of large size (its diameter is 20 times that of the Sun, and its mass is about 30 times larger), easily visible even in your telescope, and an invisible second star, the mass of which is estimated at several solar masses (up to 10). Located 6,500 light-years away, the second star would be perfectly visible if it were an ordinary star. Its invisibility, the powerful X-ray emission produced by the system and, finally, the mass estimate lead most astronomers to believe that this is the first confirmed discovery of a stellar black hole.
Doubts
However, there are also skeptics. Among them is one of the largest researchers of black holes, physicist Stephen Hawking. He even made a bet with his American colleague Keel Thorne, an ardent supporter of classifying the Cygnus X-1 object as a black hole.
The debate over the identity of the Cygnus X-1 object is not Hawking's only bet. Having devoted several nine years to theoretical studies of black holes, he became convinced of the fallacy of his previous ideas about these mysterious objects. In particular, Hawking assumed that matter, after falling into a black hole, disappears forever, and with it all of its information luggage disappears. He was so sure of this that he made a bet on this topic in 1997 with his American colleague John Preskill.
Admitting a mistake
On July 21, 2004, in his speech at the Congress on the Theory of Relativity in Dublin, Hawking admitted that Preskill was right. Black holes do not lead to the complete disappearance of matter. Moreover, they have a certain kind of “memory”. They may well contain traces of what they have consumed. Thus, by “evaporating” (that is, slowly emitting radiation due to the quantum effect), they can return this information to our Universe.
Black holes in the Galaxy
Astronomers still have many doubts about the presence of stellar black holes (like the one belonging to the binary system Cygnus X-1) in our Galaxy; but there is much less doubt about supermassive black holes.
In the center
Our Galaxy has at least one supermassive black hole. Its source, known as Sagittarius A*, is precisely localized in the center of the plane of the Milky Way. Its name is explained by the fact that it is the most powerful radio source in the constellation Sagittarius. It is in this direction that both the geometric and physical centers of our galactic system are located. Located about 26,000 light-years away, the supermassive black hole associated with radio wave source Sagittarius A* has a mass estimated at about 4 million solar masses, contained in a space the volume of which is comparable to the volume of the solar system. Its relative proximity to us (it is by far the closest supermassive black hole to Earth) has led to the object being studied particularly closely in recent years by the Chandra space observatory. It turned out, in particular, that it is also a powerful source of X-ray radiation (but not as powerful as sources in active galactic nuclei). Sagittarius A* may be a dormant remnant of what was the active core of our Galaxy millions or billions of years ago.
Second black hole?
However, some astronomers believe that there is another surprise in our Galaxy. We are talking about a second black hole of average mass, holding together a cluster of young stars and preventing them from falling into a supermassive black hole located in the center of the Galaxy itself. How can it be that at a distance of less than one light year from it there could be a star cluster that is barely 10 million years old, that is, by astronomical standards, very young? According to the researchers, the answer is that the cluster was not born there (the environment around the central black hole is too hostile for star formation), but was “pulled” there due to the existence of a second black hole inside it, which has an average mass.
In orbit
Individual stars in the cluster, attracted by the supermassive black hole, began to shift towards the galactic center. However, instead of scattering into space, they remain gathered together thanks to the gravitational pull of a second black hole located at the center of the cluster. The mass of this black hole can be estimated based on its ability to hold an entire star cluster on a leash. A medium-sized black hole apparently takes about 100 years to orbit the central black hole. This means that long-term observations over many years will allow us to “see” it.
In order for a black hole to form, it is necessary to compress a body to a certain critical density so that the radius of the compressed body is equal to its gravitational radius. The value of this critical density is inversely proportional to the square of the black hole's mass.
For a typical stellar mass black hole ( M=10M sun) gravitational radius is 30 km, and the critical density is 2·10 14 g/cm 3, that is, two hundred million tons per cubic centimeter. This density is very high compared to the average density of the Earth (5.5 g/cm3), it is equal to the density of the substance of the atomic nucleus.
For a black hole at the galactic core ( M=10 10 M sun) gravitational radius is 3·10 15 cm = 200 AU, which is five times the distance from the Sun to Pluto (1 astronomical unit - the average distance from the Earth to the Sun - is equal to 150 million km or 1.5·10 13 cm). The critical density in this case is equal to 0.2·10 –3 g/cm 3 , which is several times less than the density of air, equal to 1.3·10 –3 g/cm 3 (!).
