Stars are the hottest objects in the universe. Thermonuclear fusion processes take place in their depths, as a result of which an incredibly large amount of energy is released. The surface temperatures of stars range from 2,000 to 60,000 degrees Celsius, and the light they emit can be seen billions of light years away. But not all stars are the same, there are completely different ones - cold stars that, like ghosts, wander through the endless space, hiding from everyone.

Theory

These stars are brown dwarfs(brown dwarfs). Although according to the latest definition approved by the scientific community, brown dwarfs are substellar objects small mass (from 12 to 80 Jupiter masses or from 0.012 to 0.075 solar masses), but still these are stars, albeit not quite ordinary ones.

artist's concept of a brown dwarf

For the first time, brown dwarfs were discussed in the 1960s, but then their existence was assumed only hypothetically. The hypothesis of the existence of small, cold and dark stars intrigued many scientists and after a while the search for such objects began. However, 35 years of observations did not allow us to reveal at least something similar to a hypothetical brown dwarf. On the other hand, this was quite natural, because, as it turns out later, such stars are mostly do not emit light(or their luminosity is negligible), and all ground-based telescopes of that time had too low a sensitivity.

First brown dwarf

Only in 1995, thanks to the use of telescopes infrared range with increased sensitivity managed to find the first brown dwarf - Teide 1. After that, a very large number of similar stars were discovered, which led physicists to the hypothesis of a high prevalence of brown dwarfs in the Universe, which is now becoming increasingly plausible.

the second discovered brown dwarf, Gliese 229B, orbits the red dwarf Gliese 229 in a two-star system

In the depths of brown dwarfs, as well as in other stars, processes thermonuclear fusion, but they are not stable and do not last long, resulting in a rapid cooling of the star. Over time, the luminosity and temperature of brown dwarfs is constantly decreasing.

Spectral classes

In terms of surface temperature, brown dwarfs differ quite a lot from each other, so it was proposed to divide them into 4 spectral classes (at first there were 3 classes, class Y was rejected by the scientific world for a long time):

Spectral class M- rather massive substellar objects, close both in size and surface temperature (up to 2000 °C) to red dwarfs.

Spectral class L- surface temperature 1500-1000 °C, mass more than 70 masses of Jupiter. First class L dwarf discovered - GD 165B. In total, more than 400 substars of this type have been found.

Spectral class T- surface temperature 1000-400 °C, mass less than 70 masses of Jupiter. The first found T-dwarf - Gliese 229B. More than 200 T stars have been found so far.

Spectral type Y- until 2011, this type existed only in theory. Temperature below 400 °C.

The coldest stars

In August 2011, 7 supercold Y-class brown dwarfs were discovered.

The surface temperature of the dwarf CFBDSIR 1458+10 is approximately 97 °С.

brown dwarf WISE 1828+2650 detected by the space infrared telescope WISE has an even lower temperature - about 25 °С.

Artist's view of Y-class brown dwarf WISE 1828+2650

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. These objects include brown dwarfs. Until 1995, they existed only "at the tip of the pen."

let's get acquainted

Brown dwarfs are rather 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 actual luminaries and planets. These have a relatively small mass - from 12.57 to 80.35 of the analogous parameter of Jupiter. In their interiors, 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. "Fuel" runs out relatively quickly, and the brown dwarf begins to cool. After 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 are distinguished by several other noteworthy characteristics. They are wandering stars not associated with any galaxy. Theoretically, such cosmic bodies can surf 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 the use of special equipment. Astrophysicists did not have suitable equipment for a sufficiently long period.

First discoveries

The strongest radiation from brown dwarfs is 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 M8 spectral class 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 revolves around the red dwarf Gliese 229A. Discoveries began to follow one after another. To date, more than a hundred brown dwarfs are known.

