White dwarfs is one of the most fascinating topics in the history of astronomy: were first opened the heavenly bodies possessing properties that are very far from those with whom we deal in terrestrial conditions. And, in all probability, the resolution of the mysteries of white dwarfs started to research the mysterious nature of the substance, hidden away somewhere in different parts of the Universe.

In the Universe, a lot of white dwarfs. One time they were considered to be rare, but a careful examination of photographic plates, received at the Observatory mount Palomar (USA), showed that their number exceeds 1500. Managed to assess the spatial density of white dwarfs: it turns out that in the sphere with a radius of 30 light years should be about 100 of such stars. The history of discovery of white dwarfs dates back to the early 19th century, when Friedrich Wilhelm Bessel, tracing the movement of most of the bright star Sirius, discovered that her path is not straight line, and has a wave-like character. The proper motion of the stars was not in a straight line; it was barely noticeable was moving from side to side. To 1844., about ten years later after the first observations of Sirius, the Bessel came to the conclusion that near Sirius is the second star, which is invisible, has on Sirius gravitational influence; it is found on fluctuations in the movement of Sirius. Even more interesting was the fact that if a dark component does exist, then the period of the two stars about their common centre of gravity of approximately 50 years.

Fast forward to 1862. and from Germany in Cambridge, Massachusetts (USA). The Alvan Clark, the largest Builder of the telescopes in the United States, the University of Mississippi, was commissioned to build a telescope lens diameter 18.5 inches (46 cm), which was to become the largest telescope in the world. After Clark has finished processing the lens of a telescope, you would have to check, whether provided the necessary accuracy of the form's surface. With this aim the lens set in the mobile pipe and sent to Sirius - the brightest star, which is the best object to scan lens and the identification of their defects. Fixing the position of the telescope tube, Alvan Clark saw weak Ghost", which appeared on the Eastern edge of the field of view of the telescope in the glare of Sirius. Then, as the movement of the sky, in the sight has got himself Sirius. His image was distorted - it seemed that Ghost" is a defect lenses that should be resolved before to take the lens into operation. However, this occurred in the field of view of the telescope weak asterisk was a component of Sirius predicted by Bessel. In conclusion it should be added that due to the outbreak of world war telescope Clarke never was sent in Mississippi - its set in Gerbanovskiy Observatory, near Chicago, and the lens in use to this day, but in another setting.

Thus, Sirius was the subject of General interest and of many studies, for the physical characteristics of a dual system was intrigued by astronomers. With regard to the movement of Sirius, its distance to the earth and the amplitude deviations from rectilinear motion astronomers were able to determine the characteristics of the two stars in the system, called Sirius a and Sirius Century the total mass of the two stars were 3.4 times greater than the mass of the Sun. It was found that the distance between the stars is almost 20 times the distance between the sun and the Earth, that is approximately equal to the distance between the Sun and Uranus; obtained on the basis of measurement of parameters of the orbit mass Sirius And was 2.5 times the mass of the Sun, and the weight of Sirius b was 95% of the mass of the Sun. After you have identified the luminosity of the two stars, it was discovered that Sirius And almost 10,000 times brighter than Sirius Century absolute value of Sirius And we know that he is about 35.5 times shines stronger than the Sun. It follows that the luminosity of the Sun in 300 times the luminosity of Sirius Century Luminosity of any star depends on the temperature of the surface of the star and its size, i.e. diameter. The proximity of the second component to the brighter Sirius And complicates the determination of the spectrum that you need to set the temperature of a star. In 1915. using all technical equipment which had the biggest Observatory of the time mount Wilson (USA), were received good pictures of the spectrum of Sirius.

This led to an unexpected discovery: the temperature of the satellite was To 8000, while the Sun has a temperature of 5700 K. Thus, the satellites have been revealed in the hot Sun, and this meant that the luminosity of a unit surface is also larger. In fact, a simple calculation shows that every inch the star emits four times more energy than a square inch of the surface of the Sun. It follows that the surface of the satellite must be 300*104 times less than the Sun's surface, and Sirius In must have a diameter of about 40 000 km However, the mass of the star is 95% of the mass of the Sun. This means that a lot of substance shall be Packed in a very small volume, in other words, the star must be dense. In the result of simple arithmetic we find that the density of the satellite is nearly 100 000 times the density of water. Cubic centimeter of this substance on Earth weighed in at least 100 kg and 0,5 l such substance is about 50 tons

This is the story of the opening of the first white dwarf. Now let's ask the question: how is the substance can be compressed so that one cubic inch of it weighed 100 kg? When the high pressure of matter compressed to a much greater densities, as white dwarfs, it is in effect a different type of pressure, the so-called "degenerate pressure". It appears the strongest contraction of matter in the interior of a star. It is compression, but not the high temperature is the cause of degenerate pressure.

