When we look at the night sky, it seems to us that all the stars are the same. The human eye with great difficulty distinguishes the visible spectrum of light emitted by distant heavenly bodies. The star, which is still barely visible, may have been extinguished long ago, and we observe only its light. Each of the stars has its own life. Some shine with a steady white light, others look pulsating neon light bright dots. Still others are dim luminous spots, barely visible in the sky.
Each of the stars is at a certain stage of its evolution and over time turns into a heavenly body of a different class. Instead of a bright and dazzling dot in the night sky, a new space object appears - a white dwarf - an aging star. This stage of evolution is characteristic of most ordinary stars. A similar fate cannot be avoided for our Sun.
What is a white dwarf: a star or a phantom?
Only recently, in the 20th century, it became clear to scientists that a white dwarf is all that is left in space from an ordinary star. The study of stars from the point of view of thermonuclear physics gave an idea of the processes that rage in the bowels of celestial bodies. Stars, formed as a result of the interaction of gravitational forces, are a colossal thermonuclear reactor in which chain reactions of fission of hydrogen and helium nuclei are constantly taking place. Such complex systems the rates of evolution of the components are not the same. Huge reserves of hydrogen ensure the life of a star for billions of years to come. Thermonuclear hydrogen reactions contribute to the formation of helium and carbon. Following thermonuclear fusion, the laws of thermodynamics come into play.
After the star has used up all the hydrogen, its core begins to shrink under the influence of gravitational forces and colossal internal pressure. Losing the main part of its shell, the celestial body reaches the limit of the star's mass, at which it can exist as a white dwarf, devoid of energy sources, continuing to radiate heat by inertia. In fact, white dwarfs are stars from the class of red giants and supergiants that have lost their outer shell.
Fusion depletes a star. Hydrogen dries up, and helium, as a more massive component, can evolve further, reaching a new state. All this leads to the fact that at first red giants are formed in the place of an ordinary star, and the star leaves the main sequence. Thus, the heavenly body, having embarked on the path of its slow and inevitable aging, is gradually transformed. Star age is long way into oblivion. All this happens very slowly. white dwarf is a celestial body with which, outside the main sequence, an inevitable process of extinction occurs. The helium fusion reaction leads to the fact that the core of an aging star is compressed, the luminary finally loses its shell.
The evolution of white dwarfs
Outside the main sequence, the process of star extinction occurs. Under the influence of gravitational forces, the heated gas of red giants and supergiants scatters throughout the Universe, forming a young planetary nebula. After hundreds of thousands of years, the nebula dissipates, and in its place remains the degenerate core of a white red giant. The temperatures of such an object are quite high from 90,000 K, estimated from the absorption line of the spectrum, and up to 130,000 K, when the assessment is carried out within the X-ray spectrum. However, due to their small size, cooling heavenly body happens very slowly.
That picture of the starry sky that we observe has an age of tens to hundreds of billions of years. Where we see white dwarfs, something else may already exist in space. heavenly body. The star has passed into the class of a black dwarf, the final stage of evolution. In reality, a clot of matter remains in place of the star, the temperature of which is equal to the temperature of the surrounding space. main feature of this object is the complete absence of visible light. It is quite difficult to notice such a star in a conventional optical telescope due to its low luminosity. The main criterion for detecting white dwarfs is the presence of powerful ultraviolet radiation and X-rays.
All known white dwarfs, depending on their spectrum, are divided into two groups:
- hydrogen objects, spectral class DA, in the spectrum of which there are no helium lines;
- helium dwarfs, spectral type DB. The main lines in the spectrum are for helium.
White dwarfs of the hydrogen type make up the majority of the population, up to 80% of all currently known objects of this type. Helium dwarfs account for the remaining 20%.
The stage of evolution, as a result of which a white dwarf appears, is the last for non-massive stars, which include our star the Sun. At this stage, the star has the following characteristics. Despite such a small and compact size of a star, its stellar matter weighs exactly as much as is required for its existence. In other words, white dwarfs, which have radii 100 times smaller than the radius of the solar disk, have a mass equal to the mass of the Sun or even weigh more than our star.
This suggests that the density of a white dwarf is millions of times higher than the density of ordinary stars located within the main sequence. For example, the density of our star is 1.41 g/cm³, while the density of white dwarfs can reach colossal values of 105-110 g/cm³.
In the absence of their own energy sources, such objects gradually cool down, respectively, have a low temperature. Temperatures in the range of 5000-50000 degrees Kelvin have been recorded on the surface of white dwarfs. The older the star, the lower its temperature.
For example, the neighbor of the brightest star in our sky Sirius A, the white dwarf Sirius B, has a surface temperature of only 2100 degrees Kelvin. Inside this celestial body is much hotter, almost 10,000°K. Sirius B was the first of the white dwarfs discovered by astronomers. The color of white dwarfs discovered after Sirius B turned out to be the same white, which was the reason for giving such a name to this class of stars.
In terms of brightness, Sirius A is 22 times the brightness of our Sun, but its sister Sirius B shines with a dim light, noticeably inferior in brightness to its dazzling neighbor. It was possible to detect the presence of a white dwarf thanks to images of Sirius taken by the Chandra X-ray telescope. White dwarfs do not have a pronounced light spectrum; therefore, such stars are commonly considered to be rather cold dark cosmic objects. In infrared and X-rays, Sirius B shines much brighter, while continuing to radiate a huge amount of thermal energy. Unlike ordinary stars, where the corona serves as the source of X-ray waves, the source of radiation in white dwarfs is the photosphere.
Being out of the main sequence in terms of abundance, these stars are not the most common objects in the universe. In our galaxy, white dwarfs account for only 3-10% of the heavenly bodies. For this part of the stellar population of our galaxy, the uncertainty of the estimate makes it difficult for the radiation to be weak in the visible region of the polar. In other words, the light of white dwarfs is not able to overcome the large accumulations of cosmic gas that make up the arms of our galaxy.
A scientific look at the history of the appearance of white dwarfs
Further in the celestial bodies, in place of the dried up main sources of thermonuclear energy, a new source of thermonuclear energy arises, the triple helium reaction, or the triple alpha process, which ensures helium burnout. These assumptions were fully confirmed when it became possible to observe the behavior of stars in the infrared range. The light spectrum of an ordinary star differs significantly from the picture that we observe when looking at red giants and white dwarfs. For the degenerate cores of such stars, there is an upper mass limit, otherwise the celestial body becomes physically unstable and collapse may occur.
It is almost impossible to explain such a high density that white dwarfs have in terms of physical laws. The ongoing processes became clear only thanks to quantum mechanics, which made it possible to study the state of the electron gas of stellar matter. Unlike an ordinary star, where gas is used to study the state standard model, in white dwarfs, scientists are dealing with the pressure of the relativistic degenerate electron gas. Speaking in plain language, the following is observed. With a huge compression of 100 or more times, the stellar substance becomes like one big atom, in which all atomic bonds and chains merge together. In this state, the electrons form a degenerate electron gas, the new quantum formation of which can resist the forces of gravity. This gas forms a dense core, devoid of a shell.
A detailed study of white dwarfs using radio telescopes and X-ray optics turned out that these celestial objects are not as simple and boring as it might seem at first glance. Given the absence of such stars inside thermonuclear reactions, the question involuntarily arises - where does the huge pressure come from, which managed to balance the forces of gravity and the forces of internal attraction.
As a result of research physicists in the field of quantum mechanics, a white dwarf model was created. Under the influence of gravitational forces, the stellar matter is compressed to such an extent that the electron shells of atoms are destroyed, the electrons begin their own chaotic movement, passing from one state to another. The nuclei of atoms in the absence of electrons form a system, forming a strong and stable bond between themselves. There are so many electrons in stellar matter that many states are formed, respectively, the speed of electrons is preserved. The high speed of elementary particles creates a colossal internal pressure of the electron degenerate gas, which is able to resist the forces of gravity.
When did white dwarfs become known?
