Dwarf star ore. Stars are dwarfs

Any star is a huge ball of gas, which consists of helium and hydrogen, as well as traces of other chemical elements. There are a huge number of stars and they all differ in size and temperature, and some of them consist of two or more stars that are connected by gravity. From Earth, some stars are visible to the naked eye, while others can only be seen through a telescope. However, even with special equipment, not every star can be viewed the way you want, and even in powerful telescopes, some stars will look like nothing more than just luminous points.

Thus, an ordinary person with fairly good visual acuity, in clear weather in the night sky, can see about 3000 stars from one earthly hemisphere, however, in fact, there are much more of them in the Galaxy. All stars are classified according to size, color, temperature. Thus, there are dwarfs, giants and supergiants.

Dwarf stars are of the following types:

• yellow dwarf. This type is a small main sequence star of spectral class G. Their mass ranges from 0.8 to 1.2 solar masses.
• orange dwarf. This type includes small main sequence stars of spectral class K. Their mass is 0.5 - 0.8 solar masses. Unlike yellow dwarfs, orange dwarfs have longer lifespans.
• red dwarf. This type unites small and relatively cool main sequence stars of spectral class M. Their differences from other stars are quite pronounced. They have a diameter and mass that is no more than 1/3 of the Solar one.
• blue dwarf This type of star is hypothetical. Blue dwarfs evolve from red dwarfs before burning out all their hydrogen, after which they presumably evolve into white dwarfs.
• white dwarf. This is a type of already evolved stars. They have a mass that is not more than the mass of Chandrasekhar. White dwarfs do not have their own source of thermonuclear energy. They belong to the spectral class DA.
• black dwarf. This type is a cooled white dwarf, which, accordingly, does not emit energy, i.e. do not glow, or emit it very, very weakly. They represent the final stage of the evolution of white dwarfs in the absence of accretion. The mass of black dwarfs, like white dwarfs, does not exceed the mass of Chandrasekhar.
• brown dwarf. These stars are substellar objects that have a mass from 12.57 to 80.35 Jupiter masses, which, in turn, corresponds to 0.012 - 0.0767 solar masses. Brown dwarfs differ from main sequence stars in that the nuclear fusion reaction that converts hydrogen into helium in other stars does not occur in their cores.
• subbrown dwarfs or brown subdwarfs. They are absolutely cold formations, the mass of which is below the limit of brown dwarfs. To a greater extent, they are considered to be planets.

So, it can be noted that stars classified as white dwarfs are those stars that are initially small in size and are at their final stage of evolution. The history of the discovery of white dwarfs goes back to the relatively recent year 1844. It was at that time that the German astronomer and mathematician Friedrich Bessel, while observing Sirius, discovered a slight deviation of the star from linear motion. As a result of this, Friedrich suggested that Sirius had an invisible massive companion star. This assumption was confirmed in 1862 by the American astronomer and telescope builder Alvan Graham Clark during the adjustment of the largest refractor at that time. A dim star was discovered near Sirius, which was later named Sirius B. This star is characterized by low luminosity, and its gravitational field affects its bright partner quite noticeably. This, in turn, confirms that this star has a very small radius with a significant mass.

Which stars are dwarfs

Dwarfs are evolved stars that have a mass that does not exceed the Chandrasekhar limit. The formation of a white dwarf occurs as a result of the burning of all hydrogen. When hydrogen burns out, the core of the star contracts to high densities, while at the same time the outer layers expand greatly and are accompanied by a general dimming of luminosity. Thus, the star first turns into a red giant, which sheds its shell. The shedding of the shell occurs due to the fact that the outer layers of the star have an extremely weak connection with the central hot and very dense core. Subsequently, this shell becomes an expanding planetary nebula. It is worth paying attention to the fact that red giants and white dwarfs have a very close relationship.

All white dwarfs are divided into two spectral groups. The first group includes dwarfs that have the “hydrogen” spectral class DA, in which there are no spectral lines of helium. This type is the most common. The second type of white dwarf is DB. It is rarer and is called a helium white dwarf. No hydrogen lines were detected in the spectrum of stars of this type.

According to the American astronomer Iko Iben, these types of white dwarfs are formed in completely different ways. This is due to the fact that helium combustion in red giants is unstable and the development of layered helium flares periodically occurs. Iko Iben also suggested a mechanism by which the shell is shed at different stages of the development of a helium flash - at its peak and between flashes. Accordingly, its formation is influenced by the membrane shedding mechanism.

With the exception of the Moon and all planets, every seemingly stationary object in the sky is a star - a thermonuclear energy source, and the types of stars range from dwarfs to supergiants.

