The more 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 cosmic bodies have a relatively small mass - from 12.57 to 80.35 of the analogous parameter of Jupiter. In their bowels, as in the centersother 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 remarkable 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 have not had suitable equipment for quite a long period.
First discoveries
The strongest radiation of brown dwarfs falls on 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 light years from the Sun. It revolves around the red dwarf Gliese 229A. Discoveries began to follow one after another. To date it is knownover a hundred brown dwarfs.
Differences
Brown dwarfs are not easy to identify because of 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.
Besides, not all dwarfs of this type are capable of supporting thermonuclear reactions. The lightest of them (up to 13 Jupiter masses) 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 maintaining thermonuclear reactions. Scientists use two main methods to distinguish brown dwarfs. The first one is density measurement. 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 belongs to this type of object.
The second way is to detect x-ray and infrared radiation. Only brown dwarfs, whose temperature has dropped to the planetary level (up to 1000 K), cannot boast of such a noticeable characteristic.
The way 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 everything gradually burns out.light elements. 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 deplete all the reserves of this element in a period of half a billion years.
Methane can also be a hallmark. At 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 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's dwell on the characteristics of the objects of each of the classes.
Class L
Stars belonging to the first type of those mentioned differ from representatives of the previous class M by 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 discovered.
Class T
T-dwarfs are characterized by the presence of methane bands in the near infrared range. Similar properties were previously found only in the gas giants of the solar system, as well as 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 type has long existed 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 (the photo will not work at all).
However, in 2011American 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 twin of the sun is a brown dwarf
A story about such interesting space objects would be incomplete without mentioning 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 luminary and passes by the Earth every 26 million years.
The hypothesis is related to the assumption of paleontologists David Raup and Jack Sepkowski about the periodic mass extinction of biological species on our planet. It was expressed in 1984. In general, the theory is rather controversial, but there are arguments in its favor.
The Death Star is one possible explanation for these 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.
"Death Star", or Nemesis, asit is also called, it 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 still unknown giant planet that 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.
