RMC 136a1 is a Wolf–Rayet star located at the center of R136, the central concentration of stars of the large NGC 2070 open cluster in the Tarantula Nebula. It lies at a distance of about 49.97 kiloparsecs in a neighbouring galaxy known as the Large Magellanic Cloud. It has the highest mass and luminosity of any known star, at 315 M☉ and 8.7 million L☉, is one of the hottest, at around 53,000 K. In 1960, a group of astronomers working at the Radcliffe Observatory in Pretoria made systematic measurements of the brightness and spectra of bright stars in the Large Magellanic Cloud. Among the objects cataloged was RMC 136, the central "star" of the Tarantula Nebula, which the observers concluded was a multiple star system. Subsequent observations showed that R136 was located in the middle of a giant region of ionized interstellar hydrogen, known as an H II region, a center of intense star formation in the immediate vicinity of the observed stars. In 1979, ESO's 3.6 m telescope was used to resolve R136 into three components.
The exact nature of R136a was unclear and a subject of intense discussion. Estimates that the brightness of the central region would require as many as 100 hot O class stars within half a parsec at the centre of the cluster led to speculation that a star 3,000 times the mass of the Sun was the more explanation; the first demonstration that R136a was a star cluster was provided by Weigelt and Beier in 1985. Using the speckle interferometry technique, R136a was shown to be made up of 8 stars within 1 arcsecond at the centre of the cluster, with R136a1 being the brightest. Final confirmation of the nature of R136a came after the launch of the Hubble Space Telescope, its Wide Field and Planetary Camera resolved R136a into at least 12 components and showed that R136 contained over 200 luminous stars. The more advanced WFPC2 allowed the study of 46 massive luminous stars within half a parsec of R136a and over 3,000 stars within a 4.7 parsec radius. In the night sky, R136 appears as a 10th magnitude object at the core of the NGC 2070 cluster embedded in the Tarantula Nebula in the Large Magellanic Cloud.
It required a 3.6 metre telescope to detect R136a as a component of R136 in 1979, resolving R136a to detect R136a1 requires a space telescope or sophisticated techniques such as adaptive optics or speckle interferometry. South of about the 20th parallel south, the LMC is circumpolar, meaning that it can be seen all night every night of the year and light pollution permitting. In the Northern Hemisphere, it can be visible south of the 20th parallel north; this excludes North America, northern Africa and northern Asia. The R136a system at the core of R136 is a dense luminous knot of stars containing at least 12 stars, the most prominent being R136a1, R136a2, R136a3, all of which are luminous and massive WN5h stars. R136a1 is separated from R136a2, the second brightest star in the cluster, by 5,000 AU. R136 is located 157,000 light-years away from Earth in the Large Magellanic Cloud, positioned on the south-east corner of the galaxy at the centre of the Tarantula Nebula known as 30 Doradus. R136 itself is just the central condensation of the much larger NGC 2070 open cluster.
For such a distant star, R136a1 is unobscured by interstellar dust. The reddening causes the visual brightness to be reduced by about 1.8 magnitudes, but only around 0.22 magnitudes in the near infrared. The distance to R136a1 cannot be determined directly, but is assumed to be at the same distance as the Large Magellanic Cloud at around 50 kiloparsecs. Although binary systems are common among the most massive stars, R136a1 appears to be a single star as no evidence of a massive companion has been detected. X-ray emission was detected from R136 using the Chandra X-ray Observatory. R136a and R136c were both detected, but R136a could not be resolved. Another study separated the R136a1/2 pair from R136a3. R136a1/2 showed soft x-rays not thought to indicate a colliding winds binary. Rapid Doppler radial velocity variations would be expected from a pair of equal mass stars in a close orbit, but this has not been seen in the R136a1 spectrum. A high orbital inclination, a more distant binary, or a chance alignment of two distant stars cannot be ruled out but is thought to be unlikely.
