:: wikimiki.org ::

Iron

Iron

Iron is a chemical element with the symbol Fe (L.: Ferrum) and atomic number 26. Iron is a group 8 and period 4 metal. Iron is notable for being the final element produced by stellar nucleosynthesis, and thus the heaviest element which does not require a supernova or similarly cataclysmic event for its formation. It is therefore the most abundant heavy metal in the universe.

Notable characteristics

Iron is the most abundant metal on Earth, and is believed to be the tenth most abundant element in the universe. Iron is also the most abundant (by mass, 34.6%) element making up the Earth; the concentration of iron in the various layers of the Earth ranges from high at the inner core to about 5% in the outer crust; it is possible the Earth's inner core consists of a single iron crystal although it is more likely to be a mixture of iron and nickel; the large amount of iron in the Earth is thought to contribute to its magnetic field. Iron is a metal extracted from iron ore, and is hardly ever found in the free (elemental) state. In order to obtain elemental iron, the impurities must be removed by chemical reduction. Iron is used in the production of steel, which is not an element but an alloy, a solution of different metals (and some non-metals, particularly carbon). Nuclei of iron have some of the highest binding energies per nucleon, superseded only by the nickel isotope ⁶²Ni. The universally most abundant of the highly stable nucleides is, however, ⁵⁶Fe. This is formed by nuclear fusion in the stars. Although a further tiny energy gain could be extracted by synthesizing ⁶²Ni, conditions in stars are not right for this process to be favoured. When a very large star contracts at the end of its life, internal pressure and temperature rise, allowing the star to produce progressively heavier elements, despite these being less stable than the elements around mass number 60 (the "iron group"). This leads to a supernova. Some cosmological models with an open universe predict that there will be a phase where as a result of slow fusion and fission reactions, everything will become iron.

Applications

Iron is the most used of all the metals, comprising 95 percent of all the metal tonnage produced worldwide. Its combination of low cost and high strength make it indispensable, especially in applications like automobiles, the hulls of large ships, and structural components for buildings. Steel is the best known alloy of iron, and some of the forms that iron takes include:
- Pig iron has 4% – 5% carbon and contains varying amounts of contaminants such as sulfur, silicon and phosphorus. Its only significance is that of an intermediate step on the way from iron ore to cast iron and steel.
- Cast iron contains 2% – 4.0% carbon , 1% – 6% silicon , and small amounts of manganese. Contaminants present in pig iron that negatively affect the material properties, such as sulfur and phosphorus, have been reduced to an acceptable level. It has a melting point in the range of 1420–1470 K, which is lower than either of its two main components, and makes it the first product to be melted when carbon and iron are heated together. Its mechanical properties vary greatly, dependant upon the form carbon takes in the alloy. 'White' cast irons contain their carbon in the form of cementite, or iron carbide. This hard, brittle compound dominates the mechanical properties of white cast irons, rendering them hard, but unresistant to shock. The broken surface of a white cast iron is full of fine facets of the broken carbide, a very pale, silvery, shiny material, hence the appellation. In 'grey' cast iron, the carbon exists free as fine flakes of graphite , and also, renders the material brittle due to the stress-raising nature of the sharp edged flakes of graphite. A newer variant of grey iron, referred to as 'ductile iron' is specially treated with trace amounts of magnesium to alter the shape of graphite to sheroids, or nodules, vastly increasing the toughness and strength of the material.
- Carbon steel contains between 0.5% and 1.5% carbon, with small amounts of manganese, sulfur, phosphorus, and silicon.
- Wrought iron contains less than 0.2% carbon. It is a tough, malleable product, not as fusible as pig iron. It has a very small amount of carbon, a few tenths of a percent. If honed to an edge, it loses it quickly. Wrought iron is characterised, especially in old samples, by the presence of fine 'stringers' or filaments of slag entrapped in the metal.
- Alloy steels contain varying amounts of carbon as well as other metals, such as chromium, vanadium, molybdenum, nickel, tungsten, etc. They are used for structural purposes, as their alloy content raises their cost and necessitates justification of their use. Recent developments in ferrous metallurgy have produced a growing range of microalloyed steels, also termed 'HSLA' or high-strength, low alloy steels, containing tiny additions to produce high strengths and often spectacular toughness at minimal cost.
- Iron(III) oxides are used in the production of magnetic storage in computers. They are often mixed with other compounds, and retain their magnetic properties in solution.