Ours is a star, but it appears so bright and large because it is so close to us. Most stars look like luminous points even in powerful telescopes and, nevertheless, we know something about them. So, we know that they come in different sizes and that at least half of them consist of two or more stars bound by gravity.

What is a star?

Stars- These are huge gas balls of hydrogen and helium with traces of other chemical elements. Gravity pulls the substance in, and the pressure of the hot gas pushes it out, establishing equilibrium. The source of a star's energy lies in its core, where millions of tons of hydrogen fuse every second to form helium. And although this process has been going on continuously in the depths of the Sun for almost 5 billion years, only a very small part of all hydrogen reserves has been used up.

Types of stars

Main sequence stars. At the beginning of the 20th century. Dutchman Einar Hertzsprung and Henry Norris Russell from the USA constructed a Hertzsprung-Russell (HR) diagram, along the axes of which the luminosity of a star is plotted depending on the temperature on its surface, which makes it possible to determine the distance to the stars.

Most stars, including the Sun, fall into a band that cuts diagonally across the HR diagram, called the main sequence. These stars are often called dwarfs, although some of them are 20 times larger than the Sun and shine 20 thousand times brighter.

Red dwarfs

At the cool, dim end of the main sequence are red dwarfs, the most common type of star. Being smaller than the Sun, they use their fuel reserves sparingly to extend their existence by tens of billions of years. If all red dwarfs could be seen, the sky would be literally littered with them. However, red dwarfs shine so faintly that we can only observe the closest ones, such as Proxima Centauri.

White dwarfs

Even smaller in size than red dwarfs are white dwarfs. Typically, their diameter is approximately equal to that of the Earth, but their mass can be equal to that of the Sun. A volume of white dwarf matter equal to the volume of this book would have a mass of about 10 thousand tons! Their position on the HR diagram shows that they are very different from red dwarfs. Their nuclear source has become depleted.

Red giants

After main sequence stars, the most common are red giants. They have about the same surface temperature as red dwarfs, but they are much brighter and larger, so they are located above the main sequence on the HR diagram. The mass of these giants is usually approximately equal to the sun, however, if one of them took the place of our star, the inner planets of the solar system would end up in its atmosphere.

Supergiants

At the top of the GR diagram are rare supergiants. Betelgeuse, in Orion's shoulder, is almost 1 billion km across. Another bright object in Orion is Rigel, one of the brightest stars visible to the naked eye. It is almost ten times smaller than Betelgeuse and at the same time almost 100 times larger than the size of the Sun.

White dwarfs are stars with a large mass (on the order of the Sun) and a small radius (the radius of the Earth), which is less than the Chandrasekhar limit for the selected mass, and are a product of the evolution of red giants. The process of producing thermonuclear energy in them has been stopped, which leads to the special properties of these stars. According to various estimates, in our Galaxy their number ranges from 3 to 10% of the total stellar population.

In 1844, the German astronomer and mathematician Friedrich Bessel, during his observations, discovered a slight deviation of the star from rectilinear motion, and made the assumption that Sirius had an invisible massive companion star.

His assumption was confirmed already in 1862, when the American astronomer and telescope builder Alvan Graham Clark, while adjusting the largest refractor at that time, discovered a dim star near Sirius, which was later dubbed Sirius B.

The white dwarf Sirius B has a low luminosity, and the gravitational field affects its bright companion quite noticeably, indicating that this star has an extremely small radius and a significant mass. This is how a type of object called white dwarfs was discovered for the first time. The second similar object was the star Maanen, located in the constellation Pisces.

Education mechanism

White dwarfs represent the final stage of evolution of a small star with a mass comparable to the mass of the Sun. When do they appear? When all the hydrogen in the center of a star, like our Sun, burns out, its core contracts to high densities, while the outer layers expand greatly, and, accompanied by a general dimming of luminosity, the star turns into a red giant. The pulsating red giant then sheds its envelope as the outer layers of the star are loosely connected to the central hot and very dense core. This shell subsequently becomes an expanding planetary nebula. As you can see, red giants and white dwarfs are very closely related.

The compression of the core occurs to extremely small sizes, but, nevertheless, does not exceed the Chandrasekhar limit, that is, the upper limit of the mass of a star at which it can exist as a white dwarf.

Types of white dwarfs

Spectrally, they are divided into two groups. The emission from a white dwarf is divided into the most common “hydrogen” spectral type DA (up to 80% of the total), which lacks helium spectral lines, and the rarer “helium white dwarf” type DB, whose stellar spectra lack hydrogen lines.

American astronomer Iko Iben proposed various scenarios for their origin: due to the fact that helium combustion in red giants is unstable, a layered helium flare periodically develops. He successfully suggested a mechanism for shedding the shell at different stages of the development of a helium flash - at its peak and in the period between two flashes. Its formation depends on the shell shedding mechanism, respectively.

