The more extensive the theoretical knowledge and technical capabilities of scientists become, the more discoveries they make. It would seem that all the objects of space are already known and it is only necessary to explain their features. However, the Universe every time such an idea arises among astrophysicists gives them another surprise. Often, however, such innovations are predicted theoretically. These 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 of their main parameters are very different from the characteristics of the luminaries familiar to us, however, there are similarities. Strictly speaking, the 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 from the same parameter of Jupiter. In their bowels, 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" ends relatively quickly, and the brown dwarf begins to cool. After completing this process, 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 more remarkable characteristics. They are wandering stars that are not associated with any galaxy. Theoretically, such cosmic bodies can plow open spaces 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 a sufficiently long period.
First discoveries
The most intense radiation of brown dwarfs is in the infrared spectral region. The search for such traces was successful in 1995, when the first such object was discovered, Teide 1. It belongs to the spectral class M8 and is located in the Pleiades cluster. In the same year, another 20 stars, Gliese 229B, were discovered 20 light-years 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 because of their similarity in various parameters to planets and light stars. In their radius, they come closer to one degree or another to Jupiter. Approximately the same value of this parameter is preserved for the entire mass range of brown dwarfs. In such conditions, it becomes extremely difficult to distinguish them from the planets.
In addition, not all dwarfs of this type are capable of supporting thermonuclear reactions. The lightest of them (up to 13 masses of Jupiter) are so cold that even processes using deuterium are impossible in their bowels. The most massive very quickly (in space over 10 million years) cool down and also become incapable of supporting thermonuclear reactions. Scientists use two main methods to distinguish brown dwarfs. The first of these is a 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 is most likely related to this type of object.
The second method is the detection of x-ray and infrared radiation. Only brown dwarfs whose temperature has dropped to a planetary level (up to 1000 K) cannot boast of such a noticeable characteristic.
The method of difference from light stars
A star with a small mass is another object from which it is not easy to distinguish a brown dwarf. What is a star? This is a thermonuclear boiler, where all light elements gradually burn out. One of them is lithium. On the one hand, in the bowels of most stars it quickly ends. On the other hand, a relatively low temperature is required for a reaction with his 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.
A distinguishing feature may be methane. At the final stages of the life cycle, the brown dwarf is a star whose temperature allows you to accumulate an impressive amount. Other luminaries cannot cool to this state.
To distinguish brown dwarfs and stars, their brightness is also measured. The luminaries dim at the end of their existence. Dwarfs cool down the whole "life". At the final stages, they become so dark that it is impossible to confuse them with the stars.
Brown dwarfs: spectral class
The surface temperature of the described objects varies depending on mass and age. The possible values range from planetary to characteristic for the coldest class M stars. For these reasons, two additional spectral types, L and T, were initially identified for brown dwarfs. In addition, class Y 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 the above, differ from the representatives of the previous class M by the presence of absorption bands of not only titanium oxide and vanadium, but also metal hydrides. It was this feature that made it possible to distinguish a new class L. Also, in the spectrum of some brown dwarfs belonging to it, lines of alkali metals and iodine were found. By 2005, 400 such facilities were 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 the gas giants of the solar system, as well as Saturn’s satellite Titan. Hydrides FeH and CrH, 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 in many ways similar to gas giants. Their characteristic surface temperature varies in the range from 700 to 1300 K. If such brown dwarfs once 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 for a long time existed only in theory. The surface temperature of such objects should be below 700 K, i.e. 400 º. In the visible range, such brown dwarfs are not detected (the photo will not work at all).
However, in 2011, American astrophysicists announced the discovery of several such cold objects with temperatures ranging from 300 to 500 K. One of them, WISE 1541-2250, is located 13.7 light-years from the Sun. Another, WISE J1828 + 2650, is characterized by a surface temperature of 25 º.
Double of the sun - a brown dwarf
The story of such interesting space objects will be incomplete, if not to mention the "Death Star". This is the name of the 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 alleged object is a pair of our luminary and passes by the Earth every 26 million years.
The hypothesis is connected with the assumption of paleontologists David Raup and Jack Sepkowski about the periodic mass extinction of biological species on our planet. It was expressed in 1984. In general, the theory is quite controversial, but there are arguments in its favor.
The Death Star is one of the likely explanations 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 in a very elongated orbit. When approaching our luminary, she revolts the comets, which in large numbers “inhabit” 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. It is speculated that in the zone of the Oort cloud is still an 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 still no evidence of the existence of the planet Tyuche (that is, the sister of Nemesis).
Brown dwarfs for astronomers are relatively new objects. A lot of information about them remains to be obtained and analyzed. Already today it is assumed that such objects can be companions of many famous stars. The difficulties of research and detection of dwarfs of this type set a new high standard for scientific equipment and theoretical understanding.