Brown dwarfs are the coolest stars. Dwarf stars, giants and supergiants The smallest stars among dwarfs were

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 stated 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 about 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.

If you look closely at the night sky, it is easy to notice that the stars looking at us differ in color. Bluish, white, red, they shine evenly or flicker like a Christmas tree garland. Through a telescope, color differences become more obvious. The reason that led to such diversity lies in the temperature of the photosphere. And, contrary to logical assumption, the hottest stars are not red, but blue, blue-white and white stars. But first things first.

Spectral classification

Stars are huge, hot balls of gas. How we see them from Earth depends on many parameters. For example, stars don't actually twinkle. It is very easy to verify this: just remember the Sun. The flickering effect occurs due to the fact that light coming from cosmic bodies to us overcomes the interstellar medium full of dust and gas. Another thing is color. It is a consequence of heating the shells (especially the photosphere) to certain temperatures. The actual color may differ from the apparent color, but the difference is usually small.

Today, the Harvard spectral classification of stars is used throughout the world. It is temperature based and is based on the type and relative intensity of the spectrum lines. Each class corresponds to stars of a certain color. The classification was developed at the Harvard Observatory in 1890-1924.

One Shaved Englishman Chewed Dates Like Carrots

There are seven main spectral classes: O—B—A—F—G—K—M. This sequence reflects a gradual decrease in temperature (from O to M). To remember it, there are special mnemonic formulas. In Russian, one of them sounds like this: “One Shaved Englishman Chewed Dates Like Carrots.” Two more classes are being added to these classes. The letters C and S denote cold luminaries with bands of metal oxides in the spectrum. Let's take a closer look at the star classes:

• Class O is characterized by the highest surface temperature (from 30 to 60 thousand Kelvin). Stars of this type exceed the Sun by 60 times in mass and 15 times in radius. Their visible color is blue. In terms of luminosity, they are more than a million times greater than our star. The blue star HD93129A, which belongs to this class, is characterized by one of the highest luminosities among known cosmic bodies. According to this indicator, it is 5 million times ahead of the Sun. The blue star is located at a distance of 7.5 thousand light years from us.
• Class B has a temperature of 10-30 thousand Kelvin, a mass 18 times greater than that of the Sun. These are blue-white and white stars. Their radius is 7 times greater than that of the Sun.
• Class A is characterized by a temperature of 7.5-10 thousand Kelvin, a radius and mass that are 2.1 and 3.1 times higher, respectively, than those of the Sun. These are white stars.
• Class F: temperature 6000-7500 K. Mass is 1.7 times greater than the sun, radius is 1.3. From Earth, such stars also appear white; their true color is yellowish-white.
• Class G: temperature 5-6 thousand Kelvin. The Sun belongs to this class. The visible and true color of such stars is yellow.
• Class K: temperature 3500-5000 K. The radius and mass are less than solar, 0.9 and 0.8 from the corresponding parameters of the luminary. The color of these stars visible from Earth is yellowish-orange.
• Class M: temperature 2-3.5 thousand Kelvin. Mass and radius are 0.3 and 0.4 from similar parameters of the Sun. From the surface of our planet they appear red-orange. Beta Andromedae and Alpha Chanterelles belong to class M. A bright red star familiar to many is Betelgeuse (alpha Orionis). It is best to look for it in the sky in winter. The red star is located above and slightly to the left

Each class is divided into subclasses from 0 to 9, that is, from the hottest to the coldest. Star numbers indicate membership in a specific spectral type and the degree of heating of the photosphere compared to other stars in the group. For example, the Sun belongs to class G2.

Visual whites

Thus, star classes B through F may appear white from Earth. And only objects belonging to the A-type actually have this color. Thus, the star Saif (constellation Orion) and Algol (beta Persei) will appear white to an observer not armed with a telescope. They belong to spectral class B. Their true color is blue-white. Also Mithrac and Procyon, the brightest stars in the celestial patterns Perseus and Canis Minor, appear white. However, their true color is closer to yellow (grade F).

Why are stars white to an observer on Earth? The color is distorted due to the enormous distance separating our planet from such objects, as well as the voluminous clouds of dust and gas that are often found in space.

Class A

White stars are not characterized by such a high temperature as representatives of class O and B. Their photosphere heats up to 7.5-10 thousand Kelvin. Stars of spectral class A are much larger than the Sun. Their luminosity is also greater - about 80 times.