For the Earth ( M=3·10 –6 M sun), the gravitational radius is close to 9 mm, and the corresponding critical density is monstrously high: ρ cr = 2·10 27 g/cm 3, which is 13 orders of magnitude higher than the density of the atomic nucleus.
If we take some imaginary spherical press and compress the Earth, maintaining its mass, then when we reduce the radius of the Earth (6370 km) by four times, its second escape velocity will double and become equal to 22.4 km/s. If we compress the Earth so that its radius becomes approximately 9 mm, then the second cosmic velocity will take on a value equal to the speed of light c= 300000 km/s.
Further, a press will not be needed - the Earth, compressed to such a size, will already compress itself. In the end, a black hole will form in place of the Earth, the radius of the event horizon of which will be close to 9 mm (if we neglect the rotation of the resulting black hole). In real conditions, of course, there is no super-powerful press - gravity “works”. This is why black holes can only form when the interiors of very massive stars collapse, in which gravity is strong enough to compress matter to a critical density.
Evolution of stars
Black holes form at the final stages of the evolution of massive stars. In the depths of ordinary stars, thermonuclear reactions occur, enormous energy is released and a high temperature is maintained (tens and hundreds of millions of degrees). Gravitational forces tend to compress the star, and the pressure forces of hot gas and radiation resist this compression. Therefore, the star is in hydrostatic equilibrium.
In addition, a star can exist in thermal equilibrium, when the energy release due to thermonuclear reactions at its center is exactly equal to the power emitted by the star from the surface. As the star contracts and expands, the thermal equilibrium is disrupted. If the star is stationary, then its equilibrium is established in such a way that the negative potential energy of the star (the energy of gravitational compression) in absolute value is always twice the thermal energy. Because of this, the star has an amazing property - negative heat capacity. Ordinary bodies have a positive heat capacity: a heated piece of iron, cooling down, that is, losing energy, lowers its temperature. For a star, the opposite is true: the more energy it loses in the form of radiation, the higher the temperature at its center becomes.
This strange, at first glance, feature has a simple explanation: the star, as it radiates, slowly contracts. During compression, potential energy is converted into kinetic energy of falling layers of the star, and its interior heats up. Moreover, the thermal energy acquired by the star as a result of compression is twice as much as the energy lost in the form of radiation. As a result, the temperature of the star’s interior increases, and continuous thermonuclear synthesis of chemical elements occurs. For example, the reaction of converting hydrogen into helium in the current Sun occurs at a temperature of 15 million degrees. When, after 4 billion years, in the center of the Sun, all hydrogen turns into helium, for the further synthesis of carbon atoms from helium atoms, a significantly higher temperature will be required, about 100 million degrees (the electrical charge of helium nuclei is twice that of hydrogen nuclei, and to bring the nuclei closer together helium at a distance of 10–13 cm requires a much higher temperature). It is precisely this temperature that will be ensured due to the negative heat capacity of the Sun by the time the thermonuclear reaction of converting helium into carbon is ignited in its depths.
White dwarfs
If the mass of the star is small, so that the mass of its core affected by thermonuclear transformations is less than 1.4 M sun, thermonuclear fusion of chemical elements may cease due to the so-called degeneracy of the electron gas in the star's core. In particular, the pressure of a degenerate gas depends on density, but does not depend on temperature, since the energy of quantum motions of electrons is much greater than the energy of their thermal motion.
The high pressure of the degenerate electron gas effectively counteracts the forces of gravitational compression. Since pressure does not depend on temperature, the loss of energy by a star in the form of radiation does not lead to compression of its core. Consequently, gravitational energy is not released as additional heat. Therefore, the temperature in the evolving degenerate core does not increase, which leads to the interruption of the chain of thermonuclear reactions.
The outer hydrogen shell, unaffected by thermonuclear reactions, separates from the star's core and forms a planetary nebula, glowing in the emission lines of hydrogen, helium and other elements. The central compact and relatively hot core of an evolved low-mass star is a white dwarf - an object with a radius on the order of the Earth's radius (~10 4 km), a mass of less than 1.4 M sun and an average density of about a ton per cubic centimeter. White dwarfs are observed in large numbers. Their total number in the Galaxy reaches 10 10, that is, about 10% of the total mass of the observable matter of the Galaxy.