Despite the fact that Sirius B is considered the first white dwarf discovered by astrophysicists, there are supporters of the version of the earlier acquaintance of the scientific community with stellar objects of this class. Back in 1785, the astronomer Herschel first included a triple star system in the constellation Eridanus in the star catalog, dividing all the stars separately. Only 125 years later, astronomers discovered an anomalously low luminosity of 40 Eridani V at a high color temperature, which was the reason for the allocation of such objects into a separate class.
The object had a faint brilliance corresponding to a magnitude of +9.52m. The white dwarf had a mass ½ solar and had a diameter smaller than the earth. These parameters contradicted the theory of the internal structure of stars, where the luminosity, radius and temperature of the star's surface were the key parameters for determining the class of a star. The small diameter, low luminosity from the point of view of physical processes did not correspond to the high color temperature. This discrepancy raised many questions.
The situation with another white dwarf, Sirus B, looked similar. Being a satellite of the brightest star, the white dwarf has a small size and a huge density of stellar matter - 106 g / cm3. For comparison, the substance of this celestial body with the amount of a matchbox would weigh more than a million tons on our planet. The temperature of this dwarf is 2.5 times higher main star Sirius systems.
Latest scientific findings
The heavenly bodies with which we are dealing are a natural testing ground, thanks to which a person can study the structure of stars, the stages of their evolution. If the birth of stars can be explained by physical laws that operate in the same way in any environment, then the evolution of stars is represented by completely different processes. scientific explanation many of them goes into the category of quantum mechanics, the science of elementary particles.
White dwarfs look like the most mysterious objects in this light:
- Firstly, the process of degeneration of the star's core looks very curious, as a result of which the stellar matter does not fly apart in space, but, on the contrary, shrinks to unimaginable sizes;
- Secondly, in the absence of thermonuclear reactions, white dwarfs remain rather hot space objects;
- Thirdly, these stars, having a high color temperature, have a low luminosity.
Scientists of all stripes, astrophysicists, physicists and nuclear scientists have yet to give answers to these and many other questions that will allow us to predict the fate of our native luminary. The Sun is destined to become a white dwarf, but it remains questionable whether a person will be able to observe the Sun in this role.
If you have any questions - leave them in the comments below the article. We or our visitors will be happy to answer them.
For a number of years, the German astronomer Friedrich Wilhelm Bessel observed the proper movements in the sky of two bright stars - Sirius and Procyon - and in 1844 established that both of them did not move along straight lines, but along characteristic wavy trajectories. The discovery prompted the scientist to the idea that each of these stars has a companion invisible to us, that is, it is a physically binary star system.
But the Sun, nearly as massive as Sirius B, would shine almost as brightly as the North Star from its distance. So why was Sirius B considered an "invisible satellite" for 18 years? Maybe because of the small angular distance between it and Sirius A? Not only. As it turned out later, it is obviously inaccessible to the naked eye due to its low luminosity, 400 times inferior to the luminosity of the Sun. True, at the very beginning of the 20th century. this discovery did not seem particularly strange, since quite a few stars of low luminosity were known, and the relationship between the mass of a star and its luminosity had not yet been established. Only when the emission spectra of Sirius B and Procyon B were obtained, as well as measurements of their temperature, did the "abnormality" of these stars become apparent.
In physics, there is such a concept - absolutely black body. No, this is not a synonym for black holes- unlike her, a completely black body can shine dazzlingly! It is called absolutely black because, by definition, it absorbs all electromagnetic radiation falling on it. The theory states that the total luminous flux (over the entire wavelength range) from a unit surface of an absolutely black body does not depend on its structure or chemical composition, but is determined only by temperature. According to the Stefan-Boltzmann law, its luminosity is proportional to the fourth power of temperature. An absolutely black body, like an ideal gas, is only a physical model that is never strictly implemented in practice. However, the spectral composition of stellar light in the visible region of the spectrum is quite close to "blackbody". Therefore, we can assume that the blackbody model as a whole correctly describes the radiation of a real star.
effective temperature The temperature of a star is the temperature of an absolutely black body that radiates the same amount of energy with it from a unit surface. Generally speaking, it is not equal to the temperature of the star's photosphere. Nevertheless, this is an objective characteristic that can be used to evaluate other characteristics of a star: luminosity, size, etc.
In the 10s. In the 20th century, the American astronomer Walter Adams made an attempt to determine the effective temperature of Sirius B. It was 8000 K, and later it turned out that the astronomer was mistaken and in fact it is even higher (about 10,000 K). Consequently, the luminosity of this star, if it had the size of the Sun, should have been at least 10 times greater than the solar one. The observed luminosity of Sirius B, as we know, is 400 times less than the solar one, i.e., it turns out to be more than 4 thousand times lower than expected! The only way out of this contradiction is to assume that Sirius B has a much smaller visible surface area, and hence a smaller diameter. Calculations have shown that Sirius B is only 2.5 times the size more earth. But it retains the solar mass - it turns out that its average density should be almost 100 thousand times greater than that of the Sun! Many astronomers refused to believe in the existence of such exotic objects.
Only in 1924, mainly due to the efforts of the English astrophysicist Arthur Eddington, who developed the theory of the internal structure of the star. The compact satellites of Sirius and Procyon were finally recognized by the astronomical community as real representatives of a completely new class of stars, which are now known as white dwarfs. "White" - because the first representatives of this type were hot blue-white luminaries, "dwarfs" - because they have very small luminosities and sizes.
As we have already found out, the density of white dwarfs is many thousand times higher than that of ordinary stars. This means that their substance must be in some special, previously unknown physical state. This was also indicated by the unusual spectra of white dwarfs.
First, their absorption lines are many times wider than those of normal stars. Secondly, hydrogen lines can be present in the spectra of white dwarfs at such high temperatures at which they are absent in the spectra of ordinary stars, since all hydrogen is ionized. All this was theoretically explained by the very high pressure of matter in the atmospheres of white dwarfs.
The next feature of the spectra of these exotic stars is that the lines of all chemical elements are slightly redshifted compared to the corresponding lines in the spectra obtained in terrestrial laboratories. This is the effect of the so-called gravitational redshift, due to the fact that the acceleration of gravity on the surface of a white dwarf is many times greater than on Earth.
Indeed, from the law of universal gravitation it follows that the acceleration of gravity on the surface of a star is directly proportional to its mass and inversely proportional to the square of the radius. The masses of white dwarfs are close to the masses of normal stars, and the radii are many times smaller. Therefore, the acceleration of gravity on the surface of white dwarfs is very large: about 10 5 - 10 6 m/s 2 . Recall that on Earth it is 9.8 m / s 2, i.e., 10,000 - 100,000 times less.
According to the identified chemical composition, the spectra of white dwarfs are divided into two categories: some with hydrogen lines, others without hydrogen lines, but with lines of neutral or ionized helium or heavy elements. "Hydrogen" dwarfs sometimes have a significantly higher temperature (up to 60,000 K and higher) than "helium" ones (11,000 - 20,000 K). Based on this, scientists came to the conclusion that the substance of the latter is practically devoid of hydrogen.
In addition, white dwarfs were discovered, the spectra of which could not be identified with chemical elements and compounds known to science. Later, these stars were found to have magnetic fields 1,000 to 100,000 times stronger than those on the Sun. At such strengths of magnetic fields, the spectra of atoms and molecules are unrecognizably distorted, so it is difficult to identify them.
In the depths of white dwarfs, the density can reach values of the order of 10 10 kg/m 3 . At such density values (and even at lower ones, typical for the outer layers of white dwarfs) physical properties gas change significantly and the laws of an ideal gas are no longer applicable to it. In the mid 20s. Italian physicist Enrico Fermi developed a theory that describes the properties of gases with densities characteristic of white dwarfs. It turned out that the pressure of such a gas is not determined by its temperature. It remains high even if the matter cools down to absolute zero! A gas with these properties is called degenerate.
