Brown dwarfs are cosmic bodies with a mass of 1-8% of the solar mass. They are too massive for planets, gravitational compression makes possible thermonuclear reactions with the participation of "highly combustible" elements. But their mass is insufficient to "ignite" hydrogen, and therefore, unlike full-fledged stars, brown dwarfs do not shine for long.
Astronomers don't experiment - they get information through observations. As one of the representatives of this profession said, there are no devices that are long enough to reach the stars. However, astronomers have at their disposal physical laws that allow not only explaining the properties of already known objects, but also predicting the existence of those that have not yet been observed.
Shiva Kumar's foresight
Many have heard about neutron stars, black holes, dark matter and other cosmic exotics calculated by theorists. However, there are many other curiosities in the universe discovered in the same way. These include bodies that are intermediate between stars and gas planets. They were predicted in 1962 by Shiv Kumar, a 23-year-old Indian-American astronomer who had just completed his doctoral dissertation at the University of Michigan. Kumar called these objects black dwarfs. Later names such as black stars, Kumar objects, infrared stars appeared in the literature, but in the end the phrase "brown dwarfs", proposed in 1974 by a graduate student at the University of California, Jill Tarter, won out.
For four years, an international team of astronomers "weighed" the ultracold L-class dwarf (6.6% of the solar mass) using the Hubble telescope, VLT and the. Keck.
Kumar has been going to his opening for four years. In those days, the basics of the dynamics of star birth were already known, but there were significant gaps in the details. However, Kumar as a whole so correctly described the properties of his "black dwarfs" that later even supercomputers agreed with his conclusions. After all, the human brain has been and remains the best scientific instrument.
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The birth of understars
Stars arise from the gravitational collapse of cosmic gas clouds, which are mostly molecular hydrogen. It also contains helium (one for every 12 hydrogen atoms) and trace amounts of heavier elements. The collapse ends with the birth of a protostar, which becomes a full-fledged luminary when its core heats up to such an extent that a steady thermonuclear combustion of hydrogen begins there (helium does not participate in this, since temperatures ten times higher are needed to ignite it). The minimum temperature required to ignite hydrogen is about 3 million degrees.
Kumar was interested in the lightest protostars with a mass not exceeding one tenth of the mass of our Sun. He realized that in order to trigger the thermonuclear combustion of hydrogen, they must thicken to a higher density than the predecessors of solar-type stars. The center of the protostar is filled with a plasma of electrons, protons (hydrogen nuclei), alpha particles (helium nuclei) and nuclei of heavier elements. It happens that even before the hydrogen ignition temperature is reached, electrons give rise to a special gas, the properties of which are determined by the laws of quantum mechanics. This gas successfully resists the compression of the protostar and thus prevents the heating of its central zone. Therefore, hydrogen either does not ignite at all, or goes out long before complete burnout. In such cases, instead of a failed star, a brown dwarf is formed.
The possibility of a degenerate Fermi gas to resist gravitational compression is by no means unlimited, and it is easy to show it on one hand. As electrons fill ever higher energy levels, their speeds increase and eventually approach light. In this situation, the force of gravity prevails and the gravitational collapse resumes. The mathematical proof is more difficult, but the conclusion is similar. So it turns out that the quantum pressure of the electron gas stops the gravitational collapse only if the mass of the collapsing system remains below a certain limit, corresponding to 1.41 solar masses. It is called the chandrasekhar limit - in honor of the outstanding Indian astrophysicist and cosmologist who calculated it in 1930. The chandrasekhar limit specifies the maximum mass of white dwarfs,which our readers probably know about. However, the precursors of brown dwarfs are tens of times lighter and don't have to worry about the chandrasekhar limit.
Kumar calculated that the minimum mass of a nascent star is 0.07 solar masses when it comes to relatively young luminaries of population I, which give rise to clouds with an increased content of elements heavier than helium. For stars of population II, which arose more than 10 billion years ago, at a time when helium and heavier elements in outer space were much less, it is equal to 0.09 solar masses. Kumar also found that the formation of a typical brown dwarf takes about a billion years, and its radius does not exceed 10% of the radius of the Sun. Our Galaxy, like other star clusters, should contain a great variety of such bodies, but they are difficult to detect due to their weak luminosity.