The spectra of A stars show strong hydrogen lines of the Balmer series. The lines of other elements are noticeably weaker, but they become more significant as we move from subclass A0 to A9. Giants and supergiants belonging to spectral class A are characterized by slightly less pronounced hydrogen lines than main sequence stars. In the case of these luminaries, the lines of heavy metals become more noticeable.

Many peculiar stars belong to spectral class A. This term refers to luminaries that have noticeable features in their spectrum and physical parameters, which makes their classification difficult. For example, quite rare stars such as Lambda Boötes are characterized by a lack of heavy metals and very slow rotation. Peculiar luminaries also include white dwarfs.

Class A includes such bright night sky objects as Sirius, Mencalinan, Alioth, Castor and others. Let's get to know them better.

Alpha Canis Majoris

Sirius is the brightest, although not the closest, star in the sky. The distance to it is 8.6 light years. To an observer on Earth, it appears so bright because it has an impressive size and yet is not as far away as many other large and bright objects. The closest star to the Sun is Sirius, which is in fifth place on this list.

It refers to and is a system of two components. Sirius A and Sirius B are separated by a distance of 20 astronomical units and rotate with a period of just under 50 years. The first component of the system, a main sequence star, belongs to spectral class A1. Its mass is twice that of the Sun, and its radius is 1.7 times. This is what can be observed with the naked eye from Earth.

The second component of the system is a white dwarf. The star Sirius B is almost equal in mass to our star, which is not typical for such objects. Typically, white dwarfs are characterized by a mass of 0.6-0.7 solar. At the same time, the dimensions of Sirius B are close to those on Earth. It is believed that the white dwarf stage began for this star approximately 120 million years ago. When Sirius B was located on the main sequence, it was probably a star with a mass of 5 solar masses and belonged to spectral class B.

Sirius A, according to scientists, will move to the next stage of evolution in about 660 million years. Then it will turn into a red giant, and a little later - into a white dwarf, like its companion.

Alpha Eagle

Like Sirius, many of the white stars, the names of which are given below, are well known not only to people interested in astronomy due to their brightness and frequent mention in the pages of science fiction literature. Altair is one of these luminaries. Alpha Eagle is found, for example, in Stephen King. This star is clearly visible in the night sky due to its brightness and relatively close location. The distance separating the Sun and Altair is 16.8 light years. Of the stars of spectral class A, only Sirius is closer to us.

Altair is 1.8 times more massive than the Sun. Its characteristic feature is very fast rotation. The star completes one revolution around its axis in less than nine hours. The rotation speed near the equator is 286 km/s. As a result, the “nimble” Altair will be flattened from the poles. In addition, due to the elliptical shape, the temperature and brightness of the star decreases from the poles to the equator. This effect is called "gravitational darkening."

Another feature of Altair is that its shine changes over time. It belongs to the Scuti delta type variables.

Alpha Lyrae

Vega is the most studied star after the Sun. Alpha Lyrae is the first star to have its spectrum determined. She became the second luminary after the Sun, captured in the photograph. Vega was also one of the first stars to which scientists measured the distance using the Parlax method. For a long period, the brightness of the star was taken as 0 when determining the magnitudes of other objects.

Alpha Lyrae is well known to both amateur astronomers and ordinary observers. It is the fifth brightest among the stars and is included in the Summer Triangle asterism along with Altair and Deneb.

The distance from the Sun to Vega is 25.3 light years. Its equatorial radius and mass are 2.78 and 2.3 times greater than the similar parameters of our star, respectively. The star's shape is far from a perfect sphere. The diameter at the equator is noticeably larger than at the poles. The reason is the enormous rotation speed. At the equator it reaches 274 km/s (for the Sun this parameter is slightly more than two kilometers per second).

One of the features of Vega is the dust disk surrounding it. It is believed that it was created as a result of a large number of collisions of comets and meteorites. The dust disk rotates around the star and is heated by its radiation. As a result, the intensity of Vega's infrared radiation increases. Not long ago, asymmetries were discovered in the disk. A likely explanation is that the star has at least one planet.

Alpha Gemini

The second brightest object in the constellation Gemini is Castor. He, like the previous luminaries, belongs to spectral class A. Castor is one of the brightest stars in the night sky. In the corresponding list it is located in 23rd place.

Castor is a multiple system consisting of six components. The two main elements (Castor A and Castor B) rotate around a common center of mass with a period of 350 years. Each of the two stars is a spectral binary. The Castor A and Castor B components are less bright and presumably belong to the spectral class M.

