Brown dwarf
Encyclopedia
Brown dwarfs are sub-stellar objects
which are too low in mass to sustain hydrogen-1 fusion
reactions in their cores, which is characteristic of stars on the main sequence
. Brown dwarfs have fully convective surfaces and interiors, with no chemical differentiation by depth. Brown dwarfs occupy the mass range between that of large gas giant
planet
s and the lowest-mass stars; this upper limit is between 75 and 80 Jupiter mass
es (
). Currently there is some debate as to what criterion to use to define the separation between a brown dwarf and a giant planet at very low brown dwarf masses (~13 
), and whether brown dwarfs are required to have experienced fusion at some point in their history. In any event, brown dwarfs heavier than 13 
do fuse deuterium
and those above ~65
also fuse lithium
. Some planets are known to orbit brown dwarfs: 2M1207b
, MOA-2007-BLG-192Lb
, and 2MASS J044144b.
in 1975, were originally called black dwarfs, a classification for dark substellar objects floating freely in space which were too low in mass to sustain stable hydrogen fusion (the term black dwarf
currently refers to a white dwarf
that has cooled down so that it no longer emits significant heat or visible light). Alternative names have been proposed, including planetar
and substar
.
Early theories concerning the nature of the lowest-mass stars and the hydrogen-burning limit suggested that objects with a mass less than 0.07 solar masses for Population I objects or objects with a mass less than 0.09 solar masses for Population II objects would never go through normal stellar evolution
and would become a completely degenerate star (Kumar 1963). The role of deuterium
-burning down to 0.012 solar mass
es and the impact of dust formation in the cool outer atmosphere
s of brown dwarfs was understood by the late 1980s. They would, however, be hard to find in the sky, as they would emit almost no light. Their strongest emissions would be in the infrared
(IR) spectrum, and ground-based IR detectors were too imprecise at that time to readily identify any brown dwarfs.
Since those earlier times, numerous searches involving various methods have been conducted to find these objects. Some of those methods included multi-color imaging surveys around field stars, imaging surveys for faint companions to main sequence
dwarfs and white dwarfs, surveys of young star clusters, and radial velocity
monitoring for close companions.
For many years, efforts to discover brown dwarfs were frustrating and searches to find them seemed fruitless. In 1988, however, University of California, Los Angeles
professors Eric Becklin
and Ben Zuckerman identified a faint companion to GD 165 in an infrared search of white dwarfs. The spectrum of GD 165B was very red and enigmatic, showing none of the features expected of a low-mass red dwarf
star. It became clear that GD 165B would need to be classified as a much cooler object than the latest M dwarfs then known. GD 165B remained unique for almost a decade until the advent of the Two Micron All Sky Survey (2MASS
) when Davy Kirkpatrick, out of the California Institute of Technology
, and others discovered many objects with similar colors and spectral features.
Today, GD 165B is recognized as the prototype of a class of objects now called "L dwarfs". While the discovery of the coolest dwarf was highly significant at the time, it was debated whether GD 165B would be classified as a brown dwarf or simply a very low-mass star, since observationally, it is very difficult to distinguish between the two.
Soon after the discovery of GD 165B, other brown dwarf candidates were reported. Most failed to live up to their candidacy, however, and with further checks for substellar nature, such as the lithium test, many turned out to be stellar objects and not true brown dwarfs. When still young, up to a billion years (a gigayear) old, brown dwarfs can have temperatures and luminosities similar to some stars, so other distinguishing characteristics are necessary, such as the presence of lithium. Stars will burn lithium
in a little over 100 Myr
, at most, while most brown dwarfs will never acquire high enough core temperatures to do so. Thus, the detection of lithium in the atmosphere of a candidate object ensures its status as a brown dwarf.
In 1995 the study of brown dwarfs changed dramatically with the discovery of three incontrovertible substellar objects, some of which were identified by the presence of the 670.8 nm lithium line. The most notable of these objects was Gliese 229
B, which was found to have a temperature and luminosity well below the stellar range. Remarkably, its near-infrared spectrum clearly exhibited a methane absorption band at 2 micrometres, a feature that had previously only been observed in gas giant atmospheres and the atmosphere of Saturn
's moon, Titan
. Methane absorption is not expected at the temperatures of main-sequence stars. This discovery helped to establish yet another spectral class even cooler than L dwarfs known as "T dwarfs" for which Gl 229B is the prototype.
Since 1995, when the first brown dwarf was confirmed by Chilean astronomer María Teresa Ruiz, over a thousand have been identified. Brown dwarfs close to Earth include Epsilon Indi
Ba and Bb, a pair of dwarfs gravitationally bound to a sunlike star, around 12 light-years from the Sun
.
is through the gravitational collapse of a cold interstellar cloud of gas and dust. As the cloud contracts it heats up from the release of gravitational potential energy
. Early in the process the contracting gas quickly radiates away much of the energy, allowing the collapse to continue. Eventually, the central region becomes sufficiently dense to trap radiation. Consequently, the central temperature and density of the collapsed cloud increases dramatically with time, slowing the contraction, until the conditions are hot and dense enough for thermonuclear reactions to occur in the core of the protostar
. For most stars, gas and radiation pressure generated by the thermonuclear fusion reactions within the core of the star will support it against any further gravitational contraction. Hydrostatic equilibrium
is reached and the star will spend most of its lifetime fusing hydrogen into helium as a main-sequence star.
If, however, the mass of the protostar is less than about 0.08 solar mass, normal hydrogen thermonuclear fusion reactions will not ignite in the core. Gravitational contraction does not heat the small protostar
very effectively, and before the temperature in the core can increase enough to trigger fusion, the density reaches the point where electrons become closely packed enough to create quantum electron degeneracy pressure
. According to the brown dwarf interior models, typical conditions in the core for density, temperature and pressure are expected to be the following:
Further gravitational contraction is prevented and the result is a "failed star", or brown dwarf that simply cools off by radiating away its internal thermal energy.
: Lithium
is generally present in brown dwarfs and not in low-mass stars. Stars, which achieve the high temperature necessary for fusing hydrogen, rapidly deplete their lithium. This occurs by a collision of Lithium-7 and a proton
producing two Helium-4
nuclei. The temperature necessary for this reaction is just below the temperature necessary for hydrogen fusion. Convection in low-mass stars ensures that lithium in the whole volume of the star is depleted. Therefore, the presence of the lithium line
in a candidate brown dwarf's spectrum is a strong indicator that it is indeed substellar. The use of lithium to distinguish candidate brown dwarfs from low-mass stars is commonly referred to as the lithium test, and was pioneered by Rafael Rebolo, Eduardo Martin and Antonio Magazzu.
Methane
: Unlike stars, older brown dwarfs are sometimes cool enough that over very long periods of time their atmospheres can gather observable quantities of methane
. Dwarfs confirmed in this fashion include Gliese 229B.
Luminosity: Main sequence stars cool, but eventually reach a minimum luminosity
which they can sustain through steady fusion. This varies from star to star, but is generally at least 0.01% the luminosity of our Sun. Brown dwarfs cool and darken steadily over their lifetimes: sufficiently old brown dwarfs will be too faint to be detectable.
Iron rain as part of atmospheric convection processes is possible only with brown dwarfs, and not with small stars. The spectroscopy research into iron rain is still ongoing–and not all brown dwarfs will always have this atmospheric anomaly.
pressure, as it is in white dwarfs; at the low end of the range (10 Jupiter masses), their volume is governed primarily by Coulomb pressure
, as it is in planets. The net result is that the radii of brown dwarfs vary by only 10–15% over the range of possible masses. This can make distinguishing them from planets difficult.
In addition, many brown dwarfs undergo no fusion; those at the low end of the mass range
(under 13 Jupiter masses) are never hot enough to fuse even deuterium, and even those at the high end of the mass range (over 60 Jupiter masses) cool quickly enough that they no longer undergo fusion after a period of time on the order of 10 million years. However, there are ways to distinguish dwarfs from planets:
Mass, if over 10 Jupiter masses, means a body is unlikely to be a planet.
X-ray and infrared spectra are telltale signs. Some brown dwarfs emit X-ray
s; and all "warm" dwarfs continue to glow tellingly in the red and infrared
spectra until they cool to planetlike temperatures (under 1000 K).
Gas giant
planets have some of the characteristics of brown dwarfs. For example, Jupiter
and Saturn
are both made primarily of hydrogen and helium, like the Sun. Saturn is nearly as large as Jupiter, despite having only 30% the mass. Three of the giants in our solar system (Jupiter, Saturn, and Neptune
) emit more heat than they receive from the Sun. And all four giant planets have their own "planetary systems"—their moons. Brown dwarfs form independently, like stars, but lack sufficient mass to "ignite" as stars do. Like all stars, they can occur singly or in close proximity to other stars. Some orbit stars and can, like planets, have eccentric orbits.
Currently, the International Astronomical Union
considers an object with a mass above the limiting mass for thermonuclear fusion of deuterium (currently calculated to be 13 Jupiter masses for objects of solar metallicity) to be a brown dwarf, whereas an object under that mass (and orbiting a star or stellar remnant) is considered a planet.