History

The first signs of use of iron come from the Sumerians and the Egyptians, where around 4000 BC, a few items, such as the tips of spears, daggers and ornaments, were being fashioned from iron recovered from meteorites. Because meteorites fall from the sky some linguists have conjectured that the English word iron (OE īsern), which has cognates in many northern and western European languages, derives from the Etruscan aisar which means "the gods". By 3000 BC to 2000 BC, increasing numbers of smelted iron objects (distinguishable from meteoric iron by the lack of nickel in the product) appear in Mesopotamia, Anatolia, and Egypt. However, their use appears to be ceremonial, and iron was an expensive metal, more expensive than gold. In the Iliad, weaponry is mostly bronze, but iron ingots are used for trade. Some resources (see the reference What Caused the Iron Age? below) suggest that iron was being created then as a by-product of copper refining, as sponge iron, and was not reproducible by the metallurgy of the time. By 1600 BC to 1200 BC, iron was used increasingly in the Middle East, but did not supplant the dominant use of bronze. bronze In the period from the 12th to 10th century BC, there was a rapid transition in the Middle East from bronze to iron tools and weapons. The critical factor in this transition does not appear to be the sudden onset of a superior ironworking technology, but instead the disruption of the supply of tin. This period of transition, which occurred at different times in different parts of the world, is the ushering in of an age of civilization called the Iron Age. Concurrent with the transition from bronze to iron was the discovery of carburization, which was the process of adding carbon to the irons of the time. Iron was recovered as sponge iron, a mix of iron and slag with some carbon and/or carbide, which was then repeatedly hammered and folded over to free the mass of slag and oxidise out carbon content, so creating the product wrought iron. Wrought iron was very low in carbon content and was not easily hardened by quenching. The people of the Middle East found that a much harder product could be created by the long term heating of a wrought iron object in a bed of charcoal, which was then quenched in water or oil. The resulting product, which had a surface of steel, was harder and less brittle than the bronze it began to replace. In China the first irons used were also meteoric iron, with archeological evidence for items made of wrought iron appearing in the northwest, near Xinjiang, in the 8th century BC. These items were made of wrought iron, created by the same processes used in the Middle East and Europe, and were thought to be imported by non-Chinese people. In the later years of the Zhou Dynasty (ca 550 BC), a new iron manufacturing capability began because of a highly developed kiln technology. Producing blast furnaces capable of temperatures exceeding 1300 K, the Chinese developed the manufacture of cast, or pig iron. Iron was used in India as early as 250 BCE. The famous iron pillar in the Qutb complex in Delhi is made of very pure iron (98%) and has not rusted or eroded till this day. Delhi of wood annually from 1827 to 1891.]] If iron ores are heated with carbon to 1420–1470 K, a molten liquid is formed, an alloy of about 96.5% iron and 3.5% carbon. This product is strong, can be cast into intricate shapes, but is too brittle to be worked, unless the product is decarburized to remove most of the carbon. The vast majority of Chinese iron manufacture, from the Zhou dynasty onward, was of cast iron. Iron, however, remained a pedestrian product, used by farmers for hundreds of years, and did not really affect the nobility of China until the Qin dynasty (ca 221 BC). Cast iron development lagged in Europe, as the smelters could only achieve temperatures of about 1000 K. Through a good portion of the Middle Ages, in Western Europe, iron was still being made by the working of sponge iron into wrought iron. Some of the earliest casting of iron in Europe occurred in Sweden, in two sites, Lapphyttan and Vinarhyttan, between 1150 and 1350 AD. There are suggestions by scholars that the practice may have followed the Mongols across Russia to these sites, but there is no clear proof of this hypothesis. In any event, by the late fourteenth century, a market for cast iron goods began to form, as a demand developed for cast iron cannonballs. Early iron smelting (as the process is called) used charcoal as both the heat source and the reducing agent. In 18th century England, wood supplies ran down and coke, a fossil fuel, was used as an alternative. This innovation by Abraham Darby supplied the energy for the Industrial Revolution.