Unequal binary components are possible, but would not affect the modelling of R136a1's properties. R136a1 is a high-luminosity WN5h star, placing it on the extreme top left corner of the Hertzsprung-Russell diagram. A Wolf–Rayet star is distinguished by the strong, broad emission lines in its spectrum; this includes ionized nitrogen, carbon and silicon, but with hydrogen lines weak or absent. A WN5 star is classified on the basis of ionised helium emission being stronger than the neutral helium lines, having equal emission strength from NIII, NIV, NV; the "h" in the spectral type indicates significant hydrogen emission in the spectrum, hydrogen is calculated to make up 40% of the surface abundance by mass. WNh stars; the emission spectrum is produced in a powerful dense stellar wind, the enhanced levels of helium and nitrogen arise from convectional mixing of CNO cycle products to the surface. R136a1 is the most massive star known. An evolutionary mass of 265 M☉ is found from near infrared spectra using a combination of non-LTE line-blanketed CMFGEN and TLUSTY model atmosphere code.
The models were validated against the dynamical masses derived
In astrophysics, accretion is the accumulation of particles into a massive object by gravitationally attracting more matter gaseous matter, in an accretion disk. Most astronomical objects, such as galaxies and planets, are formed by accretion processes; the accretion model that Earth and the other terrestrial planets formed from meteoric material was proposed in 1944 by Otto Schmidt, followed by the protoplanet theory of William McCrea and the capture theory of Michael Woolfson. In 1978, Andrew Prentice resurrected the initial Laplacian ideas about planet formation and developed the modern Laplacian theory. None of these models proved successful, many of the proposed theories were descriptive; the 1944 accretion model by Otto Schmidt was further developed in a quantitative way in 1969 by Viktor Safronov. He calculated, in detail, the different stages of terrestrial planet formation. Since the model has been further developed using intensive numerical simulations to study planetesimal accumulation.
It is now accepted. Prior to collapse, this gas is in the form of molecular clouds, such as the Orion Nebula; as the cloud collapses, losing potential energy, it heats up, gaining kinetic energy, the conservation of angular momentum ensures that the cloud forms a flatted disk—the accretion disk. A few hundred thousand years after the Big Bang, the Universe cooled to the point where atoms could form; as the Universe continued to expand and cool, the atoms lost enough kinetic energy, dark matter coalesced sufficiently, to form protogalaxies. As further accretion occurred, galaxies formed. Indirect evidence is widespread. Galaxies grow through smooth gas accretion. Accretion occurs inside galaxies, forming stars. Stars are thought to form inside giant clouds of cold molecular hydrogen—giant molecular clouds of 300,000 M☉ and 65 light-years in diameter. Over millions of years, giant molecular clouds are prone to fragmentation; these fragments form small, dense cores, which in turn collapse into stars.
The cores range in mass from a fraction to several times that of the Sun and are called protostellar nebulae. They possess diameters of 2,000–20,000 astronomical units and a particle number density of 10,000 to 100,000/cm3. Compare it with the particle number density of the air at the sea level—2.8×1019/cm3. The initial collapse of a solar-mass protostellar nebula takes around 100,000 years; every nebula begins with a certain amount of angular momentum. Gas in the central part of the nebula, with low angular momentum, undergoes fast compression and forms a hot hydrostatic core containing a small fraction of the mass of the original nebula; this core forms the seed of. As the collapse continues, conservation of angular momentum dictates that the rotation of the infalling envelope accelerates, which forms a disk; as the infall of material from the disk continues, the envelope becomes thin and transparent and the young stellar object becomes observable in far-infrared light and in the visible. Around this time the protostar begins to fuse deuterium.