The 13 Jupiter-mass cutoff is a rule of thumb rather than something of precise physical significance. Larger objects will burn most of their deuterium and smaller ones will burn only a little, and the 13 Jupiter mass value is somewhere in between. The amount of deuterium burnt also depends to some extent on the composition of the object, specifically on the amount of helium
and deuterium
present and on the fraction of heavier elements, which determines the atmospheric opacity and thus the radiative cooling rate.
The Extrasolar Planets Encyclopaedia
includes objects up to 25 Jupiter masses, and the Exoplanet Data Explorer
up to 24 Jupiter masses. Objects below 13 Jupiter-mass are sometimes studied under the label "sub-brown dwarf
".
The defining characteristic of spectral class M, the coolest type in the long-standing classical stellar sequence, is an optical spectrum dominated by absorption bands of titanium(II) oxide
(TiO) and vanadium(II) oxide
(VO) molecules. However, GD 165B, the cool companion to the white dwarf GD 165, had none of the hallmark TiO features of M dwarfs. The subsequent identification of many field counterparts to GD 165B ultimately led Kirkpatrick and others to the definition of a new spectral class, the L dwarfs, defined in the red optical region not by weakening metal-oxide bands (TiO, VO), but strong metal hydride
bands (FeH, CrH, MgH
, CaH
) and prominent alkali metal
lines (Na I, K I, Cs I, Rb I). , over 600 L dwarfs have been
identified, most by wide-field surveys: the Two Micron All Sky Survey (2MASS
), the Deep Near Infrared Survey of the Southern Sky (DENIS), and the Sloan Digital Sky Survey (SDSS
).
As GD 165B is the prototype of the L dwarfs, Gliese 229
B is the prototype of a second new spectral class, the T dwarfs. Whereas near-infrared (NIR) spectra of L dwarfs show strong absorption bands of H2O and carbon monoxide
(CO), the NIR spectrum of Gliese 229B is dominated by absorption bands from methane
(CH4), features that were only found in the giant planets of the solar system and Titan
. CH4, H2O, and molecular hydrogen
(H2) collision-induced absorption (CIA) give Gliese 229B blue near-infrared colors. Its steeply sloped red optical spectrum also lacks the FeH and CrH bands that characterize L dwarfs and instead is influenced by exceptionally broad absorption features from the alkali
metals Na
and K
. These differences led Kirkpatrick to propose the T spectral class for objects exhibiting H- and K-band CH4 absorption. , 207 T dwarfs are now known. NIR classification schemes for T dwarfs have recently been developed by Adam Burgasser and Tom Geballe. Theory suggests that L dwarfs are a mixture of very low-mass stars and sub-stellar objects (brown dwarfs), whereas the T dwarf class is composed entirely of brown dwarfs. Because of the absorption of sodium
and potassium
in the green part of the spectrum of T dwarfs, the actual appearance of T dwarfs to human visual perception
is estimated to be not brown, but the color of magenta coal tar dye.
In 2009, the coolest known brown dwarfs had estimated effective temperatures between 500 and 600 K
, and have been assigned the spectral class T9. Three examples are the brown dwarfs CFBDS J005910.90-011401.3, ULAS J133553.45+113005.2
, and ULAS J003402.77−005206.7
. The spectra of these objects display absorption around 1.55 micrometers. Delorme et al. have suggested that this feature is due to absorption from ammonia
and that this should be taken as indicating the T-Y transition, making these objects of type Y0. However, the feature is difficult to distinguish from absorption by water and methane
, and other authors have stated that the assignment of class Y0 is premature.
In April 2010, two newly discovered ultracool brown subdwarfs (UGPS 0722-05 and SDWFS 1433+35) were proposed as prototypes for spectral class Y0.
In February 2011, Luhman et al. reported the discovery of a ~300 K, 7 Jupiter mass brown dwarf companion to a nearby white dwarf. Though of 'planetary' mass, Rodriguez et al. suggest it is unlikely to have formed in the same manner as planets.
Shortly after that, Liu et al. published an account of a "very cold" (~370 K) brown dwarf orbiting another very low mass brown dwarf and noted that "[g]iven its low luminosity, atypical colors and cold temperature, CFBDS J1458+10B is a promising candidate for the hypothesized Y spectral class."
In August, 2011, scientists using data from NASA's Wide-field Infrared Survey Explorer
(WISE) have discovered six "Y dwarfs"—star-like bodies with temperatures as cool as the human body.
So far, WISE data have revealed 100 new brown dwarfs. Of these, six are classified as cool Y's. One of the Y dwarfs, called WISE 1828+2650, is the record holder for the coldest brown dwarf with an estimated atmospheric temperature cooler than room temperature, or less than 298 K (25°C, 80°F ). It emits no visible light at all, making it resemble a planet rather than a star.
containing a wide variety of features, from relatively narrow lines of neutral atomic species to broad molecular bands, all of which have different dependencies on temperature, gravity, and metallicity
. Furthermore, these low temperature conditions favor condensation out of the gas state and the formation of grains.
Typical atmospheres of known brown dwarfs range in temperature from 2200 down to 750 K
. Compared to stars, which warm themselves with steady internal fusion, brown dwarfs cool quickly over time; more massive dwarfs cool slower than less massive ones.
Coronagraph
s have recently been used to detect faint objects orbiting bright visible stars, including Gliese 229B.
Sensitive telescopes equipped with charge-coupled devices (CCDs) have been used to search distant star clusters for faint objects, including Teide 1.
Wide-field searches have identified individual faint objects, such as Kelu-1 (30 ly away)
With no strong central nuclear energy source, the interior of a brown dwarf is in a rapid boiling, or convective state. When combined with the rapid rotation that most brown dwarfs exhibit, convection
sets up conditions for the development of a strong, tangled magnetic field
near the surface. The flare observed by Chandra
from LP 944-20 could have its origin in the turbulent magnetized hot material beneath the brown dwarf's surface. A sub-surface flare could conduct heat to the atmosphere, allowing electric currents to flow and produce an X-ray flare, like a stroke of lightning
. The absence of X-rays from LP 944-20 during the non flaring period is also a significant result. It sets the lowest observational limit on steady X-ray power produced by a brown dwarf star, and shows that coronas cease to exist as the surface temperature of a brown dwarf cools below about 2500°C and becomes electrically neutral.
Using NASA's Chandra X-ray Observatory
, scientists have detected X-rays from a low-mass brown dwarf in a multiple star system. This is the first time that a brown dwarf this close to its parent star(s) (Sun-like stars TWA 5A) has been resolved in X-rays. "Our Chandra data show that the X-rays originate from the brown dwarf's coronal plasma which is some 3 million degrees Celsius", said Yohko Tsuboi of Chuo University
in Tokyo. "This brown dwarf is as bright as the Sun today in X-ray light, while it is fifty times less massive than the Sun", said Tsuboi. "This observation, thus, raises the possibility that even massive planets might emit X-rays by themselves during their youth!"
The brown dwarf Cha 110913-773444
, located 500 light years away in the constellation Chamaeleon, may be in the process of forming a mini solar system. Astronomers from Pennsylvania State University
have detected what they believe to be a disk of gas and dust similar to the one hypothesized to have formed our own solar system. Cha 110913-773444 is the smallest brown dwarf found to date (8 Jupiter masses), and if it formed a solar system, it would be the smallest known object to have one. Their findings were published in the December 10, 2005 issue of Astrophysical Journal Letters.
NASA
's WISE
mission has detected 100 new brown dwarfs, and could possibly discover some closer to the Earth than Proxima Centauri
.
, GQ Lupi b
and 2MASS J044144 that are orbiting brown-dwarfs, may have formed by cloud collapse rather than accretion and so may be sub-brown dwarf
s rather than planet
s.
Disks
around brown dwarfs have been found to have many of the same features as disks around stars; therefore, it is expected that there will be accretion-formed planets around brown dwarfs. Given the small mass of brown dwarf disks, most planets will be terrestrial planets rather than gas giants. If a giant planet orbits a brown dwarf across our line of sight then since they have approximately the same diameter this would give a large signal for detection by transit. The accretion zone for planets around a brown dwarf is very close to the brown dwarf itself, so tidal forces would have a strong effect.
Substellar object
A substellar object, sometimes called a lump, is an astronomical object whose mass is smaller than the smallest mass, approximately 0.08 solar masses, at which a star can sustain hydrogen fusion...
which are too low in mass to sustain hydrogen-1 fusion
Nuclear fusion
Nuclear fusion is the process by which two or more atomic nuclei join together, or "fuse", to form a single heavier nucleus. This is usually accompanied by the release or absorption of large quantities of energy...
reactions in their cores, which is characteristic of stars on the main sequence
Main sequence
The main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Hertzsprung and Henry Norris Russell...