Occurrence

Industrial Revolution Iron is one of the more common elements on Earth, making up about 5% of the Earth's crust. Most of this iron is found in various iron oxides, such as the minerals hematite, magnetite, and taconite. The earth's core is believed to consist largely of a metallic iron-nickel alloy. About 5% of the meteorites similarly consist of iron-nickel alloy. Although rare, these are the major form of natural metallic iron on the earth's surface. Iron is also one of the least reactive metals, and therefore, it is sometimes found pure in nature.

Extraction from ore

Industrially, iron is extracted from its ores, principally hematite (nominally Fe₂O₃) and magnetite (Fe₃O₄) by a carbothermic reaction (reduction with carbon) in a blast furnace at temperatures of about 2000°C. In a blast furnace, iron ore, carbon in the form of coke, and a flux such as limestone are fed into the top of the furnace, while a blast of heated air is forced into the furnace at the bottom. In the furnace, the coke reacts with oxygen in the air blast to produce carbon monoxide: :6 C + 3 O₂ → 6 CO The carbon monoxide reduces the iron ore (in the chemical equation below, hematite) to molten iron, becoming carbon dioxide in the process: :6 CO + 2 Fe₂O₃ → 4 Fe + 6 CO₂ The flux is present to melt impurities in the ore, principally silicon dioxide sand and other silicates. Common fluxes include limestone (principally calcium carbonate) and dolomite (magnesium carbonate). Other fluxes may be used depending on the impurities that need to be removed from the ore. In the heat of the furnace the limestone flux decomposes to calcium oxide (quicklime): :CaCO₃ → CaO + CO₂ Then calcium oxide combines then with silicon dioxide to form a slag. :CaO + SiO₂ → CaSiO₃ The slag melts in the heat of the furnace, which silicon dioxide would not have. In the bottom of the furnace, the molten slag floats on top of the more dense liquid iron, and spouts in the side of the furnace may be opened to drain off either the iron or the slag. The iron, once cooled, is called pig iron, while the slag can be used as a material in road construction or to improve mineral-poor soils for agriculture. Approximately 1100Mt (million tons) of iron ore was produced in the world in 2000, with a gross market value of approximately 25 billion US dollars. While ore production occurs in 48 countries, the five largest producers were China, Brazil, Australia, Russia and India, accounting for 70% of world iron ore production. The 1100Mt of iron ore was used to produce approximately 572Mt of pig iron.

Compounds

2000 production.]] Common oxidation states of iron include:
- the Iron(-II) state, Fe^2- (e.g. Fe(CO)₄^2-,Fe(CO)₂(NO)₂.
- the Iron(0) state, Fe(CO)₅, Fe(PF₃)₅.
- the Iron(I) state, [Fe(H₂O)₅NO]²⁺.
- the Iron(II) state, Fe²⁺, previously ferrous is very common.
- the Iron(III) state, Fe³⁺, previously ferric, is also very common, for example in rust.
- the Iron(IV) state, Fe⁴⁺, previously ferryl, stabilized in some enzymes (e.g. peroxidases).
- the Iron(VI) state, Fe⁶⁺ is also known, if rare, in potassium ferrate. Iron carbide Fe₃C is known as cementite.