How they light up
These estimates have not changed much over time. It is now believed that the temporary ignition of hydrogen in a protostar, born from relatively young molecular clouds, occurs in the range of 0.07-0.075 solar masses and lasts from 1 to 10 billion years (for comparison, red dwarfs, the lightest of real stars, are capable of shining tens of billions of years!). As Adam Burrows, professor of astrophysics at Princeton University, noted in an interview with PM, thermonuclear fusion compensates for no more than half of the loss of radiant energy from the surface of a brown dwarf, while in real main sequence stars, the degree of compensation is 100%. Therefore, the failed star cools down even when the "hydrogen furnace" is operating, and even more so it continues to cool down after its plugging.
A protostar with a mass of less than 0.07 solar masses is not capable of igniting hydrogen at all. True, deuterium can flare up in its depths, since its nuclei merge with protons already at temperatures of 600-700 thousand degrees, giving rise to helium-3 and gamma quanta. But there is not much deuterium in space (there is only one deuterium atom for 200,000 hydrogen atoms), and its reserves last only a few million years. The nuclei of gas bunches that have not reached 0.012 solar masses (which is 13 Jupiter masses) do not heat up even to this threshold and therefore are not capable of any thermonuclear reactions. As the professor at the University of California at San Diego Adam Burgasser emphasized, many astronomers believe that this is where the border between the brown dwarf and the planet passes. According to representatives of another camp,A lighter gas bunch can also be considered a brown dwarf if it arose as a result of the collapse of the primary cloud of cosmic gas, and was not born from a gas-dust disk surrounding a normal star that had just flared up. However, any such definitions are a matter of taste.
Another clarification is related to lithium-7, which, like deuterium, was formed in the first minutes after the Big Bang. Lithium enters into thermonuclear fusion at slightly less heating than hydrogen, and therefore ignites if the mass of the protostar exceeds 0.055-0.065 solar. However, lithium in space is 2500 times less than deuterium, and therefore, from an energy point of view, its contribution is absolutely negligible.
What do they have inside
What happens in the interior of a protostar if the gravitational collapse did not end with a thermonuclear ignition of hydrogen, and the electrons have united into a single quantum system, the so-called degenerate Fermi gas? The proportion of electrons in this state increases gradually, and does not jump in a single instant from zero to 100%. However, for simplicity, we will assume that this process has already been completed.
Pauli's principle states that two electrons entering the same system cannot be in the same quantum state. In a Fermi gas, the state of an electron is determined by its momentum, position, and spin, which takes on only two values. This means that in the same place there can be no more than a pair of electrons with the same momenta (and, naturally, opposite spins). And since in the course of gravitational collapse electrons are packed into an ever decreasing volume, they occupy states with increasing momenta and, accordingly, energies. This means that as the protostar contracts, the internal energy of the electron gas increases. This energy is determined by purely quantum effects and is not related to thermal motion; therefore, in the first approximation, it does not depend on temperature (in contrast to the energy of a classical ideal gas,the laws of which are studied in the school physics course). Moreover, at a sufficiently high compression ratio, the energy of the Fermi gas is many times greater than the thermal energy of the chaotic motion of electrons and atomic nuclei.
An increase in the energy of the electron gas also increases its pressure, which also does not depend on temperature and grows much stronger than the thermal pressure. It is precisely this that opposes the gravitation of the protostar matter and stops its gravitational collapse. If this happened before the hydrogen ignition temperature was reached, the brown dwarf cools down immediately after a short cosmic deuterium burnout. If the proto-star is in the border zone and has a mass of 0.07-0.075 solar, it burns hydrogen for billions of years, but this does not affect its final. Eventually, the quantum pressure of the degenerate electron gas lowers the temperature of the stellar core to such an extent that hydrogen combustion stops. And although its reserves would be enough for tens of billions of years, the brown dwarf will no longer be able to set fire to them. This is what makes it different from the lightest red dwarf, which turns off the nuclear furnace only when all the hydrogen has turned into helium.