. Brown dwarfs have fully convective surfaces and interiors, with no chemical differentiation by depth. Brown dwarfs occupy the mass range between that of large gas giant
Gas giant
A gas giant is a large planet that is not primarily composed of rock or other solid matter. There are four gas giants in the Solar System: Jupiter, Saturn, Uranus, and Neptune...
planet
Planet
A planet is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.The term planet is ancient, with ties to history, science,...
s and the lowest-mass stars; this upper limit is between 75 and 80 Jupiter mass
Jupiter mass
Jupiter mass , is the unit of mass equal to the total mass of the planet Jupiter . Jupiter mass is used to describe masses of the gas giants, such as the outer planets and extrasolar planets. It is also used in describing brown dwarfs....
es (
). Currently there is some debate as to what criterion to use to define the separation between a brown dwarf and a giant planet at very low brown dwarf masses (~13 ), and whether brown dwarfs are required to have experienced fusion at some point in their history. In any event, brown dwarfs heavier than 13 do fuse deuteriumDeuterium burning
Deuterium burning is a nuclear fusion reaction that occurs in stars and some substellar objects, in which a deuterium nucleus and a proton combine to form a helium-3 nucleus...
and those above ~65
also fuse lithiumLithium burning
Lithium is generally present in brown dwarfs and not in low-mass stars. Stars, which achieve the high temperature necessary for fusing hydrogen, rapidly deplete their lithium. This occurs by a collision of lithium-7 and a proton producing two helium-4 nuclei. The temperature necessary for this...
. Some planets are known to orbit brown dwarfs: 2M1207b
2M1207b
2M1207b is a planetary-mass object orbiting the brown dwarf 2M1207, in the constellation Centaurus, approximately 170 light-years from Earth...
, MOA-2007-BLG-192Lb
MOA-2007-BLG-192Lb
MOA-2007-BLG-192Lb, occasionally shortened to MOA-192 b, is an extrasolar planet approximately 3,000 light-years away in the constellation of Sagittarius. The planet was discovered orbiting the brown dwarf or low-mass star MOA-2007-BLG-192L. At a mass of approximately 3.3 times Earth, it is one of...
, and 2MASS J044144b.
History
Brown dwarfs, a term coined by Jill TarterJill Tarter
Jill Cornell Tarter is an American astronomer and the current director of the Center for SETI Research, holding the Bernard M. Oliver Chair for SETI at the SETI Institute.-Education:...
in 1975, were originally called black dwarfs, a classification for dark substellar objects floating freely in space which were too low in mass to sustain stable hydrogen fusion (the term black dwarf
Black dwarf
A black dwarf is a hypothetical stellar remnant, created when a white dwarf becomes sufficiently cool to no longer emit significant heat or light...
currently refers to a white dwarf
White dwarf
A white dwarf, also called a degenerate dwarf, is a small star composed mostly of electron-degenerate matter. They are very dense; a white dwarf's mass is comparable to that of the Sun and its volume is comparable to that of the Earth. Its faint luminosity comes from the emission of stored...
that has cooled down so that it no longer emits significant heat or visible light). Alternative names have been proposed, including planetar
Planetar (astronomy)
Planetar is a term used in astronomy that refers to one of two things:* Brown dwarfs — objects intermediate in size between planets and stars — but having formed similarly to planets....
and substar
Substellar object
A substellar object, sometimes called a lump, is an astronomical object whose mass is smaller than the smallest mass, approximately 0.08 solar masses, at which a star can sustain hydrogen fusion...
.
Early theories concerning the nature of the lowest-mass stars and the hydrogen-burning limit suggested that objects with a mass less than 0.07 solar masses for Population I objects or objects with a mass less than 0.09 solar masses for Population II objects would never go through normal stellar evolution
Stellar evolution
Stellar evolution is the process by which a star undergoes a sequence of radical changes during its lifetime. Depending on the mass of the star, this lifetime ranges from only a few million years to trillions of years .Stellar evolution is not studied by observing the life of a single...
and would become a completely degenerate star (Kumar 1963). The role of deuterium
Deuterium
Deuterium, also called heavy hydrogen, is one of two stable isotopes of hydrogen. It has a natural abundance in Earth's oceans of about one atom in of hydrogen . Deuterium accounts for approximately 0.0156% of all naturally occurring hydrogen in Earth's oceans, while the most common isotope ...
-burning down to 0.012 solar mass
Solar mass
The solar mass , , is a standard unit of mass in astronomy, used to indicate the masses of other stars and galaxies...
es and the impact of dust formation in the cool outer atmosphere
Atmosphere
An atmosphere is a layer of gases that may surround a material body of sufficient mass, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere's temperature is low...
s of brown dwarfs was understood by the late 1980s. They would, however, be hard to find in the sky, as they would emit almost no light. Their strongest emissions would be in the infrared
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...
(IR) spectrum, and ground-based IR detectors were too imprecise at that time to readily identify any brown dwarfs.
Since those earlier times, numerous searches involving various methods have been conducted to find these objects. Some of those methods included multi-color imaging surveys around field stars, imaging surveys for faint companions to main sequence
Main sequence
The main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Hertzsprung and Henry Norris Russell...
dwarfs and white dwarfs, surveys of young star clusters, and radial velocity
Radial velocity
Radial velocity is the velocity of an object in the direction of the line of sight . In astronomy, radial velocity most commonly refers to the spectroscopic radial velocity...
monitoring for close companions.
For many years, efforts to discover brown dwarfs were frustrating and searches to find them seemed fruitless. In 1988, however, University of California, Los Angeles
University of California, Los Angeles
The University of California, Los Angeles is a public research university located in the Westwood neighborhood of Los Angeles, California, USA. It was founded in 1919 as the "Southern Branch" of the University of California and is the second oldest of the ten campuses...
professors Eric Becklin
Eric Becklin
Eric E. Becklin is an American astrophysicist, best known for his pioneering study of infra-red sources at the center of our galaxy....
and Ben Zuckerman identified a faint companion to GD 165 in an infrared search of white dwarfs. The spectrum of GD 165B was very red and enigmatic, showing none of the features expected of a low-mass red dwarf
Red dwarf
According to the Hertzsprung-Russell diagram, a red dwarf star is a small and relatively cool star, of the main sequence, either late K or M spectral type....
star. It became clear that GD 165B would need to be classified as a much cooler object than the latest M dwarfs then known. GD 165B remained unique for almost a decade until the advent of the Two Micron All Sky Survey (2MASS
2MASS
Observations for the Two Micron All-Sky Survey began in 1997 and were completed in 2001 at two telescopes located one each in the northern and southern hemispheres to ensure coverage of the entire sky...
) when Davy Kirkpatrick, out of the California Institute of Technology
California Institute of Technology
The California Institute of Technology is a private research university located in Pasadena, California, United States. Caltech has six academic divisions with strong emphases on science and engineering...
, and others discovered many objects with similar colors and spectral features.
Today, GD 165B is recognized as the prototype of a class of objects now called "L dwarfs". While the discovery of the coolest dwarf was highly significant at the time, it was debated whether GD 165B would be classified as a brown dwarf or simply a very low-mass star, since observationally, it is very difficult to distinguish between the two.
Soon after the discovery of GD 165B, other brown dwarf candidates were reported. Most failed to live up to their candidacy, however, and with further checks for substellar nature, such as the lithium test, many turned out to be stellar objects and not true brown dwarfs. When still young, up to a billion years (a gigayear) old, brown dwarfs can have temperatures and luminosities similar to some stars, so other distinguishing characteristics are necessary, such as the presence of lithium. Stars will burn lithium
Lithium burning
Lithium is generally present in brown dwarfs and not in low-mass stars. Stars, which achieve the high temperature necessary for fusing hydrogen, rapidly deplete their lithium. This occurs by a collision of lithium-7 and a proton producing two helium-4 nuclei. The temperature necessary for this...
in a little over 100 Myr
Myr
The symbol myr was formerly used in English-language geology, and remains as the standard usage in astronomy, as a unit of one million years.It is an abbreviation for 'million years' and lower case is used in geology, while upper case is used in astronomy....
, at most, while most brown dwarfs will never acquire high enough core temperatures to do so. Thus, the detection of lithium in the atmosphere of a candidate object ensures its status as a brown dwarf.
In 1995 the study of brown dwarfs changed dramatically with the discovery of three incontrovertible substellar objects, some of which were identified by the presence of the 670.8 nm lithium line. The most notable of these objects was Gliese 229
Gliese 229
Gliese 229 is a red dwarf star about 19 light years away in the constellation Lepus. It has 58% of the mass of the Sun, 69% of the Sun's radius, and a very low projected rotation velocity of 1 km/s at the stellar equator.The star is known to be a low activity flare star, which means it...
B, which was found to have a temperature and luminosity well below the stellar range. Remarkably, its near-infrared spectrum clearly exhibited a methane absorption band at 2 micrometres, a feature that had previously only been observed in gas giant atmospheres and the atmosphere of Saturn
Saturn
Saturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn is named after the Roman god Saturn, equated to the Greek Cronus , the Babylonian Ninurta and the Hindu Shani. Saturn's astronomical symbol represents the Roman god's sickle.Saturn,...