In 1926, the English physicist Ralph Fowler successfully applied the degenerate gas theory to white dwarfs (and only later did Fermi's theory find numerous applications in "terrestrial" physics). Based on this theory, two important conclusions were drawn. First, the radius of a white dwarf for a given chemical composition of matter is uniquely determined by its mass. Secondly, the mass of a white dwarf cannot exceed a certain critical value, the value of which is approximately 1.4 solar masses.
Further observations and studies confirmed these theoretical premises and made it possible to draw the final conclusion that there is practically no hydrogen in the depths of white dwarfs. Since the theory of degenerate gas explained well the observed properties of white dwarfs, they began to be called degenerate stars. The next step was the construction of a theory of their formation.
AT modern theory In stellar evolution, white dwarfs are considered as the final stage in the evolution of stars of medium and low mass (less than 3–4 solar masses).
After all the hydrogen in the central regions of an aging star burns out, its core should shrink and warm up. In this case, the outer layers expand greatly, the effective temperature of the star drops, and it becomes a red giant. The resulting rarefied shell of the star is very weakly bound to the core; it eventually dissipates in space. In place of the former red giant, a very hot and compact star, consisting mainly of helium, remains - a white dwarf. Due to its high temperature, it emits mainly in the ultraviolet range and ionizes the gas of the expanding envelope.
The expanding shells surrounding hot stars have been known for a long time. They're called planetary nebulae and were discovered in the 18th century. William Herschel. Their observed number is in good agreement with the number of red giants and white dwarfs, and, consequently, with the fact that the main mechanism for the formation of white dwarfs is the evolution of ordinary stars with the ejection of a gaseous envelope at the red giant stage.
In close binary star systems, the components are located so close to each other that there is an exchange of matter between them. The swollen shell of a red giant constantly flows onto a neighboring star until only a white dwarf is left of it. Probably the first discovered representatives of white dwarfs - Sirius B and Procyon B - were formed in this way.
At the end of the 40s. Soviet astrophysicist Samuil Aronovich Kaplan showed that the radiation of white dwarfs leads to their cooling. This means that these stars have no internal sources of energy. Kaplan also built a quantitative theory of the cooling of white dwarfs, and in the early 50s. British and French scientists came to similar conclusions. True, due to the small surface area, these stars cool extremely slowly.
So, most of the observed properties of white dwarfs could be explained by the huge values of the density of their matter and very strong gravitational field on their surfaces. This makes white dwarfs unique objects: it is not yet possible to reproduce the conditions in which their matter is located in terrestrial laboratories.
If you look closely at the night sky, it is easy to notice that the stars looking at us differ in color. Bluish, white, red, they shine evenly or flicker like a Christmas tree garland. In a telescope, color differences become more apparent. The reason for this diversity lies in the temperature of the photosphere. And, contrary to a logical assumption, the hottest are not red, but blue, white-blue and white stars. But first things first.
Spectral classification
Stars are huge hot balls of gas. The way we see them from Earth depends on many parameters. For example, stars don't actually twinkle. It is very easy to be convinced of this: it is enough to remember the Sun. The flickering effect occurs due to the fact that the light coming from cosmic bodies to us overcomes the interstellar medium, full of dust and gas. Another thing is color. It is a consequence of the heating of the shells (especially the photosphere) to certain temperatures. The true color may differ from the visible one, but the difference is usually small.
Today, the Harvard spectral classification of stars is used all over the world. It is a temperature one and is based on the shape and relative intensity of the spectrum lines. Each class corresponds to the stars of a certain color. The classification was developed at the Harvard Observatory in 1890-1924.
One Shaved Englishman Chewing Dates Like Carrots
There are seven main spectral classes: O-B-A-F-G-K-M. This sequence reflects a gradual decrease in temperature (from O to M). To remember it, there are special mnemonic formulas. In Russian, one of them sounds like this: "One Shaved Englishman Chewed Dates Like Carrots." Two more are added to these classes. The letters C and S denote cold luminaries with metal oxide bands in the spectrum. Consider the star classes in more detail:
- Class O is characterized by the highest surface temperature (from 30 to 60 thousand Kelvin). Stars of this type exceed the Sun in mass by 60, and in radius by 15 times. Their visible color is blue. In terms of luminosity, they are ahead of our star by more than a million times. The blue star HD93129A, belonging to this class, is characterized by one of the highest luminosity among known cosmic bodies. According to this indicator, it is ahead of the Sun by 5 million times. The blue star is located at a distance of 7.5 thousand light years from us.
- Class B has a temperature of 10-30 thousand Kelvin, a mass 18 times greater than the same parameter of the Sun. These are white-blue and white stars. Their radius is 7 times greater than that of the Sun.
- Class A is characterized by a temperature of 7.5-10 thousand Kelvin, a radius and mass exceeding 2.1 and 3.1 times, respectively, the similar parameters of the Sun. These are white stars.
- Class F: temperature 6000-7500 K. The mass is 1.7 times greater than the sun, the radius is 1.3. From Earth, such stars also look white, their true color is yellowish white.
- Class G: temperature 5-6 thousand Kelvin. The Sun belongs to this class. The visible and true color of such stars is yellow.
- Class K: temperature 3500-5000 K. The radius and mass are less than solar, they are 0.9 and 0.8 of the corresponding parameters of the star. The color of these stars seen from Earth is yellowish-orange.
- Class M: temperature 2-3.5 thousand Kelvin. The mass and radius are 0.3 and 0.4 of the similar parameters of the Sun. From the surface of our planet, they look red-orange. Beta Andromedae and Alpha Chanterelles belong to the M class. The bright red star familiar to many is Betelgeuse (Alpha Orionis). It is best to look for it in the sky in winter. The red star is located above and slightly to the left
Each class is divided into subclasses from 0 to 9, that is, from the hottest to the coldest. The numbers of stars indicate belonging to a certain spectral type and the degree of heating of the photosphere in comparison with other luminaries in the group. For example, the Sun belongs to the class G2.
visual whites
Thus, star classes B through F can look white from Earth. And only objects belonging to the A-type actually have this coloration. So, the star Saif (the constellation Orion) and Algol (beta Perseus) to an observer not armed with a telescope will seem white. They belong to spectral class B. Their true color is blue-white. Also appearing white are Mythrax and Procyon, the brightest stars in the celestial drawings of Perseus and Canis Minor. However, their true color is closer to yellow (class F).
Why are stars white to an earthly observer? The color is distorted due to the vast distance separating our planet from similar objects, as well as voluminous clouds of dust and gas, often found in space.
Class A
White stars are characterized by a not so high temperature as representatives of the O and B classes. Their photosphere heats up to 7.5-10 thousand Kelvin. Spectral class A stars are much larger than the Sun. Their luminosity is also greater - about 80 times.
In the spectra of A stars, hydrogen lines of the Balmer series are strongly pronounced. The lines of other elements are noticeably weaker, but they become more significant as you move from subclass A0 to A9. Giants and supergiants belonging to the spectral class A are characterized by slightly less pronounced hydrogen lines than main sequence stars. In the case of these luminaries, the lines of heavy metals become more noticeable.
Many peculiar stars belong to the spectral class A. This term refers to luminaries that have noticeable features in the spectrum and physical parameters, which makes it difficult to classify them. For example, quite rare stars Boötes lambda types are characterized by a lack of heavy metals and very slow rotation. Peculiar luminaries also include white dwarfs.
Class A includes such bright objects in the night sky as Sirius, Menkalinan, Aliot, Castor and others. Let's get to know them better.
Alpha Canis Major
Sirius is the brightest, though not the closest, star in the sky. Its distance is 8.6 light years. For an earthly observer, it seems so bright because it has an impressive size and yet is not as far removed as many other large and bright objects. The closest star to the Sun is Sirius in this list is in fifth place.
It refers to and is a system of two components. Sirius A and Sirius B are separated by 20 astronomical units and rotate with a period of just under 50 years. The first component of the system, a main-sequence star, belongs to the spectral type A1. Its mass is twice that of the sun, and its radius is 1.7 times. It can be observed with the naked eye from Earth.