Biological role

Iron is essential to all organisms, except for a few bacteria. It is mostly stably incorporated in the inside of metalloproteins, because in exposed or in free form it causes production of free radicals that are generally toxic to cells. To say that iron is free doesn't mean that it is free floating in the bodily fluids. Iron binds avidly to virtually all biomolecules so it will adhere nonspecifically to cell membranes, nucleic acids, proteins etc. Many animals incorporate iron into the heme complex, an essential component of cytochromes, which are proteins involved in redox reactions (including but not limited to cellular respiration), and of oxygen carrying proteins hemoglobin and myoglobin. Inorganic iron involved in redox reactions is also found in the iron-sulfur clusters of many enzymes, such as nitrogenase (involved in the synthesis of ammonia from nitrogen and hydrogen) and hydrogenase. A class of non-heme iron proteins is responsible for a wide range of functions within several life forms, such as enzymes methane monooxygenase (oxidizes methane to methanol), ribonucleotide reductase (reduces ribose to deoxyribose; DNA biosynthesis), hemerythrins (oxygen transport and fixation in marine invertebrates) and purple acid phosphatase (hydrolysis of phosphate esters). When the body is fighting a bacterial infection, the body sequesters iron inside of cells (mostly stored in the storage molecule ferritin so that it cannot be used by bacteria. Iron distribution is heavily regulated in mammals, both as a defense against bacterial infection as well as the potential biological toxicity of iron. The iron absorbed from the duodenum binds to transferrin, and is carried by blood to different cells. There it gets by an as yet unknown mechanism incorporated into target proteins. [http://www.plosbiology.org/plosonline/?request=get-document&doi=10.1371%2Fjournal.pbio.0000079]. A lengthier article on the system of human iron regulation can be found in the article on human iron metabolism. Good sources of dietary iron include meat, fish, poultry, lentils, beans, leaf vegetables, tofu, chickpeas, black-eyed pea, strawberries and farina. Iron provided by dietary supplements is often found as Iron (II) fumarate. The RDA for iron varies considerably based on the age, gender, and source of dietary iron (heme-based iron has higher bioavailability)[http://www.iom.edu/Object.File/Master/7/294/0.pdf]. Also note the section below on precautions.

Isotopes

Naturally occurring iron consists of four isotopes: 5.845% of radioactive ⁵⁴Fe (half-life: >3.1E22 years), 91.754% of stable ⁵⁶Fe, 2.119% of stable ⁵⁷Fe and 0.282% of stable ⁵⁸Fe. ⁶⁰Fe is an extinct radionuclide of long half-life (1.5 million years). Much of the past work on measuring the isotopic composition of Fe has centered on determining ⁶⁰Fe variations due to processes accompanying nucleosynthesis (i.e., meteorite studies) and ore formation. The isotope ⁵⁶Fe is of particular interest to nuclear scientists. A common misconception is that this isotope represents the most stable nucleus possible, and that it thus would be impossible to perform fission or fusion on ⁵⁶Fe and still liberate energy. This is not true, as both ⁶²Ni and ⁵⁸Fe are more stable. In phases of the meteorites Semarkona and Chervony Kut a correlation between the concentration of ⁶⁰Ni, the daughter product of ⁶⁰Fe, and the abundance of the stable iron isotopes could be found which is evidence for the existence of ⁶⁰Fe at time formation of solar system. Possibly the energy released by the decay of ⁶⁰Fe contributed, together with the energy released by decay of the radionuclide ²⁶Al, to the remelting and differentiation of asteroids after their formation 4.6 billion years ago. The abundance of ⁶⁰Ni present in extraterrestrial material may also provide further insight into the origin of the solar system and its early history. Of the stable isotopes, only ⁵⁷Fe has a nuclear spin (−1/2). For this reason, ⁵⁷Fe has application as a spin isotope in chemistry and biochemistry.

Precautions

Excessive dietary iron is toxic, because excess ferrous iron reacts with peroxides in the body, producing free radicals. When iron is in normal quantity, the body's own antioxidant mechanisms can control this process. In excess, uncontrollable quantities of free radicals are produced. The lethal dose of iron in a two-year-old is about three grams of iron. One gram can induce severe poisoning. There are reported cases of children being poisoned by consuming between 10 and 50 tablets of ferrous sulfate over a period of several hours. Over-consumption of iron is the single highest cause of death in children by unintentional ingestion of pharmaceuticals. The DRI lists the Tolerable Upper Intake Level (UL) for adults as 45 mg/day. For children under fourteen years old the UL is 40 mg/day. If iron intake is excessive a number of iron overload disorders can result, such as hemochromatosis. For this reason, people should not take iron supplements unless they suffer from iron deficiency and have consulted a doctor. Blood donors are at special risk of low iron levels and are often recommended to supplement their iron intake. A specific chelating agent called Desferrioxime is used to expell excess iron from the body in case of iron toxicity.