's moon, Titan
Titan (moon)
Titan , or Saturn VI, is the largest moon of Saturn, the only natural satellite known to have a dense atmosphere, and the only object other than Earth for which clear evidence of stable bodies of surface liquid has been found....
. Methane absorption is not expected at the temperatures of main-sequence stars. This discovery helped to establish yet another spectral class even cooler than L dwarfs known as "T dwarfs" for which Gl 229B is the prototype.
Since 1995, when the first brown dwarf was confirmed by Chilean astronomer María Teresa Ruiz, over a thousand have been identified. Brown dwarfs close to Earth include Epsilon Indi
Epsilon Indi
Epsilon Indi is a K-type main-sequence star approximately 12 light-years away in the constellation of Indus. Two brown dwarfs, found in 2003, orbit the star.- Observation :...
Ba and Bb, a pair of dwarfs gravitationally bound to a sunlike star, around 12 light-years from the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
.
Theory
The standard mechanism for star birthStellar evolution
Stellar evolution is the process by which a star undergoes a sequence of radical changes during its lifetime. Depending on the mass of the star, this lifetime ranges from only a few million years to trillions of years .Stellar evolution is not studied by observing the life of a single...
is through the gravitational collapse of a cold interstellar cloud of gas and dust. As the cloud contracts it heats up from the release of gravitational potential energy
Potential energy
In physics, potential energy is the energy stored in a body or in a system due to its position in a force field or due to its configuration. The SI unit of measure for energy and work is the Joule...
. Early in the process the contracting gas quickly radiates away much of the energy, allowing the collapse to continue. Eventually, the central region becomes sufficiently dense to trap radiation. Consequently, the central temperature and density of the collapsed cloud increases dramatically with time, slowing the contraction, until the conditions are hot and dense enough for thermonuclear reactions to occur in the core of the protostar
Protostar
A protostar is a large mass that forms by contraction out of the gas of a giant molecular cloud in the interstellar medium. The protostellar phase is an early stage in the process of star formation. For a one solar-mass star it lasts about 100,000 years...
. For most stars, gas and radiation pressure generated by the thermonuclear fusion reactions within the core of the star will support it against any further gravitational contraction. Hydrostatic equilibrium
Hydrostatic equilibrium
Hydrostatic equilibrium or hydrostatic balance is the condition in fluid mechanics where a volume of a fluid is at rest or at constant velocity. This occurs when compression due to gravity is balanced by a pressure gradient force...
is reached and the star will spend most of its lifetime fusing hydrogen into helium as a main-sequence star.
If, however, the mass of the protostar is less than about 0.08 solar mass, normal hydrogen thermonuclear fusion reactions will not ignite in the core. Gravitational contraction does not heat the small protostar
Protostar
A protostar is a large mass that forms by contraction out of the gas of a giant molecular cloud in the interstellar medium. The protostellar phase is an early stage in the process of star formation. For a one solar-mass star it lasts about 100,000 years...
very effectively, and before the temperature in the core can increase enough to trigger fusion, the density reaches the point where electrons become closely packed enough to create quantum electron degeneracy pressure
Electron degeneracy pressure
Electron degeneracy pressure is a particular manifestation of the more general phenomenon of quantum degeneracy pressure. The Pauli Exclusion Principle disallows two half integer spin particles from occupying the same quantum state at a given time. The resulting emergent repulsive force is...
. According to the brown dwarf interior models, typical conditions in the core for density, temperature and pressure are expected to be the following:
Further gravitational contraction is prevented and the result is a "failed star", or brown dwarf that simply cools off by radiating away its internal thermal energy.
Distinguishing high-mass brown dwarfs from low-mass stars
LithiumLithium
Lithium is a soft, silver-white metal that belongs to the alkali metal group of chemical elements. It is represented by the symbol Li, and it has the atomic number 3. Under standard conditions it is the lightest metal and the least dense solid element. Like all alkali metals, lithium is highly...
: Lithium
Lithium
Lithium is a soft, silver-white metal that belongs to the alkali metal group of chemical elements. It is represented by the symbol Li, and it has the atomic number 3. Under standard conditions it is the lightest metal and the least dense solid element. Like all alkali metals, lithium is highly...
is generally present in brown dwarfs and not in low-mass stars. Stars, which achieve the high temperature necessary for fusing hydrogen, rapidly deplete their lithium. This occurs by a collision of Lithium-7 and a proton
Proton
The proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....
producing two Helium-4
Helium-4
Helium-4 is a non-radioactive isotope of helium. It is by far the most abundant of the two naturally occurring isotopes of helium, making up about 99.99986% of the helium on earth. Its nucleus is the same as an alpha particle, consisting of two protons and two neutrons. Alpha decay of heavy...
nuclei. The temperature necessary for this reaction is just below the temperature necessary for hydrogen fusion. Convection in low-mass stars ensures that lithium in the whole volume of the star is depleted. Therefore, the presence of the lithium line
Spectral line
A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from a deficiency or excess of photons in a narrow frequency range, compared with the nearby frequencies.- Types of line spectra :...
in a candidate brown dwarf's spectrum is a strong indicator that it is indeed substellar. The use of lithium to distinguish candidate brown dwarfs from low-mass stars is commonly referred to as the lithium test, and was pioneered by Rafael Rebolo, Eduardo Martin and Antonio Magazzu.
- However, lithium is also seen in very young stars, which have not yet had enough time to burn it all. Heavier stars like our sun can retain lithium in their outer atmospheres, which never get hot enough for lithium depletion, but those are distinguishable from brown dwarfs by their size.
- Contrariwise, brown dwarfs at the high end of their mass range can be hot enough to deplete their lithium when they are young. Dwarfs of mass greater than 65 Jupiter masses can burn off their lithium by the time they are half a billion years old[Kulkarni], thus this test is not perfect.
Methane
Methane
Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...
: Unlike stars, older brown dwarfs are sometimes cool enough that over very long periods of time their atmospheres can gather observable quantities of methane
Methane
Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...
. Dwarfs confirmed in this fashion include Gliese 229B.
Luminosity: Main sequence stars cool, but eventually reach a minimum luminosity
Luminosity
Luminosity is a measurement of brightness.-In photometry and color imaging:In photometry, luminosity is sometimes incorrectly used to refer to luminance, which is the density of luminous intensity in a given direction. The SI unit for luminance is candela per square metre.The luminosity function...
which they can sustain through steady fusion. This varies from star to star, but is generally at least 0.01% the luminosity of our Sun. Brown dwarfs cool and darken steadily over their lifetimes: sufficiently old brown dwarfs will be too faint to be detectable.
Iron rain as part of atmospheric convection processes is possible only with brown dwarfs, and not with small stars. The spectroscopy research into iron rain is still ongoing–and not all brown dwarfs will always have this atmospheric anomaly.
Distinguishing low-mass brown dwarfs from high-mass planets
A remarkable property of brown dwarfs is that they are all roughly the same radius as Jupiter. At the high end of their mass range (60–90 Jupiter masses), the volume of a brown dwarf is governed primarily by electron degeneracyDegenerate matter
Degenerate matter is matter that has such extraordinarily high density that the dominant contribution to its pressure is attributable to the Pauli exclusion principle. The pressure maintained by a body of degenerate matter is called the degeneracy pressure, and arises because the Pauli principle...
pressure, as it is in white dwarfs; at the low end of the range (10 Jupiter masses), their volume is governed primarily by Coulomb pressure
Coulomb barrier
The Coulomb barrier, named after Coulomb's law, which is named after physicist Charles-Augustin de Coulomb , is the energy barrier due to electrostatic interaction that two nuclei need to overcome so they can get close enough to undergo a nuclear reaction...
, as it is in planets. The net result is that the radii of brown dwarfs vary by only 10–15% over the range of possible masses. This can make distinguishing them from planets difficult.
In addition, many brown dwarfs undergo no fusion; those at the low end of the mass range
Sub-brown dwarf
A sub-brown dwarf is an astronomical object of planetary mass that is not orbiting a star and is not considered to be a brown dwarf because its mass is below the limiting mass for thermonuclear fusion of deuterium ....
(under 13 Jupiter masses) are never hot enough to fuse even deuterium, and even those at the high end of the mass range (over 60 Jupiter masses) cool quickly enough that they no longer undergo fusion after a period of time on the order of 10 million years. However, there are ways to distinguish dwarfs from planets:
Mass, if over 10 Jupiter masses, means a body is unlikely to be a planet.
X-ray and infrared spectra are telltale signs. Some brown dwarfs emit X-ray
X-ray
X-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and longer than gamma...
s; and all "warm" dwarfs continue to glow tellingly in the red and infrared
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...
spectra until they cool to planetlike temperatures (under 1000 K).