Differences

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

In addition, far from all dwarfs of this type are able to support the lightest of them (up to 13 are so cold that even processes using deuterium are impossible in their depths. The most massive very quickly (on a cosmic scale - in 10 million years) cool down and also become incapable of sustaining thermonuclear reactions.Scientists use two main methods to distinguish brown dwarfs.The first of them is to measure the density.Brown dwarfs are characterized by approximately the same values ​​​​of radius and volume, and therefore a cosmic body with a mass of 10 Jupiters and above, most likely refers to 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.

How to distinguish from light stars

A luminary with a small mass is another object from which it can be difficult to distinguish a brown dwarf. What is a star? This is a thermonuclear boiler, where all the light elements gradually burn out. One of them is lithium. On the one hand, in the depths of most stars, it ends rather quickly. On the other hand, a relatively low temperature is required for the reaction with its participation. It turns out that the object with lithium lines in the 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 the reserves of this element over a period of half a billion years.

Methane can also be a distinguishing feature. In the final stages of its life cycle, a brown dwarf is a star whose temperature allows it to accumulate an impressive amount. Other luminaries cannot cool down 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 all "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 the mass and age. Possible values ​​are in the range from planetary to those characteristic of the coldest class M stars. For these reasons, two additional spectral types, L and T, were originally identified for brown dwarfs. In addition to them, the Y class also existed in theory. To date, its reality has been confirmed . Let us dwell on the characteristics of the objects of each of the classes.

Class L

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

T class

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

According to the assumptions of scientists, such objects should have a relatively small mass - no more than 70 Jupiter masses. Brown T-dwarfs are similar in many ways to gas giants. Their characteristic surface temperature varies from 700 to 1300 K. If such brown dwarfs ever fall into the camera lens, the photo will show pinkish-blue objects. 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 for a long time only in theory. The surface temperature of such objects should be below 700 K, i.e. 400 ºС. In the visible range, such brown dwarfs are not detected (it will not be possible 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, has a surface temperature of 25°C.

The sun's twin is a brown dwarf

A story about such interesting ones would be incomplete if not to mention the Death Star. This is the name of the hypothetically existing twin of the Sun, according to the assumptions of some scientists, located at a distance of 50-100 astronomical units from it, outside the Oort cloud. According to astrophysicists, the alleged object is a pair of our star 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 rather controversial, but there are arguments in its favor.

The Death Star is one possible explanation for such extinctions. A similar assumption arose simultaneously in two different groups of astronomers. According to their calculations, the twin of the Sun should move along a highly elongated orbit. When approaching our luminary, it perturbs comets, in large numbers "inhabiting" the Oort cloud. 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. There are suggestions that in the zone of the Oort cloud there is a yet unknown giant planet, which affects the orbits of comets. It attracts ice blocks to itself, thereby preventing their possible collision with the Earth, that is, it does not act at all like the hypothetical Death Star. However, there is no evidence of the existence of the planet Tyche (that is, the sister of Nemesis) either.

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 today that such objects can be companions of many known stars. The difficulties of researching and detecting this type of dwarfs set a new high bar for scientific equipment and theoretical understanding.

Each star has its own destiny and its own lifespan. There comes a point when it starts to fade.

White dwarfs are unusual stars. They consist of a substance whose density is extremely high. In the theory of stellar evolution, they are considered as the final stage in the evolution of stars of low and medium mass, comparable to the mass of the Sun. According to various estimates, there are 3-4% of such stars in our Galaxy.

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 effective temperature of the star drops, and it turns into a red giant. The rarefied shell of the star, very weakly connected with the core, eventually dissipates in space, flowing to neighboring planets, and a very compact star, called a white dwarf, remains in place of the red giant.

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 dependence - mass-luminosity - for white dwarfs, which distinguishes them from other stars. A huge amount of matter is “packed” in an extremely small volume, which is why the density of a white dwarf is almost 100 times that of water.