Thermonuclear burning in a degenerate white dwarf can be unstable and lead to a nuclear explosion of a sufficiently massive white dwarf with a mass close to the so-called Chandrasekhar limit (1.4 M sun). Such explosions look like Type I supernovae, which have no hydrogen lines in their spectrum, but only lines of helium, carbon, oxygen and other heavy elements.
Neutron stars
If the star’s core is degenerate, then as its mass approaches the limit of 1.4 M sun, the usual degeneracy of the electron gas in the nucleus is replaced by the so-called relativistic degeneracy.
The quantum motions of degenerate electrons become so fast that their speeds approach the speed of light. In this case, the elasticity of the gas decreases, its ability to counteract the forces of gravity decreases, and the star experiences gravitational collapse. During collapse, electrons are captured by protons, and neutronization of the substance occurs. This leads to the formation of a neutron star from a massive degenerate core.
If the initial mass of the star's core exceeds 1.4 M sun, then a high temperature is reached in the core, and electron degeneration does not occur throughout its evolution. In this case, negative heat capacity works: as the star loses energy in the form of radiation, the temperature in its depths increases, and there is a continuous chain of thermonuclear reactions converting hydrogen into helium, helium into carbon, carbon into oxygen, and so on, up to the elements of the iron group. The reaction of thermonuclear fusion of nuclei of elements heavier than iron no longer occurs with the release, but with the absorption of energy. Therefore, if the mass of the star's core, consisting mainly of iron group elements, exceeds the Chandrasekhar limit of 1.4 M sun , but less than the so-called Oppenheimer–Volkov limit ~3 M sun, then at the end of the nuclear evolution of the star, gravitational collapse of the core occurs, as a result of which the outer hydrogen shell of the star is shed, which is observed as a type II supernova explosion, in the spectrum of which powerful hydrogen lines are observed.
The collapse of the iron core leads to the formation of a neutron star.
When the massive core of a star that has reached a late stage of evolution is compressed, the temperature rises to gigantic values of the order of a billion degrees, when the nuclei of atoms begin to break apart into neutrons and protons. Protons absorb electrons and turn into neutrons, emitting neutrinos. Neutrons, according to the quantum mechanical Pauli principle, with strong compression begin to effectively repel each other.
When the mass of the collapsing core is less than 3 M sun, neutron speeds are significantly less than the speed of light and the elasticity of matter due to the effective repulsion of neutrons can balance the gravitational forces and lead to the formation of a stable neutron star.
The possibility of the existence of neutron stars was first predicted in 1932 by the outstanding Soviet physicist Landau immediately after the discovery of the neutron in laboratory experiments. The radius of a neutron star is close to 10 km, its average density is hundreds of millions of tons per cubic centimeter.
When the mass of the collapsing stellar core is greater than 3 M sun, then, according to existing ideas, the resulting neutron star, cooling, collapses into a black hole. The collapse of a neutron star into a black hole is also facilitated by the reverse fall of part of the star's shell, ejected during a supernova explosion.
A neutron star typically rotates rapidly because the normal star that gave birth to it can have significant angular momentum. When the core of a star collapses into a neutron star, the characteristic dimensions of the star decrease from R= 10 5 –10 6 km to R≈ 10 km. As the size of a star decreases, its moment of inertia decreases. To maintain angular momentum, the speed of axial rotation must increase sharply. For example, if the Sun, rotating with a period of about a month, is compressed to the size of a neutron star, then the rotation period will decrease to 10 -3 seconds.
Single neutron stars with a strong magnetic field manifest themselves as radio pulsars - sources of strictly periodic pulses of radio emission that arise when the energy of the rapid rotation of a neutron star is converted into directed radio emission. In binary systems, accreting neutron stars exhibit the phenomenon of X-ray pulsar and type 1 X-ray burster.
One cannot expect strictly periodic pulsations of radiation from a black hole, since the black hole has no observable surface and no magnetic field. As physicists often say, black holes do not have “hair” - all fields and all inhomogeneities near the event horizon are emitted when the black hole is formed from collapsing matter in the form of a stream of gravitational waves. As a result, the resulting black hole has only three characteristics: mass, angular momentum and electric charge. All individual properties of the collapsing substance are forgotten during the formation of a black hole: for example, black holes formed from iron and from water have, other things being equal, the same characteristics.