Degenerate gas

Before Ralph Fowler explained the density and pressure characteristics inside white dwarfs in his 1922 work Dense Matter, the high density and physical features of such a structure seemed paradoxical. Fowler suggested that, unlike main sequence stars, for which the equation of state is described by the properties of an ideal gas, in white dwarfs it is determined by the properties of a degenerate gas.

A graph of the radius of a white dwarf versus its mass. Note that the ultrarelativistic Fermi gas limit is the same as the Chandrasekhar limit

A degenerate gas is formed when the distance between its particles becomes less than the de Broglie wave, which means that quantum mechanical effects caused by the identity of gas particles begin to affect its properties.

In white dwarfs, due to their enormous densities, the shells of atoms are destroyed under the force of internal pressure, and the matter becomes electron-nuclear plasma, and the electronic part is described by the properties of a degenerate electron gas, similar to the behavior of electrons in metals.

Among them, the most common are carbon-oxygen ones with a shell consisting of helium and hydrogen.

Statistically, the radius of the white dwarf is comparable to the radius of the Earth, and its mass varies from 0.6 to 1.44 solar masses. The surface temperature is in the range of up to 200,000 K, which also explains their color.

Core

The main characteristic of the internal structure is the very high density of the core, in which gravitational equilibrium is caused by a degenerate electron gas. The temperature in the interior of the white dwarf and gravitational compression are balanced by the pressure of the degenerate gas, which ensures the relative stability of the diameter, and its luminosity mainly occurs due to the cooling and compression of the outer layers. The composition depends on how far the mother star has evolved; it is mainly carbon with oxygen and small admixtures of hydrogen and helium, which turn into degenerate gas.

Evolution

The helium burst and shedding of the outer shells by the red giant propels the star along the Hertzsprung-Russell diagram, determining its prevailing chemical composition. The life cycle of a white dwarf then remains stable until it cools, when the star loses its luminosity and becomes invisible, entering the stage of the so-called “black dwarf” - the end result of evolution, although this term is used less and less in modern literature.

The flow of matter from a star to a white dwarf, which is not visible due to low luminosity

The presence of nearby stellar companions prolongs their lives due to the fall of matter to the surface through the formation of an accretion disk. Features of the accretion of matter in paired systems can lead to the accumulation of matter on the surface of white dwarfs, which ultimately leads to the explosion of a nova or supernova (in the case of particularly massive ones) of type Ia.

An artist's impression of a supernova explosion

If accretion in the “white dwarf – red dwarf” system is nonstationary, the result may be a kind of explosion of a white dwarf (for example, U Gem (UG)) or nova-like variable stars, the explosion of which is catastrophic.

Supernova remnant SN 1006 is an exploded white dwarf that was located in a binary system. It gradually captured the matter of the companion star and the increasing mass provoked a thermonuclear explosion that tore apart the dwarf

Position on the Hertzsprung-Russell diagram

In the diagram, they occupy the lower left part, belonging to the branch of stars that left the main sequence from the state of red giants.

There is a region of hot stars with low luminosity, which is the second largest among stars in the observable Universe.

Spectral classification

Many White Dwarfs in the M4 globular cluster, Hubble image

They are allocated to a special spectral class D (from the English Dwarfs - dwarfs, gnomes). But in 1983, Edward Zion proposed a more precise classification that takes into account the differences in their spectra, namely: D (subclass) (spectral feature) (temperature index).

There are the following subclasses of spectra DA, DB, DC, DO, DZ and DQ, which specify the presence or absence of lines of hydrogen, helium, carbon and metals. And the spectral features of P, H, V and X clarify the presence or absence of polarization, a magnetic field in the absence of polarization, variability, peculiarity or unclassifiability of white dwarfs.

1. What is the closest white dwarf to the Sun? The closest is van Maanen's star, which is a dim object located only 14.4 light years from the Sun. It is located in the center of the constellation Pisces.

   

   Van Maanen's Star is the closest, single white dwarf

   Van Maanen's Star is too faint for us to see with the naked eye, with a magnitude of 12.2. However, if we consider a white dwarf in a system with a star, then the closest one is Sirius B, distant from us at a distance of 8.5 light years. By the way, the most famous white dwarf is Sirius B.

   

   Comparison of the sizes of Sirius B and Earth

2. The largest white dwarf is located in the center of the planetary nebula M27 (NGC 6853), which is better known as the Dumbbell Nebula. It is located in the constellation Vulpecula, at a distance of about 1360 light years from us. Its central star is larger than any other known white dwarf at the moment.

