Proton
A proton is a subatomic particle, symbol p or p+, with a positive electric charge of +1e elementary charge and a mass less than that of a neutron. Protons and neutrons, each with masses of one atomic mass unit, are collectively referred to as "nucleons". One or more protons are present in the nucleus of every atom; the number of protons in the nucleus is the defining property of an element, is referred to as the atomic number. Since each element has a unique number of protons, each element has its own unique atomic number; the word proton is Greek for "first", this name was given to the hydrogen nucleus by Ernest Rutherford in 1920. In previous years, Rutherford had discovered that the hydrogen nucleus could be extracted from the nuclei of nitrogen by atomic collisions. Protons were therefore a candidate to be a fundamental particle, hence a building block of nitrogen and all other heavier atomic nuclei. In the modern Standard Model of particle physics, protons are hadrons, like neutrons, the other nucleon, are composed of three quarks.
Although protons were considered fundamental or elementary particles, they are now known to be composed of three valence quarks: two up quarks of charge +2/3e and one down quark of charge –1/3e. The rest masses of quarks contribute only about 1% of a proton's mass, however; the remainder of a proton's mass is due to quantum chromodynamics binding energy, which includes the kinetic energy of the quarks and the energy of the gluon fields that bind the quarks together. Because protons are not fundamental particles, they possess a physical size, though not a definite one. At sufficiently low temperatures, free protons will bind to electrons. However, the character of such bound protons does not change, they remain protons. A fast proton moving through matter will slow by interactions with electrons and nuclei, until it is captured by the electron cloud of an atom; the result is a protonated atom, a chemical compound of hydrogen. In vacuum, when free electrons are present, a sufficiently slow proton may pick up a single free electron, becoming a neutral hydrogen atom, chemically a free radical.
Such "free hydrogen atoms" tend to react chemically with many other types of atoms at sufficiently low energies. When free hydrogen atoms react with each other, they form neutral hydrogen molecules, which are the most common molecular component of molecular clouds in interstellar space. Protons are composed of three valence quarks, making them baryons; the two up quarks and one down quark of a proton are held together by the strong force, mediated by gluons. A modern perspective has a proton composed of the valence quarks, the gluons, transitory pairs of sea quarks. Protons have a positive charge distribution which decays exponentially, with a mean square radius of about 0.8 fm. Protons and neutrons are both nucleons, which may be bound together by the nuclear force to form atomic nuclei; the nucleus of the most common isotope of the hydrogen atom is a lone proton. The nuclei of the heavy hydrogen isotopes deuterium and tritium contain one proton bound to one and two neutrons, respectively. All other types of atomic nuclei are composed of two or more protons and various numbers of neutrons.
The concept of a hydrogen-like particle as a constituent of other atoms was developed over a long period. As early as 1815, William Prout proposed that all atoms are composed of hydrogen atoms, based on a simplistic interpretation of early values of atomic weights, disproved when more accurate values were measured. In 1886, Eugen Goldstein discovered canal rays and showed that they were positively charged particles produced from gases. However, since particles from different gases had different values of charge-to-mass ratio, they could not be identified with a single particle, unlike the negative electrons discovered by J. J. Thomson. Wilhelm Wien in 1898 identified the hydrogen ion as particle with highest charge-to-mass ratio in ionized gases. Following the discovery of the atomic nucleus by Ernest Rutherford in 1911, Antonius van den Broek proposed that the place of each element in the periodic table is equal to its nuclear charge; this was confirmed experimentally by Henry Moseley in 1913 using X-ray spectra.
In 1917, Rutherford proved that the hydrogen nucleus is present in other nuclei, a result described as the discovery of protons. Rutherford had earlier learned to produce hydrogen nuclei as a type of radiation produced as a product of the impact of alpha particles on nitrogen gas, recognize them by their unique penetration signature in air and their appearance in scintillation detectors; these experiments were begun when Rutherford had noticed that, when alpha particles were shot into air, his scintillation detectors showed the signatures of typical hydrogen nuclei as a product. After experimentation Rutherford traced the reaction to the nitrogen in air, found that when alphas were produced into pure nitrogen gas, the effect was larger. Rutherford determined that this hydrogen could have come only from the nitrogen, therefore nitrogen must contain hydrogen nuclei. One hydrogen nucleus was being knocked off by the impact of the alpha particle, producing oxygen-17 in the process; this was 14N + α → 17O + p.