References

- [http://periodic.lanl.gov/elements/26.html Los Alamos National Laboratory — Iron]

External links

- [http://www.webelements.com/webelements/elements/text/Fe/index.html WebElements.com – Iron]
- [http://education.jlab.org/itselemental/ele026.html It's Elemental – Iron]
- [http://hyperphysics.phy-astr.gsu.edu/hbase/nucene/nucbin2.html The Most Tightly Bound Nuclei] Category:Chemical elements Category:Transition metals ko:철 ms:Besi ja:鉄 simple:Iron th:เหล็ก

Chemical element

A chemical element, often called simply element, is a chemical substance that canot be divided or changed into other chemical substances by any ordinary chemical technique. The smallest unit of this kind of chemical substances is an atom. An element is a class of substances that contain the same number of protons in all its atoms.

Chemistry terminology

Earlier an element or pure element was defined as a substance which "cannot be further broken down into another compound with different chemical properties" -- which should be taken to mean it consists of atoms of one element. However, due to allotropy, the isotope effect, and the confusion with the more useful term referring to the general class of atoms (irrespective of what compound it may be in), this usage is in disfavor amongst contemporary chemists, and sees restricted, mostly historical, use. This definition was motivated by the observation that these elements could not be dissociated by chemical means into other compounds. For example, water could be converted into hydrogen and oxygen, but hydrogen and oxygen could not be further decomposed, thus "elemental". There are also many counterexamples (for example "elemental oxygen" (O₂) can be decomposed by solely chemical means into oxygen ions and atoms which have drastically different chemical properties). The remainder of this article will concern itself with the first definition.

Description

The atomic number of an element, Z, is equal to the number of protons which defines the element. For example, all carbon atoms contain 6 protons in their nucleus, so for carbon Z=6. These atoms may have different amounts of neutrons, and are known as isotopes of the element. The atomic mass of an element, A, is measured in unified atomic mass units (u) is the average mass of all the atoms of the element in an environment of interest (usually the earth's crust and atmosphere). Since electrons are light, and neutrons are barely more than the mass of the proton, this usually corresponds to the sum of the protons and neutrons in the nucleus of the most abundant isotope, though this is not always the case (notably chlorine, which is about three-quarters ³⁵Cl and a quarter ³⁷Cl). Some isotopes are radioactive and decay into other elements upon radiating an alpha or beta particle. Some elements have no nonradioactive isotopes, in particular all elements with Z >= 84. The lightest elements are hydrogen and helium. Hydrogen is thought to be the first element to appear after the Big Bang. All the heavier elements, are made naturally and artificially through various methods of nucleosynthesis. As of 2005, there are 116 known elements: 93 occur naturally on earth (including technetium and plutonium), and 94 (including promethium) have been detected so far in the universe. The 23 elements not found on earth are derived artificially; the first purportedly synthesized element was technetium, in 1937, although the trace amounts of naturally occurring technetium were not known then. All artificially derived elements are radioactive with short half-lives so that any such atoms that were present at the formation of Earth are extremely likely to have already decayed. Lists of the elements by name, by symbol, by atomic number, by density, by melting point and by boiling point are available. The most convenient presentation of the elements is in the periodic table, which groups elements with similar chemical properties together.