As predicted by the General Theory of Relativity (GR), stars whose iron core masses at the end of their evolution exceed 3 M sun, experience unlimited compression (relativistic collapse) with the formation of a black hole. This is explained by the fact that in general relativity the gravitational forces tending to compress a star are determined by the energy density, and at the enormous densities of matter achieved during the compression of such a massive star core, the main contribution to the energy density is no longer made by the rest energy of the particles, but by the energy of their movement and interaction . It turns out that in general relativity the pressure of a substance at very high densities seems to “weigh” itself: the greater the pressure, the greater the energy density and, consequently, the greater the gravitational forces tending to compress the substance. In addition, under strong gravitational fields, the effects of space-time curvature become fundamentally important, which also contributes to the unlimited compression of the star’s core and its transformation into a black hole (Fig. 3).
In conclusion, we note that black holes formed in our era (for example, the black hole in the Cygnus X-1 system), strictly speaking, are not one hundred percent black holes, since due to relativistic time dilation for a distant observer, their event horizons still have not formed. The surfaces of such collapsing stars appear to an observer on Earth as frozen, endlessly approaching their event horizons.
In order for black holes from such collapsing objects to finally form, we must wait the entire infinitely long time of the existence of our Universe. It should be emphasized, however, that already in the first seconds of relativistic collapse, the surface of the collapsing star for an observer from Earth approaches very close to the event horizon, and all processes on this surface slow down infinitely.
« Science fiction can be useful - it stimulates the imagination and relieves fear of the future. However, scientific facts can be much more surprising. Science fiction never even imagined the existence of such things as black holes»
Stephen Hawking
In the depths of the universe there are countless mysteries and secrets hidden for humans. One of them is black holes - objects that even the greatest minds of mankind cannot understand. Hundreds of astrophysicists are trying to uncover the nature of black holes, but at this stage we have not even proven their existence in practice.
Film directors dedicate their films to them, and among ordinary people black holes have become such a cult phenomenon that they are identified with the end of the world and inevitable death. They are feared and hated, but at the same time they are idolized and worshiped by the unknown that these strange fragments of the Universe conceal within themselves. Agree, being swallowed up by a black hole is such a romantic thing. With their help, it is possible, and they can also become guides for us in.
The yellow press often speculates on the popularity of black holes. Finding headlines in newspapers related to the end of the world due to another collision with a supermassive black hole is not a problem. Much worse is that the illiterate part of the population takes everything seriously and raises a real panic. To bring some clarity, we will take a journey to the origins of the discovery of black holes and try to understand what it is and how to approach it.
Invisible stars
It just so happens that modern physicists describe the structure of our Universe using the theory of relativity, which Einstein carefully provided to humanity at the beginning of the 20th century. Black holes become even more mysterious, at the event horizon of which all the laws of physics known to us, including Einstein’s theory, cease to apply. Isn't this wonderful? In addition, the conjecture about the existence of black holes was expressed long before Einstein himself was born.
In 1783 there was a significant increase in scientific activity in England. In those days, science went side by side with religion, they got along well together, and scientists were no longer considered heretics. Moreover, priests were engaged in scientific research. One of these servants of God was the English pastor John Michell, who wondered not only about questions of existence, but also completely scientific problems. Michell was a very titled scientist: initially he was a teacher of mathematics and ancient linguistics at one of the colleges, and after that he was accepted into the Royal Society of London for a number of discoveries.
John Michell studied seismology, but in his spare time he liked to think about the eternal and the cosmos. So he came up with the idea that somewhere in the depths of the Universe there could be supermassive bodies with such powerful gravity that in order to overcome the gravitational force of such a body it is necessary to move at a speed equal to or higher than the speed of light. If we accept such a theory as true, then even light will not be able to develop a second escape velocity (the speed necessary to overcome the gravitational attraction of the leaving body), so such a body will remain invisible to the naked eye.
Michell called his new theory “dark stars,” and at the same time tried to calculate the mass of such objects. He expressed his thoughts on this matter in an open letter to the Royal Society of London. Unfortunately, in those days such research was not of particular value for science, so Michell’s letter was sent to the archives. Only two hundred years later, in the second half of the 20th century, it was discovered among thousands of other records carefully stored in the ancient library.