Gas giant
Gas giant
A gas giant is a large planet that is not primarily composed of rock or other solid matter. There are four gas giants in the Solar System: Jupiter, Saturn, Uranus, and Neptune...
planets have some of the characteristics of brown dwarfs. For example, Jupiter
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...
and Saturn
Saturn
Saturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn is named after the Roman god Saturn, equated to the Greek Cronus , the Babylonian Ninurta and the Hindu Shani. Saturn's astronomical symbol represents the Roman god's sickle.Saturn,...
are both made primarily of hydrogen and helium, like the Sun. Saturn is nearly as large as Jupiter, despite having only 30% the mass. Three of the giants in our solar system (Jupiter, Saturn, and Neptune
Neptune
Neptune is the eighth and farthest planet from the Sun in the Solar System. Named for the Roman god of the sea, it is the fourth-largest planet by diameter and the third largest by mass. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times...
) emit more heat than they receive from the Sun. And all four giant planets have their own "planetary systems"—their moons. Brown dwarfs form independently, like stars, but lack sufficient mass to "ignite" as stars do. Like all stars, they can occur singly or in close proximity to other stars. Some orbit stars and can, like planets, have eccentric orbits.
Currently, the International Astronomical Union
International Astronomical Union
The International Astronomical Union IAU is a collection of professional astronomers, at the Ph.D. level and beyond, active in professional research and education in astronomy...
considers an object with a mass above the limiting mass for thermonuclear fusion of deuterium (currently calculated to be 13 Jupiter masses for objects of solar metallicity) to be a brown dwarf, whereas an object under that mass (and orbiting a star or stellar remnant) is considered a planet.
The 13 Jupiter-mass cutoff is a rule of thumb rather than something of precise physical significance. Larger objects will burn most of their deuterium and smaller ones will burn only a little, and the 13 Jupiter mass value is somewhere in between. The amount of deuterium burnt also depends to some extent on the composition of the object, specifically on the amount of helium
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...
and deuterium
Deuterium
Deuterium, also called heavy hydrogen, is one of two stable isotopes of hydrogen. It has a natural abundance in Earth's oceans of about one atom in of hydrogen . Deuterium accounts for approximately 0.0156% of all naturally occurring hydrogen in Earth's oceans, while the most common isotope ...
present and on the fraction of heavier elements, which determines the atmospheric opacity and thus the radiative cooling rate.
The Extrasolar Planets Encyclopaedia
Extrasolar Planets Encyclopaedia
The Extrasolar Planets Encyclopedia is an astronomy website, founded in Paris, France at the Meudon Observatory by Jean Schneider in February 1995, which maintains a database of all the currently known and candidate extrasolar planets, with individual "note" pages for each planet and a full list...
includes objects up to 25 Jupiter masses, and the Exoplanet Data Explorer
Exoplanet Data Explorer
The Exoplanet Data Explorer / Exoplanet Orbit Database lists extrasolar planets up to 24 Jupiter masses."We have retained the generous upper mass limit of 24 Jupiter masses in our definition of a “planet”, for the same reasons as in the Catalog: at the moment, any mass limit is arbitrary and will...
up to 24 Jupiter masses. Objects below 13 Jupiter-mass are sometimes studied under the label "sub-brown dwarf
Sub-brown dwarf
A sub-brown dwarf is an astronomical object of planetary mass that is not orbiting a star and is not considered to be a brown dwarf because its mass is below the limiting mass for thermonuclear fusion of deuterium ....
".
Spectral class M
There are brown dwarfs with a spectral class of M6.5 or later. They are also called Late-M dwarfs.Spectral class L
Titanium(II) oxide
Titanium oxide is an inorganic chemical compound of titanium and oxygen. It can be prepared from titanium dioxide and titanium metal at 1500°C. It is non-stoichiometric in a range TiO0.7 to TiO1.3 and this is caused by vacancies of either Ti or O in the defect rock salt structure . In pure TiO...
(TiO) and vanadium(II) oxide
Vanadium(II) oxide
Vanadium oxide, VO, is one of the many oxides of vanadium. VO is a long-lived, electronically neutral reagent chemical. It adopts a distorted NaCl structure and contains weak V-V metal to metal bonds. As shown by band theory, VO is a conductor of electricity due to its partially filled conduction...
(VO) molecules. However, GD 165B, the cool companion to the white dwarf GD 165, had none of the hallmark TiO features of M dwarfs. The subsequent identification of many field counterparts to GD 165B ultimately led Kirkpatrick and others to the definition of a new spectral class, the L dwarfs, defined in the red optical region not by weakening metal-oxide bands (TiO, VO), but strong metal hydride
Hydride
In chemistry, a hydride is the anion of hydrogen, H−, or, more commonly, a compound in which one or more hydrogen centres have nucleophilic, reducing, or basic properties. In compounds that are regarded as hydrides, hydrogen is bonded to a more electropositive element or group...
bands (FeH, CrH, MgH
Magnesium hydride
Magnesium hydride is the chemical compound MgH2. It contains 7.66% by weight of hydrogen and has been studied as a potential hydrogen storage medium.- Preparation :...
, CaH
Calcium hydride
Calcium hydride is the chemical compound with the formula CaH2. This grey powder reacts vigorously with water liberating hydrogen gas. CaH2 is thus used as a drying agent, i.e. a desiccant....
) and prominent alkali metal
Alkali metal
The alkali metals are a series of chemical elements in the periodic table. In the modern IUPAC nomenclature, the alkali metals comprise the group 1 elements, along with hydrogen. The alkali metals are lithium , sodium , potassium , rubidium , caesium , and francium...
lines (Na I, K I, Cs I, Rb I). , over 600 L dwarfs have been
identified, most by wide-field surveys: the Two Micron All Sky Survey (2MASS
2MASS
Observations for the Two Micron All-Sky Survey began in 1997 and were completed in 2001 at two telescopes located one each in the northern and southern hemispheres to ensure coverage of the entire sky...
), the Deep Near Infrared Survey of the Southern Sky (DENIS), and the Sloan Digital Sky Survey (SDSS
Sloan Digital Sky Survey
The Sloan Digital Sky Survey or SDSS is a major multi-filter imaging and spectroscopic redshift survey using a dedicated 2.5-m wide-angle optical telescope at Apache Point Observatory in New Mexico, United States. The project was named after the Alfred P...
).
Spectral class T
Gliese 229
Gliese 229 is a red dwarf star about 19 light years away in the constellation Lepus. It has 58% of the mass of the Sun, 69% of the Sun's radius, and a very low projected rotation velocity of 1 km/s at the stellar equator.The star is known to be a low activity flare star, which means it...
B is the prototype of a second new spectral class, the T dwarfs. Whereas near-infrared (NIR) spectra of L dwarfs show strong absorption bands of H2O and carbon monoxide
Carbon monoxide
Carbon monoxide , also called carbonous oxide, is a colorless, odorless, and tasteless gas that is slightly lighter than air. It is highly toxic to humans and animals in higher quantities, although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal...
(CO), the NIR spectrum of Gliese 229B is dominated by absorption bands from methane
Methane
Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...
(CH4), features that were only found in the giant planets of the solar system and Titan
Titan (moon)
Titan , or Saturn VI, is the largest moon of Saturn, the only natural satellite known to have a dense atmosphere, and the only object other than Earth for which clear evidence of stable bodies of surface liquid has been found....
. CH4, H2O, and molecular hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
(H2) collision-induced absorption (CIA) give Gliese 229B blue near-infrared colors. Its steeply sloped red optical spectrum also lacks the FeH and CrH bands that characterize L dwarfs and instead is influenced by exceptionally broad absorption features from the alkali
Alkali
In chemistry, an alkali is a basic, ionic salt of an alkali metal or alkaline earth metal element. Some authors also define an alkali as a base that dissolves in water. A solution of a soluble base has a pH greater than 7. The adjective alkaline is commonly used in English as a synonym for base,...
metals Na
Sodium
Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...
and K
Potassium
Potassium is the chemical element with the symbol K and atomic number 19. Elemental potassium is a soft silvery-white alkali metal that oxidizes rapidly in air and is very reactive with water, generating sufficient heat to ignite the hydrogen emitted in the reaction.Potassium and sodium are...
. These differences led Kirkpatrick to propose the T spectral class for objects exhibiting H- and K-band CH4 absorption. , 207 T dwarfs are now known. NIR classification schemes for T dwarfs have recently been developed by Adam Burgasser and Tom Geballe. Theory suggests that L dwarfs are a mixture of very low-mass stars and sub-stellar objects (brown dwarfs), whereas the T dwarf class is composed entirely of brown dwarfs. Because of the absorption of sodium
Sodium
Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...
and potassium
Potassium
Potassium is the chemical element with the symbol K and atomic number 19. Elemental potassium is a soft silvery-white alkali metal that oxidizes rapidly in air and is very reactive with water, generating sufficient heat to ignite the hydrogen emitted in the reaction.Potassium and sodium are...
in the green part of the spectrum of T dwarfs, the actual appearance of T dwarfs to human visual perception
Visual perception
Visual perception is the ability to interpret information and surroundings from the effects of visible light reaching the eye. The resulting perception is also known as eyesight, sight, or vision...
is estimated to be not brown, but the color of magenta coal tar dye.