The second component of the system is a white dwarf. The star Sirius B is almost equal to our luminary in mass, which is not typical for such objects. Typically, white dwarfs are characterized by a mass of 0.6-0.7 solar masses. At the same time, the dimensions of Sirius B are close to those of the earth. It is assumed that the white dwarf stage began for this star about 120 million years ago. When Sirius B was located on the main sequence, it was probably a luminary with a mass of 5 solar masses and belonged to the spectral class B.
Sirius A, according to scientists, will move to the next stage of evolution in about 660 million years. Then it will turn into a red giant, and a little later - into a white dwarf, like its companion.
Alpha Eagle
Like Sirius, many white stars, whose names are given below, are well known not only to people who are fond of astronomy because of their brightness and frequent mention in the pages of science fiction literature. Altair is one of those luminaries. Alpha Eagle is found, for example, in Steven King. In the night sky, this star is clearly visible due to its brightness and relatively close proximity. The distance separating the Sun and Altair is 16.8 light years. Of the stars of spectral class A, only Sirius is closer to us.
Altair is 1.8 times as massive as the Sun. Its characteristic feature is a very fast rotation. The star makes one rotation around its axis in less than nine hours. The rotation speed near the equator is 286 km/s. As a result, the "nimble" Altair will be flattened from the poles. In addition, due to the elliptical shape, the temperature and brightness of the star decrease from the poles to the equator. This effect is called "gravitational darkening".
Another feature of Altair is that its brilliance changes over time. It belongs to the Delta Shield type variables.
Alpha Lyrae
Vega is the most studied star after the Sun. Alpha Lyrae is the first star to have its spectrum determined. She also became the second luminary after the Sun, captured in the photograph. Vega was also among the first stars to which scientists measured the distance using the parlax method. For a long period, the brightness of the star was taken as 0 when determining the magnitudes of other objects.
Lyra's alpha is well known to both the amateur astronomer and the simple observer. It is the fifth brightest among the stars, and is included in the Summer Triangle asterism along with Altair and Deneb.
The distance from the Sun to Vega is 25.3 light years. Its equatorial radius and mass are 2.78 and 2.3 times larger than the similar parameters of our star, respectively. The shape of a star is far from being a perfect ball. The diameter at the equator is noticeably larger than at the poles. The reason is the huge speed of rotation. At the equator, it reaches 274 km / s (for the Sun, this parameter is slightly more than two kilometers per second).
One of the features of Vega is the disk of dust that surrounds it. It is presumed that it resulted from a large number collisions of comets and meteorites. The dust disk revolves around the star and is heated by its radiation. As a result, the intensity of the infrared radiation of Vega increases. Not so long ago, asymmetries were discovered in the disk. Their likely explanation is that the star has at least one planet.
Alpha Gemini
The second brightest object in the constellation Gemini is Castor. He, like the previous luminaries, belongs to the spectral class A. Castor is one of the brightest stars in the night sky. In the corresponding list, he is on the 23rd place.
Castor is a multiple system consisting of six components. The two main elements (Castor A and Castor B) revolve around a common center of mass with a period of 350 years. Each of the two stars is a spectral binary. The Castor A and Castor B components are less bright and presumably belong to the M spectral type.
Castor C was not immediately connected to the system. Initially, it was designated as an independent star YY Gemini. In the process of researching this region of the sky, it became known that this luminary was physically connected with the Castor system. The star revolves around a center of mass common to all components with a period of several tens of thousands of years and is also a spectral binary.
Beta Aurigae
The celestial drawing of the Charioteer includes about 150 "points", many of them are white stars. The names of the luminaries will say little to a person far from astronomy, but this does not detract from their significance for science. The brightest object in the celestial pattern, belonging to the spectral class A, is Mencalinan or Beta Aurigae. The name of the star in Arabic means "shoulder of the owner of the reins."
Mencalinan is a ternary system. Its two components are subgiants of spectral class A. The brightness of each of them exceeds the similar parameter of the Sun by 48 times. They are separated by a distance of 0.08 astronomical units. The third component is a red dwarf at a distance of 330 AU from the pair. e.
Epsilon Ursa Major
The brightest "point" in perhaps the most famous constellation of the northern sky (Ursa Major) is Aliot, also belonging to class A. The apparent magnitude is 1.76. In the list of the brightest luminaries, the star takes 33rd place. Alioth enters the asterism of the Big Dipper and is located closer to the bowl than other luminaries.
The Aliot spectrum is characterized by unusual lines that fluctuate with a period of 5.1 days. It is assumed that the features are associated with the influence of the magnetic field of the star. Fluctuations in the spectrum, according to the latest data, may occur due to the proximity of a cosmic body with a mass of almost 15 Jupiter masses. Whether this is so is still a mystery. Her, like other secrets of the stars, astronomers are trying to understand every day.
white dwarfs
The story about white stars will be incomplete if we do not mention that stage in the evolution of the stars, which is designated as the "white dwarf". Such objects got their name due to the fact that the first discovered of them belonged to the spectral class A. It was Sirius B and 40 Eridani B. Today, white dwarfs are called one of the options for the final stage of a star's life.
Let us dwell in more detail on the life cycle of the luminaries.
Star evolution
Stars are not born in one night: any of them goes through several stages. First, a cloud of gas and dust begins to shrink under the influence of its own. Slowly, it takes the form of a ball, while the energy of gravity turns into heat - the temperature of the object rises. At the moment when it reaches a value of 20 million Kelvin, the reaction of nuclear fusion begins. This stage is considered the beginning of the life of a full-fledged star.
Suns spend most of their time on the main sequence. Hydrogen cycle reactions are constantly going on in their depths. The temperature of the stars may vary. When all the hydrogen in the nucleus ends, a new stage of evolution begins. Now helium is the fuel. At the same time, the star begins to expand. Its luminosity increases, while the surface temperature, on the contrary, decreases. The star leaves the main sequence and becomes a red giant.
The mass of the helium core gradually increases, and it begins to shrink under its own weight. The red giant stage ends much faster than the previous one. The path that further evolution will take depends on the initial mass of the object. Low-mass stars at the red giant stage begin to swell. As a result of this process, the object sheds its shells. The bare core of the star is also formed. In such a nucleus, all fusion reactions are completed. It is called a helium white dwarf. More massive red giants (up to a certain limit) evolve into carbon white dwarfs. Their nuclei contain more heavy elements than helium.
Characteristics
White dwarfs are bodies that are usually very close in mass to the Sun. At the same time, their size corresponds to the earth. The colossal density of these cosmic bodies and the processes taking place in their depths are inexplicable from the point of view of classical physics. The secrets of the stars helped to reveal quantum mechanics.
The substance of white dwarfs is an electron-nuclear plasma. It is almost impossible to design it even in a laboratory. Therefore, many characteristics of such objects remain incomprehensible.
Even if you study the stars all night long, you will not be able to detect at least one white dwarf without special equipment. Their luminosity is much less than that of the sun. According to scientists, white dwarfs make up approximately 3 to 10% of all objects in the Galaxy. However, to date, only those of them have been found that are located no further than 200-300 parsecs from the Earth.
White dwarfs continue to evolve. Immediately after formation, they have a high surface temperature, but cool quickly. A few tens of billions of years after formation, according to the theory, a white dwarf turns into a black dwarf - a body that does not emit visible light.
A white, red or blue star for the observer differs primarily in color. The astronomer looks deeper. Color for him immediately tells a lot about the temperature, size and mass of the object. A blue or bright blue star is a giant hot ball, far ahead of the Sun in all respects. White luminaries, examples of which are described in the article, are somewhat smaller. Star numbers in various catalogs also tell professionals a lot, but not all. A large amount of information about the life of distant space objects has either not yet been explained, or remains not even discovered.
Stars: their birth, life and death [Third edition, revised] Shklovsky Iosif Samuilovich
Chapter 10 How are white dwarfs arranged?
Chapter 10 How are white dwarfs arranged?