(In the picture, a comparison of the sizes of two white dwarfs with the planet Earth)

The temperature of white dwarfs remains almost constant, despite the absence of thermonuclear reactions inside them. What explains this? Due to the strong compression, the electron shells of the 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 more and more as it contracts: 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 dramatically, which does not lead to a change in the diameter of the star. Here you can make a comparison with a cannonball - cooling down, it does not decrease in volume.

(In the picture, van Maanen's star is a dim white dwarf located in the constellation Pisces.)

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

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 their size and temperature, and some of them consist of two or more stars, which are interconnected by the force of 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 with powerful telescopes, some stars will look like nothing more than just luminous dots.

Thus, a simple person with a fairly good visual acuity in clear weather in the night sky can see about 3000 stars from one earth's 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 stars of the main sequence of the spectral class K. Their mass is 0.5 - 0.8 solar masses. Unlike yellow dwarfs, orange dwarfs have a longer lifespan.
• red dwarf. This type combines small and relatively cold main sequence stars of spectral type M. Their differences from other stars are quite pronounced. They have a diameter and mass that is no more than 1/3 of the Sun's.
• blue dwarf. This type of star is hypothetical. Blue dwarfs evolve from red dwarfs before all hydrogen burns out, after which they presumably evolve into white dwarfs.
• white dwarf. This is the type of already evolved stars. They have a mass that is no more than the mass of Chandrasekhar. White dwarfs are deprived of their own source of thermonuclear energy. They belong to the DA spectral class.
• black dwarf. This type is a cooled white dwarfs, which, accordingly, do not radiate energy, i.e. do not glow, or emit it very, very weakly. They represent the final stage in the evolution of white dwarfs in the absence of accretion. The mass of black dwarfs, as well as white ones, does not exceed the mass of Chandrasekhar.
• brown dwarf. These stars are substellar objects that have a mass of 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 they do not contain the fusion reaction that causes other stars to convert hydrogen into helium.
• 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 the stars belonging to 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 rectilinear 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 designer Alvan Graham Clarke during the adjustment of the largest refractor at that time. A dim star was discovered near Sirius, later called Sirius B. This star is characterized by low luminosity, and its gravitational field affects its bright partner quite noticeably. This, in turn, is a confirmation that this star has a very small radius with a significant mass.

What 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 out of all hydrogen. When the hydrogen burns out, the core of the star is compressed to high densities, while the outer layers expand greatly and are accompanied by a general dimming of the luminosity. Thus, the star first turns into a red giant, which sheds its shell. The ejection 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 a "hydrogen" spectral type DA, in which there are no helium spectral lines. This type is the most common. The second type of white dwarfs is DB. It is rarer and is called a "helium white dwarf". No hydrogen lines have been found 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 a helium sheet flash develops periodically. Iko Iben also suggested a mechanism by which the shell is ejected at different stages of the development of a helium flash - at its peak and between flashes. Accordingly, its formation is affected by the shell ejection mechanism.

Relatively bright and massive luminaries are quite easy to see with the naked eye, but there are much more dwarf stars in the Galaxy, which are visible only in powerful telescopes, even if they are located close to the solar system. Among them there are both modest centenarians - red dwarfs, and brown dwarfs that did not reach full-fledged stellar status and retired white dwarfs, gradually turning into black ones. Photo above SPL/EAST NEWS

The fate of a star depends entirely on the size, or rather on the mass. To better imagine the mass of a star, we can give the following example. If you put 333,000 earth globes on one side of the scales, and the Sun on the other, then they will balance each other. In the world of stars, our Sun is average. It is 100 times smaller in mass than the largest stars and 20 times larger than the lightest. It would seem that the range is small: approximately like from a whale (15 tons) to a cat (4 kilograms). But stars are not mammals, their physical properties are much more dependent on mass. Compare at least the temperature: for a whale and a cat, it is almost the same, but for stars it differs by dozens of times: from 2,000 kelvins for dwarfs to 50,000 for massive stars. Even stronger - billions of times the power of their radiation differs. That is why we easily notice distant giant stars in the sky, and we do not see dwarfs even in the vicinity of the Sun.