If the protostar is sufficiently massive, hydrogen fusion follows. Otherwise, if its mass is too low, the object becomes a brown dwarf; this birth of a new star occurs 100,000 years after the collapse begins. Objects at this stage are known as Class I protostars, which are called young T Tauri stars, evolved protostars, or young stellar objects. By this time, the forming star has accreted much of its mass. At the next stage, the envelope disappears, having been gathered up by the disk, the protostar becomes a classical T Tauri star; the latter have accretion disks and continue to accrete hot gas, which manifests itself by strong emission lines in their spectrum. The former do not possess accretion disks. Classical T Tauri stars evolve into weakly lined T Tauri stars; this happens after about 1 million years. The mass of the disk around a classical T Tauri star is about 1–3% of the stellar mass, it is accreted at a rate of 10−7 to 10−9 M☉ per year. A pair of bipolar jets is present as well; the accretion explains all peculiar properties of classical T Tauri stars: strong flux in the emission lines, magnetic activity, photometric variability and jets.
The emission lines form as the accreted gas hits the "surface" of the star, which happens around its magnetic poles. The jets are byproducts of accretion: they carry away excessive angular momentum; the classical T Tauri stage lasts about 10 million years. The disk disappears due to accretion onto the central star, planet formation, ejection by jets, photoevaporation by ultraviolet radiation from the central star and nearby stars; as a result, the young star becomes a weakly lined T Tauri star, over hundreds of millions of years, evolves into an ordinary Sun-like star, dependent on its initial mass. Self-accretion of cosmic dust accelerates the growth of the particles into boulder-sized planetesimals; the more massive planetesimals accrete some smaller ones. Accretion disks are common around smaller stars, or stellar remnants in a close binary, or black holes surrounded by material, such as those at the centers of galaxies; some dynamics in the disk, such as dynamical friction, are necessary to allow orbiting gas to lose angular momentum and fall onto the central mas
Lithium is a chemical element with symbol Li and atomic number 3. It is a silvery-white alkali metal. Under standard conditions, it is the lightest solid element. Like all alkali metals, lithium is reactive and flammable, is stored in mineral oil; when cut, it exhibits a metallic luster, but moist air corrodes it to a dull silvery gray black tarnish. It never occurs in nature, but only in compounds, such as pegmatitic minerals, which were once the main source of lithium. Due to its solubility as an ion, it is present in ocean water and is obtained from brines. Lithium metal is isolated electrolytically from a mixture of lithium chloride and potassium chloride; the nucleus of the lithium atom verges on instability, since the two stable lithium isotopes found in nature have among the lowest binding energies per nucleon of all stable nuclides. Because of its relative nuclear instability, lithium is less common in the solar system than 25 of the first 32 chemical elements though its nuclei are light: it is an exception to the trend that heavier nuclei are less common.
For related reasons, lithium has important uses in nuclear physics. The transmutation of lithium atoms to helium in 1932 was the first man-made nuclear reaction, lithium deuteride serves as a fusion fuel in staged thermonuclear weapons. Lithium and its compounds have several industrial applications, including heat-resistant glass and ceramics, lithium grease lubricants, flux additives for iron and aluminium production, lithium batteries, lithium-ion batteries; these uses consume more than three quarters of lithium production. Lithium is present in biological systems in trace amounts. Lithium salts have proven to be useful as a mood-stabilizing drug in the treatment of bipolar disorder in humans. Like the other alkali metals, lithium has a single valence electron, given up to form a cation; because of this, lithium is a good conductor of heat and electricity as well as a reactive element, though it is the least reactive of the alkali metals. Lithium's low reactivity is due to the proximity of its valence electron to its nucleus.
However, molten lithium is more reactive than its solid form. Lithium metal is soft enough to be cut with a knife; when cut, it possesses a silvery-white color that changes to gray as it oxidizes to lithium oxide. While it has one of the lowest melting points among all metals, it has the highest melting and boiling points of the alkali metals. Lithium has a low density, comparable with pine wood, it is the least dense of all elements. Furthermore, apart from helium and hydrogen, it is less dense than any liquid element, being only two thirds as dense as liquid nitrogen. Lithium can float on the lightest hydrocarbon oils and is one of only three metals that can float on water, the other two being sodium and potassium. Lithium's coefficient of thermal expansion is twice that of aluminium and four times that of iron. Lithium is superconductive below 400 μK at standard pressure and at higher temperatures at high pressures. At temperatures below 70 K, like sodium, undergoes diffusionless phase change transformations.