All known stars on the Hertzsprung-Russell diagram are not evenly distributed, but are combined into several spectral classes taking into account the luminosity (Yerkes classification, or MCC, by the names of the astronomers who developed it from the Yerkes Observatory - William Morgan, Philip Keenan and Edith Kellman). The modern classification distinguishes eight such main groups on the Hertzsprung-Russell diagram. Class 0 - these are hypergiants, massive and very bright stars, exceeding the Sun in mass by 100-200 times, and in terms of luminosity - in millions and tens of millions. Class Ia and Ib - these are supergiants, tens of times more massive than the Sun and tens of thousands of times superior in luminosity. Class II - bright giants that are intermediate between supergiants and class III giants. Class V - this is the so-called the main sequence (dwarfs) on which most of the stars lie, including our Sun. When a main sequence star runs out of hydrogen and starts burning helium in its core, it will become a class IV subgiant. Just below the main sequence is class VI - subdwarfs. And class VII includes compact white dwarfs, the final stage in the evolution of stars that do not exceed the Chandrasekhar mass limit. And class VII includes compact white dwarfs, the final stage in the evolution of stars that do not exceed the Chandrasekhar mass limit. And class VII includes compact white dwarfs, the final stage in the evolution of stars that do not exceed the Chandrasekhar mass limit.
Professor Burrows notes one more difference between the star and the brown dwarf. An ordinary star not only does not cool down, losing radiant energy, but, paradoxically, heats up. This happens because the star compresses and heats up its core, and this greatly increases the rate of thermonuclear combustion (for example, during the existence of our Sun, its luminosity has increased by at least a quarter). A brown dwarf is a different matter, whose compression is prevented by the quantum pressure of the electron gas. Due to radiation from the surface, it cools down like a stone or a piece of metal, although it consists of hot plasma, like a normal star.
Long searches
The pursuit of brown dwarfs dragged on for a long time. Even in the most massive representatives of this family, which emit a purple glow in their youth, the surface temperature usually does not exceed 2000 K, and in those that are lighter and older, sometimes it does not even reach 1000 K. The radiation of these objects also contains an optical component, although very weak. Therefore, high-resolution infrared equipment, which appeared only in the 1980s, is best suited for finding them. At the same time, infrared space telescopes began to be launched, without which it is almost impossible to detect cold brown dwarfs (the peak of their radiation occurs at waves with a length of 3-5 micrometers, which are mainly delayed by the earth's atmosphere).
It was during these years that reports of possible candidates appeared. At first, such statements did not stand up to verification, and the real discovery of the first of the pseudo stars predicted by Shiv Kumar took place only in 1995. The palm here belongs to a group of astronomers led by professor at the University of California at Berkeley Gibor Basri. The researchers studied the extremely faint object PPl 15 in the Pleiades star cluster, about 400 light-years away, which was previously discovered by the team of Harvard astronomer John Stauffer. According to preliminary data, the mass of this celestial body was 0.06 solar masses, and it could well turn out to be a brown dwarf. However, this estimate was very rough and could not be relied upon. Professor Basri and his colleagues were able to solve this problem using a lithium sample,which was recently invented by the Spanish astrophysicist Rafael Rebolo.
“Our group worked on the Keck Observatory's first 10-meter telescope, which went into operation in 1993,” recalls Professor Basri. - We decided to use the lithium test, since it made it possible to distinguish between brown dwarfs and red dwarfs close to them in mass. Red dwarfs burn lithium-7 very quickly, and almost all brown dwarfs are not capable of this. Then it was believed that the age of the Pleiades is about 70 million years, and even the lightest red dwarfs during this time should have completely got rid of lithium. If we found lithium in the PPl 15 spectrum, then we would have every reason to assert that we are dealing with a brown dwarf. The task was not easy. The first spectrographic test in November 1994 did indeed reveal lithium, but the second, control one, in March 1995, did not confirm this. Naturally,we were disappointed - the discovery slipped right out of our hands. However, the initial conclusion was correct. PPl 15 turned out to be a pair of brown dwarfs orbiting a common center of mass in just six days. That is why the spectral lines of lithium sometimes merged, then diverged - so we did not see them during the second test. Along the way, we discovered that the Pleiades are older than previously thought.