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Accretion (astrophysics)
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
Chromosphere
The chromosphere is the second of the three main layers in the Sun's atmosphere and is 3,000 to 5,000 kilometers deep. The chromosphere's rosy red color is only apparent during eclipses; the chromosphere sits just below the solar transition region. The layer of the chromosphere atop the photosphere is homogeneous. A forest of hairy-appearing spicules rise from the homogeneous layer, some of which extend 10,000 km into the corona above; the density of the chromosphere is only 10−4 times that of the photosphere, the layer beneath, 10−8 times that of the atmosphere of Earth at sea level. This makes the chromosphere invisible and it can be seen only during a total eclipse, where its reddish color is revealed; the color hues are anywhere between red. Without special equipment, the chromosphere cannot be seen due to the overwhelming brightness of the photosphere beneath; the density of the chromosphere decreases with distance from the center of the Sun. This decreases logarithmically from 1017 particles per cubic centimeter, or 2×10−4 kg/m3 to under 1.6×10−11 kg/m3 at the outer boundary.
The temperature decreases from the inner boundary at about 6,000 K to a minimum of 3,800 K, before increasing to upwards of 35,000 K at the outer boundary with the transition layer of the corona. Chromospheres have been observed in stars other than the Sun; the Sun's chromosphere has been hard to examine and decipher, although observations continue with the help of the electromagnetic spectrum. Whilst the photosphere has an absorption line spectrum, the chromosphere's spectrum is dominated by emission lines. In particular, one of its strongest lines is the Hα at a wavelength of 656.3 nm. A wavelength of 656.3 nm is in the red part of the spectrum, which causes the chromosphere to have its characteristic reddish colour. By analysing the spectrum of the chromosphere, it was found that the temperature of this layer of the solar atmosphere increases with increasing height in the chromosphere itself; the temperature at the top of photosphere is only about 4,400 K, while at the top of chromosphere, some 2,000 km higher, it reaches 25,000 K.
This is however the opposite of what we find in the photosphere, where the temperature drops with increasing height. It is not yet understood what phenomenon causes the temperature of the chromosphere to paradoxically increase further from the Sun's interior. However, it seems to be explained or by magnetic reconnection. Many interesting phenomena can be observed in the chromosphere, complex and dynamic: Filaments underlie many coronal mass ejections and hence are important to the prediction of space weather. Solar prominences rise up through the chromosphere from the photosphere, sometimes reaching altitudes of 150,000 km; these gigantic plumes of gas are the most spectacular of solar phenomena, aside from the less frequent solar flares. The most common feature is the presence of spicules, long thin fingers of luminous gas which appear like the blades of a huge field of fiery grass growing upwards from the photosphere below. Spicules rise to the top of the chromosphere and sink back down again over the course of about 10 minutes.
There are horizontal wisps of gas called fibrils, which last about twice as long as spicules. Images taken in typical chromospheric lines show the presence of brighter cells called as network, while the surrounding darker regions are named internetwork, they look similar to granules observed on the photosphere due to the heat convection. Periodic oscillations have been found since the first observations with the instrument SUMER on board SOHO with a frequency from 3 mHz to 10 mHz, corresponding to a characteristic periodic time of three minutes. Oscillations of the radial component of the plasma velocity are typical of the high chromosphere. Now we know that the photospheric granulation pattern has no oscillations above 20 mHz while higher frequency waves were detected in the solar atmosphere by TRACE. Cool loops can be seen at the border of the solar disk, they are different from prominences because they look as concentric arches with maximum temperature of the order 0,1 MK. These cool loops show an intense variability: they appear and disappear in some UV lines in a time less than an hour, or they expand in 10–20 minutes.
Foukal studied these cool loops in detail from the observations taken with the EUV spectrometer on Skylab in 1976. Otherwise, when the plasma temperature of these loops becomes coronal, these features appear more stable and evolve on longer times. See the flash spectrum of the solar chromosphere. A spectroscopic measure of chromospheric activity on other stars is the Mount Wilson S-index. See Superflare#Spectroscopic observations of superflare stars. Plage Orders of magnitude Moreton wave Animated explanation of the Chromosphere. Animated explanation of the temperature of the Chromosphere
Nebular hypothesis
The nebular hypothesis is the most accepted model in the field of cosmogony to explain the formation and evolution of the Solar System. It suggests; the theory was developed by Immanuel Kant and published in his Allgemeine Naturgeschichte und Theorie des Himmels, published in 1755. Applied to the Solar System, the process of planetary system formation is now thought to be at work throughout the universe; the accepted modern variant of the nebular hypothesis is the solar nebular disk model or solar nebular model. It offered explanations for a variety of properties of the Solar System, including the nearly circular and coplanar orbits of the planets, their motion in the same direction as the Sun's rotation; some elements of the original nebular hypothesis are echoed in modern theories of planetary formation, but most elements have been superseded. According to the nebular hypothesis, stars form in massive and dense clouds of molecular hydrogen—giant molecular clouds; these clouds are gravitationally unstable, matter coalesces within them to smaller denser clumps, which rotate and form stars.