But when careful calculations were made, it turned out that the prevalence of giants and dwarfs in the Galaxy strongly resembles the situation with whales and cats on Earth. There is a rule in the biosphere: the smaller the organism, the more its individuals in nature. It turns out that this is also true for stars, but explaining this analogy is not so easy. In wildlife, food chains operate: large ones eat small ones. If there were more foxes in the forest than hares, what would these foxes eat? However, the stars generally do not eat each other. Then why are there fewer giant stars than dwarfs? Astronomers already know half the answer to this question.

The fact is that the life of a massive star is thousands of times shorter than that of a dwarf one. To keep their own body from gravitational collapse, heavy stars have to heat up to a high temperature - hundreds of millions of degrees at the center. Thermonuclear reactions are very intense in them, which leads to enormous radiation power and rapid combustion of the "fuel". A massive star spends all its energy in a few million years, and economical dwarfs, slowly smoldering, stretch their thermonuclear age for tens or more billions of years. So, whenever a dwarf is born, he is still alive, because the age of the Galaxy is only about 13 billion years. But the massive stars that were born more than 10 million years ago have long since died.

However, this is only half the answer to the question of why giants are so rare in space. And the other half is that massive stars are born much less frequently than dwarf ones. For a hundred newborn stars like our Sun, only one star appears with a mass 10 times greater than that of the Sun. The reason for this "environmental regularity" of astrophysics has not yet been unraveled.

Degenerate stars

Usually, during the formation of a star, its gravitational contraction continues until the density and temperature in the center reach the values ​​necessary to start thermonuclear reactions, and then, due to the release of nuclear energy, the pressure of the gas balances its own gravitational attraction. Massive stars have a higher temperature and reactions begin at a relatively low density of matter, but the smaller the mass, the higher the “ignition density” turns out to be. For example, in the center of the Sun, the plasma is compressed to 150 grams per cubic centimeter. However, at a density even hundreds of times greater, the matter begins to resist pressure regardless of the increase in temperature, and as a result, the compression of the star stops before the energy yield in thermonuclear reactions becomes significant. The reason for stopping the compression is a quantum mechanical effect, which physicists call the pressure of a degenerate electron gas.

The fact is that electrons belong to the type of particles that obeys the so-called "Pauli principle", established by the physicist Wolfgang Pauli in 1925. This principle states that identical particles, such as electrons, cannot be in the same state at the same time. That is why electrons in an atom move in different orbits. There are no atoms in the bowels of a star: at high density they are crushed and there is a single "electronic sea". For him, the Pauli principle sounds like this: electrons located nearby cannot have the same speed. If one electron is at rest, the other must move, and the third must move even faster, and so on. This state of the electron gas is called degeneracy by physicists.

Even if a small star has burned all its fusion fuel and lost its energy source, its contraction can be stopped by the pressure of the degenerate electron gas. No matter how much the matter is cooled, at high density, the movement of electrons will not stop, which means that the pressure of the substance will resist compression regardless of temperature: the greater the density, the higher the pressure. The contraction of a dying star with a mass equal to the sun will stop when it decreases to about the size of the Earth, that is, 100 times, and the density of its matter becomes a million times higher than the density of water. This is how white dwarfs are formed. A star of lesser mass stops collapsing at a lower density, because its gravitational force is not so strong. A very small failing star can become degenerate and stop contracting even before the temperature in its interior rises to the threshold of "thermonuclear ignition." Such a body will never become a real star.