In § 1, when we discussed the physical properties of various stars plotted on the Hertzsprung-Russell diagram, attention was already drawn to the so-called "white dwarfs". A typical representative of this class of stars is the famous satellite of Sirius, the so-called "Sirius B". At the same time, it was emphasized that these strange stars are by no means a rare category of some pathological "freaks" in our Galaxy. On the contrary, it is a very numerous group of stars. There should be at least several billion of them in the Galaxy, and perhaps even ten billion, that is, up to 10% of all the stars in our giant star system. Therefore, white dwarfs must have been formed as a result of some natural process that took place in a noticeable part of the stars. And from this it follows that our understanding of the world of stars will be very far from complete if we do not understand the nature of white dwarfs and do not clarify the question of their origin. However, in this section we will not discuss issues related to the problem of the formation of white dwarfs - this will be done in § 13. Our task for now is to try to understand the nature of these amazing objects. The main features of white dwarfs are:
a. The mass is not too different from the mass of the Sun at a radius a hundred times smaller than that of the Sun. The dimensions of white dwarfs are of the same order as the dimensions of the globe.
b. From this follows a huge average density of the substance, reaching up to 10 6 -10 7 g / cm 3 (ie, up to ten tons, "pressed" in a cubic centimeter!).
c. The luminosity of white dwarfs is very low: hundreds and thousands of times less than the sun.
At the first attempt to analyze the conditions in the depths of white dwarfs, we immediately encounter a very great difficulty. In § 6, a relationship was established between the mass of a star, its radius, and central temperature (see formula (6.2)). Since the latter must be inversely proportional to the radius of the star, it would seem that the central temperatures of white dwarfs should reach enormous values of the order of many hundreds of millions of kelvins. At such monstrous temperatures, an unreasonably large amount of nuclear energy should have been released there. Even assuming that all the hydrogen there is "burned out", the triple helium reaction should be quite efficient. The energy released during nuclear reactions must "seep" to the surface and go into interstellar space in the form of radiation, which should have been extremely powerful. Meanwhile, the luminosity of white dwarfs is completely negligible, several orders of magnitude less than that of "ordinary" stars of the same mass. What's the matter here?
Let's try to understand this paradox.
First of all, such a strong discrepancy between the expected and observed luminosity means that formula (6.2) § 6 is simply not applicable to white dwarfs. Let us now recall what basic assumptions were made in the derivation of this formula. First of all, it was assumed that the star is in a state of equilibrium under the action of two forces: gravity and gas pressure. There is no doubt that white dwarfs are in a state of hydrostatic equilibrium, which we discussed in detail in § 6. Otherwise, they would cease to exist in a short time: they would disperse in interstellar space if the pressure exceeded gravity, or they would shrink “to a point”, if gravity were not compensated by gas pressure. There is also no doubt about the universality of the law of universal gravitation: the force of gravity acts everywhere and it does not depend on any other properties of matter, except for its quantity. Then there is only one possibility: to doubt the dependence of gas pressure on temperature, which we obtained using the well-known Clapeyron's law.
This law is valid for an ideal gas. In § 6 we have seen that the matter of the interiors of ordinary stars can be regarded with sufficient accuracy as an ideal gas. Therefore, the logical conclusion is that very dense matter bowels of white dwarfs already is not an ideal gas.
True, it is reasonable to doubt at all whether this substance is a gas? Maybe it's a liquid or a solid? It is easy to verify that this is not the case. Indeed, in liquids and solids are densely packed atoms, which are in contact with their electron shells, which are not so small in size: about 10 -8 cm. Closer than this distance, atomic nuclei, in which almost the entire mass of atoms is concentrated, cannot “move” to each other. It follows directly from this that the average density of a solid or liquid substance cannot significantly exceed
20 g/cm3. The fact that the average density of matter in white dwarfs can be tens of thousands of times greater means that the nuclei there are located at distances much less than 10 -8 cm from each other. It follows that the electron shells of atoms are, as it were, "crushed and the nuclei are separated from the electrons. In this sense, we can speak of the matter in the interior of white dwarfs as a very dense plasma. But plasma is primarily a gas, i.e., such a state of matter when the distance between the particles that form it significantly exceeds the size of the latter. In our case, the distance between the nuclei is not less than
10 -10 cm, while the size of the nuclei is negligible - about 10 -12 cm.
So, the substance of the bowels of white dwarfs is a very dense ionized gas. However, due to its enormous density, its physical properties differ sharply from those of an ideal gas. Do not confuse this difference between properties and properties. real gases, about which a lot is said in the course of physics.
The specific properties of an ionized gas at ultrahigh densities are determined by degeneration. This phenomenon can only be explained in terms of quantum mechanics. The concept of "degeneration" is alien to classical physics. What is it? To answer this question, we will first have to dwell a little on the features of the motion of electrons in an atom, described by the laws of quantum mechanics. The state of each electron in an atomic system is determined by the assignment of quantum numbers. These numbers are the main thing quantum number n, which determines the energy of an electron in an atom, the quantum number l, giving the value of the orbital angular momentum of the electron, the quantum number m, which gives the value of the projection of this moment on a physically selected direction (for example, the direction of the magnetic field), and, finally, the quantum number s, giving the value own torque electron (spin). The fundamental law of quantum mechanics is Pauli principle, which forbids for any quantum system (for example, complex atom) to any two electrons to have all quantum numbers the same. Let us explain this principle using a simple semiclassical Bohr model of the atom. The totality of three quantum numbers (except spin) determines the orbit of an electron in an atom. The Pauli principle, as applied to this model of the atom, prohibits more than two electrons from being in the same quantum orbit. If there are two electrons in such an orbit, then they must have oppositely oriented spins. This means that although the three quantum numbers of such electrons may coincide, the quantum numbers characterizing the spins of the electrons must be different.
The Pauli principle is of great importance for all atomic physics. In particular, only on the basis of this principle can one understand all the features periodic system elements of Mendeleev. The Pauli principle has a universal meaning and is applicable to all quantum systems consisting of a large number of identical particles. An example of such a system, in particular, are ordinary metals at room temperature. As is known, in metals, the outer electrons are not associated with "own" nuclei, but are, as it were, "socialized". They move in the complex electric field of the ionic metal lattice. In a rough, semiclassical approximation, one can imagine that the electrons move along some, however, very complex trajectories. And of course, for such trajectories, the Pauli principle must also be satisfied. This means that no more than two electrons can move along each of the electron trajectories mentioned above, which must differ in their spins. It must be emphasized that, according to quantum mechanical laws, the number of such possible trajectories, although very large, is finite. Consequently, not all geometrically possible orbits are realized.
In fact, of course, our reasoning is quite simplistic. We talked about "trajectories" above for clarity. Instead of the classical picture of motion along a trajectory, quantum mechanics speaks only of able electron, described by several completely defined (“quantum”) parameters. In each of the possible states, the electron has some specific energy. Within the framework of our model of movement along trajectories, the Pauli principle can also be formulated as follows: no more than two electrons can move along the same “allowed” trajectory with the same speed (i.e., have the same energy).
As applied to complex, many-electron atoms, the Pauli principle makes it possible to understand why their electrons did not “fall” into the “deepest” orbits, the energy of which is minimal. In other words, it gives the key to understanding the structure of the atom. The situation is exactly the same in the case of electrons in a metal, and in the case of matter in the interiors of white dwarfs. If the same number of electrons and atomic nuclei filled a sufficiently large volume, then "there would be enough room for everyone." But imagine now that this volume limited. Then only a small part of the electrons would occupy all possible trajectories for their movement, the number of which is necessarily limited. The rest of the electrons would have to move along the same trajectories that are already "occupied". But by virtue of the Pauli principle, they will move along these trajectories at high speeds and, therefore, have greater energy. The situation is exactly the same as in a many-electron atom, where, due to the same principle, the "excess" electrons obliged move in orbits with more energy.