At 4.2 K it has a rhombohedral crystal system. At liquid-helium temperatures the rhombohedral structure is prevalent. Multiple allotropic forms have been identified for lithium at high pressures. Lithium has a mass specific heat capacity of 3.58 kilojoules per kilogram-kelvin, the highest of all solids. Because of this, lithium metal is used in coolants for heat transfer applications. Lithium reacts with water but with noticeably less vigor than other alkali metals; the reaction forms hydrogen lithium hydroxide in aqueous solution. Because of its reactivity with water, lithium is stored in a hydrocarbon sealant petroleum jelly. Though the heavier alkali metals can be stored in more dense substances, such as mineral oil, lithium is not dense enough to be submerged in these liquids. In moist air, lithium tarnishes to form a black coating of lithium hydroxide, lithium nitride and lithium carbonate; when placed over a flame, lithium compounds give off a striking crimson color, but when it burns the flame becomes a brilliant silver.
Lithium will burn in oxygen when exposed to water or water vapors. Lithium is flammable, it is explosive when exposed to air and to water, though less so than the other alkali metals; the lithium-water reaction at normal temperatures is brisk but nonviolent because the hydrogen produced does not ignite on its own. As with all alkali metals, lithium fires are difficult to extinguish, requiring dry powder fire extinguishers. Lithium is one of the few metals. Lithium has a diagonal relationship with an element of similar atomic and ionic radius. Chemical resemblances between the two metals include the formation of a nitride by reaction with N2, the formation of an oxide and peroxide when burnt in O2, salts with similar solubilities, thermal instability of the carbonates and nitrides; the metal reacts with hy
The Eddington luminosity referred to as the Eddington limit, is the maximum luminosity a body can achieve when there is balance between the force of radiation acting outward and the gravitational force acting inward. The state of balance is called hydrostatic equilibrium; when a star exceeds the Eddington luminosity, it will initiate a intense radiation-driven stellar wind from its outer layers. Since most massive stars have luminosities far below the Eddington luminosity, their winds are driven by the less intense line absorption; the Eddington limit is invoked to explain the observed luminosity of accreting black holes such as quasars. Sir Arthur Stanley Eddington took only the electron scattering into account when calculating this limit, something that now is called the classical Eddington limit. Nowadays, the modified Eddington limit counts on other radiation processes such as bound-free and free-free radiation interaction; the limit is obtained by setting the outward radiation pressure equal to the inward gravitational force.
Both forces decrease by inverse square laws, so once equality is reached, the hydrodynamic flow is different throughout the star. From Euler's equation in hydrostatic equilibrium, the mean acceleration is zero, d u d t = − ∇ p ρ − ∇ Φ = 0 where u is the velocity, p is the pressure, ρ is the density, Φ is the gravitational potential. If the pressure is dominated by radiation pressure associated with a radiation flux F r a d, − ∇ p ρ = κ c F r a d. Here κ is the opacity of the stellar material, defined as the fraction of radiation energy flux absorbed by the medium per unit density and unit length. For ionized hydrogen κ = σ T / m p, where σ T is the Thomson scattering cross-section for the electron and m p is the mass of a proton. Note that F r a d = d 2 E / d A d t is defined at the energy flux over a surface, which can be expressed with the momentum flux using E = p c for radiation; therefore the rate of momentum transfer from the radiation to the gaseous medium per unit density is κ F r a d / c, which explains the right hand side of the above equation.