Missing link

Until recently, there was a big hole in the classification of astronomical objects: the smallest known stars were 10 times lighter than the Sun, and the most massive planet, Jupiter, was 1000 times lighter. Are there intermediate objects in nature - not stars and not planets with a mass from 1/1000 to 1/10 solar? What should this "missing link" look like? Can it be detected? These questions have long worried astronomers, but the answer began to take shape only in the mid-1990s, when programs to search for planets outside the solar system bore the first fruits. In orbits around several sun-like stars, giant planets were found, all of which turned out to be more massive than Jupiter. The mass gap between stars and planets began to shrink. But is a bond possible, and where to draw the line between a star and a planet?

Until recently, it seemed that it is quite simple: the star shines with its own light, and the planet - reflected. Therefore, those objects fall into the category of planets, in the depths of which, for the entire time of their existence, thermonuclear fusion reactions do not occur. If, at some stage of evolution, their power was comparable to luminosity (that is, thermonuclear reactions served as the main source of energy), then such an object deserves to be called a star. But it turned out that there may be intermediate objects in which thermonuclear reactions occur, but never serve as the main source of energy. They were discovered in 1996, but long before that they were called brown dwarfs. The discovery of these strange objects was preceded by a thirty-year search, which began with a remarkable theoretical prediction.

In 1963, a young American astrophysicist of Indian origin, Shiv Kumar, calculated models of the most low-mass stars and found that if the mass of a cosmic body exceeds 7.5% of the Sun, then the temperature in its core reaches several million degrees and thermonuclear reactions begin in it, converting hydrogen into helium. With a smaller mass, the compression stops before the temperature in the center reaches the value necessary for the helium fusion reaction to proceed. Since then, this critical mass value has been called the "hydrogen ignition limit", or the Kumar limit. The closer a star is to this limit, the slower its nuclear reactions go. For example, with a mass of 8% of the solar star will "smolder" for about 6 trillion years - 400 times more than the current age of the universe! So, in whatever era such stars are born, they are all still in their infancy.

However, in the life of less massive objects there is a brief episode when they resemble a normal star. We are talking about bodies with masses from 1% to 7% of the mass of the Sun, that is, from 13 to 75 masses of Jupiter. During the formation period, shrinking under the influence of gravity, they warm up and begin to glow with infrared and even a little red - visible light. The temperature of their surface can rise to 2500 kelvins, and in the depths exceed 1 million kelvins. This is enough to start the reaction of thermonuclear fusion of helium, but not from ordinary hydrogen, but from a very rare heavy isotope - deuterium, and not ordinary helium, but a light isotope of helium-3. Since there is very little deuterium in cosmic matter, all of it quickly burns out, without giving a significant release of energy. It's like throwing a sheet of paper into a cooling fire: it will burn instantly, but it will not give heat. A “stillborn” star cannot warm up more strongly - its compression stops under the influence of the internal pressure of the degenerate gas. Deprived of heat sources, it only cools down in the future, like an ordinary planet. Therefore, these failed stars can be noticed only during their short youth, while they are warm. They are not destined to reach the stationary regime of thermonuclear combustion.

nearest neighbors

Of the several thousand stars visible in the sky with the naked eye, only a couple of hundred have been honored with their own name. It would seem that there is nothing to talk about dim luminaries, hardly visible even through a telescope. But no! Astronomical books often mention such objects as Proxima Centauri, Barnard's Flying Star, the stars of Kapteyn, Przybylsky, van Maanen, Leuthen ... Usually they are named after the astronomers who studied them. These names have established themselves in science in the same way as a Petri dish or X-rays - spontaneously, without any formal decisions, simply as a form of recognition of the merits of scientists. And what is curious, almost all the stars bearing the names of scientists turned out to be nondescript, very small and dim.