In the same 1995, there were reports of the discovery of two more brown dwarfs. Raphael Rebolo and his colleagues at the Astrophysical Institute of the Canary Islands discovered the dwarf Teide 1 in the Pleiades, which was also identified using the lithium method. And at the very end of 1995, researchers from the California Institute of Technology and Johns Hopkins University reported that the red dwarf Gliese 229, which is only 19 light years from the solar system, has a companion. This moon is 20 times heavier than Jupiter and contains methane lines in its spectrum. Methane molecules are destroyed if the temperature exceeds 1500K, while the atmospheric temperature of the coldest normal stars is always above 1700K. This allowed Gliese 229-B to be recognized as a brown dwarf without even using a lithium test. Now it is already knownthat its surface is heated to only 950 K, so this dwarf is very cold.
Astronomers are constantly learning something new about brown dwarfs. So, at the end of November 2010, scientists from Chile, England and Canada reported the discovery in the constellation Virgo, just 160 light years from the Sun, a stellar pair of two dwarfs of different color categories - white and brown. The latter is one of the hottest T-class dwarfs (its atmosphere is heated to 1300 K) and is 70 Jupiters in mass. Both celestial bodies are gravitationally bound, despite the fact that they are separated by a huge distance - approximately 1 light year. Astronomers observed a stellar pair of brown dwarfs using the UKIRT (United Kingdom Infrared Telescope) telescope with a 3.8-meter mirror. This telescope, located near the summit of Mauna Kea in Hawaii at an altitude of 4200 m above sea level - - one of the largest instruments in the world,working in the infrared range.
L-dwarfs, E-dwarfs - what's next?
At present, there are twice as many brown dwarfs known as exoplanets - about 1000 versus 500. The study of these bodies forced scientists to expand the classification of stars and star-like objects, since the previous one was insufficient.
Astronomers have long classified stars into groups according to the spectral characteristics of radiation, which, in turn, are primarily determined by the temperature of the atmosphere. Today, the system is mainly used, the foundations of which were laid by the staff of the Harvard University Observatory more than a hundred years ago. In its simplest version, stars are divided into seven classes, denoted by the Latin letters O, B, A, F, G, K and M. Class O includes extremely massive blue stars with surface temperatures above 33,000K, while class M includes red dwarfs, red giants, and even a number of red supergiants, whose atmosphere is heated to less than 3700 K. Each class, in turn, is divided into ten subclasses - from the hottest zero to the coldest ninth. For example, our Sun belongs to the G2 class. The Harvard system also has more complex variants (for example, lately white dwarfs have been allocated to a special class D), but these are subtleties.
The discovery of brown dwarfs resulted in the introduction of new spectral types L and T. The class L includes objects with surface temperatures from 1300 to 2000 K. Among them are not only brown dwarfs, but also the dimmest red dwarfs, which were previously classified as M-class. Class T includes only one brown dwarf, whose atmospheres are heated from 700 to 1300 K. Methane lines are abundant in their spectra, so these bodies are often called methane dwarfs (this is exactly what Gliese 229 B is).
“By the late 1990s, we had amassed a lot of information about the spectra of the faintest stars, including brown dwarfs,” Caltech astronomer Davey Kirkpatrick, who is part of a group of scientists who initiated the new classes, told PM. - It turned out that they have a number of features not previously encountered. The spectral marks of vanadium and titanium oxides, typical of red M-dwarfs, disappeared, but lines of alkali metals - sodium, potassium, rubidium and cesium - appeared. So we decided that the Harvard classification should be expanded. First, the class L was added, it was I who suggested this letter - simply because nothing was listed for it yet. However, Gliese 229 B did not correspond to class L due to the presence of methane. I had to use one more free letter - T, so the T-class appeared."
Most likely, this will not end there. It has already been proposed to introduce the class y, which is reserved for hypothetical ultracold brown dwarfs heated below 600K. Their spectra should also have characteristic features, such as clear absorption lines of ammonia (and at temperatures below 400 K, water vapor will also appear). Since all brown dwarfs are doomed to cool down, y-class bodies must exist, although they have not yet been discovered. It is possible that they will be opened after the launch of the giant james webb infrared telescope, which will go into space in 2014. Perhaps this observatory will even find planets in brown dwarfs, the existence of which, in principle, is quite acceptable. There are still many interesting things ahead for astronomers.
Alexey Levin