Spectral class Y
Class Y dwarfs are expected to be much cooler than T-dwarfs. They have been modelled, though there is no well-defined spectral sequence yet with prototypes.- Y: < 500 K, ultra-cool brown dwarfs
In 2009, the coolest known brown dwarfs had estimated effective temperatures between 500 and 600 K
Kelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...
, and have been assigned the spectral class T9. Three examples are the brown dwarfs CFBDS J005910.90-011401.3, ULAS J133553.45+113005.2
ULAS J133553.45+113005.2
ULAS J133553.45+113005.2 is a T-type brown dwarf in the constellation of Virgo. It was discovered in data from the UK Infrared Telescope Infrared Deep Sky Survey Large Area Survey...
, and ULAS J003402.77−005206.7
ULAS J003402.77−005206.7
ULAS J003402.77-005206.7 is a T-type brown dwarf in the constellation of Cetus.ULAS J0034-00 is one of the coolest brown dwarfs known. It was first identified in data from the UK Infrared Telescope Infrared...
. The spectra of these objects display absorption around 1.55 micrometers. Delorme et al. have suggested that this feature is due to absorption from ammonia
Ammonia
Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...
and that this should be taken as indicating the T-Y transition, making these objects of type Y0. However, the feature is difficult to distinguish from absorption by water and methane
Methane
Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...
, and other authors have stated that the assignment of class Y0 is premature.
In April 2010, two newly discovered ultracool brown subdwarfs (UGPS 0722-05 and SDWFS 1433+35) were proposed as prototypes for spectral class Y0.
In February 2011, Luhman et al. reported the discovery of a ~300 K, 7 Jupiter mass brown dwarf companion to a nearby white dwarf. Though of 'planetary' mass, Rodriguez et al. suggest it is unlikely to have formed in the same manner as planets.
Shortly after that, Liu et al. published an account of a "very cold" (~370 K) brown dwarf orbiting another very low mass brown dwarf and noted that "[g]iven its low luminosity, atypical colors and cold temperature, CFBDS J1458+10B is a promising candidate for the hypothesized Y spectral class."
In August, 2011, scientists using data from NASA's Wide-field Infrared Survey Explorer
Wide-field Infrared Survey Explorer
Wide-field Infrared Survey Explorer is a NASA infrared-wavelength astronomical space telescope launched on December 14, 2009, and decommissioned/hibernated on February 17, 2011 when its transmitter was turned off...
(WISE) have discovered six "Y dwarfs"—star-like bodies with temperatures as cool as the human body.
So far, WISE data have revealed 100 new brown dwarfs. Of these, six are classified as cool Y's. One of the Y dwarfs, called WISE 1828+2650, is the record holder for the coldest brown dwarf with an estimated atmospheric temperature cooler than room temperature, or less than 298 K (25°C, 80°F ). It emits no visible light at all, making it resemble a planet rather than a star.
Spectral and atmospheric properties of brown dwarfs
The majority of flux emitted by L and T dwarfs is in the 1 to 2.5 micrometre near-infrared range. Low and decreasing temperatures through the late M, L, and T dwarf sequence result in a rich near-infrared spectrumSpectrum
A spectrum is a condition that is not limited to a specific set of values but can vary infinitely within a continuum. The word saw its first scientific use within the field of optics to describe the rainbow of colors in visible light when separated using a prism; it has since been applied by...
containing a wide variety of features, from relatively narrow lines of neutral atomic species to broad molecular bands, all of which have different dependencies on temperature, gravity, and metallicity
Metallicity
In astronomy and physical cosmology, the metallicity of an object is the proportion of its matter made up of chemical elements other than hydrogen and helium...
. Furthermore, these low temperature conditions favor condensation out of the gas state and the formation of grains.
Typical atmospheres of known brown dwarfs range in temperature from 2200 down to 750 K
Kelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...
. Compared to stars, which warm themselves with steady internal fusion, brown dwarfs cool quickly over time; more massive dwarfs cool slower than less massive ones.
Observational techniques
Coronagraph
A coronagraph is a telescopic attachment designed to block out the direct light from a star so that nearby objects – which otherwise would be hidden in the star's bright glare – can be resolved...
s have recently been used to detect faint objects orbiting bright visible stars, including Gliese 229B.
Sensitive telescopes equipped with charge-coupled devices (CCDs) have been used to search distant star clusters for faint objects, including Teide 1.
Wide-field searches have identified individual faint objects, such as Kelu-1 (30 ly away)
Milestones
- 1995: First brown dwarf verified. Teide 1Teide 1Teide 1 was the first brown dwarf to be verified in 1995. This brown dwarf is located in the Pleiades open star cluster located approximately 400 light years from Earth....
, an M8 object in the PleiadesPleiades (star cluster)In astronomy, the Pleiades, or Seven Sisters , is an open star cluster containing middle-aged hot B-type stars located in the constellation of Taurus. It is among the nearest star clusters to Earth and is the cluster most obvious to the naked eye in the night sky...
clusterStar clusterStar clusters or star clouds are groups of stars. Two types of star clusters can be distinguished: globular clusters are tight groups of hundreds of thousands of very old stars which are gravitationally bound, while open clusters, more loosely clustered groups of stars, generally contain less than...
, is picked out with a CCD in the Spanish Observatory of Roque de los Muchachos of the Instituto de Astrofísica de CanariasInstituto de Astrofísica de CanariasThe Instituto de Astrofísica de Canarias is an astrophysical research institute located in Tenerife in the Canary Islands, Spain. It was founded in 1975 at the University of La Laguna.It operates two astronomical observatories in the Canary Islands:...
.
- First methane brown dwarf verified. Gliese 229B is discovered orbiting red dwarf Gliese 229Gliese 229Gliese 229 is a red dwarf star about 19 light years away in the constellation Lepus. It has 58% of the mass of the Sun, 69% of the Sun's radius, and a very low projected rotation velocity of 1 km/s at the stellar equator.The star is known to be a low activity flare star, which means it...
A (20 ly away) using an adaptive opticsAdaptive opticsAdaptive optics is a technology used to improve the performance of optical systems by reducing the effect of wavefront distortions. It is used in astronomical telescopes and laser communication systems to remove the effects of atmospheric distortion, and in retinal imaging systems to reduce the...
coronagraph to sharpen images from the 60 inch (1.5 m) reflecting telescope at Palomar ObservatoryPalomar ObservatoryPalomar Observatory is a privately owned observatory located in San Diego County, California, southeast of Pasadena's Mount Wilson Observatory, in the Palomar Mountain Range. At approximately elevation, it is owned and operated by the California Institute of Technology...
on Southern California's Mt. Palomar; followup infrared spectroscopy made with their 200 inch (5 m) Hale telescopeHale telescopeThe Hale Telescope is a , 3.3 reflecting telescope at the Palomar Observatory in California, named after astronomer George Ellery Hale. With funding from the Rockefeller Foundation, he orchestrated the planning, design, and construction of the observatory, but did not live to see its commissioning...
shows an abundance of methane.
- 1998: First X-ray-emitting brown dwarf found. Cha Halpha 1, an M8 object in the Chamaeleon I dark cloud, is determined to be an X-ray source, similar to convective late-type stars.
- December 15, 1999: First X-ray flare detected from a brown dwarf. A team at the University of California monitoring LP 944-20 (60 Jupiter masses, 16 ly away) via the Chandra X-ray ObservatoryChandra X-ray ObservatoryThe Chandra X-ray Observatory is a satellite launched on STS-93 by NASA on July 23, 1999. It was named in honor of Indian-American physicist Subrahmanyan Chandrasekhar who is known for determining the maximum mass for white dwarfs. "Chandra" also means "moon" or "luminous" in Sanskrit.Chandra...
, catches a 2-hour flare. - 27 July 2000: First radio emission (in flare and quiescence) detected from a brown dwarf. A team of students at the Very Large ArrayVery Large ArrayThe Very Large Array is a radio astronomy observatory located on the Plains of San Agustin, between the towns of Magdalena and Datil, some fifty miles west of Socorro, New Mexico, USA...
reported their observations of LP 944-20 in the 15 March 2001 issue of the journal NatureNature (journal)Nature, first published on 4 November 1869, is ranked the world's most cited interdisciplinary scientific journal by the Science Edition of the 2010 Journal Citation Reports...
.
Brown dwarf as an X-ray source
X-ray flares detected from brown dwarfs since late 1999 suggest changing magnetic fields within them, similar to those in very low-mass stars.With no strong central nuclear energy source, the interior of a brown dwarf is in a rapid boiling, or convective state. When combined with the rapid rotation that most brown dwarfs exhibit, convection
Convection
Convection is the movement of molecules within fluids and rheids. It cannot take place in solids, since neither bulk current flows nor significant diffusion can take place in solids....
sets up conditions for the development of a strong, tangled magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
near the surface. The flare observed by Chandra
Chandra X-ray Observatory
The Chandra X-ray Observatory is a satellite launched on STS-93 by NASA on July 23, 1999. It was named in honor of Indian-American physicist Subrahmanyan Chandrasekhar who is known for determining the maximum mass for white dwarfs. "Chandra" also means "moon" or "luminous" in Sanskrit.Chandra...
from LP 944-20 could have its origin in the turbulent magnetized hot material beneath the brown dwarf's surface. A sub-surface flare could conduct heat to the atmosphere, allowing electric currents to flow and produce an X-ray flare, like a stroke of lightning
Lightning
Lightning is an atmospheric electrostatic discharge accompanied by thunder, which typically occurs during thunderstorms, and sometimes during volcanic eruptions or dust storms...