   

3. The smallest white dwarf has the cacophonous name GRW +70 8247 and is located approximately 43 light years from Earth in the constellation Draco. Its magnitude is about 13 and is only visible through a large telescope.
4. The lifespan of a white dwarf depends on how slowly it cools. Sometimes enough gas accumulates on its surface and it turns into a Type Ia supernova. Life expectancy is very long - billions of years, or rather 10 to the 19th power and even more. Their long life expectancy is due to the fact that they cool down very slowly and they have every chance of surviving until the end of the Universe. And the cooling time is proportional to the fourth power of temperature.

   

5. The average white dwarf is 100 times smaller in size than our Sun, and with a density of 29,000 kg/cubic centimeter, the weight of 1 cubic cm is 29 tons. But it is worth considering that the density can vary depending on the size, from 10*5 to 10*9 g/cm3.
6. Our Sun will eventually turn into a white dwarf. No matter how sad it may sound, the mass of our star does not allow it to turn into a neutron star or black hole. The sun will turn into a white dwarf and will exist in this form for billions of years.
7. How does a star turn into a white dwarf? Basically everything depends on mass, let's look at the example of our Sun. A few more billion years will pass and the Sun will begin to increase in size, turning into a red giant, due to the fact that all the hydrogen will burn out in its core. After the hydrogen burns out, the synthesis reaction of helium and carbon begins.

   As a result of these processes, the star becomes unstable and stellar winds may form. Since combustion reactions of heavier elements than helium lead to greater heat release. With the synthesis of helium, some sections of the expanded outer shell of the Sun will be able to break away and a planetary nebula will form around our star. As a result, only one core will remain from our star, and when the Sun turns into a white dwarf, nuclear fusion reactions will cease in it.

8. A planetary nebula that forms as a result of the expansion and shedding of its outer shells often glows very brightly. The reason is that the core remaining from the star (consider a white dwarf) cools very slowly, and the high surface temperature of hundreds of thousands and millions of degrees Kelvin emits mainly in the far ultraviolet. The gases of the nebula, absorbing these UV quanta, re-emit them in the visible part of the light, simultaneously absorbing part of the quantum energy and shining very brightly, in contrast to the remainder, which is very dim in the visible range.

   

Answers on questions

1. What is the difference between a white dwarf and a white dwarf? The entire evolution of a star is based on its initial mass; its luminosity, life expectancy and what it will turn into in the end will depend on this parameter. For a star with a mass of 0.5-1.44 solar, life will end with the star expanding and turning into a red giant, which, having shed its outer shells, forms a planetary nebula, leaving behind only one core consisting of degenerate gas.

   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   

   This is a simplified mechanism of how a white dwarf is formed. If the mass of the star is greater than 1.44 solar masses (the so-called Chandrasekhar limit, at which the star can exist as a white dwarf. If the mass exceeds it, then it will become a neutron star), then the star, having consumed all the hydrogen in the core, begins the synthesis of heavier elements , right down to the iron. Further synthesis of elements that are heavier than iron is impossible because requires more energy than is released during the fusion process and the star's core collapses into a neutron star. Electrons escape from their orbits and fall into the nucleus, where they merge with protons and eventually form neutrons. Neutron matter weighs hundreds and millions of times more than any other.

2. Difference between a white dwarf and a pulsar. All the same differences as in the case of a neutron star, only it is worth considering that a pulsar (and this is a neutron star) also rotates very quickly, tens of times per second, and the rotation period of a white dwarf is, in the example of a star, 40 Eri B, 5 hours 17 minutes. The difference is noticeable!

   

   Pulsar PSR J0348 +0432 - neutron star and white dwarf

3. Why do white dwarfs glow? So thermonuclear reactions no longer occur; all available radiation is thermal energy, so why do they glow? Essentially, it cools slowly, like a hot iron that starts out bright white and then turns red. The degenerate gas conducts heat very well from the center and it cools by 1% over hundreds of millions of years. Over time, the cooling slows down and it can last for trillions of years.
4. What do white dwarfs turn into? The age of the Universe is too small for the formation of so-called black dwarfs, the final stage of evolution. So we don’t have any visible evidence yet. Based on calculations of its cooling, we know only one thing: their life expectancy is truly enormous, exceeding the age of the Universe (13.7 billion years) and theoretically amounting to trillions of years.
5. Is there a white dwarf with a strong magnetic field like a neutron star? Some of them have powerful magnetic fields, much stronger than any we have created on Earth. For example, the magnetic field strength at the Earth's surface is only 30 to 60 ppm of a tesla, while the magnetic field strength of a white dwarf can be as high as 100,000 tesla.