The luminosity of a source bounded by a surface S is L = ∫ S F r a d ⋅ d S = ∫ S c κ ∇ Φ ⋅ d S. Now assuming that the opacity is a constant, it can be brought outside of the integral. Using Gauss's theorem and Poisson's equation gives L = c κ ∫ S ∇ Φ ⋅ d S = c κ ∫ V ∇ 2 Φ d V = 4 π G c κ ∫ V ρ d V = 4 π G M c κ where M is the mass of the central object; this is called the Eddington Luminosity. For pure ionized hydrogen L E d d = 4 π G M m p c σ T ≅ 1.26 × 10 31 W = 1.26 × 10 38 e r g / s = 3.2 × 10
A molecular cloud, sometimes called a stellar nursery, is a type of interstellar cloud, the density and size of which permit the formation of molecules, most molecular hydrogen. This is in contrast to other areas of the interstellar medium that contain predominantly ionized gas. Molecular hydrogen is difficult to detect by infrared and radio observations, so the molecule most used to determine the presence of H2 is carbon monoxide; the ratio between CO luminosity and H2 mass is thought to be constant, although there are reasons to doubt this assumption in observations of some other galaxies. Within molecular clouds are regions with higher density, where lots of dust and gas cores reside, called clumps; these clumps are the beginning of star formation, if gravity can overcome the high density and force the dust and gas to collapse. Within the Milky Way, molecular gas clouds account for less than one percent of the volume of the interstellar medium, yet it is the densest part of the medium, comprising half of the total gas mass interior to the Sun's galactic orbit.
The bulk of the molecular gas is contained in a ring between 3.5 and 7.5 kiloparsecs from the center of the Milky Way. Large scale CO maps of the galaxy show that the position of this gas correlates with the spiral arms of the galaxy; that molecular gas occurs predominantly in the spiral arms suggests that molecular clouds must form and dissociate on a timescale shorter than 10 million years—the time it takes for material to pass through the arm region. Vertically to the plane of the galaxy, the molecular gas inhabits the narrow midplane of the galactic disc with a characteristic scale height, Z, of 50 to 75 parsecs, much thinner than the warm atomic and warm ionized gaseous components of the ISM; the exception to the ionized-gas distribution are H II regions, which are bubbles of hot ionized gas created in molecular clouds by the intense radiation given off by young massive stars and as such they have the same vertical distribution as the molecular gas. This distribution of molecular gas is averaged out over large distances.
A vast assemblage of molecular gas with a mass of 103 to 107 times the mass of the Sun is called a giant molecular cloud. GMCs are around 15 to 600 light-years in diameter. Whereas the average density in the solar vicinity is one particle per cubic centimetre, the average density of a GMC is a hundred to a thousand times as great. Although the Sun is much more dense than a GMC, the volume of a GMC is so great that it contains much more mass than the Sun; the substructure of a GMC is a complex pattern of filaments, sheets and irregular clumps. The densest parts of the filaments and clumps are called "molecular cores", while the densest molecular cores are called "dense molecular cores" and have densities in excess of 104 to 106 particles per cubic centimeter. Observationally, typical molecular cores are traced with CO and dense molecular cores are traced with ammonia; the concentration of dust within molecular cores is sufficient to block light from background stars so that they appear in silhouette as dark nebulae.
GMCs are so large. These local GMCs are arrayed in a ring in the neighborhood of the Sun coinciding with the Gould Belt; the most massive collection of molecular clouds in the galaxy forms an asymmetrical ring about the galactic center at a radius of 120 parsecs. The Sagittarius region is chemically rich and is used as an exemplar by astronomers searching for new molecules in interstellar space. Isolated gravitationally-bound small molecular clouds with masses less than a few hundred times that of the Sun are called Bok globules; the densest parts of small molecular clouds are equivalent to the molecular cores found in GMCs and are included in the same studies. In 1984 IRAS identified a new type of diffuse molecular cloud; these were diffuse filamentary clouds. These clouds have a typical density of 30 particles per cubic centimeter; the formation of stars occurs within molecular clouds. This is a natural consequence of their low temperatures and high densities, because the gravitational force acting to collapse the cloud must exceed the internal pressures that are acting "outward" to prevent a collapse.
