White dwarf is a star that is quite common in our space. Scientists call it the result of the evolution of stars, the final stage of development. In total, there are two scenarios for the modification of a stellar body, in one case the final stage is a neutron star, in the other a black hole. Dwarfs are the final evolutionary step. They have planetary systems around them. Scientists were able to determine this by examining metal-enriched specimens.
Background
White dwarfs are stars that attracted the attention of astronomers in 1919. For the first time, such a celestial body was discovered by a scientist from the Netherlands, Maanen. For his time, the specialist made a rather atypical and unexpected discovery. The dwarf he saw looked like a star, but had non-standard small sizes. The spectrum, however, was as if it were a massive and large celestial body.
The reasons for such a strange phenomenon have attracted scientists for quite some time, so a lot of effort has been made to study the structure of white dwarfs. The breakthrough was made when they expressed and proved the assumption of the abundance of various metal structures in the atmosphere of a celestial body.
It is necessary to clarify that metals in astrophysics are all kinds of elements, the molecules of which are heavier than hydrogen, helium, and their chemical composition is more progressive than these two compounds. Helium, hydrogen, as scientists managed to establish, are more widespread in our universe than any other substances. Based on this, it was decided to designate everything else as metals.
Theme development
Although white dwarfs very different in size from the Sun were first seen in the twenties, only half a century later people discovered that the presence of metallic structures in the stellar atmosphere is not a typical phenomenon. As it turned out, when included in the atmosphere, in addition to the two most common substances, heavier ones, they are displaced into the deeper layers. Heavy substances, being among the molecules of helium, hydrogen, must eventually move to the core of the star.
There were several reasons for this process. The radius of a white dwarf is small, such stellar bodies are very compact - it is not for nothing that they got their name. On average, the radius is comparable to that of the earth, while the weight is similar to the weight of a star that illuminates our planetary system. This ratio of dimensions and weight causes an exceptionally large gravitational surface acceleration. Consequently, the deposition of heavy metals in the hydrogen and helium atmosphere occurs only a few Earth days after the molecule enters the total gaseous mass.
Features and duration
Sometimes characteristics of white dwarfsare such that the process of sedimentation of molecules of heavy substances can be delayed for a long time. The most favorable options, from the point of view of an observer from the Earth, are processes that take millions, tens of millions of years. Yet such time spans are exceptionally short compared to the lifetime of the stellar body itself.
The evolution of a white dwarf is such that most of the formations observed by man at the moment are already several hundred million Earth years old. If we compare this with the slowest process of absorption of metals by the nucleus, the difference is more than significant. Therefore, the detection of metal in the atmosphere of a certain observable star allows us to conclude with certainty that the body did not initially have such an atmospheric composition, otherwise all metal inclusions would have disappeared long ago.
Theory and practice
The observations described above, as well as information collected over many decades about white dwarfs, neutron stars, black holes, suggested that the atmosphere receives metallic inclusions from external sources. Scientists first decided that this is the medium between the stars. A celestial body moves through such matter, accretes the medium onto its surface, thereby enriching the atmosphere with heavy elements. But further observations showed that such a theory is untenable. As the experts specified, if the change in the atmosphere occurred in this way, the dwarf would mainly receive hydrogen from the outside, since the medium between the stars was formed in its bulk by hydrogen andhelium molecules. Only a small percentage of the medium is heavy compounds.
If the theory formed from primary observations of white dwarfs, neutron stars, black holes would justify itself, dwarfs would consist of hydrogen as the lightest element. This would not allow the existence of even helium celestial bodies, because helium is heavier, which means that hydrogen accretion would completely hide it from the eye of an external observer. Based on the presence of helium dwarfs, scientists came to the conclusion that the interstellar medium cannot serve as the only and even the main source of metals in the atmosphere of stellar bodies.
How to explain?
Scientists who studied black holes and white dwarfs in the 70s of the last century suggested that metallic inclusions can be explained by the fall of comets on the surface of a celestial body. True, at one time such ideas were considered too exotic and did not receive support. This was largely due to the fact that people did not yet know about the presence of other planetary systems - only our "home" solar system was known.
A significant step forward in the study of black holes, white dwarfs was made at the end of the next, the eighth decade of the last century. Scientists have at their disposal especially powerful infrared instruments for observing the depths of space, which made it possible to detect infrared radiation around one of the known white dwarf astronomers. This was revealed precisely around the dwarf, the atmosphere of which contained metallicinclusion.