   

   But a neutron star has a truly strong magnetic field - 10 * 11 Tesla and is called a magnetar! Shocks can form on the surface of some magnetars, which create oscillations in the star. These fluctuations often result in huge bursts of gamma rays from the magnetar. For example, the magnetar SGR 1900+14, which is located 20,000 light years away in the constellation Aquila, exploded on August 27, 1998. The powerful burst of gamma rays was so strong that it forced the NEAR Shoemaker spacecraft to turn off its equipment in order to preserve it .

Popular science film about the heroes of our article

The more extensive the theoretical knowledge and technical capabilities of scientists become, the more discoveries they make. It would seem that all space objects are already known and it is only necessary to explain their features. However, every time astrophysicists have such a thought, the Universe presents them with another surprise. Often, however, such innovations are predicted theoretically. Such objects include brown dwarfs. Until 1995, they existed only “at the tip of the pen.”

let's get acquainted

Brown dwarfs are quite unusual stars. All their main parameters are very different from the characteristics of the luminaries familiar to us, however, there are similarities. Strictly speaking, a brown dwarf is a substellar object; it occupies an intermediate position between the luminaries themselves and the planets. These have a relatively small mass - from 12.57 to 80.35 of that of Jupiter. In their depths, as in the centers of other stars, thermonuclear reactions take place. The difference between brown dwarfs is the extremely insignificant role of hydrogen in this process. Such stars use deuterium, boron, lithium and beryllium as fuel. The “fuel” runs out relatively quickly, and the brown dwarf begins to cool. Once this process is completed, it becomes a planet-like object. Thus, brown dwarfs are stars that never fall on the main sequence of the Hertzsprung-Russell diagram.

Invisible Wanderers

These interesting objects have several other notable characteristics. They are wandering stars not associated with any galaxy. Theoretically, such cosmic bodies can roam the expanses of space for many millions of years. However, one of their most significant properties is the almost complete absence of radiation. It is impossible to notice such an object without using special equipment. Astrophysicists did not have suitable equipment for quite a long period.

First discoveries

The strongest emission from brown dwarfs occurs in the infrared spectral region. The search for such traces was crowned with success in 1995, when the first such object, Teide 1, was discovered. It belongs to the spectral class M8 and is located in the Pleiades cluster. In the same year, another such star, Gliese 229B, was discovered at a distance of 20 from the Sun. It orbits the red dwarf Gliese 229A. Discoveries began to follow one after another. Today, more than a hundred brown dwarfs are known.

Differences

Brown dwarfs are not easy to identify due to their similarity in various parameters to planets and light stars. In their radius, they approach Jupiter to one degree or another. Approximately the same value of this parameter is maintained for the entire range of masses of brown dwarfs. Under such conditions, it becomes extremely difficult to distinguish them from planets.

In addition, not all dwarfs of this type are capable of supporting The lightest of them (up to 13 are so cold that even processes using deuterium are impossible in their depths. The most massive ones cool very quickly (on a cosmic scale - within 10 million years) and also become incapable of maintaining thermonuclear reactions. Scientists use two main methods to distinguish brown dwarfs. The first of them is by measuring density. Brown dwarfs are characterized by approximately the same radius and volume, and therefore a cosmic body with a mass of 10 Jupiters or more is most likely classified as this type of object.

The second method is the detection of X-rays. Only brown dwarfs whose temperature has dropped to the planetary level (up to 1000 K) cannot boast of such a noticeable characteristic.

Method of distinguishing from light stars

A low-mass star is another object from which a brown dwarf can be difficult to distinguish. What is a star? This is a thermonuclear boiler where all light elements are gradually burned. One of them is lithium. On the one hand, in the depths of most stars it ends quite quickly. On the other hand, a reaction involving it requires a relatively low temperature. It turns out that an object with lithium lines in its spectrum probably belongs to the class of brown dwarfs. This method has its limitations. Lithium is often present in the spectrum of young stars. In addition, brown dwarfs can exhaust all reserves of this element over a period of half a billion years.

Methane may also be a distinctive feature. In the final stages of its life cycle, a brown dwarf is a star whose temperature allows it to accumulate an impressive amount of it. Other luminaries cannot cool to such a state.

To distinguish between brown dwarfs and stars, their brightness is also measured. The luminaries dim at the end of their existence. Dwarfs cool down throughout their “life.” At the final stages they become so dark that it is impossible to confuse them with stars.

Brown dwarfs: spectral type

The surface temperature of the described objects varies depending on mass and age. Possible values ​​range from planetary to those characteristic of the coldest class M stars. For these reasons, two additional spectral types were initially identified for brown dwarfs - L and T. In addition to them, in theory there was also a class Y. To date, its reality has been confirmed . Let us dwell on the characteristics of objects of each class.