There is observed evidence that the large, star-forming clouds are confined to a large degree by their own gravity rather than by external pressure. The evidence comes from the fact that the "turbulent" velocities inferred from CO linewidth scale in the same manner as the orbital velocity; the physics of molecular clouds is poorly much debated. Their internal motions are governed by turbulence in a cold, magnetized gas, for which the turbulent motions are supersonic but comparable to the speeds of magnetic disturbances; this state is thought to lose energy requiring either an overall collapse or a steady reinjection of energy. At the same time, the clouds are known to be disrupted by some process—most the effects of massive stars—before a significant fraction of their mass has become stars. Molecular clouds, GMCs, are
European Southern Observatory
The European Southern Observatory, formally the European Organisation for Astronomical Research in the Southern Hemisphere, is a 16-nation intergovernmental research organization for ground-based astronomy. Created in 1962, ESO has provided astronomers with state-of-the-art research facilities and access to the southern sky; the organisation employs about 730 staff members and receives annual member state contributions of €162 million. Its observatories are located in northern Chile. ESO has operated some of the largest and most technologically advanced telescopes; these include the 3.6 m New Technology Telescope, an early pioneer in the use of active optics, the Very Large Telescope, which consists of four individual 8.2 m telescopes and four smaller auxiliary telescopes which can all work together or separately. The Atacama Large Millimeter Array observes the universe in the millimetre and submillimetre wavelength ranges, is the world's largest ground-based astronomy project to date, it was completed in March 2013 in an international collaboration by Europe, North America, East Asia and Chile.
Under construction is the Extremely Large Telescope. It will use a 39.3-metre-diameter segmented mirror, become the world's largest optical reflecting telescope when operational in 2024. Its light-gathering power will allow detailed studies of planets around other stars, the first objects in the universe, supermassive black holes, the nature and distribution of the dark matter and dark energy which dominate the universe. ESO's observing facilities have made astronomical discoveries and produced several astronomical catalogues, its findings include the discovery of the most distant gamma-ray burst and evidence for a black hole at the centre of the Milky Way. In 2004, the VLT allowed astronomers to obtain the first picture of an extrasolar planet orbiting a brown dwarf 173 light-years away; the High Accuracy Radial Velocity Planet Searcher instrument installed on the older ESO 3.6 m telescope led to the discovery of extrasolar planets, including Gliese 581c—one of the smallest planets seen outside the solar system.
The idea that European astronomers should establish a common large observatory was broached by Walter Baade and Jan Oort at the Leiden Observatory in the Netherlands in spring 1953. It was pursued by Oort, who gathered a group of astronomers in Leiden to consider it on June 21 that year. Thereafter, the subject was further discussed at the Groningen conference in the Netherlands. On January 26, 1954, an ESO declaration was signed by astronomers from six European countries expressing the wish that a joint European observatory be established in the southern hemisphere. At the time, all reflector telescopes with an aperture of 2 metres or more were located in the northern hemisphere; the decision to build the observatory in the southern hemisphere resulted from the necessity of observing the southern sky. Although it was planned to set up telescopes in South Africa, tests from 1955 to 1963 demonstrated that a site in the Andes was preferable. On November 15, 1963 Chile was chosen as the site for ESO's observatory.
The decision was preceded by the ESO Convention, signed 5 October 1962 by Belgium, France, the Netherlands and Sweden. Otto Heckmann was nominated as the organisation's first director general on 1 November 1962. A preliminary proposal for a convention of astronomy organisations in these five countries was drafted in 1954. Although some amendments were made in the initial document, the convention proceeded until 1960 when it was discussed during that year's committee meeting; the new draft was examined in detail, a council member of CERN highlighted the need for a convention between governments. The convention and government involvement became pressing due to rising costs of site-testing expeditions; the final 1962 version was adopted from the CERN convention, due to similarities between the organisations and the dual membership of some members. In 1966, the first ESO telescope at the La Silla site in Chile began operating; because CERN had sophisticated instrumentation, the astronomy organisation turned to the nuclear-research body for advice and a collaborative agreement between ESO and CERN was signed in 1970.