Castor S was not immediately associated with the system. Initially it was designated as an independent star YY Gemini. In the process of studying this area of ​​​​the sky, it became known that this luminary is physically connected with the Castor system. The star rotates around a center of mass common to all components with a period of several tens of thousands of years and is also a spectral binary.

Beta Aurigae

The celestial pattern of Auriga includes approximately 150 “dots,” many of them white stars. The names of the luminaries will tell little to a person far from astronomy, but this does not detract from their importance for science. The brightest object in the celestial pattern, belonging to spectral class A, is Mencalinan or beta Aurigae. The name of the star translated from Arabic means “shoulder of the owner of the reins.”

Mencalinan is a triple system. Its two components are subgiants of spectral class A. The brightness of each of them exceeds that of the Sun by 48 times. They are separated by a distance of 0.08 astronomical units. The third component is a red dwarf, 330 AU away from the pair. e.

Epsilon Ursa Major

The brightest “point” in perhaps the most famous constellation of the northern sky (Ursa Major) is Alioth, also classified as class A. Apparent magnitude - 1.76. The star occupies 33rd place in the list of the brightest luminaries. Alioth is included in the Big Dipper asterism and is located closer than other luminaries to the bowl.

Aliot's spectrum is characterized by unusual lines that fluctuate with a period of 5.1 days. It is assumed that the features are associated with the influence of the star's magnetic field. Spectral fluctuations, according to the latest data, may arise due to the close proximity of a cosmic body with a mass of almost 15 times the mass of Jupiter. Whether this is so is still a mystery. Astronomers try to understand it, like other mysteries of the stars, every day.

White dwarfs

The story about white stars will be incomplete without mentioning that stage of the evolution of luminaries, which is designated as a “white dwarf”. Such objects received their name due to the fact that the first ones discovered belonged to spectral class A. These were Sirius B and 40 Eridani B. Today, white dwarfs are called one of the options for the final stage of a star’s life.

Let us dwell in more detail on the life cycle of luminaries.

Stellar evolution

Stars are not born overnight: each of them goes through several stages. First, the cloud of gas and dust begins to shrink under the influence of its own. Slowly it takes the shape of a ball, while the gravitational energy turns into heat - the temperature of the object increases. At the moment when it reaches a value of 20 million Kelvin, the nuclear fusion reaction begins. This stage is considered the beginning of the life of a full-fledged star.

The luminaries spend most of their time on the main sequence. Hydrogen cycle reactions are constantly taking place in their depths. The temperature of the stars may vary. When all the hydrogen in the core runs out, a new stage of evolution begins. Now helium becomes the fuel. At the same time, the star begins to expand. Its luminosity increases, and the surface temperature, on the contrary, decreases. The star leaves the main sequence and becomes a red giant.

The mass of the helium core gradually increases, and it begins to compress under its own weight. The red giant stage ends much faster than the previous one. The path that further evolution will take depends on the initial mass of the object. Low-mass stars at the red giant stage begin to inflate. As a result of this process, the object sheds its shells. The bare core of the star is also formed. In such a nucleus all fusion reactions were completed. It is called a helium white dwarf. More massive red giants (to a certain extent) evolve into carbon-based white dwarfs. Their cores contain elements heavier than helium.

Characteristics

White dwarfs are bodies that are usually very close in mass to the Sun. Moreover, their size corresponds to that of the earth. The colossal density of these cosmic bodies and the processes occurring in their depths are inexplicable from the point of view of classical physics. Quantum mechanics helped reveal the secrets of the stars.

The matter of white dwarfs is electron-nuclear plasma. It is almost impossible to construct it even in a laboratory. Therefore, many characteristics of such objects remain unclear.

Even if you study the stars all night, you will not be able to detect at least one white dwarf without special equipment. Their luminosity is significantly less than that of the sun. According to scientists, white dwarfs make up approximately 3 to 10% of all objects in the Galaxy. However, to date, only those of them have been found that are located no further than at a distance of 200-300 parsecs from the Earth.

White dwarfs continue to evolve. Immediately after formation, they have a high surface temperature, but cool quickly. A few tens of billions of years after formation, according to the theory, a white dwarf turns into a black dwarf - a body that does not emit visible light.