. The absence of X-rays from LP 944-20 during the non flaring period is also a significant result. It sets the lowest observational limit on steady X-ray power produced by a brown dwarf star, and shows that coronas cease to exist as the surface temperature of a brown dwarf cools below about 2500°C and becomes electrically neutral.
Using NASA's Chandra X-ray Observatory
Chandra X-ray Observatory
The Chandra X-ray Observatory is a satellite launched on STS-93 by NASA on July 23, 1999. It was named in honor of Indian-American physicist Subrahmanyan Chandrasekhar who is known for determining the maximum mass for white dwarfs. "Chandra" also means "moon" or "luminous" in Sanskrit.Chandra...
, scientists have detected X-rays from a low-mass brown dwarf in a multiple star system. This is the first time that a brown dwarf this close to its parent star(s) (Sun-like stars TWA 5A) has been resolved in X-rays. "Our Chandra data show that the X-rays originate from the brown dwarf's coronal plasma which is some 3 million degrees Celsius", said Yohko Tsuboi of Chuo University
Chuo University
Chuo University is a one of the Japanese leading universities. Thus it is competitive in several rankings such as shown below.-General Rankings:The university has been ranked 27th, 25th, 34th during 2008-2010 respectively in the ranking "Truly Strong Universities" by Toyo Keizai.-Research...
in Tokyo. "This brown dwarf is as bright as the Sun today in X-ray light, while it is fifty times less massive than the Sun", said Tsuboi. "This observation, thus, raises the possibility that even massive planets might emit X-rays by themselves during their youth!"
Recent developments
Recent observations of known brown dwarf candidates have revealed a pattern of brightening and dimming of infrared emissions that suggests relatively cool, opaque cloud patterns obscuring a hot interior that is stirred by extreme winds. The weather on such bodies is thought to be extremely violent, comparable to but far exceeding Jupiter's famous storms.The brown dwarf Cha 110913-773444
Cha 110913-773444
Cha 110913-773444 is an astronomical object surrounded by what appears to be a protoplanetary disk...
, located 500 light years away in the constellation Chamaeleon, may be in the process of forming a mini solar system. Astronomers from Pennsylvania State University
Pennsylvania State University
The Pennsylvania State University, commonly referred to as Penn State or PSU, is a public research university with campuses and facilities throughout the state of Pennsylvania, United States. Founded in 1855, the university has a threefold mission of teaching, research, and public service...
have detected what they believe to be a disk of gas and dust similar to the one hypothesized to have formed our own solar system. Cha 110913-773444 is the smallest brown dwarf found to date (8 Jupiter masses), and if it formed a solar system, it would be the smallest known object to have one. Their findings were published in the December 10, 2005 issue of Astrophysical Journal Letters.
NASA
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...
's WISE
Wide-field Infrared Survey Explorer
Wide-field Infrared Survey Explorer is a NASA infrared-wavelength astronomical space telescope launched on December 14, 2009, and decommissioned/hibernated on February 17, 2011 when its transmitter was turned off...
mission has detected 100 new brown dwarfs, and could possibly discover some closer to the Earth than Proxima Centauri
Proxima Centauri
Proxima Centauri is a red dwarf star about 4.2 light-years distant in the constellation of Centaurus. It was discovered in 1915 by Robert Innes, the Director of the Union Observatory in South Africa, and is the nearest known star to the Sun, although it is too faint to be seen with the naked eye...
.
Planets around brown dwarfs
The planetary-mass objects 2M1207b2M1207b
2M1207b is a planetary-mass object orbiting the brown dwarf 2M1207, in the constellation Centaurus, approximately 170 light-years from Earth...
, GQ Lupi b
GQ Lupi b
GQ Lupi b is a possible extrasolar planet orbiting the star GQ Lupi. Its discovery was announced in April 2005. Along with 2M1207b, this was one of the first extrasolar planet candidates to be directly imaged...
and 2MASS J044144 that are orbiting brown-dwarfs, may have formed by cloud collapse rather than accretion and so may be sub-brown dwarf
Sub-brown dwarf
A sub-brown dwarf is an astronomical object of planetary mass that is not orbiting a star and is not considered to be a brown dwarf because its mass is below the limiting mass for thermonuclear fusion of deuterium ....
s rather than planet
Planet
A planet is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.The term planet is ancient, with ties to history, science,...
s.
Disks
Circumstellar disk
A circumstellar disk is a torus, pancake or ring-shaped accumulation of matter composed of gas, dust, planetesimals, asteroids or collision fragments in orbit around a star. Around the youngest stars, they are the reservoirs of material out of which planets may form...
around brown dwarfs have been found to have many of the same features as disks around stars; therefore, it is expected that there will be accretion-formed planets around brown dwarfs. Given the small mass of brown dwarf disks, most planets will be terrestrial planets rather than gas giants. If a giant planet orbits a brown dwarf across our line of sight then since they have approximately the same diameter this would give a large signal for detection by transit. The accretion zone for planets around a brown dwarf is very close to the brown dwarf itself, so tidal forces would have a strong effect.
Notable brown dwarfs
- WD 0137-349WD 0137-349|-! style="background-color: #FFFFC0;" colspan="2" | Astrometry|- style="vertical-align: top;"| Distance | 333 Ly|-! style="background-color: #FFFFC0;" colspan="2" | Binary orbit|- style="vertical-align: top;"| Period | 2 hs...
B: first confirmed brown dwarf to have survived the primary's red giantRed giantA red giant is a luminous giant star of low or intermediate mass in a late phase of stellar evolution. The outer atmosphere is inflated and tenuous, making the radius immense and the surface temperature low, somewhere from 5,000 K and lower...
phase. - In 1984 it was postulated by some astronomers that the Sun may be orbited by an undetected brown dwarf (sometimes referred to as NemesisNemesis (star)Nemesis is a hypothetical hard-to-detect red dwarf star, white dwarf star or brown dwarf, originally postulated in 1984 to be orbiting the Sun at a distance of about 95,000 AU , somewhat beyond the Oort cloud, to explain a perceived cycle of mass extinctions in the geological record, which seem to...
) which could interact with the Oort cloudOort cloudThe Oort cloud , or the Öpik–Oort cloud , is a hypothesized spherical cloud of comets which may lie roughly 50,000 AU, or nearly a light-year, from the Sun. This places the cloud at nearly a quarter of the distance to Proxima Centauri, the nearest star to the Sun...
just as passing stars can. But this dated theory has fallen out of favor.