Due to high compression atoms are so tightly Packed that the electron shell begin to penetrate one another. Gravitational compression of the white dwarf is happening for a long time, and the electron shell continue to flow into each other as long as the distance between the nuclei will not become the order of the radius of the smallest of the electron shell. Inner electron shells are impermeable barrier preventing further compression. At maximum compression, the electrons are not already associated with an individual cores and move freely about them. The process of separation of electrons from nuclei occurs as a result of ionization by pressure. When ionization becomes full, the cloud of electrons moving relative to the array of heavier nuclei, so that the substance of the white dwarf becomes certain physical properties characteristic of metals. In this matter, energy is transferred to the surface electrons, similar to the way heat is distributed on an iron bar, heated at one end.

But electron gas shows and unusual properties. As the compression of electrons their speed is increasingly important because, as we know, according to the fundamental physical principle, two electrons in one element of the phase space, can not have the same energy. Therefore, in order not to be one and the same element of volume, they have to move with great speed. The smallest allowable amount depends on the range of velocities of the electrons. However, on average, the lower the velocity of the electrons, the more the minimum amount that they can take. In other words, the fast electrons occupy the smallest volume.

Although individual electrons are worn with speeds corresponding internal temperature of millions of degrees, the temperature of the full ensemble of electrons in General remains low. It is set that the gas atoms plain white dwarf form a lattice tightly Packed heavy nuclei through which moves a degenerate electron gas. Closer to the surface of the star degeneration is weakening, and on the surface atoms are ionized not completely so that part of the substance is in a normal gaseous state. Knowing the physical characteristics of white dwarfs, we can construct a clear model. Let's start with the fact that white dwarfs are the atmosphere. The analysis of spectra of dwarf leads to the conclusion that their thickness of the atmosphere is only a few hundred meters. In this atmosphere, astronomers detect various familiar chemical elements. Known white dwarfs are two types of cold and hot. In the atmospheres more hot white dwarfs contains a supply of hydrogen, though, he probably does not exceed 0,05%. However, on the lines in the spectra of these stars were discovered hydrogen, helium, calcium, iron, carbon, and even titanium oxide. The atmosphere of cold white dwarfs are composed almost entirely of helium, hydrogen, perhaps, accounts for less than one atom out of a million. The surface temperature of white dwarfs vary from 5000 To the "cold" stars up to 50 000 To the "hot". Under the atmosphere of the white dwarf region is non-degenerate matter, which contains a small number of free electrons. The thickness of this layer is 160 kilometres, approximately 1% of the radius of the star. Layer this may change with time, but the diameter of the white dwarf remains constant and equal to about 40 000 km

As a rule, white dwarfs, not reduced in size after you have reached this state. They behave like a cannon-ball, heated to high temperature; the kernel can change the temperature of radiating energy, but its size remains unchanged. What determines the final diameter of a white dwarf? Is its mass. Than larger the mass of the white dwarf, the less its radius; minimally possible radius is 10 000 km Theoretically, if the mass of the white dwarf exceeds the mass of the Sun 1.2 times its radius can be infinitely small. It is the pressure of a degenerate electron gas protects the star from all further compression, and, although temperatures can vary from millions of degrees at the core of the star is up to scratch on the surface, diameter not changed. Over time, the star becomes a dark body with the same diameter, which she had entered the stage of a white dwarf. Under the top layer of the star degenerate gas of almost isotermico, i.e. the temperature is almost constant until the very center of the star; it amounts to several million degrees is the most real figure 6 million K.

Now that we have some ideas about the structure of the white dwarf, the question arises: why is he on? One thing is clear: thermonuclear reactions are excluded. Inside the white dwarf is missing hydrogen, which would support this mechanism of generation of energy. The only type of energy, which has a white dwarf, is the heat energy. The nuclei of atoms are in a disorderly movement, as they disperse a degenerate electron gas. Over time, the motion of the nuclei is slowing, which is equivalent to the cooling process. Electronic gas, which is unlike any known on Earth gases with outstanding thermal conductivity, and electrons conduct heat energy to the surface, where through the atmosphere this energy is radiated into space.

Astronomers compare the process of cooling of a hot white dwarf to the cooling of the iron rod, taken out of the fire. First white dwarf cools quickly, but with the drop of the temperature inside the cooling slows down. According to estimates, for the first hundreds of millions of years white dwarf luminosity decreases by 1% from the luminosity of the Sun.