For an observer, a white, red or blue star differs primarily in color. The astronomer looks deeper. The color immediately tells a lot about the temperature, size and mass of the object. A blue or light blue star is a giant hot ball, in all respects far ahead of the Sun. White luminaries, examples of which are described in the article, are somewhat smaller. Star numbers in various catalogs also tell professionals a lot, but not everything. A large amount of information about the life of distant space objects has either not yet been explained or remains undetected.

There are a huge number of stars in space. Bright and huge ones can be seen with the naked eye, even if they are very far away, even by cosmic standards. But there are many more dwarf stars. It is almost impossible to see them with the naked eye. Among the dwarf stars there are red dwarfs that are already outliving their useful life. And brown dwarfs, which can hardly even be called stars. And already almost cooled white dwarfs, which will eventually turn into black ones.

On our planet there is a certain law of nature that the smaller the organism, the more of its individuals. This law also applies to the stars. This state of affairs raises many questions. After all, with living beings on Earth everything is extremely clear, but with the stars it’s not entirely clear. Scientists have solved this riddle halfway. In order to maintain themselves from gravitational collapse, stars with enormous weight need to heat up to high temperatures and, as a result, in a few million years they simply exhaust their energy supply, since in order to maintain a temperature in the center of hundreds of millions of degrees, very large expenditures of this energy are needed the energy itself. Dwarfs quietly smolder, stretching their “fuel” for tens of billions of years. Our Galaxy is only thirteen billion years old, therefore, whenever a dwarf appeared, it lives to this day. The second half of the question is that giant stars are born much less frequently than dwarfs. For every 100 stars like our Sun, only one star appears ten times more massive than . This is precisely the question that scientists have not yet answered. For a long time, among astronomical classifications there was no place for objects that were neither stars nor planets. The question of whether such objects exist has worried astronomers for decades. But in the mid-nineties, such planets were discovered outside the solar system. They turned out to be more massive than Jupiter, the largest planet in the solar system.
But the question arose of where to draw the line between a planet and a star. It was believed that the star uses its main source of energy, i.e. thermonuclear reactions. Planets glow due to reflected Sveta and thermonuclear reactions do not occur in it. But it turned out that there are objects of thermonuclear reactions in which they occur, but are not the main source of energy. Astrophysicist Kumar calculated that if the mass of a cosmic body is 7.5% or more of the mass of the Sun, then at the center of such an object the temperature will be sufficient for the reaction to occur. This value was called the “hydrogen flammability limit.” For example, if a star has 8% the mass of the Sun, it will smolder for about six trillion years, which is 400 times the age of the Universe.

The search for brown dwarfs invented by Shiv Kumar continued for three decades. Although this scientist was a theorist, he also took up the telescope in the hope of finding such a star. It was immediately clear that we needed to search near other stars, to which the distance was already known. But this star should not be bright, since it will simply blind the telescope and prevent it from seeing the dim dwarf. Consequently, it was necessary to look near red stars, or already cooling white ones. But at that time these searches were unsuccessful.

It was only when more sensitive instruments became available that astronomers were able to detect very dim red dwarfs. Over time, it became clear that to detect the so-called “failed stars” it is not necessary to have huge telescopes.

From 1995 to 1997, many such objects were discovered, which made it possible to classify new objects located between planets and stars.

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

The fate of a star depends entirely on its size, or more precisely on its mass. To better imagine the mass of a star, we can give the following example. If you put 333 thousand earthly globes on one scale and the Sun on the other, they will balance each other. In the world of stars, our Sun is average. It is 100 times less massive than the largest stars and 20 times greater than the lightest. It would seem that the range is small: approximately the same as from a whale (15 tons) to a cat (4 kilograms). But stars are not mammals; their physical properties depend much more strongly on mass. Just compare the temperature: for a whale and a cat it is almost the same, but for stars it differs tenfold: from 2000 Kelvin for dwarfs to 50,000 for massive stars. Even stronger - the power of their radiation differs billions of times. That is why we easily notice distant giant stars in the sky, but we do not see dwarfs even in the vicinity of the Sun.