| Brown Dwarfs | |||||
|---|---|---|---|---|---|
| Title | Brown Dwarf Name | Spectral Type | RA/Dec | Constellation | Notes |
| First discovered | Teide Pleiades 1 | M8 | 3h47m18.0s +24°22'31" | Taurus Taurus (constellation) Taurus is one of the constellations of the zodiac. Its name is a Latin word meaning 'bull', and its astrological symbol is a stylized bull's head:... |
Imaged in 1989 and 1994 |
| First imaged with coronography | Gliese 229 B | T6.5 | 06h10m34.62s -21°51'52.1" | Lepus Lepus (constellation) Lepus is a constellation lying just south of the celestial equator, immediately south of Orion. Its name is Latin for hare. Although the hare does not represent any particular figure in Greek mythology, Lepus was one of the 48 constellations listed by the 2nd century astronomer Ptolemy, and it... |
Discovered 1994 |
| First with planemo | 2MASSW J1207334-393254 2M1207 2M1207, 2M1207A or 2MASS J12073346-3932539 is a brown dwarf located in the constellation Centaurus; a companion object, 2M1207b, may be the first extrasolar planetary mass companion to be directly imaged, and is the first discovered orbiting a brown dwarf.2M1207 was discovered during the course of... |
M8 | 12h07m33.47s -39°32'54.0" | Centaurus Centaurus Centaurus is a bright constellation in the southern sky. One of the largest constellations, Centaurus was included among the 48 constellations listed by the 2nd century astronomer Ptolemy, and it remains one of the 88 modern constellations.-Stars:... |
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| First with a planetary mass in orbit about it | 2M1207 2M1207 2M1207, 2M1207A or 2MASS J12073346-3932539 is a brown dwarf located in the constellation Centaurus; a companion object, 2M1207b, may be the first extrasolar planetary mass companion to be directly imaged, and is the first discovered orbiting a brown dwarf.2M1207 was discovered during the course of... |
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| First with a dust disk Protoplanetary disk A protoplanetary disk is a rotating circumstellar disk of dense gas surrounding a young newly formed star, a T Tauri star, or Herbig Ae/Be star... |
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| First with bipolar outflow Bipolar outflow A bipolar outflow represents two continuous flows of gas from the poles of a star. Bipolar outflows may be associated with protostars , or with evolved post-AGB stars .... |
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| First field type (solitary) | Teide 1 | M8 | 3h47m18.0s +24°22'31" | Taurus | 1995 |
| First as a companion to a normal star | Gliese 229 B | T6.5 | 06h10m34.62s -21°51'52.1" | Lepus | 1995 |
| First spectroscopic binary brown dwarf | PPL 15 A, B http://arxiv.org/abs/astro-ph/9908015 | M6.5 | Taurus | Basri and Martin 1999 | |
| First binary brown dwarf of T Type | Epsilon Indi Ba, Bb Epsilon Indi Epsilon Indi is a K-type main-sequence star approximately 12 light-years away in the constellation of Indus. Two brown dwarfs, found in 2003, orbit the star.- Observation :... http://www.eso.org/outreach/press-rel/pr-2003/pr-01-03.html |
T1 + T6 | Indus | Distance: 3.626pc | |
| First trinary brown dwarf | DENIS-P J020529.0-115925 DENIS-P J020529.0-115925 |-! style="background-color: #FFFFC0;" colspan="2" | Binary orbit|- style="vertical-align: top;"| Orbit period of B and C: | 8 years|- style="vertical-align: top;"| Space between A and B:| 10 AU|- style="vertical-align: top;"... A/B/C |
L5, L8 and T0 | 02h05m29.40s -11°59'29.7" | Cetus Cetus Cetus is a constellation. Its name refers to Cetus, a sea monster in Greek mythology, although it is often called 'the whale' today. Cetus is located in the region of the sky that contains other water-related constellations such as Aquarius, Pisces, and Eridanus.-Ecliptic:Although Cetus is not... |
Delfosse et al. 1997, mentions |
| First halo brown dwarf | 2MASS J05325346+8246465 | sd Subdwarf star A subdwarf star, sometimes denoted by "sd", is luminosity class VI under the Yerkes spectral classification system. They are defined as stars with luminosity 1.5 to 2 magnitudes lower than that of main-sequence stars of the same spectral type... L7 |
05h32m53.46s +82°46'46.5" | Gemini Gemini (constellation) Gemini is one of the constellations of the zodiac. It was one of the 48 constellations described by the 2nd century astronomer Ptolemy and it remains one of the 88 modern constellations today. Its name is Latin for "twins", and it is associated with the twins Castor and Pollux in Greek mythology... |
Adam J. Burgasser, et al. 2003 |
| First Late-M spectra | Teide 1 | M8 | 3h47m18.0s +24°22'31" | Taurus | 1995 |
| First L spectra | |||||
| First T spectra | Gliese 229 B | T6.5 | 06h10m34.62s -21°51'52.1" | Lepus | 1995 |
| Latest T spectrum | ULAS J0034-00 ULAS J003402.77−005206.7 ULAS J003402.77-005206.7 is a T-type brown dwarf in the constellation of Cetus.ULAS J0034-00 is one of the coolest brown dwarfs known. It was first identified in data from the UK Infrared Telescope Infrared... |
T9 | Cetus | 2007 | |
| First Y spectrum | CFBDS0059 - pending. This is also classified as a T9 dwarf, due to its close resemblance to other T dwarfs | ~Y0 | 2008 | ||
| First X-ray-emitting | Cha Halpha 1 | M8 | Chamaeleon Chamaeleon Chamaeleon is a small constellation in the southern sky. It is named after the chameleon, a form of lizard. It was first defined in the sixteenth century.-History:... |
1998 | |
| First X-ray flare | LP 944-20 | M9V | 03h39m35.22s -35°25'44.1" | Fornax | 1999 |
| First radio emission (in flare and quiescence) | LP 944-20 | M9V | 03h39m35.22s -35°25'44.1" | Fornax | 2000 |
| Brown Dwarfs | |||||
|---|---|---|---|---|---|
| Title | Brown Dwarf Name | Spectral Type | RA/Dec | Constellation | Notes |
| Oldest | |||||
| Youngest | |||||
| Heaviest | |||||
| Metal-rich | |||||
| Metal-poor | 2MASS J05325346+8246465 | sd Subdwarf star A subdwarf star, sometimes denoted by "sd", is luminosity class VI under the Yerkes spectral classification system. They are defined as stars with luminosity 1.5 to 2 magnitudes lower than that of main-sequence stars of the same spectral type... L7 |
05h32m53.46s +82°46'46.5" | Gemini Gemini (constellation) Gemini is one of the constellations of the zodiac. It was one of the 48 constellations described by the 2nd century astronomer Ptolemy and it remains one of the 88 modern constellations today. Its name is Latin for "twins", and it is associated with the twins Castor and Pollux in Greek mythology... |
distance is ~10–30pc, metallicity is 0.1–0.01ZSol Sun The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields... |
| Lightest | |||||
| Largest | |||||
| Smallest | |||||
| Furthest | |||||
| Nearest | WISE 1541-2250 WISE 1541-2250 WISE 1541-2250 is a brown dwarf, located in constellation . It is one of six Y-type brown dwarfs among 106 brown dwarfs , discovered in 2011 by the Wide-field Infrared... |
Y | Distance: 9 ly | ||
| Nearest binary | Epsilon Indi Ba, Bb Epsilon Indi Epsilon Indi is a K-type main-sequence star approximately 12 light-years away in the constellation of Indus. Two brown dwarfs, found in 2003, orbit the star.- Observation :... http://www.eso.org/outreach/press-rel/pr-2003/pr-01-03.html |
T1 + T6 | Indus | Distance: 12 ly | |
| Brightest | |||||
| Dimmest | 2MASS J09393548-2448279 | ||||
| Hottest | |||||
| Coolest | WISE 1828+2650 | Y | Temperature 300 K Kelvin The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all... |
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| Most dense | COROT-3b | Transiting brown dwarf COROT-3b has with a diameter 1.01±0.07 times that of Jupiter. This makes it twice as dense as the metal platinum. | |||
| Least dense | |||||
See also
- Dark matterDark matterIn astronomy and cosmology, dark matter is matter that neither emits nor scatters light or other electromagnetic radiation, and so cannot be directly detected via optical or radio astronomy...
- Brown-dwarf desertBrown-dwarf desertThe brown-dwarf desert is a theorized range of orbits around a star on which brown dwarfs cannot exist as a companion object. This is usually up to 5 AU around solar mass stars....
- Stellar objectsStarA star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...
- Blue dwarfBlue dwarf (red dwarf stage)A blue dwarf is a hypothetical class of star that develops from a red dwarf star after it has exhausted much of its hydrogen fuel supply. Since red dwarf stars fuse their hydrogen slowly and are fully convective , the universe is currently not old enough for any blue dwarfs to have formed yet...
- Orange dwarfOrange dwarfA K-type main-sequence star , also referred to orange dwarf, are main-sequence stars of spectral type K and luminosity class V. These stars are intermediate in size between red M-type main-sequence stars and yellow G-type main-sequence stars...
- Red dwarfRed dwarfAccording to the Hertzsprung-Russell diagram, a red dwarf star is a small and relatively cool star, of the main sequence, either late K or M spectral type....
- Yellow dwarfYellow dwarfA G-type main-sequence star , often called a yellow dwarf, is a main-sequence star of spectral type G and luminosity class V. Such a star has about 0.8 to 1.2 solar masses and surface temperature of between 5,300 and 6,000 K., Tables VII, VIII...
- Blue dwarf
History
- S. S. Kumar, Low-Luminosity Stars. Gordon and Breach, London, 1969—an early overview paper on brown dwarfs
- The Columbia Encyclopedia
Details
- A current list of L and T dwarfs
- A geological definition of brown dwarfs, contrasted with stars and planets (via Berkeley)
- Neill Reid's pages at the Space Telescope Science InstituteSpace Telescope Science InstituteThe Space Telescope Science Institute is the science operations center for the Hubble Space Telescope and for the James Webb Space Telescope...
:- On spectral analysis of M dwarfs, L dwarfs, and T dwarfs
- Temperature and mass characteristics of low-temperature dwarfs
- First X-ray from brown dwarf observed, Spaceref.com, 2000
- Brown Dwarfs and ultracool dwarfs (late-M, L, T) - D. Montes, UCM
- Wild Weather: Iron Rain on Failed Stars - scientists are investigating astonishing weather patterns on brown dwarfs, Space.com, 2006
- NASA Brown dwarf detectives - Detailed information in a simplified sense.
- Brown Dwarfs - Website with general information about brown dwarfs (has many detailed and colorful artist's impressions).
Stars
- Cha Halpha 1 stats and history
- A census of observed brown dwarfs (not all confirmed), ca 1998
- Epsilon Indi Ba and Bb, a pair of brown dwarfs 12 ly away
- Luhman et al., Discovery of a Planetary-Mass Brown Dwarf with a Circumstellar Disk
- Discovery Narrows the Gap Between Planets and Brown Dwarfs, 2007
- Y-Spectral class for Ultra-Cool Dwarfs, N.R.Deacon and N.C.Hambly, 2006