Why are these tiny stars so attractive to astronomers? First of all, the fact that our Sun is one of them. According to the combination of properties, it can be attributed to large dwarfs. Therefore, by studying the life of small stars, we are trying to understand its past and future. In addition, dwarf stars are our closest neighbors. And this is not surprising, since there are more kids in the Galaxy. Proxima in the constellation Centaurus is located four light years away from us - the closest of all other stars, as indicated by its name (Latin proxima - "nearest"). But, despite the proximity, it is visible only through a telescope. And this is not surprising, because its optical luminosity is 18 thousand times less than that of the sun. In size, it is only 1.5 times larger than Jupiter, and its surface temperature is about 3000 K - half that of the Sun. Proxima is 7 times lighter than the Sun and is located very close to the Kumar limit - the lower limit of stellar masses. It is barely capable of supporting thermonuclear reactions in its depths.

A little further than Proxima, but in a gravitational link with it, is the double star Alpha Centauri. Both of its components are almost exact copies of our Sun. True, they are about 200 million years older, which means that by studying them, we predict the future of the Sun for millions of years ahead.

The more distant future of the Sun is represented, for example, by van Maanen's star - this is the closest single white dwarf to us, the remnant of a star that once looked like the Sun. In 6-7 billion years, our luminary will have the same fate: having shed its outer layers, it will shrink to the size of the globe, turning into a super-dense cooling "cinder" of a star - first white from high temperature, then gradually reddening and finally almost invisible cold black dwarf. How this transformation will take place is told by another "named" star, which appears in astronomical articles as "Sakurai's object". Japanese amateur astronomer Yukio Sakurai discovered it on February 20, 1996 at the moment of a sudden increase in its brightness. At first it seemed that it was an ordinary young white dwarf, but in six months it swelled up hundreds of times, demonstrating the “death convulsions” of a star burning out the last drops of its nuclear fuel. Astronomers call it a helium flash. If you believe the calculations, then a few more such outbreaks, and the dwarf should calm down forever.

Discovery of "stillborn" stars

Physicists are sure that what is not forbidden by the conservation laws is allowed. Astronomers add to this: nature is richer than our imagination. If Shiv Kumar was able to invent brown dwarfs, then it would seem that nature would not be difficult to create them. For three decades, fruitless searches for these dim luminaries continued. More and more researchers were included in the work. Even the theorist Kumar clung to the telescope in the hope of finding the objects he discovered on paper. His idea was simple: detecting a single brown dwarf is very difficult, because you need not only to fix its radiation, but also to prove that it is not a distant giant star with a cold (by stellar standards) atmosphere or even a galaxy surrounded by dust at the edge of the Universe. The most difficult thing in astronomy is to determine the distance to an object. Therefore, it is necessary to look for dwarfs near normal stars, the distances to which are already known. But a bright star will blind the telescope and make it impossible to see a dim dwarf. Therefore, you need to look for them next to other dwarfs! For example, with red - stars of extremely small mass or white - cooling remnants of normal stars. In the 1980s, searches by Kumar and other astronomers came up empty. Although there have been reports of the discovery of brown dwarfs more than once, a detailed study each time showed that these are small stars. However, the idea of ​​the search was the right one, and a decade later it worked.

In the 1990s, astronomers got new sensitive radiation detectors - CCD arrays and large telescopes up to 10 meters in diameter with adaptive optics, which compensates for the distortions introduced by the atmosphere and makes it possible to obtain almost the same clear images from the Earth's surface as from space. This immediately bore fruit: extremely dim red dwarfs were discovered, literally bordering on brown ones.

And the first brown dwarf was found in 1995 by a group of astronomers led by Rafael Rebolo from the Institute of Astrophysics in the Canary Islands. Using a telescope on the island of La Palma, they found an object in the Pleiades star cluster, which they called Teide Pleiades 1, borrowing the name from the Pico de Teide volcano on the island of Tenerife. True, some doubts remained about the nature of this object, and while Spanish astronomers proved that it was indeed a brown dwarf, their American colleagues announced their discovery in the same year. A team led by Tadashi Nakajima from the Palomar Observatory, using telescopes at the Palomar Observatory, discovered at a distance of 19 light years from Earth in the constellation Hare, next to the very small and cold star Gliese 229, its even smaller and colder satellite Gliese 229B. The temperature of its surface is only 1000 K, and the radiation power is 160 thousand times lower than that of the sun.