Class L

Stars belonging to the first type of the above differ from representatives of the previous class M by the presence of absorption bands not only of titanium and vanadium oxide, but also of metal hydrides. It was this feature that made it possible to identify a new class L. Also, lines of alkali metals and iodine were discovered in the spectrum of some brown dwarfs belonging to it. By 2005, 400 such facilities had been opened.

Class T

T dwarfs are characterized by the presence of methane bands in the near-infrared range. Similar properties were previously discovered only in Saturn’s moon Titan. The FeH and CrH hydrides characteristic of L-dwarfs are replaced in the T-class by alkali metals such as sodium and potassium.

According to scientists, such objects should have a relatively small mass - no more than 70 masses of Jupiter. Brown T dwarfs are similar to gas giants in many ways. Their characteristic surface temperature varies in the range from 700 to 1300 K. If such brown dwarfs ever fall into the camera lens, the photo will show objects of a pinkish-blue color. This effect is associated with the influence of the spectra of sodium and potassium, as well as molecular compounds.

Class Y

The last spectral class existed only in theory for a long time. The surface temperature of such objects should be below 700 K, that is, 400 ºС. Such brown dwarfs are not detected in the visible range (it is impossible to take a photo at all).

However, in 2011, American astrophysicists announced the discovery of several similar cold objects with temperatures ranging from 300 to 500 K. One of them, WISE 1541-2250, is located at a distance of 13.7 light years from the Sun. The other, WISE J1828+2650, is characterized by a surface temperature of 25 ºС.

The sun's twin is a brown dwarf

A story about such interesting ones would be incomplete without mentioning the Death Star. This is the name given to a hypothetically existing twin of the Sun, which, according to some scientists, is located at a distance of 50-100 astronomical units from it, outside the Oort cloud. According to astrophysicists, the proposed object is a partner to our luminary and passes by the Earth every 26 million years.

The hypothesis is related to the assumption of paleontologists David Raup and Jack Sepkowski about the periodic mass extinction of biological species on our planet. It was expressed in 1984. In general, the theory is quite controversial, but there are arguments in its favor.

The Death Star is one likely explanation for such extinctions. A similar assumption simultaneously arose among two different groups of astronomers. According to their calculations, the Sun's twin should move in a highly elongated orbit. As it approaches our star, it disturbs the comets that “inhabit” the Oort cloud in large numbers. As a result, the number of their collisions with the Earth increases, which leads to the death of organisms.

The Death Star, or Nemesis as it is also called, can be a brown, white or red dwarf. To date, however, no objects suitable for this role have been found. It is suggested that in the Oort cloud zone there is an as yet unknown giant planet that influences the orbits of comets. It attracts ice blocks to itself, thereby preventing their possible collision with the Earth, that is, it acts completely differently than the hypothetical Death Star. However, there is also no evidence of the existence of the planet Tyche (that is, the sister of Nemesis).

Brown dwarfs are relatively new objects for astronomers. There is still a lot of information about them to be obtained and analyzed. It is already assumed that such objects may be companions of many known stars. The difficulties of studying and detecting dwarfs of this type set a new high bar for scientific equipment and theoretical understanding.

There are many different stars in the Universe. Big and small, hot and cold, charged and uncharged. In this article we will name the main types of stars, and also give a detailed description of Yellow and White dwarfs.