In a piece of metal or in some volume inside a white dwarf, the number of electrons is greater than the number of allowed trajectories of motion. The situation is different in ordinary gas, in particular, in the interiors of main-sequence stars. There the number of electrons is always smaller number of allowed trajectories. Therefore, electrons can move along different trajectories with different speeds, as if "without interfering" with each other. The Pauli principle in this case does not affect their movement. In such a gas, the Maxwellian distribution of velocities is established and the laws of the gaseous state of matter, well known from school physics, are fulfilled, in particular, Clapeyron's law. If the "ordinary" gas is strongly compressed, then the number of possible trajectories for electrons will become much smaller and, finally, a state will come when each trajectory will have more than two electrons. By virtue of the Pauli principle, these electrons must have different velocities exceeding a certain critical value. If we now strongly cool this compressed gas, then the electron velocities will by no means decrease. Otherwise, as is easy to understand, the Pauli principle would cease to hold. Even near absolute zero, the velocities of electrons in such a gas would remain high. A gas with such extraordinary properties is called degenerate. The behavior of such a gas is entirely explained by the fact that its particles (in our case, electrons) occupy all possible trajectories and move along them “by necessity” at very high speeds. In contrast to a degenerate gas, the velocities of particles in an "ordinary" gas become very small as its temperature decreases. Accordingly, its pressure also decreases. What about the pressure of the degenerate gas? To do this, remember what we call gas pressure. This is the impulse that gas particles transfer in one second of time when they collide with some “wall” that limits its volume. From this it is clear that the pressure of the degenerate gas must be very high, since the velocities of the particles forming it are high. Even with very low temperatures the pressure of the degenerate gas must remain high, since the velocities of its particles, in contrast to ordinary gas, almost do not decrease with decreasing temperature. It should be expected that the pressure of a degenerate gas depends little on its temperature, since the velocity of the particles forming it is determined primarily by the Pauli principle.
Along with electrons in the depths of white dwarfs, there should be “bare” nuclei, as well as highly ionized atoms that have retained “inner” electron shells. It turns out that for them the number of "allowed" trajectories is always greater than the number of particles. Therefore, they form not a degenerate, but a "normal" gas. Their speeds are determined by the temperature of the matter of white dwarfs and are always much less than the speeds of electrons, due to the Pauli principle. Therefore, in the depths of white dwarfs, the pressure is due only to the degenerate electron gas. It follows that the equilibrium of white dwarfs is almost independent of their temperature.
As quantum mechanical calculations show, the pressure of a degenerate electron gas, expressed in atmospheres, is determined by the formula
| (10.1) |
where is the constant K = 3
10 6 and the density
expressed, as usual, in grams per cubic centimeter. Formula (10.1) replaces the Clapeyron equation for a degenerate gas and is its "equation of state". A characteristic feature of this equation is that temperature is not included in it. In addition, in contrast to the Clapeyron equation, where the pressure is proportional to the first power of density, here the dependence of pressure on density is stronger. This is not difficult to understand. After all, pressure is proportional to the concentration of particles and their speed. The concentration of particles, naturally, is proportional to the density, and the velocity of particles of a degenerate gas increases with increasing density, since, according to the Pauli principle, the number of "excess" particles that are forced to move at high speeds increases.
The condition for the applicability of formula (10.1) is the smallness of the thermal velocities of electrons in comparison with the velocities due to "degeneracy". At very high temperatures, formula (10.1) should go over into Clapeyron's formula (6.2). If the pressure obtained for a gas with density
by formula (10.1), more than according to formula (6.2), which means that the gas is degenerate. This results in the "degeneracy condition"
|
(10.2) |
Average molecular weight. What does it matter
in the depths of white dwarfs? First of all, there should be practically no hydrogen there: at such enormous densities and rather high temperatures, it has long been “burned out” in nuclear reactions. The main element in the depths of white dwarfs should be helium. Since its atomic mass is 4 and it gives two electrons during ionization (it must also be taken into account that only electrons are the particles that produce pressure), then the average molecular mass should be very close to 2. Numerically, the degeneracy condition (10.2) is written like this:
|
(10.3) |
If, for example, the temperature T= 300 K (room temperature), then
> 2, 510 -4 g/cm 3 . This is a very low density, from which it immediately follows that the electrons in metals must be degenerate (in fact, in this case, the constants K and
have a different meaning, but the essence of the matter remains the same). If the temperature T close to the temperature of the stellar interior, i.e., about 10 million kelvins, then > 1000 g/cm3. Two conclusions immediately follow from this:
a. In the interiors of ordinary stars, where the density, although high, is certainly below 1000 g/cm 3 , the gas is not degenerate. This justifies the applicability of the usual laws of the gaseous state, which we made extensive use of in § 6.
b. White dwarfs have average, and even more so, central densities that are obviously greater than 1000 g/cm 3 . Therefore, the usual laws of the gaseous state are not applicable to them. To understand white dwarfs, it is necessary to know the properties of a degenerate gas, described by its equation of state (10.1). First of all, it follows from this equation that the structure of white dwarfs is practically independent of their temperature. Since, on the other hand, the luminosity of these objects is determined by their temperature (for example, the rate of thermonuclear reactions depends on temperature), we can conclude that the structure of white dwarfs does not depend on luminosity either. In principle, a white dwarf can exist (i.e., be in an equilibrium configuration) even at a temperature close to absolute zero. We thus come to the conclusion that for white dwarfs, unlike "ordinary" stars, there is no "mass - luminosity" dependence.
For these unusual stars, however, there is a specific mass-radius relationship. Just as balls of the same mass made from the same metal must have the same diameter, white dwarfs with the same mass must also have the same size. This statement is obviously not true for other stars: giant stars and main sequence stars can have the same masses, but significantly different diameters. This difference between white dwarfs and other stars is explained by the fact that temperature plays almost no role in their hydrostatic equilibrium, which determines the structure.
Since this is the case, there must be some universal relationship between the masses of white dwarfs and their radii. It is not our task to derive this important dependence, which is far from being elementary. The dependence itself (on a logarithmic scale) is shown in fig. 10.1. In this figure, circles and squares mark the position of some white dwarfs with known masses and radii. The dependence of mass and radius shown in this figure for white dwarfs has two interesting features. First, it follows from it that the greater the mass of a white dwarf, the smaller its radius. In this regard, white dwarfs behave differently than balls made of a single block of metal ... Secondly, white dwarfs have a limit permissible value masses[ 27 ]. The theory predicts that white dwarfs cannot exist in nature, the mass of which would exceed 1.43 solar masses [28]. If the mass of a white dwarf approaches this critical value from the side of smaller masses, then its radius will tend to zero. In practice, this means that starting from a certain mass, the pressure of the degenerate gas can no longer balance the gravitational force and the star will collapse catastrophically.
This result is exclusively great importance for the whole problem of stellar evolution. Therefore, it is worth dwelling on it in more detail. As the mass of a white dwarf increases, its central density will increase more and more. The degeneracy of the electron gas will become stronger and stronger. This means that one "allowed" trajectory will have an increasing number of particles. They will be very “cramped” and they will (in order not to violate the Pauli principle!) move with greater and greater speeds. These speeds will become quite close to the speed of light. A new state of matter will arise, which is called "relativistic degeneracy". The equation of state for such a gas will change - it will no longer be described by formula (10.1). Instead of (10.1), we will have the relation
| (10.4) |
To assess the situation that has arisen, let us, as was done in § 6,
M/R 3 . Then under relativistic degeneracy P M 4/ 3 /R 4 , and the force opposing gravity and equal to the pressure drop,Meanwhile, the gravitational force is
GM/R 2 M 2 /R 5 . We see that both forces - gravity and pressure drop - depend on the size of the star in the same way: as R-5 , and depend differently on the mass. Therefore, there must be some, quite definite value of the mass of the star, at which both forces are balanced. If the mass exceeds a certain critical value, then the gravitational force will always prevail over the force due to the pressure drop, and the star will shrink catastrophically.Let us now assume that the mass is less than the critical mass. Then the force due to pressure will be greater than the gravitational force, therefore, the star will begin to expand. In the process of expansion, the relativistic degeneracy will be replaced by the usual "non-relativistic" degeneracy. In this case, from the equation of state P
5/ 3 it follows that P/R M 5/ 3 /R 6 , i.e., the dependence of the force opposing gravity on R will be stronger. Therefore, at a certain value of the radius, the expansion of the star will stop.