Star formation is a complex process, which always produces a gaseous protoplanetary disk, around the young star. This may give birth to planets in certain circumstances, thus the formation of planetary systems is thought to be a natural result of star formation. A Sun-like star takes 1 million years to form, with the protoplanetary disk evolving into a planetary system over the next 10–100 million years; the protoplanetary disk is an accretion disk. Hot, the disk cools in what is known as the T Tauri star stage; the grains may coagulate into kilometer-sized planetesimals. If the disk is massive enough, the runaway accretions begin, resulting in the rapid—100,000 to 300,000 years—formation of Moon- to Mars-sized planetary embryos. Near the star, the planetary embryos go through a stage of violent mergers, producing a few terrestrial planets; the last stage takes 100 million to a billion years. The formation of giant planets is a more complicated process, it is thought to occur beyond the frost line, where planetary embryos are made of various types of ice.
As a result, they are several times more massive than in the inner part of the protoplanetary disk. What follows after the embryo formation is not clear; some embryos appear to continue to grow and reach 5–10 Earth masses—the threshold value, necessary to begin accretion of the hydrogen–helium gas from the disk. The accumulation of gas by the core is a slow process, which continues for several million years, but after the forming protoplanet reaches about 30 Earth masses it accelerates and proceeds in a runaway manner. Jupiter- and Saturn-like planets are thought to accumulate the bulk of their mass during only 10,000 years; the accretion stops. The formed planets can migrate over long distances after their formation. Ice giants such as Uranus and Neptune are thought to be failed cores, which formed too late when the disk had disappeared. There is evidence that Emanuel Swedenborg first proposed parts of the nebular hypothesis in 1734. Immanuel Kant, familiar with Swedenborg's work, developed the theory further in 1755, publishing his own Universal Natural History and Theory of the Heavens, wherein he argued that gaseous clouds rotate collapse and flatten due to gravity forming stars and planets.
Pierre-Simon Laplace independently developed and proposed a similar model in 1796 in his Exposition du systeme du monde. He envisioned that the Sun had an extended hot atmosphere throughout the volume of the Solar System, his theory featured. As this cooled and contracted, it flattened and spun more throwing off a series of gaseous rings of material, his model was similar to Kant's. While the Laplacian nebular model dominated in the 19th century, it encountered a number of difficulties; the main problem involved angular momentum distribution between planets. The planets have 99% of the angular momentum, this fact could not be explained by the nebular model; as a result, astronomers abandoned this theory of planet formation at the beginning of the 20th century. A major critique came during the 19th century from James Clerk Maxwell, who maintained that different rotation between the inner and outer parts of a ring could not allow condensation of material. Astronomer Sir David Brewster rejected Laplace, writing in 1876 that "those who believe in the Nebular Theory consider it as certain that our Earth derived its solid matter and its atmosphere from a ring thrown from the Solar atmosphere, which afterwards contracted into a solid terraqueous sphere, from which the Moon was thrown off by the same process".
He argued that under such view, "the Moon must have carried off water and air from the watery and aerial parts of the Earth and must have an atmosphere". Brewster claimed that Sir Isaac Newton's religious beliefs had considered nebular ideas as tending to atheism, quoted him as saying that "the growth of new systems out of old ones, without the mediation of a Divine power, seemed to him absurd"; the perceived deficiencies of the Laplacian mod
Planetary migration
Planetary migration occurs when a planet or other stellar satellite interacts with a disk of gas or planetesimals, resulting in the alteration of the satellite's orbital parameters its semi-major axis. Planetary migration is the most explanation for hot Jupiters: exoplanets with jovian masses but orbits of only a few days; the accepted theory of planet formation from a protoplanetary disk predicts such planets cannot form so close to their stars, as there is insufficient mass at such small radii and the temperature is too high to allow the formation of rocky or icy planetesimals. It has become clear that terrestrial-mass planets may be subject to rapid inward migration if they form while the gas disk is still present; this may affect the formation of the cores of the giant planets, if those planets form via the core-accretion mechanism. Protoplanetary gas disks around young stars are observed to have lifetimes of a few million years. If planets with masses of around an Earth mass or greater form while the gas is still present, the planets can exchange angular momentum with the surrounding gas in the protoplanetary disk so that their orbits change gradually.
Although the sense of migration is inwards in locally isothermal disks, outward migration may occur in disks that possess entropy gradients. During the late phase of planetary system formation, massive protoplanets and planetesimals gravitationally interact in a chaotic manner causing many planetesimals to be thrown into new orbits; this results in angular-momentum exchange between the planets and the planetesimals, leads to migration. Outward migration of Neptune is believed to be responsible for the resonant capture of Pluto and other Plutinos into the 3:2 resonance with Neptune; this type of orbital migration arises from the gravitational force exerted by a sufficiently massive body embedded in a disk on the surrounding disk's gas, which perturbs its density distribution. By the reaction principle of classical mechanics, the gas exerts an equal and opposite gravitational force on the body, which can be expressed in terms of a torque; this torque alters the angular momentum of the planet's orbit, resulting in a variation of the orbital elements, such as the semi-major axis.