Nomenclature

The naming of elements precedes the atomic theory of matter, although at the time it was not known which chemicals were elements and which compounds. When it was learned, existing names (e.g., gold, mercury, iron) were kept in most countries, and national differences emerged over the names of elements either for convenience, linguistic niceties, or nationalism. For example, the Germans use "Wasserstoff" for "hydrogen" and "Sauerstoff" for "oxygen," while some romance languages use "natrium" for "sodium" and "kalium" for "potassium," and the French prefer the obsolete but historic term "azote" for "nitrogen." But for international trade, the official names of the chemical elements both ancient and recent are decided by the International Union of Pure and Applied Chemistry, which has decided on a sort of international English language. That organization has recently prescribed that "aluminium" and "caesium" take the place of the US spellings "aluminum" and "cesium," while the US "sulfur" takes the place of the British "sulphur." But chemicals which are practicable to be sold in bulk within many countries, however, still have national names, and those which do not use the Latin alphabet cannot be expected to use the IUPAC name. According to IUPAC, the full name of an element is not capitalized, even if it is derived from a proper noun (unless it would be capitalized by some other rule, for instance if it begins a sentence). And in the second half of the twentieth century physics laboratories became able to produce nuclei of chemical elements that have too quick a decay rate to ever be sold in bulk. These are also named by IUPAC, which generally adopts the name chosen by the discoverer. This can lead to the controversial question of which research group actually discovered an element, a question which delayed the naming of elements with atomic number of 104 and higher for a considerable time. (See element naming controversy). Precursors of such controversies involved the nationalistic namings of elements in the late nineteenth century (e.g., as "lutetium" refers to Paris, France, the Germans were reticent about relinquishing naming rights to the French, often calling it "cassiopeium"). And notably, the British discoverer of "niobium" originally named it "columbium," after the New World, though this did not catch on in Europe. The Americans had to accept the international name just when it was becoming an economically important material late in the twentieth century.

Chemical symbols

Specific chemical elements

Before chemistry became a science, alchemists had designed arcane symbols for both metals and common compounds. These were however used as abbreviations in diagrams or procedures; there was no concept of one atoms combining to form molecules. With his advances in the atomic theory of matter, John Dalton devised his own simpler symbols, based on circles, which were to be used to depict molecules. These were superseded by the current typographical system in which chemical symbols are not used as mere abbreviations though each consists letters of the Latin alphabet - they are symbols intended to be used by peoples of all languages and alphabets. The first of these symbols were intended to be fully international, for they were based on the Latin abbreviations of the names of metals: Fe comes from Ferrum; Ag from Argentum. The symbols were not followed by a period (full stop) as abbreviations were. Besides a name, later chemical elements are also given a unique chemical symbol, based on the name of the element, not necessarily derived from the colloquial English name. (e.g., sodium has chemical symbol 'Na' after the Latin natrium). The same applies to "W" (wolframium) for Tungsten , "Hg" (Hydrargyrum) for mercury and "K" for potassium. Stricly taken, a symbol like Tu for tungsten or M or Me for mercury seems to be more logical. Chemical symbols are understood internationally when element names might need to be translated. There are sometimes differences; for example, the Germans have used "J" instead of "I" for iodine, so the character would not be confused with a roman numeral. The first letter of a chemical symbol is always capitalized, as in the preceding examples, and the subsequent letters, if any, are always minuscule (small letters).

General chemical symbols

There are also symbols for series of chemical elements, for comparative formulas. These are one capital letter in length, and the letters are reserved so they are not permitted to be given for the names of specific elements. For example, an "X" is used to indicate a variable group amongst a class of compounds (though usually a halogen), while "R" is used for a radical (not to be confused with radical_(chemistry), meaning a compound structure such as a hydrocarbon chain. The letter "Q" is reserved for "heat" in a chemical reaction. "Y" is also often used as a general chemical symbol, although it is also the symbol of Yttrium. "Z" is also frequently used as a general variable group. "L" is used to represent a general ligand in inorganic and organometallic chemistry. "M" is also often used in place of a general metal.

Nonelement symbols

Nonelements, especially in organic and organometallic chemistry, often acquire symbols which are inspired by the elemental symbols. A few examples: Cy - cyclohexyl; Ph - phenyl; Bz - benzoyl; Bn - benzyl; Cp - Cyclopentadiene; Pr - propyl; Me - methyl; Et - ethyl; Tf - triflate; Ts - tosyl.