Infrared radiation, which made it possible to estimate the temperature of the white dwarf, also told scientists that the stellar body is surrounded by some substance that can absorb stellar radiation. This substance is heated to a specific temperature level, less than that of a star. This allows you to gradually redirect the absorbed energy. Radiation occurs in the infrared range.
Science moves forward
The spectra of the white dwarf have become the object of study of the advanced minds of the world of astronomers. As it turned out, from them you can get quite a lot of information about the features of celestial bodies. Of particular interest were observations of stellar bodies with excess infrared radiation. At present, it has been possible to identify about three dozen systems of this type. Their main percentage was studied using the most powerful Spitzer telescope.
Scientists, observing celestial bodies, found that the density of white dwarfs is significantly less than this parameter, characteristic of giants. It was also found that excess infrared radiation is due to the presence of disks formed by a specific substance that can absorb energy radiation. It is it that then radiates energy, but in a different wavelength range.
The disks are exceptionally close and affect the mass of white dwarfs to some extent (which cannot exceed the Chandrasekhar limit). The outer radius is called the detrital disk. It has been suggested that it was formed during the destruction of some body. On average, the radius is comparable in size to the Sun.
If you pay attention to our planetary system, it becomes clear that relatively close to the "home" we can observe a similar example - these are the rings surrounding Saturn, the size of which is also comparable to the radius of our star. Over time, scientists have found that this feature is not the only one that dwarfs and Saturn have in common. For example, both the planet and the stars have very thin disks, which are not transparent when trying to shine through the light.
Conclusions and development of the theory
Because the rings of white dwarfs are comparable to those that surround Saturn, it has become possible to formulate new theories that explain the presence of metals in the atmosphere of these stars. Astronomers know that the rings around Saturn are formed by the tidal disruption of some bodies that are close enough to the planet to be affected by its gravitational field. In such a situation, the external body cannot maintain its own gravity, which leads to a violation of integrity.
About fifteen years ago, a new theory was presented that explained the formation of white dwarf rings in a similar way. It was assumed that initially the dwarf was a star in the center of the planetary system. The celestial body evolves over time, which takes billions of years, swells, loses its shell, and this causes the formation of a dwarf, which gradually cools down. By the way, the color of white dwarfs is explained precisely by their temperature. For some, it is estimated at 200,000 K.
The system of planets in the course of such an evolution can survive, which leads toexpansion of the outer part of the system simultaneously with a decrease in the mass of the star. As a result, a large system of planets is formed. Planets, asteroids and many other elements survive evolution.
What's next?
The progress of the system can lead to its instability. This leads to the bombardment of the space surrounding the planet by stones, and asteroids partially fly out of the system. Some of them, however, move into orbits, sooner or later finding themselves within the solar radius of the dwarf. Collisions do not occur, but tidal forces lead to a violation of the integrity of the body. A cluster of such asteroids takes on a shape similar to the rings surrounding Saturn. Thus, a debris disk is formed around the star. The density of the white dwarf (about 10^7 g/cm3) and its detrital disk differ significantly.
The described theory has become a fairly complete and logical explanation of a number of astronomical phenomena. Through it, one can understand why disks are compact, because a star cannot be surrounded by a disk with a radius comparable to the sun's during its entire existence, otherwise such disks would be inside its body at first.
By explaining the formation of discs and their size, one can understand where the peculiar supply of metals comes from. It could end up on the stellar surface, contaminating the dwarf with metal molecules. The described theory, without contradicting the revealed indicators of the average density of white dwarfs (of the order of 10^7 g/cm3), proves why metals are observed in the atmosphere of stars, why the measurement of the chemicalcomposition by means possibly accessible to man and for what reason the distribution of elements is similar to that characteristic of our planet and other studied objects.
Theories: is there any benefit?
The described idea was widely used as a basis for explaining why the shells of stars are contaminated with metals, why debris disks appeared. In addition, it follows from it that a planetary system exists around the dwarf. There is little surprise in this conclusion, because mankind has established that most of the stars have their own systems of planets. This is characteristic of both those that are similar to the Sun, and those that are much larger than its dimensions - namely, white dwarfs are formed from them.
Topics not exhausted
Even if we consider the theory described above to be generally accepted and proven, some questions for astronomers remain open to this day. Of particular interest is the specificity of the transfer of matter between the disks and the surface of a celestial body. As some suggest, this is due to radiation. Theories calling in this way to describe the transport of matter are based on the Poynting-Robertson effect. This phenomenon, under the influence of which particles slowly move in an orbit around a young star, gradually spiraling towards the center and disappearing in a celestial body. Presumably, this effect should manifest itself on the debris disks surrounding the stars, that is, the molecules that are present in the disks sooner or later find themselves in exceptional proximity to the dwarf. Solidsare subject to evaporation, gas is formed - such in the form of disks has been recorded around several observed dwarfs. Sooner or later, the gas reaches the surface of the dwarf, transporting metals here.