1. Yellow dwarf. A yellow dwarf is a type of small main sequence star with a mass of 0.8 to 1.2 solar masses and a surface temperature of 5000–6000 K. See below for more information about this type of star.
2. Red giant. A red giant is a large star with a reddish or orange color. The formation of such stars is possible both at the stage of star formation and at later stages of their existence. The largest of the giants turn into red supergiants. A star called Betelgeuse in the constellation Orion is the most striking example of a red supergiant.
3. White dwarf. A white dwarf is what remains of an ordinary star with a mass of less than 1.4 solar masses after it passes through the red giant stage. See below for more information about this type of star.
4. Red dwarf. Red dwarfs are the most common stellar-type objects in the Universe. Estimates of their number vary from 70 to 90% of the number of all stars in the galaxy. They are quite different from other stars.
5. Brown dwarf. Brown dwarf - substellar objects (with masses ranging from approximately 0.01 to 0.08 solar masses, or, respectively, from 12.57 to 80.35 Jupiter masses and a diameter approximately equal to the diameter of Jupiter), in the depths of which, in contrast from main sequence stars, there is no thermonuclear fusion reaction with the conversion of hydrogen into helium.
6. Subbrown dwarfs. Subbrown dwarfs, or brown subdwarfs, are cool formations that fall below the brown dwarf mass limit. Their mass is less than approximately one hundredth the mass of the Sun or, accordingly, 12.57 the mass of Jupiter, the lower limit is not defined. They are generally considered to be planets, although the scientific community has not yet come to a final conclusion about what is considered a planet and what is a sub-brown dwarf.
7. Black dwarf. Black dwarfs are white dwarfs that have cooled and, as a result, do not emit in the visible range. Represents the final stage of the evolution of white dwarfs. The masses of black dwarfs, like the masses of white dwarfs, are limited above 1.4 solar masses.
8. Double star. A binary star is two gravitationally bound stars orbiting a common center of mass.
9. New star. Stars whose luminosity suddenly increases 10,000 times. The nova is a binary system consisting of a white dwarf and a companion star located on the main sequence. In such systems, gas from the star gradually flows to the white dwarf and periodically explodes there, causing a burst of luminosity.
10. Supernova. A supernova is a star that ends its evolution in a catastrophic explosive process. The flare in this case can be several orders of magnitude larger than in the case of a nova. Such a powerful explosion is a consequence of the processes occurring in the star at the last stage of evolution.
11. Neutron star. Neutron stars (NS) are stellar formations with masses of about 1.5 solar and sizes noticeably smaller than white dwarfs, about 10-20 km in diameter. They consist mainly of neutral subatomic particles - neutrons, tightly compressed by gravitational forces. In our Galaxy, according to scientists, there may exist from 100 million to 1 billion neutron stars, that is, somewhere around one per thousand ordinary stars.
12. Pulsars. Pulsars are cosmic sources of electromagnetic radiation coming to Earth in the form of periodic bursts (pulses). According to the dominant astrophysical model, pulsars are rotating neutron stars with a magnetic field that is inclined to the rotation axis. When the Earth falls into the cone formed by this radiation, it is possible to detect a pulse of radiation repeating at intervals equal to the revolution period of the star. Some neutron stars rotate up to 600 times per second.
13. Cepheids. Cepheids are a class of pulsating variable stars with a fairly precise period-luminosity relationship, named after the star Delta Cephei. One of the most famous Cepheids is Polaris. The given list of the main types (types) of stars with their brief characteristics, of course, does not exhaust the entire possible variety of stars in the Universe.

Yellow dwarf

Being at various stages of their evolutionary development, stars are divided into normal stars, dwarf stars, and giant stars. Normal stars are main sequence stars. These, for example, include our Sun. Sometimes such normal stars are called yellow dwarfs.

Characteristic

Today we will briefly talk about yellow dwarfs, which are also called yellow stars. Yellow dwarfs are typically stars of average mass, luminosity, and surface temperature. They are main sequence stars, lying roughly in the middle on the Hertzsprung–Russell diagram and following cooler, less massive red dwarfs.

According to the Morgan-Keenan spectral classification, yellow dwarfs mainly correspond to luminosity class G, but in transition variations they sometimes correspond to class K (orange dwarfs) or class F in the case of yellow-white dwarfs.

The mass of yellow dwarfs often ranges from 0.8 to 1.2 solar masses. Moreover, their surface temperature is for the most part from 5 to 6 thousand degrees Kelvin.

The brightest and most famous representative of yellow dwarfs is our Sun.

In addition to the Sun, among the yellow dwarfs closest to Earth it is worth noting:

1. Two components in the triple system Alpha Centauri, among which Alpha Centauri A is similar in luminosity spectrum to the Sun, and Alpha Centauri B is a typical orange class K dwarf. The distance to both components is just over 4 light years.
2. The orange dwarf is the star Ran, also known as Epsilon Eridani, with luminosity class K. Astronomers estimated the distance to Ran to be about 10 and a half light years.
3. The double star 61 Cygni, located just over 11 light years from Earth. Both components of 61 Cygni are typical orange dwarfs of luminosity class K.
4. The Sun-like star Tau Ceti, approximately 12 light years distant from Earth, has a luminosity spectrum of G and an interesting planetary system consisting of at least 5 exoplanets.

Education

The evolution of yellow dwarfs is very interesting. The lifespan of a yellow dwarf is approximately 10 billion years.

Like most stars, intense thermonuclear reactions take place in their depths, in which mainly hydrogen burns into helium. After the start of reactions involving helium in the star's core, hydrogen reactions move increasingly towards the surface. This becomes the starting point in the transformation of a yellow dwarf into a red giant. The result of such a transformation may be the red giant Aldebaran.

Over time, the surface of the star will gradually cool, and the outer layers will begin to expand. At the final stages of evolution, the red giant sheds its shell, which forms a planetary nebula, and its core will turn into a white dwarf, which will further shrink and cool.