External links

- [http://www.vanderkrogt.net/elements/ Elementymology & Elements Multidict] word history and language dictionary

Chemical information

- [http://www.webelements.com/ WebElements]
- [http://www.vcs.ethz.ch/chemglobe/ptoe/ ChemGlobe]
- [http://pearl1.lanl.gov/periodic/default.htm Los Alamos National Laboratory]
- [http://www.chemicalelements.com/ ChemicalElements] ko:화학 원소 ms:Unsur kimia ja:元素 simple:Element th:ธาตุเคมี

Latin

Latin is an ancient Indo-European language originally spoken in the region around Rome called Latium. It gained great importance as the formal language of the Roman Empire. All Romance languages, those being most notably Spanish, French, Portuguese, Italian, and Romanian, are descended from Latin, and many words based on Latin are found in other modern languages such as English. The Latin alphabet, derived from the Greek, remains the most widely-used alphabet in the world. It is said that 80 percent of scholarly English words are derived from Latin (in a large number of cases by way of French). Moreover, in the Western world, Latin was a lingua franca, the learned language for scientific and political affairs, for more than a thousand years, being eventually replaced by French in the 18th century and English in the late 19th. Ecclesiastical Latin remains the formal language of the Roman Catholic Church to this day, and thus the official national language of the Vatican. The Church used Latin as its primary liturgical language until the Second Vatican Council in the 1960s. Latin is also still used (drawing heavily on Greek roots) to furnish the names used in the scientific classification of living things. The modern study of Latin, along with Greek, is known as Classics.