It quickly became clear that not the largest telescopes are suitable for searching for “failed stars”. The first single brown dwarfs were discovered on an ordinary telescope during systematic surveys of the sky. For example, the Kelu-1 object in the constellation Hydra was discovered as part of a long-term search for dwarf stars in the vicinity of the Sun, which began at the European Southern Observatory in Chile back in 1987. Using a 1-meter Schmidt telescope, University of Chile astronomer Maria Teresa Ruiz has been regularly photographing parts of the sky for many years, and then comparing the images taken at intervals of years. Among hundreds of thousands of faint stars, she looks for those that are noticeably shifted relative to others - this is an unmistakable sign of nearby luminaries. In this way, Maria Ruiz has already discovered dozens of white dwarfs, and in 1997 she finally got a brown one. Its type was determined by the spectrum, in which the lines of lithium and methane turned out to be. Maria Ruiz named it Kelu-1: in the language of the Mapuche people who once inhabited the central part of Chile, "kelu" means red. It is located at a distance of about 30 light years from the Sun and is not associated with any star.

All these findings, made in 1995-1997, became the prototypes of a new class of astronomical objects, which took its place between stars and planets. As is usually the case in astronomy, the first discoveries were immediately followed by new ones. In recent years, many dwarfs have been discovered during routine 2MASS and DENIS infrared sky surveys.

Unsuccessful stars discovered “on the tip of a pen”, Kumar called “black dwarfs”, but since they could not be detected for a long time, the new term was forgotten (now cooled white dwarfs are called so in popular science literature). In the mid-1970s, when astronomers began looking for an invisible hidden mass (now called dark matter) that manifests itself only through gravity, suspicion fell on the dim dwarf objects predicted by Kumar. New ideas for their naming began to arrive. Given that they are still not quite black, Chris Davidson of the University of Minnesota coined the term "infrared dwarfs", other astronomers tried to call them "crimson dwarfs", but in 1975 graduate student Jill Tarter of the University at Berkeley coined the term brown dwarf and he settled down. It was translated into Russian as “brown dwarf”, later the “brown dwarf” variant appeared, although in reality these objects have an infrared color, and perhaps it would be more accurate to translate brown as “dark” or “dim”. But it's too late: in our scientific literature they are called "brown dwarfs", and in the popular science literature there are also "brown" ones.

star dust

Soon after the discovery, brown dwarfs forced astronomers to make adjustments to the spectral classification of stars that had been established decades ago. The optical spectrum of a star is its face, or rather, its passport. The position and intensity of the lines in the spectrum primarily indicate the temperature of the surface, as well as other parameters, in particular the chemical composition, gas density in the atmosphere, magnetic field strength, etc. About 100 years ago, astronomers developed a classification of stellar spectra, designating each class letter of the Latin alphabet. Their order was revised many times, rearranging, removing and adding letters, until a generally accepted scheme was formed that served astronomers flawlessly for many decades. In the traditional form, the sequence of spectral classes looks like this: O-B-A-F-G-K-M. The surface temperature of stars from class O to class M decreases from 100,000 to 2000 K. English astronomy students even came up with a mnemonic rule for remembering the order of letters: “Oh! Be A Fine Girl, Kiss Me!" And at the turn of the century, this classic row had to be extended by two letters at once. It turned out that dust plays a very important role in the formation of the spectra of extremely cold stars and substars.