This qualitative analysis illustrates, on the one hand, the need for the existence of the mass-radius dependence for white dwarfs and its nature (i.e., the smaller the radius, the greater the mass), and, on the other hand, justifies the existence of a limiting mass, which is a consequence of the inevitability of the coming relativistic degeneracy. How long can stars with a mass greater than 1.2 solar masses shrink? This fascinating, which has become last years very topical, the problem will be discussed in § 24.
The substance of the interiors of white dwarfs is characterized by high transparency and thermal conductivity. The good transparency of this substance is again explained by the Pauli principle. After all, the absorption of light in matter is associated with a change in the state of electrons due to their transitions from one orbit to another. But if the overwhelming majority of "orbits" (or "trajectories") in a degenerate gas are "occupied", then such transitions are very difficult. Only a very few, especially the fast electrons in the plasma of a white dwarf, can absorb radiation quanta. The thermal conductivity of a degenerate gas is high - ordinary metals serve as an example. Due to the very high transparency and thermal conductivity, large temperature drops cannot occur in the matter of a white dwarf. Almost the entire temperature drop, if you move from the surface of a white dwarf to its center, occurs in a very thin, outer layer of matter, which is in a non-degenerate state. In this layer, the thickness of which is about 1% of the radius, the temperature rises from a few thousand kelvins at the surface to about ten million kelvins, and then almost does not change all the way to the center of the star.
White dwarfs, although weakly, still radiate. What is the energy source of this radiation? As already emphasized above, there is practically no hydrogen, the main nuclear fuel, in the bowels of white dwarfs. Almost all of it burned out at the stages of the star's evolution that preceded the white dwarf stage. But, on the other hand, spectroscopic observations clearly indicate that there is hydrogen in the outermost layers of white dwarfs. He either did not have time to burn out, or (more likely) got there from the interstellar medium. It is possible that the source of energy for white dwarfs can be hydrogen nuclear reactions occurring in a very thin spherical layer at the boundary of the dense degenerate matter of their interior and atmosphere. In addition, white dwarfs can maintain a fairly high surface temperature through normal thermal conduction. This means that white dwarfs that have no energy sources cool down, radiating at the expense of their heat reserves. And these reserves are very solid. Since the motions of electrons in the matter of white dwarfs are due to the phenomenon of degeneracy, the reserve of heat in their depths is contained in the nuclei and ionized atoms. Assuming that the matter of white dwarfs consists mainly of helium (atomic weight is 4), it is easy to find the amount of thermal energy contained in a white dwarf:
|
(10.5) |
where m H is the mass of the hydrogen atom, k is the Boltzmann constant. The cooling time of a white dwarf can be estimated by dividing E T to its luminosity L. It turns out to be of the order of several hundred million years.
On fig. 10.2 for a number of white dwarfs shows the empirical dependence of the luminosity on the surface temperature. Straight lines are locus of constant radii. The latter are expressed in fractions of the solar radius. The empirical points seem to fit well along these lines. This means that the observed white dwarfs are in different stages of cooling.
In recent years, a strong splitting of spectral absorption lines due to the Zeeman effect has been discovered for a dozen white dwarfs. It follows from the magnitude of the splitting that the magnetic field strength on the surface of these stars reaches of great importance about ten million oersteds (E). Such a large value of the magnetic field, apparently, is explained by the conditions for the formation of white dwarfs. For example, if we assume that the star contracts without significant loss of mass, we can expect that magnetic flux(i.e., the product of the surface area of the star and the magnetic field strength) retains its value. It follows that the strength of the magnetic field as the star contracts will grow in inverse proportion to the square of its radius. Therefore, it can grow hundreds of thousands of times. This mechanism of increasing the magnetic field is especially important for neutron stars, which will be discussed in § 22 [ 29 ]. It is interesting to note that most white dwarfs do not have a field stronger than a few thousand oersteds. Thus, "magnetized" white dwarfs form a special group among stars of this type. "Black" and "white holes" of the universe In March 1974, a curious announcement appeared at the entrance of the P. N. Lebedev State Astronomical Institute of the USSR Academy of Sciences. At the joint seminar, a paper was to be read entitled "Do White Holes Explode?". Scientific
From the book Prince from the Land of the Clouds author Galfar ChristophCHAPTER 4 Pressing his ear to the wall, Tristam listened to the sound of Lazurro's footsteps dying away. Meanwhile, Tom was examining the bottom door that had stopped their fall. "Is everything all right?" - Tristam asked in a whisper, returning to his friend. - No, not at all! It was better to go out and confess everything. They are
From the book Eye and Sun author Vavilov Sergey IvanovichChapter 7 At this evening hour the square was almost deserted. Tristam moved forward with a decisive step, but then he was called out. - And what are you doing here? Hey! Village! I'm telling you! Didn't Lazurro grab you in the library? It was Jerry, the son of the head of the cloud
From the book Interstellar: the science behind the scenes author Thorn Kip StevenChapter 8 Leaving Tristam at the far end of the garden, Tom went up to his room and began to put on dry clothes. The bell rang again, it was time to go to the table. One thing interfered: Tom could not forget about the book from the secret library. Even changing clothes, he did not tear away from her
From the author's bookChapter 16 The wind was blowing harder. The stalks of rice whisks mercilessly whipped Tom and Tristam, who were running away from their pursuers. Distraught with fear, the boys thought only of catching up with Mrs. Drake. It was not far to the protective fence. Near the city limits Tristam's mother
From the author's bookChapter 1 Tristam and Tom flew very high, much higher than the clouds rise natural origin. More than an hour had passed since they left behind the icy veil from which the tyrant's troops fell upon Myrtilville. The sky here was not the same as over their town:
From the author's bookChapter 2 The stars of the Milky Way twinkled in the sky. From the beginning of the flight, Tom did not utter a word, but Tristam felt that his friend was no longer as gloomy as before. “At night, the Sun illuminates the other side of the Earth,” Tom suddenly spoke. Tristam turned around. “What are you talking about?” “About the sky. You
From the author's bookChapter 3 It was dawning. Space and stars gradually disappeared. The sky was filled with light and lost transparency. It got very, very cold. And very quietly: nothing seemed to portend trouble. Tom and Tristam were asleep. They did not see that on the control panel has been blinking for a long time
From the author's bookChapter 4 - Comes to his senses, - said a female voice. Tristam opened his eyes. He was lying on a bed, next to which were three: a man and two women. The ceiling of the room he was in was painted dark green. The walls were also green, but in a lighter shade. There were no windows
From the author's bookCHAPTER 5 When the hospital door opened and let the convoy out, Tristam involuntarily closed his eyes bright light. The peaks of the cloudy hepteplums that surrounded the city sparkled with such pure and dazzling whiteness that he had to follow the policemen with his eyes closed. So,
From the author's bookCHAPTER 6 The prison, with blind walls without a single window, was located deep in the bowels of the cloud on which the White Capital was built. Once in the cell, frightened Tristam and Tom sat silently for some time on the bed assigned to them for two - in reality they were
From the author's bookChapter 7 Hours passed. Tristam and Tom lay on hard bunks in a dark, windowless cell, constantly tossing and turning from side to side. As soon as the flute stopped singing, the old man immediately dozed off, mumbling something unintelligible in his sleep. Tom began to shiver again; I understood Tristam
From the author's bookCHAPTER 8 Thick smoke from the chimneys mingled with the cool, damp dawn air. At all crossroads in the center of the White Capital, snowmen were posted. They looked less like law enforcement officers than like occupying troops. Tristam and Tom, in
From the author's book From the author's bookStar Death: White Dwarfs, Neutron Stars, and Black Holes The Sun and Earth are about 4.5 billion years old, about a third of the age of the universe. After about another 6.5 billion years, the core of the Sun will run out of nuclear fuel that keeps the Sun hot. Then will begin
neutron starCalculations show that the explosion of a supernova with M ~ 25M leaves a dense neutron core (neutron star) with a mass of ~ 1.6M . In stars with a residual mass M > 1.4M that have not reached the supernova stage, the pressure of the degenerate electron gas is also unable to balance gravitational forces and the star shrinks to a state of nuclear density. The mechanism of this gravitational collapse is the same as in a supernova explosion. The pressure and temperature inside the star reach such values at which electrons and protons seem to be “pressed” into each other and as a result of the reaction
after the ejection of neutrinos, neutrons are formed, occupying a much smaller phase volume than electrons. A so-called neutron star appears, the density of which reaches 10 14 - 10 15 g/cm 3 . characteristic size neutron star 10 - 15 km. In a sense, a neutron star is a giant atomic nucleus. Further gravitational contraction is prevented by the pressure of nuclear matter, which arises due to the interaction of neutrons. This is also the degeneracy pressure, as earlier in the case of a white dwarf, but is the degeneracy pressure of a much denser neutron gas. This pressure is able to hold masses up to 3.2M.