In the end the white dwarf should disappear and become a black dwarf, but this may need trillions of years, and, according to many scientists, is very doubtful that the age of the Universe was large enough to appear in her black dwarfs. Other astronomers believe that in the initial phase, when the white dwarf is still quite hot, cooling rate small. And when its surface temperature falls to a value of about temperature of the Sun, the cooling rate increases and extinction occurs very quickly. When the bowels of the white dwarf are cool enough, they will harden. Anyway, if we assume that the age of the Universe is more than 10 billion years, red dwarfs it should be much larger than the white. Knowing this, astronomers do a search for red dwarfs.

While they are unsuccessful. Mass white dwarfs are not well defined. Reliably you can install components in binary systems, as in the case of Sirius. But few white dwarfs are members of binary stars. In the three most well-studied cases of mass white dwarfs, measured with an accuracy of more than 10% was less than the mass of the Sun and made up about half of it. Theoretically, the maximum weight for totally degenerate non-rotating stars must be 1.2 times the mass of the Sun. However, if the stars rotate, but in all likelihood, the way it is, it is quite possible mass, several times greater than the sun.

The force of gravity on the surface of white dwarfs in about 60-70 times more than the Sun. If a person weighs on Earth 75 kg, then in the Sun he weighed would tonne, and on the surface of the white dwarf, its weight would be 120-140 tons. Given the fact that the radii of white dwarfs differ little, and their mass is almost the same, it can be concluded that the force of gravity on the surface of any white dwarf approximately the same. In the Universe, a lot of white dwarfs. One time they were considered to be rare, but a careful examination of photographic plates, received at the Observatory mount Palomar, showed that their number exceeds 1500. Astronomers believe that the frequency of occurrence of white dwarfs standing for at least the last 5 billion years. Perhaps white dwarfs are the most numerous class of objects in the sky.

Managed to assess the spatial density of white dwarfs: it turns out that in the sphere with a radius of 30 light years should be about 100 of such stars. The question arises whether all the stars become white dwarfs at the end of their evolutionary path ? If not, what part of the stars go on the stage of the white dwarf ? The most important step in solving the problem came when the astronomers paid a position of Central stars of planetary nebulae on the chart temperature - luminosity. To understand the properties of the stars, located in the center of planetary nebulae, consider these heavenly bodies. Photos planetary nebula looks like a long mass of gases ellipsoidal shape with a small but hot star in the center. In fact this mass is a complex turbulent, concentric shell, which expands with velocities 15-50 km/S. Although these formations look like rings, in fact, they are wrappers and speed of turbulent motion of the gas reaches approximately 120 km/S. it was Found that the diameters of several planetary nebulae, which are able to measure the distance up to about 1 light year, or about 10 trillion kilometers.

Expanding on the above velocity, gas shells is very low and may not be instituted and, therefore, it cannot be seen after 100,000 years. Many planetary nebula, we observe today were born in the last 50 000 years, and the typical age is close to 20 000 years. The Central star of such nebulae are the most hot objects among known in nature. The temperature of their surface varies from 50 000 to 1 million. K. due to the unusually high temperatures the most part of radiation star falls on the far ultraviolet region of the electromagnetic spectrum.

It's ultraviolet radiation is absorbed, converted and pereslushal gas shell in the visible spectrum, and that allows us to observe the shell. This means that the shell is significantly brighter than the Central star, which actually are the source of energy, because a great amount of radiation star falls on the invisible part of the spectrum. From the analysis of the characteristics of Central stars of planetary nebulae follows that the typical value of the mass concluded in the range of 0.6-1 the mass of the Sun. And for the synthesis of heavy elements in the bowels of the stars of high mass. The amount of hydrogen in these stars slightly. However, shell gas rich in hydrogen and helium.

Some astronomers believe that 50-95 % of all white dwarfs came not from planetary nebulae. Thus, although some white dwarfs entirely connected with planetary nebulae, at least half or more of them came from normal stars of the main sequence, not passing through the stage of planetary nebulae. Full picture of the formation of white dwarfs vague and uncertain. Missing so much detail that at best description of the evolutionary process can be built only through logical reasoning. Nevertheless, the General conclusion is that many stars lose some substances on the way to their final, a similar stage of the white dwarf, and then hidden in heaven "cemeteries" as black, invisible dwarfs. If the mass of the star is about twice the mass of the Sun, the stars in the final stages of its evolution lose stability. Such stars can explode as a supernova, and then shrink to the size of balls of radius of several kilometers, i.e. become a neutron star.