The revealed facts are estimated by astronomers as a significant contribution to science, as they suggest how the planets are formed. This is important, since the objects for research that attract specialists are often inaccessible. For example, planets revolving around stars larger than the Sun are extremely rare to study - it is too difficult at the technical level that is available to our civilization. Instead, people were able to study planetary systems after the transformation of stars into dwarfs. If we manage to develop in this direction, it will certainly be possible to reveal new data on the presence of planetary systems and their distinctive characteristics.
White dwarfs, in the atmosphere of which metals have been detected, allow us to get an idea of the chemical composition of comets and other cosmic bodies. In fact, scientists simply have no other way to assess the composition. For example, studying the giant planets, one can get an idea only of the outer layer, but there is no reliable information about the inner content. This also applies to our "home" system, since the chemical composition can only be studied from that celestial body that fell to the surface of the Earth or where it was possible to land the research apparatus.
How is it going?
Sooner or later, our planetary system will also become the "home" of a white dwarf. As scientists say, the stellar core hasa limited amount of matter to obtain energy, and sooner or later thermonuclear reactions are exhausted. The gas decreases in volume, the density rises to a ton per cubic centimeter, while in the outer layers the reaction is still proceeding. The star expands, becomes a red giant, the radius of which is comparable to hundreds of stars equal to the Sun. When the outer shell stops "burning", within 100,000 years there is a dispersion of matter in space, which is accompanied by the formation of a nebula.
The core of the star, freed from the shell, lowers the temperature, which leads to the formation of a white dwarf. In fact, such a star is a high-density gas. In science, dwarfs are often referred to as degenerate celestial bodies. If our star were compressed and its radius would be only a few thousand kilometers, but the weight would be completely preserved, then a white dwarf would also take place here.
Features and technical points
The type of cosmic body under consideration is capable of glowing, but this process is explained by other mechanisms than thermonuclear reactions. The glow is called residual, it is explained by a decrease in temperature. The dwarf is formed by a substance whose ions are sometimes colder than 15,000 K. Oscillatory motions are characteristic of the elements. Gradually, the celestial body becomes crystalline, its glow weakens, and the dwarf evolves into brown.
Scientists have identified a mass limit for such a celestial body - up to 1.4 the weight of the Sun, but not more than this limit. If the mass exceeds this limit,the star cannot exist. This is due to the pressure of a substance in a compressed state - it is less than the gravitational attraction that compresses the substance. There is a very strong compression, which leads to the appearance of neutrons, the substance is neutronized.
The compression process can lead to degeneration. In this case, a neutron star is formed. The second option is continued compression, sooner or later leading to an explosion.
General parameters and features
The bolometric luminosity of the category of celestial bodies under consideration is less than that of the Sun by about ten thousand times. The radius of the dwarf is less than a hundred times the sun, while the weight is comparable to that characteristic of the main star of our planetary system. To determine the mass limit for a dwarf, the Chandrasekhar limit was calculated. When it is exceeded, the dwarf evolves into another form of a celestial body. The photosphere of a star, on average, consists of dense matter, estimated at 105–109 g/cm3. Compared to the main sequence, it is about a million times denser.
Some astronomers believe that only 3% of all stars in the galaxy are white dwarfs, and some are convinced that every tenth belongs to this class. Estimates vary so much about the reason for the difficulty of observing celestial bodies - they are far from our planet and glow too faintly.
Stories and names
In 1785, a body appeared in the list of double stars, which Herschel was observing. The star was named 40 Eridani B. It is she who is considered the first person seen from the white category.dwarfs. In 1910, Russell noticed that this celestial body has an extremely low level of luminosity, although the color temperature is quite high. Over time, it was decided that celestial bodies of this class should be separated into a separate category.
In 1844, Bessel, studying the information obtained by tracking Procyon B, Sirius B, decided that both of them shifted from a straight line from time to time, which means that there are close satellites there. Such an assumption seemed unlikely to the scientific community, since no satellite could be seen, while the deviations could only be explained by a celestial body, the mass of which is exceptionally large (similar to Sirius, Procyon).
In 1962, Clark, working with the largest telescope in existence at the time, identified a very dim celestial body near Sirius. It was he who was called Sirius B, the same satellite that Bessel had suggested long before. In 1896, studies showed that Procyon also had a satellite - it was called Procyon B. Therefore, Bessel's ideas were fully confirmed.