Several months ESO's telescope division moved into a CERN building in Geneva and ESO's Sky Atlas Laboratory was established on CERN property. ESO's European departments moved into the new ESO headquarters in Garching, Germany in 1980. Although ESO is headquartered in Germany, its telescopes and observatories are in northern Chile, where the organisation operates advanced ground-based astronomical facilities: La Silla, which hosts the New Technology Telescope Paranal, where the Very Large Telescope is located Llano de Chajnantor, which hosts the APEX submillimetre telescope and where ALMA, the Atacama Large Millimeter/submillimeter Array, is locatedThese are among the best locations for astronomical observations in the southern hemisphere. An ESO project is the Extremely Large Telescope, a 40-metre-class telescope based on a five-mirror design and the planned Overwhelmingly Large Telescope; the ELT will be the near-infrared telescope in the world. ESO began its design in early 2006, aimed to begin construction in 2012.
Construction work at the ELT site started in June 2014. As decided by the ESO council on 26 April 2010, a fou
A gas giant is a giant planet composed of hydrogen and helium. Gas giants are sometimes known as failed stars because they contain the same basic elements as a star. Jupiter and Saturn are the gas giants of the Solar System; the term "gas giant" was synonymous with "giant planet", but in the 1990s it became known that Uranus and Neptune are a distinct class of giant planet, being composed of heavier volatile substances. For this reason and Neptune are now classified in the separate category of ice giants. Jupiter and Saturn consist of hydrogen and helium, with heavier elements making up between 3 and 13 percent of the mass, they are thought to consist of an outer layer of molecular hydrogen surrounding a layer of liquid metallic hydrogen, with a molten rocky core. The outermost portion of their hydrogen atmosphere is characterized by many layers of visible clouds that are composed of water and ammonia; the layer of metallic hydrogen makes up the bulk of each planet, is referred to as "metallic" because the large pressure turns hydrogen into an electrical conductor.
The gas giants' cores are thought to consist of heavier elements at such high temperatures and pressures that their properties are poorly understood. The defining differences between a low-mass brown dwarf and a gas giant are debated. One school of thought is based on formation. Part of the debate concerns whether "brown dwarfs" must, by definition, have experienced nuclear fusion at some point in their history; the term gas giant was coined in 1952 by the science fiction writer James Blish and was used to refer to all giant planets. It is, something of a misnomer because throughout most of the volume of all giant planets, the pressure is so high that matter is not in gaseous form. Other than solids in the core and the upper layers of the atmosphere, all matter is above the critical point, where there is no distinction between liquids and gases; the term has caught on, because planetary scientists use "rock", "gas", "ice" as shorthands for classes of elements and compounds found as planetary constituents, irrespective of what phase the matter may appear in.
In the outer Solar System and helium are referred to as "gases". Because Uranus and Neptune are composed of, in this terminology, not gas, they are referred to as ice giants and separated from the gas giants. Gas giants can, theoretically, be divided into five distinct classes according to their modeled physical atmospheric properties, hence their appearance: ammonia clouds, water clouds, alkali-metal clouds, silicate clouds. Jupiter and Saturn are both class I. Hot Jupiters are class IV or V. A cold hydrogen-rich gas giant more massive than Jupiter but less than about 500 M⊕ will only be larger in volume than Jupiter. For masses above 500 M⊕, gravity will cause the planet to shrink. Kelvin–Helmholtz heating can cause a gas giant to radiate more energy than it receives from its host star. Although the words "gas" and "giant" are combined, hydrogen planets need not be as large as the familiar gas giants from the Solar System. However, smaller gas planets and planets closer to their star will lose atmospheric mass more via hydrodynamic escape than larger planets and planets farther out.
A gas dwarf could be defined as a planet with a rocky core that has accumulated a thick envelope of hydrogen and other volatiles, having as result a total radius between 1.7 and 3.9 Earth-radii. The smallest known extrasolar planet, a "gas planet" is Kepler-138d, which has the same mass as Earth but is 60% larger and therefore has a density that indicates a thick gas envelope. A low-mass gas planet can still have a radius resembling that of a gas giant if it has the right temperature. List of gravitationally rounded objects of the Solar System List of planet types Hot Jupiter