A similar future awaits our Sun, which is now in the middle stage of its development. In about 4 billion years, it will begin its transformation into a red giant, the photosphere of which, when expanding, can absorb not only the Earth and Mars, but even Jupiter.

The lifespan of a yellow dwarf is on average 10 billion years. After the entire supply of hydrogen burns, the star increases in size many times and turns into a red giant. most planetary nebulae, and the core collapses into a small, dense white dwarf.

White dwarfs

White dwarfs are stars with a large mass (on the order of the Sun) and a small radius (the radius of the Earth), which is less than the Chandrasekhar limit for the selected mass, and are a product of the evolution of red giants. The process of producing thermonuclear energy in them has been stopped, which leads to the special properties of these stars. According to various estimates, in our Galaxy their number ranges from 3 to 10% of the total stellar population.

History of discovery

In 1844, the German astronomer and mathematician Friedrich Bessel, while observing Sirius, discovered a slight deviation of the star from rectilinear motion, and made the assumption that Sirius had an invisible massive companion star.

His assumption was confirmed already in 1862, when the American astronomer and telescope builder Alvan Graham Clark, while adjusting the largest refractor at that time, discovered a dim star near Sirius, which was later dubbed Sirius B.

The white dwarf Sirius B has a low luminosity, and the gravitational field affects its bright companion quite noticeably, indicating that this star has an extremely small radius and a significant mass. This is how a type of object called white dwarfs was discovered for the first time. The second similar object was the star Maanen, located in the constellation Pisces.

How are white dwarfs formed?

After all the hydrogen in an aging star burns out, its core contracts and heats up, which contributes to the expansion of its outer layers. The star's effective temperature drops and it becomes a red giant. The rarefied shell of the star, very weakly connected with the core, dissipates in space over time, flowing to neighboring planets, and in the place of the red giant there remains a very compact star, called a white dwarf.

For a long time, it remained a mystery why white dwarfs, which have a temperature exceeding the temperature of the Sun, are small compared to the size of the Sun, until it became clear that the density of matter inside them is extremely high (within 10 5 - 10 9 g/cm 3). There is no standard mass-luminosity relationship for white dwarfs, which distinguishes them from other stars. A huge amount of matter is “packed” into an extremely small volume, which is why the density of the white dwarf is almost 100 times greater than the density of water.

The temperature of white dwarfs remains almost constant, despite the absence of thermonuclear reactions inside them. What explains this? Due to strong compression, the electron shells of atoms begin to penetrate each other. This continues until the distance between the nuclei becomes minimal, equal to the radius of the smallest electron shell.

As a result of ionization, electrons begin to move freely relative to the nuclei, and the matter inside the white dwarf acquires physical properties that are characteristic of metals. In such matter, energy is transferred to the surface of the star by electrons, the speed of which increases as they compress: some of them move at a speed corresponding to a temperature of a million degrees. The temperature on the surface and inside the white dwarf can differ sharply, which does not lead to a change in the diameter of the star. Here we can make a comparison with a cannonball - as it cools, it does not decrease in volume.

The white dwarf fades extremely slowly: over hundreds of millions of years, the radiation intensity drops by only 1%. But eventually it will have to disappear, turning into a black dwarf, which could take trillions of years. White dwarfs can well be called unique objects of the Universe. No one has yet succeeded in reproducing the conditions in which they exist in earthly laboratories.

X-ray emission from white dwarfs

The surface temperature of young white dwarfs, the isotropic cores of stars after the ejection of their shells, is very high - more than 2·10 5 K, but drops quite quickly due to radiation from the surface. Such very young white dwarfs are observed in the X-ray range (for example, observations of the white dwarf HZ 43 by the ROSAT satellite). In the X-ray range, the luminosity of white dwarfs exceeds the luminosity of main sequence stars: photographs of Sirius taken by the Chandra X-ray telescope can serve as an illustration - in them the white dwarf Sirius B looks brighter than Sirius A of spectral class A1, which is ~10,000 times brighter in the optical range brighter than Sirius B.

The surface temperature of the hottest white dwarfs is 7 10 4 K, the coldest ones are less than 4 10 3 K.

A peculiarity of the radiation of white dwarfs in the X-ray range is the fact that the main source of X-ray radiation for them is the photosphere, which sharply distinguishes them from “normal” stars: the latter have an X-ray corona heated to several million kelvins, and the temperature of the photosphere is too low for X-ray emission.

In the absence of accretion, the source of luminosity for white dwarfs is the stored thermal energy of ions in their interior, so their luminosity depends on age. A quantitative theory of the cooling of white dwarfs was developed in the late 1940s by Professor Samuel Kaplan.