But when careful calculations were carried out, it turned out that the prevalence of giants and dwarfs in the Galaxy is very similar to the situation with whales and cats on Earth. There is a rule in the biosphere: the smaller the organism, the more of its individuals there are in nature. It turns out that this is also true for stars, but the analogy is not so easy to explain. In living nature there are food chains: the big ones eat the small ones. If there were more foxes in the forest than hares, what would these foxes eat? However, 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 star. To keep their own body from gravitational collapse, heavy stars have to heat up to a high temperature - hundreds of millions of degrees in the center. Thermonuclear reactions occur very intensely in them, which leads to colossal radiation power and rapid combustion of “fuel”. A massive star wastes all its energy in a few million years, while thrifty dwarfs, slowly smoldering, stretch out their thermonuclear age for tens or more billions of years. So, no matter when the dwarf was born, it is still alive, because the age of the Galaxy is only about 13 billion years. But 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 every hundred newborn stars like our Sun, only one star appears with a mass 10 times greater than that of the Sun. Astrophysicists have not yet figured out the reason for this “ecological pattern.”

Degenerate stars

Typically, during the formation of a star, its gravitational compression continues until the density and temperature in the center reach the values ​​​​necessary to trigger thermonuclear reactions, and then, due to the release of nuclear energy, the pressure of the gas balances its own gravitational attraction. Massive stars have higher temperatures and reactions begin at a relatively low density of matter, but the lower the mass, the higher the “ignition density” turns out to be. For example, at the center of the Sun the plasma is compressed to 150 grams per cubic centimeter. However, at a density hundreds of times greater, the matter begins to resist pressure regardless of the temperature increase, and as a result, the compression of the star stops before the energy output 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 are a type of particle that obeys the so-called “Pauli principle”, established by physicist Wolfgang Pauli in 1925. This principle states that identical particles, such as electrons, cannot be in the same state at the same time. This is why electrons in an atom move in different orbits. There are no atoms in the interior of a star: at high densities they are crushed and there is a single “electron sea”. For him, the Pauli principle sounds like this: electrons located nearby cannot have the same speed. If one electron is at rest, another must move, and the third must move even faster, etc. Physicists call this state of the electron gas degeneracy.

Even if a small star has burned all its thermonuclear fuel and lost its energy source, its compression can be stopped by the pressure of the degenerate electron gas. No matter how much a substance cools, 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 higher the density, the higher the pressure. The contraction of a dying star with a mass equal to the Sun will stop when it shrinks to about the size of the Earth, that is, 100 times, and its density of matter becomes a million times higher than the density of water. This is how white dwarfs are formed. A star with a lower mass stops contracting at a lower density because its gravitational force is not so strong. A very small failed star can become degenerate and stop contracting even before the temperature in its depths rises to the “thermonuclear ignition” threshold. 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 or 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 emerge only in the mid-1990s, when programs to search for planets outside the solar system bore their first fruits. Giant planets have been discovered in orbit around several Sun-like stars, all of them more massive than Jupiter. The mass gap between stars and planets began to shrink. But is a bond possible, and where to draw the boundary between a star and a planet?

Until recently, it seemed that this was quite simple: the star shines with its own light, and the planet with reflected light. Therefore, the category of planets includes those objects in the depths of which no thermonuclear fusion reactions have occurred during their entire existence. If, at some stage of evolution, their power was comparable to their 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 lowest-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 of converting hydrogen into helium begin in it. At a lower mass, compression stops before the temperature in the center reaches the value necessary for the helium fusion reaction to occur. Since then, this critical mass value has been called the “hydrogen ignition limit,” or Kumara limit. The closer a star is to this limit, the slower nuclear reactions occur in it. For example, with a mass of 8% of the Sun, a star will “smolder” for about 6 trillion years - 400 times the current age of the Universe! So, no matter what era such stars were 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 period of formation, compressing under the influence of gravity, they heat up and begin to glow with infrared and even slightly red visible light. Their surface temperature can rise to 2500 kelvins, and in their depths exceed 1 million kelvins. This is enough for the reaction of thermonuclear fusion of helium to begin, but not from ordinary hydrogen, but from a very rare heavy isotope - deuterium, and not ordinary helium, but the light isotope helium-3. Since there is very little deuterium in cosmic matter, all of it quickly burns, without providing a significant energy output. It's the same as throwing a sheet of paper into a cooling fire: it will burn instantly, but will not provide any heat. A “stillborn” star cannot heat up any more - its compression stops under the influence of the internal pressure of the degenerate gas. Deprived of heat sources, it subsequently only cools down, like an ordinary planet. Therefore, these failed stars can only be noticed during their short youth, while they are warm. They are not destined to reach a stationary regime of thermonuclear combustion.