The first scientific evidence for the existence of black holes
After Einstein's General Theory of Relativity was published, mathematicians and physicists seriously began solving the equations presented by the German scientist, which were supposed to tell us a lot of new things about the structure of the Universe. The German astronomer and physicist Karl Schwarzschild decided to do the same thing in 1916.
The scientist, using his calculations, came to the conclusion that the existence of black holes is possible. He was also the first to describe what was later called the romantic phrase "event horizon" - the imaginary boundary of space-time at a black hole, after crossing which there is a point of no return. Nothing will escape from the event horizon, not even light. It is beyond the event horizon that the so-called “singularity” occurs, where the laws of physics known to us cease to apply.
Continuing to develop his theory and solve equations, Schwarzschild discovered new secrets of black holes for himself and the world. Thus, he was able, solely on paper, to calculate the distance from the center of the black hole, where its mass is concentrated, to the event horizon. Schwarzschild called this distance the gravitational radius.
Despite the fact that mathematically, Schwarzschild's solutions were extremely correct and could not be refuted, the scientific community of the early 20th century could not immediately accept such a shocking discovery, and the existence of black holes was written off as a fantasy, which appeared every now and then in the theory of relativity. For the next decade and a half, space exploration for the presence of black holes was slow, and only a few adherents of the theory of the German physicist were engaged in it.
Stars giving birth to darkness
After Einstein's equations were sorted into pieces, it was time to use the conclusions drawn to understand the structure of the Universe. In particular, in the theory of stellar evolution. It's no secret that in our world nothing lasts forever. Even stars have their own life cycle, albeit longer than a person.
One of the first scientists to become seriously interested in stellar evolution was the young astrophysicist Subramanyan Chandrasekhar, a native of India. In 1930, he published a scientific work that described the supposed internal structure of stars, as well as their life cycles.
Already at the beginning of the 20th century, scientists guessed about such a phenomenon as gravitational compression (gravitational collapse). At a certain point in its life, a star begins to contract at tremendous speed under the influence of gravitational forces. As a rule, this happens at the moment of the death of a star, but during gravitational collapse there are several ways for the continued existence of a hot ball.
Chandrasekhar's scientific adviser, Ralph Fowler, a respected theoretical physicist in his time, assumed that during gravitational collapse any star turns into a smaller and hotter one - a white dwarf. But it turned out that the student “broke” the teacher’s theory, which was shared by most physicists at the beginning of the last century. According to the work of a young Indian, the demise of a star depends on its initial mass. For example, only those stars whose mass does not exceed 1.44 times the mass of the Sun can become white dwarfs. This number was called the Chandrasekhar limit. If the mass of the star exceeded this limit, then it dies in a completely different way. Under certain conditions, such a star at the moment of death can be reborn into a new, neutron star - another mystery of the modern Universe. The theory of relativity tells us another option - compression of the star to ultra-small values, and this is where the fun begins.
In 1932, an article appeared in one of the scientific journals in which the brilliant physicist from the USSR Lev Landau suggested that during collapse a supermassive star is compressed into a point with an infinitesimal radius and infinite mass. Despite the fact that such an event is very difficult to imagine from the point of view of an unprepared person, Landau was not far from the truth. The physicist also suggested that, according to the theory of relativity, gravity at such a point will be so great that it will begin to distort space-time.
Astrophysicists liked Landau's theory, and they continued to develop it. In 1939, in America, thanks to the efforts of two physicists - Robert Oppenheimer and Hartland Snyder - a theory emerged that described in detail a supermassive star at the time of collapse. As a result of such an event, a real black hole should have appeared. Despite the convincingness of the arguments, scientists continued to deny the possibility of the existence of such bodies, as well as the transformation of stars into them. Even Einstein distanced himself from this idea, believing that a star was not capable of such phenomenal transformations. Other physicists did not skimp on their statements, calling the possibility of such events ridiculous.
However, science always reaches the truth, you just have to wait a little. And so it happened.
The brightest objects in the Universe
Our world is a collection of paradoxes. Sometimes things coexist in it, the coexistence of which defies any logic. For example, the term “black hole” would not be associated by a normal person with the expression “incredibly bright,” but a discovery in the early 60s of the last century allowed scientists to consider this statement to be incorrect.
With the help of telescopes, astrophysicists were able to discover hitherto unknown objects in the starry sky, which behaved very strangely despite the fact that they looked like ordinary stars. While studying these strange luminaries, the American scientist Martin Schmidt drew attention to their spectrography, the data of which showed different results from scanning other stars. Simply put, these stars were not like others we are used to.
