Brown dwarfs are neither stars nor planets. They form just like stars but cannot maintain star-like luminosity by sustaining the nuclear fusion of hydrogen. Learn more about the formation of these substellar objects and some really interesting facts about them.
Did you know?
The existence of brown dwarfs was hypothesized in 1963 by Shiv Kumar, an American astronomer, who postulated the theory that very low mass gaseous objects cannot sustain hydrogen fusion reactions.
A brown dwarf is a celestial or an astronomical object having a size between a small star and a giant planet, like Jupiter. In other words, brown dwarfs are too large to be classified as planets and too small to be called a star.
As far as their origin is concerned, brown dwarfs resemble stars, as both the astronomical bodies are formed when cold interstellar clouds of gas and dust collapse. But unlike stars, brown dwarfs fail to sustain the thermonuclear fusion of hydrogen, which gives the stars their luminosity.
As a result, they do not shine like stars, but glow faintly by radiating heat at infrared wavelengths. For this reason, many scientists call them 'failed stars'. Let us find out some interesting facts about brown dwarfs and how they are formed.
Facts About Brown Dwarfs
How are Brown Dwarfs Formed?
» A star is formed due to the contraction of interstellar cloud of gas and dust. This contraction causes the release of gravitational potential energy, which in turn, heats up the cloud. As a result, the temperature in the core rises to such an extent that it paves the way for thermonuclear fusion of hydrogen.
» An enormous amount of energy is released due to thermonuclear fusion reactions, where hydrogen is converted to helium. This causes stars to shine and also prevents their further contraction, which in turn, keeps the size and luminosity of stars constant for a long time, often for billions of years.
» However, the gravitational contraction fails to effectively heat up the core of a protostar, which has a mass less than 0.08 times the mass of the Sun. The core of such a protostar becomes dense enough to stabilize and prevents further contraction, before its temperature rises to trigger the fusion of hydrogen. This is how brown dwarfs are formed.
» Brown dwarfs above 60 Jupiter masses undergo thermonuclear fusion of hydrogen, but fail to sustain the reactions due to their low mass.
When was the First Brown Dwarf Discovered?
The first brown dwarf to be discovered is Teide 1, located in the Pleiades, which is an open star cluster in the constellation of Taurus. It was verified in 1995 and was estimated to be 120 million years old.
The most popular brown dwarf Gliese 229B was discovered in the same year as a part of a binary system. A binary system can be described as a system where two stars orbit around one another. Gliese 229B was found to orbit the red dwarf Gliese 229A.
Types of Brown Dwarfs
There are basically four major types of brown dwarfs, as per the spectral classification. These are known as spectral class M, spectral class L, spectral class T, and spectral class Y.
The type M is characterized by a spectrum dominated by the bands of titanium(II) oxide (TiO) and vanadium(II) oxide (VO) molecules. An example of M brown dwarf is Teide1.
The spectral class L, on the other hand, is characterized by strong metal hydride bands (FeH, CrH, MgH, CaH) and alkali metal lines (Na I, K I, Cs I, Rb I) in its spectrum. L dwarfs are usually dark red in color.
Spectral class T is also known as 'methane dwarfs', as its spectrum is dominated by methane. SIMP 0136 is a T brown dwarf.
As far as the spectral class Y is concerned, it lacks a well-defined spectral sequence. The Y dwarfs are usually cooler than the T dwarfs. WISE 1828+2650 is a Y dwarf, and is the coolest known brown dwarf.
Other Interesting Facts
» Brown dwarfs were earlier called black dwarfs, before the term 'brown dwarf' was coined by Jill Tarter, of the SETI (Search for Extraterrestrial Intelligence) Institute, in her 1975 PhD thesis. Contrary to their name, brown dwarfs appear to be deep red or magenta in color.
» In the first few million years after their formation, both brown dwarfs as well as stars produce energy by the fusion of a rare isotope of hydrogen, known as deuterium. In case of stars, deuterium fusion is eventually replaced by hydrogen fusion when the core temperature increases, which however, does not happen in case of brown dwarfs.
» The brown dwarfs undergo deuterium fusion for a relatively short period, after which they cool down and fade. As they are much cooler than stars, they emit radiation not in the visible spectrum, but at the infrared wavelengths. This is one of the most distinguishing characteristics of these astrophysical objects.
» Another characteristic feature of brown dwarfs is, traces of lithium can be found in their spectra. While stars burn their lithium within a little over 100 Myr, brown dwarfs never become hot enough to deplete their lithium.
So, the detection of lithium in the atmosphere is an important criterion to determine whether the astrophysical object older than 100 Myr is a brown dwarf or a star.
» However, brown dwarfs above 65 Jupiter mass can burn their lithium along with deuterium, and can deplete their lithium levels within half a billion years. Therefore, the lithium test mentioned earlier may not be a conclusive test for the detection of brown dwarfs above 65 Jupiter masses.
» Both brown dwarfs and stars cool with time. However, brown dwarfs become dark or dim when they cool down, while stars can maintain a minimum luminosity by sustaining steady fusion of hydrogen.
» The twin brown dwarfs 2M 0939 are the dimmest brown dwarfs known so far. Their atmospheric temperature is 565 to 635 Kelvin.
» According to the International Astronomical Union, an object with a mass above the minimum mass required for the thermonuclear fusion of deuterium can be considered a brown dwarf, while the objects that fail to meet this criterion are regarded as planets. This minimum mass is calculated to be 13 Jupiter mass.
» As brown dwarfs do not emit light in the visible spectrum, they are not as bright as stars. They are more likely to be seen with an infrared telescope, which can detect the heat radiated by these objects.
» The older brown dwarfs have been observed to cool down sufficiently over billion of years, which facilitates the accumulation of methane in their atmosphere. This can also help distinguish brown dwarfs from stars. In fact, the brown dwarf Gliese 229B was detected this way.
» The temperature of the largest and youngest brown dwarfs can be as high as 2,800 K, while that of the oldest and smallest can be as low as 300 K.
» WISE 1506+7027 is the closest known brown dwarf to our Solar System. This was discovered by Kevin Luhman, an associate professor of astronomy and astrophysics at Penn State University and a researcher in Penn State's Center for Exoplanets and Habitable Worlds.
So far, hundreds of brown dwarfs have been discovered with the help of Infrared Sky Survey and other techniques. They hold special interest to astronomers and scientists, as many of them believe these substellar objects could help explain the 'missing mass problem' (also known as invisible dark matter), which is still a mystery to modern astronomy.