Nearest neighbors

Of the several thousand stars visible in the sky with the naked eye, only a couple hundred are worthy of their own names. It would seem that there is nothing to say about dim luminaries, barely visible even through a telescope. But no! Astronomy books often mention such objects as Proxima Centauri, Barnard's Flying Star, the stars of Kapteyn, Przybylski, van Maanen, Leuten... They are usually named after the astronomers who studied them. These names were established in science in the same way as the 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 is that almost all the stars bearing the names of scientists turned out to be inconspicuous, very small and dim.

Why are astronomers so attracted to these tiny stars? First of all, because our Sun is one of them. Based on the totality of its properties, it can be classified as a large dwarf. 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 babies in the Galaxy. Proxima in the constellation Centaurus is located four light years away from us - closer than all other stars, as indicated by its name (Latin proxima - “closest”). But, despite its proximity, it is visible only through a telescope. And this is not surprising, because its optical luminosity is 18 thousand times less than 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 very close to the Kumara limit - the lower limit of stellar masses. It is barely capable of maintaining thermonuclear reactions in its depths.

A little further than Proxima, but in a gravitational connection 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 millions of years in advance.

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 was once similar to the Sun. After 6-7 billion years, our star is destined for the same fate: having shed its outer layers, 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 an almost invisible cold black dwarf. Another “named” star, which appears in astronomical articles as the “Sakurai object,” tells how this transformation will occur. Japanese astronomy enthusiast Yukio Sakurai discovered it on February 20, 1996, when its brightness suddenly increased. At first it seemed that this was an ordinary young white dwarf, but over six months it swelled hundreds of times, demonstrating the “death convulsions” of a star burning out the last drops of its nuclear fuel. Astronomers call this a helium burst. If you believe the calculations, then a few more such outbursts, and the dwarf should calm down forever.

Discovery of "stillborn" stars

Physicists are sure that what is not prohibited by conservation laws is permitted. Astronomers add to this: nature is richer than our imagination. If Shiv Kumar was able to come up with brown dwarfs, then it would seem that nature would have no difficulty in creating them. The fruitless search for these dim luminaries continued for three decades. More and more researchers were involved 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, since it is necessary not only to detect 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 hardest thing in astronomy is determining the distance to an object. Therefore, you need to look for dwarfs near normal stars, the distances to which are already known. But the bright star will blind the telescope and will not allow you to see the dim dwarf. Therefore, you need to look for them near other dwarfs! For example, with red ones - stars of extremely low mass, or white ones - cooling remnants of normal stars. In the 1980s, searches by Kumar and other astronomers yielded no results. Although there have been reports of the discovery of brown dwarfs more than once, detailed research has shown each time that these are small stars. However, the search idea was correct and a decade later it worked.

In the 1990s, astronomers acquired new sensitive radiation detectors - CCD matrices and large telescopes with a diameter of up to 10 meters with adaptive optics, which compensate for distortions introduced by the atmosphere and allow them to receive images from the Earth's surface almost as clear 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 named Teide Pleiades 1, borrowing the name from the Pico de Teide volcano on the island of Tenerife. True, some doubts about the nature of this object remained, and while Spanish astronomers were proving 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 telescopes 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 companion Gliese 229B. Its surface temperature is only 1000 K, and the radiation power is 160 thousand times lower than the sun.

The non-stellar nature of Gliese 229B was finally confirmed in 1997 by the so-called lithium test. In normal stars, small amounts of lithium, preserved from the birth of the Universe, quickly burn up in thermonuclear reactions. However, brown dwarfs are not hot enough for this. When lithium was discovered in the atmosphere of Gliese 229B, it became the first "definite" brown dwarf. It is almost the same size as Jupiter, and its mass is estimated at 3-6% of the mass of the Sun. It orbits its more massive companion Gliese 229A in an orbit with a radius of about 40 astronomical units (like Pluto around the Sun).

It quickly became clear that not even the largest telescopes are suitable for searching for “failed stars.” The first single brown dwarfs were discovered using an ordinary telescope during systematic surveys of the sky. For example, the object Kelu-1 in the constellation Hydra was discovered as part of a long-term program of searching for dwarf stars in the vicinity of the Sun, which began at the European Southern Observatory in Chile back in 1987. Using the 1-meter Schmidt telescope, University of Chile astronomer Maria Teresa Ruiz has been regularly photographing certain areas of the sky for many years, and then comparing images taken at intervals of years. Among hundreds of thousands of faint stars, she looks for those that are noticeably displaced 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 came across a brown one. Its type was determined by the spectrum, which contained the lines of lithium and methane. Maria Ruiz named it Kelu-1: in the language of the Mapuche people who once inhabited central Chile, “quelu” means red. It is located about 30 light years from the Sun and is not associated with any star.