On the surface of most stars, due to the high temperature, no molecules can exist. However, in the coldest M-class stars (with temperatures below 3000 K), powerful absorption bands of titanium and vanadium oxides (TiO, VO) are visible in the spectra. Naturally, even cooler brown dwarfs were expected to have these molecular lines even stronger. All in the same 1997, a brown companion GD 165B was discovered near the white dwarf GD 165, with a surface temperature of 1900 K and a luminosity of 0.01% solar. It struck researchers with the fact that, unlike other cold stars, it does not have TiO and VO absorption bands, for which it was nicknamed the "strange star". The spectra of other brown dwarfs with temperatures below 2000 K turned out to be the same. Calculations showed that TiO and VO molecules in their atmospheres condense into solid particles - dust particles, and no longer manifest themselves in the spectrum, as is characteristic of gas molecules.

To take this feature into account, Davy Kirkpatrick of the California Institute of Technology proposed the following year to expand the traditional spectral classification by adding the L-class for low-mass infrared stars with a surface temperature of 1500-2000 K. Most L-class objects should be brown dwarfs, although very old low-mass stars can also cool below 2000 K.

Continuing the study of L-dwarfs, astronomers have discovered even more exotic objects. Powerful absorption bands of water, methane and molecular hydrogen are visible in their spectra, which is why they are called "methane dwarfs". The first discovered brown dwarf, Gliese 229B, is considered to be the prototype of this class. In 2000, James Liebert and colleagues from the University of Arizona singled out T-dwarfs with a temperature of 1500-1000 K and even a little lower into an independent group. Brown dwarfs pose many complex and very interesting questions for astronomers. The colder the atmosphere of a star, the more difficult it is to study it for both observers and theorists. The presence of dust makes this task even more difficult: the condensation of solid particles not only changes the composition of free chemical elements in the atmosphere, but also affects heat transfer and the shape of the spectrum. In particular, theoretical models taking dust into account have predicted the greenhouse effect in the upper atmosphere, which is confirmed by observations. In addition, calculations show that after condensation, dust particles begin to sink. It is possible that dense clouds of dust form at different levels in the atmosphere. The meteorology of brown dwarfs can be as varied as that of giant planets. But if the atmospheres of Jupiter and Saturn can be studied up close, then methane cyclones and dust storms of brown dwarfs will have to be deciphered only by their spectra.

Secrets of the "half-breeds"

Questions about the origin and abundance of brown dwarfs are still open. The first calculations of their number in young star clusters of the Pleiades type show that, compared with normal stars, the total mass of brown dwarfs is apparently not so large as to “write off” the entire hidden mass of the Galaxy to them. But this conclusion still needs to be verified.

The generally accepted theory of the origin of stars does not answer the question of how brown dwarfs are formed. Objects of such low mass could form like giant planets in circumstellar disks. But quite a few single brown dwarfs have been discovered, and it is difficult to assume that all of them were lost to their more massive companions shortly after birth. In addition, a planet was recently discovered in orbit around one of the brown dwarfs, which means that it was not subjected to the strong gravitational influence of its neighbors, otherwise the dwarf would have lost it.

A very special way of the birth of brown dwarfs has recently been outlined in the study of two close binary systems - LL Andromeda and EF Eridani. In them, a more massive companion, a white dwarf, pulls matter with its gravity from a less massive companion, the so-called donor star. Calculations show that initially in these systems, donor satellites were ordinary stars, but over several billion years their mass fell below the limit value and thermonuclear reactions in them died out. Now, according to external signs, these are typical brown dwarfs. The temperature of the donor star in the LL Andromeda system is about 1300 K, and in the EF Eridani system it is about 1650 K. They are only a few tens of times larger than Jupiter in mass, and methane lines are visible in their spectra. How much their internal structure and chemical composition are similar to those of "real" brown dwarfs is still unknown. Thus, a normal low-mass star, having lost a significant fraction of its matter, can become a brown dwarf.

The astronomers were right when they said that nature is more inventive than our imagination. Brown dwarfs, these "not stars and not planets", have already begun to surprise. As it turned out recently, despite their cold nature, some of them are sources of radio and even X-ray (!) Radiation. So in the future, this new type of space objects promises us many interesting discoveries.