The neutrinos produced at the moment of collapse cool the neutron star rather quickly. According to theoretical estimates, its temperature drops from 10 11 to 10 9 K in ~ 100 s. Further, the rate of cooling decreases somewhat. However, it is quite high in astronomical terms. The decrease in temperature from 10 9 to 10 8 K occurs in 100 years and to 10 6 K in a million years. Detecting neutron stars with optical methods is quite difficult due to their small size and low temperature.
In 1967, at the University of Cambridge, Hewish and Bell discovered cosmic sources of periodic electromagnetic radiation - pulsars. The pulse repetition periods of most pulsars lie in the range from 3.3·10 -2 to 4.3 s. According to modern concepts, pulsars are rotating neutron stars with a mass of 1 - 3M and a diameter of 10 - 20 km. Only compact objects with the properties of neutron stars can maintain their shape without collapsing at such rotational speeds. The conservation of angular momentum and magnetic field during the formation of a neutron star leads to the birth of rapidly rotating pulsars with a strong magnetic field B ~ 10 12 G.
It is believed that a neutron star has a magnetic field whose axis does not coincide with the axis of rotation of the star. In this case, the radiation of the star (radio waves and visible light) glides across the Earth like the rays of a beacon. When the beam crosses the Earth, an impulse is registered. The very radiation of a neutron star arises due to the fact that charged particles from the surface of the star move outward along lines of force magnetic field, emitting electromagnetic waves. This pulsar radio emission mechanism, first proposed by Gold, is shown in Fig. 39.
If the radiation beam hits an earthly observer, then the radio telescope detects short pulses of radio emission with a period equal to the rotation period of the neutron star. The shape of the pulse can be very complex, which is due to the geometry of the magnetosphere of a neutron star and is characteristic of each pulsar. The rotation periods of pulsars are strictly constant and the measurement accuracy of these periods reaches 14-digit figures.
Pulsars that are part of binary systems have now been discovered. If the pulsar orbits around the second component, then variations in the period of the pulsar due to the Doppler effect should be observed. When the pulsar approaches the observer, the recorded period of radio pulses decreases due to the Doppler effect, and when the pulsar moves away from us, its period increases. Based on this phenomenon, pulsars that are part of binary stars were discovered. For the first discovered pulsar PSR 1913 + 16, which is part of a binary system, the orbital period of revolution was 7 hours 45 minutes. The proper period of revolution of the pulsar PSR 1913 + 16 is 59 ms.
The radiation of the pulsar should lead to a decrease in the speed of rotation of the neutron star. Such an effect was also found. A neutron star, which is part of a binary system, can also be a source of intense X-rays.
The structure of a neutron star with a mass of 1.4M and a radius of 16 km is shown in Fig. 40.
I - thin outer layer of densely packed atoms. In regions II and III, the nuclei are arranged in the form of a body-centered cubic lattice. Region IV consists mainly of neutrons. In region V, matter can consist of pions and hyperons, forming the hadronic core of a neutron star. Individual details of the structure of a neutron star are currently being specified.
The formation of neutron stars is not always the result of a supernova explosion. Another mechanism for the formation of neutron stars during the evolution of white dwarfs in close binary star systems is also possible. The flow of matter from the companion star to the white dwarf gradually increases the mass of the white dwarf, and upon reaching the critical mass (the Chandrasekhar limit), the white dwarf turns into a neutron star. In the case when the flow of matter continues after the formation of a neutron star, its mass can increase significantly and, as a result of gravitational collapse, it can turn into a black hole. This corresponds to the so-called “silent” collapse.
Compact double stars can also appear as sources of X-rays. It also arises due to the accretion of matter falling from a “normal” star onto a more compact one. During accretion of matter onto a neutron star with B > 10 10 G, the matter falls into the region of the magnetic poles. X-ray radiation is modulated by its rotation around the axis. Such sources are called X-ray pulsars.
There are X-ray sources (called bursters) in which bursts of radiation occur periodically at intervals of several hours to days. The characteristic burst rise time is 1 sec. Burst duration from 3 to 10 sec. The intensity at the time of the burst can be 2–3 orders of magnitude greater than the luminosity at calm state. At present, several hundred such sources are known. It is believed that bursts of radiation occur as a result of thermonuclear explosions of matter accumulated on the surface of a neutron star as a result of accretion.
It is well known that at small distances between nucleons (<
0.3·10 -13 см) ядерные силы притяжения сменяются силами
оттал-кивания, т. е. противодействие ядерного вещества на малых расстояниях
сжимающей силе тяготения увеличивается. Если плотность вещества в центре
нейтронной звезды превышает ядерную плотность ρ яд
и достигает 10 15 г/см 3 , то в центре звезды наряду с
нуклонами и электронами образуются также мезоны, гипероны и другие более
массивные частицы. Исследования поведения вещества при плотностях, превышающих
ядерную плотность, в настоящее время находятся в начальной стадии и имеется
много нерешенных проблем. Расчеты показывают, что при плотностях вещества
ρ >ρ poison, such processes as the appearance of a pion condensate, the transition of a neutronized substance to a solid crystalline state, the formation of hyperon and quark-gluon plasma are possible. The formation of superfluid and superconducting states of neutron matter is possible.
In accordance with modern ideas about the behavior of matter at densities 10 2 - 10 3 times higher than the nuclear one (namely, such densities are discussed when the internal structure of a neutron star is discussed), atomic nuclei are formed inside the star near the stability limit. A deeper understanding can be achieved as a result of studying the state of matter depending on the density, temperature, stability of nuclear matter with exotic ratios of the number of protons to the number of neutrons in the nucleus n p /n n , taking into account weak processes involving neutrinos. At present, nuclear reactions between heavy ions are practically the only way to study matter at densities greater than nuclear. However, the experimental data on the collision of heavy ions do not yet provide enough information, because the achievable values of n p /n n both for the target nucleus and for the incident accelerated nucleus are small (~ 1 - 0.7).
Accurate measurements of the periods of radio pulsars have shown that the speed of rotation of a neutron star is gradually slowing down. This is due to the transition of the kinetic energy of the star's rotation into the radiation energy of the pulsar and to the emission of neutrinos. Small jumps in the periods of radio pulsars are explained by the accumulation of stresses in the surface layer of a neutron star, accompanied by “cracking” and “breaks”, which leads to a change in the speed of rotation of the star. The observed temporal characteristics of radio pulsars contain information about the properties of the "crust" of a neutron star, the physical conditions inside it, and the superfluidity of neutron matter. AT recent times a significant number of radio pulsars with periods less than 10 ms have been discovered. This requires a refinement of ideas about the processes occurring in neutron stars.
Another problem is the study of neutrino processes in neutron stars. The emission of neutrinos is one of the mechanisms of energy loss by a neutron star during 10 5 - 10 6 years after its formation.