An increase over time of the semi-major axis leads to outward migration, i.e. away from the star, whereas the opposite behavior leads to inward migration. Small planets undergo Type I migration driven by torques arising from waves launched at the locations of the Lindblad and from co-rotation resonances. Lindblad resonances excite spiral density waves in the surrounding gas and exterior of the planet's orbit. In most cases, the outer spiral wave exerts a greater torque than does the inner wave, causing the planet to lose angular momentum, hence migrate toward the star; the migration rate due to these torques is proportional to the mass of the planet and to the local gas density, results in a migration timescale that tends to be short relative to the million-year lifetime of the gaseous disk. Additional co-rotation torques are exerted by gas orbiting with a period similar to that of the planet. In a reference frame attached to the planet, this gas follows horseshoe orbits, reversing direction when it approaches the planet from ahead or from behind.
The gas reversing course ahead of the planet originates from a larger semi-major axis and may be cooler and denser than the gas reversing course behind the planet. This may result in a region of excess density ahead of the planet and of lesser density behind the planet, causing the planet to gain angular momentum; the planet mass for which migration can be approximated to Type I depends on the local gas pressure scale-height and, to a lesser extent, the kinematic viscosity of the gas. In warm and viscous disks, Type I migration may apply to larger mass planets. In locally isothermal disks and far from steep density and temperature gradients, co-rotation torques are overpowered by the Lindblad torques. Regions of outward migration may exist for some planetary mass ranges and disk conditions in both local isothermal and non-isothermal disks; the locations of these regions may vary during the evolution of the disk, in the local-isothermal case are restricted to regions with large density and/or temperature radial gradients over several pressure scale-heights.
Type I migration in a local isothermal disk was shown to be compatible with the formation and long-term evolution of some of the observed Kepler planets. The rapid accretion of solid material by the planet may produce a "heating torque" that causes the planet to gain angular momentum. A planet massive enough to open a gap in a gaseous disk undergoes a regime of migration referred to as Type II; when the mass of a perturbing planet is large enough, the tidal torque it exerts on the gas transfers angular momentum to the gas exterior of the planet's orbit, does the opposite interior to the planet, thereby repelling gas from around the orbit. In a Type I regime, viscous torques can efficiently counter this effect by resupplying gas and smoothing out sharp density gradients, but when the torques become strong enough to overcome the viscous torques in the vicinity of the planet's orbit, a lower density annular gap is created. The depth of this gap depends on the planet mass. In the simple scenario in which no gas crosses the gap, the migration of the planet follows the viscous evolution of the disk's gas.
In the inner disk, the planet spirals inward on the viscous timescale, following the accretion of gas onto the star. In this case, the migration rate is slower than would be the migration of t
Lithium
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
T Tauri
T Tauri is a variable star in the constellation Taurus, the prototype of the T Tauri stars. It was discovered in October 1852 by John Russell Hind. T Tauri appears from Earth amongst the Hyades cluster, not far from ε Tauri. Faint nebulosity around T Tauri is a Herbig–Haro object called Burnham's Nebula or HH 255. Like all T Tauri stars, it is young, being only a million years old, its distance from Earth is about 460 light years, its apparent magnitude varies unpredictably from about 9.3 to 14. The T Tauri system consists of at least three stars, only one of, visible at optical wavelengths. Through VLA radio observations, it was found that the young star changed its orbit after a close encounter with one of its companions and may have been ejected from the system. Physically nearby is NGC 1555, a reflection nebula known as Hind's Nebula or Hind's Variable Nebula, it is illuminated by T Tauri, thus varies in brightness. The nebula NGC 1554 was associated with T Tauri and was observed in 1868 by Otto Wilhelm von Struve, but soon disappeared or never existed, is known as "Struve's Lost Nebula".
The T Tauri wind, so named because this young star is in this stage, is a phase of stellar development between the accretion of material from the slowing rotating material of a solar nebula and the ignition of the hydrogen that has agglomerated into the protostar. A protostar is the denser parts of a cloud core with a mass around 104 solar masses in the form of gas and dust, that collapses under its own weight/gravity, continues to attract matter; the protostar, at first, only has about 1% of its final mass. But the envelope of the star continues to grow. After a few million years, thermonuclear fusion begins in its core a strong stellar wind is produced which stops the infall of new mass; the protostar is now considered a young star since its mass is fixed, its future evolution is now set. P Cygni profile AAVSO Variable Star of the Month Profile of T Tauri http://www.kencroswell.com/TTauri.html http://www.spaceref.com/news/viewpr.html?pid=10340 http://www.daviddarling.info/encyclopedia/T/T_Tauri.html Simbad