Suddenly it dawned on Schmidt, and he noticed a shift in the spectrum in the red range. It turned out that these objects are much further from us than the stars that we are used to observing in the sky. For example, the object observed by Schmidt was located two and a half billion light years from our planet, but shone as brightly as a star some hundred light years away. It turns out that the light from one such object is comparable to the brightness of an entire galaxy. This discovery was a real breakthrough in astrophysics. The scientist called these objects “quasi-stellar” or simply “quasar”.
Martin Schmidt continued to study new objects and found that such a bright glow can only be caused by one reason - accretion. Accretion is the process of absorption of surrounding matter by a supermassive body using gravity. The scientist came to the conclusion that at the center of quasars there is a huge black hole, which with incredible force draws in the matter surrounding it in space. As the hole absorbs matter, the particles accelerate to enormous speeds and begin to glow. A kind of luminous dome around a black hole is called an accretion disk. Its visualization was well demonstrated in Christopher Nolan's film Interstellar, which gave rise to many questions: “how can a black hole glow?”
To date, scientists have already found thousands of quasars in the starry sky. These strange, incredibly bright objects are called beacons of the Universe. They allow us to imagine the structure of the cosmos a little better and come closer to the moment from which it all began.
Although astrophysicists had been receiving indirect evidence for many years of the existence of supermassive invisible objects in the Universe, the term “black hole” did not exist until 1967. To avoid complex names, American physicist John Archibald Wheeler proposed calling such objects “black holes.” Why not? To some extent they are black, because we cannot see them. Besides, they attract everything, you can fall into them, just like into a real hole. And according to modern laws of physics, it is simply impossible to get out of such a place. However, Stephen Hawking claims that when traveling through a black hole, you can get to another Universe, another world, and this is hope.
Fear of Infinity
Due to the excessive mystery and romanticization of black holes, these objects have become a real horror story among people. The tabloid press loves to speculate on the illiteracy of the population, publishing amazing stories about how a huge black hole is moving towards our Earth, which will devour the Solar system in a matter of hours, or simply emitting waves of toxic gas towards our planet.
The topic of destroying the planet with the help of the Large Hadron Collider, which was built in Europe in 2006 on the territory of the European Council for Nuclear Research (CERN), is especially popular. The wave of panic began as someone's stupid joke, but grew like a snowball. Someone started a rumor that a black hole could form in the particle accelerator of the collider, which would swallow our planet entirely. Of course, the indignant people began to demand a ban on experiments at the LHC, fearing this outcome of events. The European Court began to receive lawsuits demanding that the collider be closed and the scientists who created it punished to the fullest extent of the law.
In fact, physicists do not deny that when particles collide in the Large Hadron Collider, objects similar in properties to black holes can arise, but their size is at the level of the size of elementary particles, and such “holes” exist for such a short time that we cannot even record their occurrence.
One of the main experts who are trying to dispel the wave of ignorance in front of people is Stephen Hawking, a famous theoretical physicist who, moreover, is considered a real “guru” regarding black holes. Hawking proved that black holes do not always absorb the light that appears in the accretion disks, and some of it is scattered into space. This phenomenon was called Hawking radiation, or black hole evaporation. Hawking also established a relationship between the size of a black hole and the rate of its “evaporation” - the smaller it is, the less time it exists. This means that all opponents of the Large Hadron Collider should not worry: black holes in it will not be able to survive even a millionth of a second.
Theory not proven in practice
Unfortunately, human technology at this stage of development does not allow us to test most of the theories developed by astrophysicists and other scientists. On the one hand, the existence of black holes has been quite convincingly proven on paper and deduced using formulas in which everything fits with each variable. On the other hand, in practice we have not yet been able to see a real black hole with our own eyes.
Despite all the disagreements, physicists suggest that in the center of each galaxy there is a supermassive black hole, which gathers stars into clusters with its gravity and forces them to travel around the Universe in a large and friendly company. In our Milky Way galaxy, according to various estimates, there are from 200 to 400 billion stars. All these stars are orbiting something that has enormous mass, something that we can't see with a telescope. It is most likely a black hole. Should we be afraid of her? – No, at least not in the next few billion years, but we can make another interesting film about it.