All these discoveries, made in 1995-1997, became the prototypes of a new class of astronomical objects, which took a place between the 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 infrared sky surveys 2MASS and DENIS.

What should we call you now?

Kumar called the failed stars discovered “at the tip of his pen” “black dwarfs,” but since they could not be discovered for a long time, the new term was forgotten (now in popular science literature this is what cooled white dwarfs are called). In the mid-1970s, as astronomers began searching for invisible hidden mass (now called dark matter) that manifests itself only through gravity, suspicion fell on the faint dwarf objects predicted by Kumar. New ideas for naming them began to come in. Given that they are still not completely black, Chris Davidson from the University of Minnesota proposed the term "infrared dwarfs", other astronomers tried to call them "raspberry dwarfs", but in 1975, graduate student Jill Tarter from the University of Berkeley coined the term brown dwarf , and he took root. It was translated into Russian as “brown dwarf”, later the variant “brown dwarf” 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 popular science there are also “brown” ones.

star dust

Soon after their 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 lines in the spectrum primarily indicate surface temperature, as well as other parameters, in particular 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 emerged 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 series had to be lengthened 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, the coldest M-class stars (with temperatures below 3000 K) show strong absorption bands of titanium and vanadium oxides (TiO, VO) in their spectra. Naturally, these molecular lines were expected to be even stronger in even cooler brown dwarfs. 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 amazed researchers by the fact that, unlike other cool stars, it does not have TiO and VO absorption bands, for which it was nicknamed a “strange star.” The spectra of other brown dwarfs with temperatures below 2000 K turned out to be the same. Calculations have shown that TiO and VO molecules in their atmospheres condense into solid particles - dust grains, and no longer manifest themselves in the spectrum, as is typical for gas molecules.

To account for this feature, Davy Kirkpatrick of the California Institute of Technology proposed the following year to expand the traditional spectral classification by adding class L 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 their studies of L-dwarfs, astronomers have discovered even more exotic objects. Their spectra show strong absorption bands of water, methane and molecular hydrogen, which is why they are called “methane dwarfs”. The prototype of this class is considered to be the first discovered brown dwarf, Gliese 229B. In 2000, James Liebert and colleagues from the University of Arizona identified T-dwarfs with temperatures of 1500-1000 K and even slightly lower as a separate group. Brown dwarfs pose many challenging and very interesting questions to astronomers. The colder a star's atmosphere, the more difficult it is for both observers and theorists to study. The presence of dust makes this task even more difficult: condensation of particulate matter 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 accounting for dust have predicted a greenhouse effect in the upper atmosphere, which is confirmed by observations. In addition, calculations show that after condensation, dust grains begin to sink. It is possible that dense clouds of dust form at different levels in the atmosphere. The meteorology of brown dwarfs may be no less diverse than that of the giant planets. But if the atmospheres of Jupiter and Saturn can be studied closely, then methane cyclones and dust storms of brown dwarfs will have to be deciphered only from their spectra.

Secrets of the "half-breeds"

Questions about the origin and abundance of brown dwarfs still remain open. The first calculations of their number in young star clusters like the Pleiades show that, compared with normal stars, the total mass of brown dwarfs is apparently not so great as to “attribute” 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 imagine that all of them were lost by their more massive companions shortly after birth. In addition, quite recently a planet was discovered in orbit around one of the brown dwarfs, which means that it was not subject to strong gravitational influence from its neighbors, otherwise the dwarf would have lost it.

A very special path for the birth of brown dwarfs has recently emerged in the study of two close binary systems - LL Andromeda and EF Eridani. In them, a more massive companion, a white dwarf, with its gravity pulls matter from a less massive companion, the so-called donor star. Calculations show that initially in these systems the donor satellites were ordinary stars, but over several billion years their mass fell below the limiting value and thermonuclear reactions in them died out. Now, in appearance, 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. Their mass is only several tens of times greater than Jupiter, and methane lines are visible in their spectra. How similar their internal structure and chemical composition are 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.

Astronomers were right when they argued that nature is more inventive than our imagination. Brown dwarfs, these “neither stars nor planets,” have already begun to present surprises. 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 object promises us many interesting discoveries.