Main features

Latin is a synthetic inflectional language: affixes (which usually encode more than one grammatical category) are attached to fixed stems to express gender, number, and case in adjectives, nouns, and pronouns, which is called declension; and person, number, tense, voice, mood, and aspect in verbs, which is called conjugation. There are five declensions (declinationes) of nouns and four conjugations of verbs. There are six noun cases: #nominative (used as the subject of the verb or the predicate nominative), #genitive (used to indicate relation or possession, often represented by the English of or the addition of s to a noun), #dative (used of the indirect object of the verb, often represented by the English to or for), #accusative (used of the direct object of the verb, or object of the preposition in some cases), #ablative (separation, source, cause, or instrument, often represented by the English by, with, from), #vocative (used of the person or thing being addressed). In addition, some nouns have a locative case used to express location (otherwise expressed by the ablative with a preposition such as in), but this survival from Proto-Indo-European is found only in the names of lakes, cities, towns, small islands, and a few other words related to locations, such as "house", "ground", and "countryside". Latin itself, being a very old language, is far closer to Proto-Indo-European than are most modern Western European languages; it has, in fact, about the same relationship with PIE as modern Italian or French has to Latin. There are six general tenses in Latin (technically they are tense/aspect/mood complexes). The indicative mood can be used with all of them. The subjunctive mood, however, has only present, imperfect, perfect, and pluperfect tenses. These tenses in the subjunctive mood do not completely correlate in meaning to the tenses in the indicative. The following examples are of the first conjugation verb "laudare" ("to praise") in the indicative mood and the active voice:
Primary sequence tenses
# present (laudo, "I praise") # imperfect (laudabam, "I was praising") # future (laudabo, "I shall praise," "I will praise")
Secondary sequence tenses
# perfect (laudavi, "I praised", "I have praised") # pluperfect (laudaveram, "I had praised") # future perfect (laudavero, "I shall have praised," "I will have praised") The future perfect tense can also imply a normal future idea (like in "When I will have run...") and so may also sometimes be included in the primary sequence.
Latin and Romance
After the collapse of the Roman Empire, Latin evolved into the various Romance languages. These were for many centuries only spoken languages, Latin still being used for writing. For example, Latin was the official language of Portugal until 1296 when it was replaced by Portuguese. The Romance languages evolved from Vulgar Latin, the spoken language of common usage, which in turn evolved from an older speech which also produced the formal classical standard. Latin and Romance differ (for example) in that Romance had distinctive stress, whereas Latin had distinctive length of vowels. In Italian and Sardo logudorese, there is distinctive length of consonants and stress, in Spanish only distinctive stress, and in French even stress is no longer distinctive. Another major distinction between Romance and Latin is that all Romance languages, excluding Romanian, have lost their case endings in most words except for some pronouns. Romanian retains a direct case (nominative/accusative), an indirect case (dative/genitive), and vocative. In Italy, Latin is still compulsory in secondary schools as Liceo Classico and Liceo Scientifico which are usually attended by people who aim to the highest level of education. In Liceo Classico Ancient Greek is a compulsory subject.
Latin and English
See Latin influence in English for a more complete exposition. English grammar is independent of Latin grammar, though prescriptive grammarians in English have been heavily influenced by Latin. Attempts to make English grammar follow Latin rules — such as the prohibition against the split infinitive — have not worked successfully in regular usage. However, as many as half the words in English were derived from Latin, including many words of Greek origin first adopted by the Romans, not to mention the thousands of French, hundreds of Spanish, Portuguese and Italian words of Latin origin that have also enriched English. During the 16th and on through the 18th century English writers created huge numbers of new words from Latin and Greek roots. These words were dubbed "inkhorn" or "inkpot" words (as if they had spilled from a pot of ink). Many of these words were used once by the author and then forgotten, but some remain. Imbibe, extrapolate, dormant and inebriation are all inkhorn terms carved from Latin words. In fact, the word etymology is derived from the Greek word etymologia, meaning "true sense of the word." Latin was once taught in many of the schools in Britain with academic leanings - perhaps 25% of the total [http://www.channel4.com/history/microsites/T/teachem2/thennow/]. However, the requirement for it was gradually abandoned in the professions such as the law and medicine, and then, from around the late 1960s, for admission to university. After the introduction of the Modern Language GCSE in the 1980s, it was gradually replaced by other languages, although it is now being taught by more schools along with other classical languages.
Latin education
The linguistic element of Latin courses offered in high schools or secondary schools, and in universities, is primarily geared toward an ability to translate Latin texts into modern languages, rather than using it in oral communication. As such, the skill of reading is heavily emphasized, whereas speaking and listening skills are barely touched upon. However, there is a growing movement, sometimes known as the Living Latin movement, whose supporters believe that Latin can, or should, be taught in the same way that modern "living" languages are taught, that is, as a means of both spoken and written communication. One of the most interesting aspects of such an approach is that it assists speculative insight into how many of the ancient authors spoke and incorporated sounds of the language stylistically; without understanding how the language is meant to be heard it is very difficult to identify patterns in Latin poetry. Institutions offering Living Latin instruction include the Vatican and the University of Kentucky. In Britain the Classical Association encourages this approach, and there has been something of a vogue for books describing the adventures of a mouse called Minimus. In the United States there is a thriving competitive organization for high school Latin students, the National Junior Classical League (the second-largest youth organization in the world after the Boy Scouts), backed up by the Senior Classical League for college students. Many would-be international auxiliary languages have been heavily influenced by Latin, and the moderately successful Interlingua considers itself to be the modernized and simplified version of the language (le latino moderne international e simplificate). Latin translations of modern literature such as Paddington Bear, Winnie the Pooh, Harry Potter and the Philosopher's Stone, Le Petit Prince, Max und Moritz, and The Cat in the Hat have also helped boost interest in the language.
See also

About the Latin language

- Latin grammar
- Latin spelling and pronunciation
- Latin declension
- Latin conjugation
- Latin alphabet
- List of Latin words with English derivatives
- Latin verbs with English derivatives
- Latin nouns with English derivatives
- ablative absolute
- Word order in Latin
About the Latin literary heritage

- Latin literature
- Romance languages
- Loeb Classical Library
- List of Latin phrases
- List of Latin proverbs
- Brocard
- List of Latin and Greek words commonly used in systematic names
- List of Latin place names in Europe
- Carmen Possum
Other related topics

- Roman Empire
- Internationalism
References

- Bennett, Charles E. Latin Grammar (Allyn and Bacon, Chicago, 1908)
- N. Vincent: "Latin", in The Romance Languages, M. Harris and N. Vincent, eds., (Oxford Univ. Press. 1990), ISBN 0195208293
- Waquet, Françoise, Latin, or the Empire of a Sign: From the Sixteenth to the Twentieth Centuries (Verso, 2003) ISBN 1859844022; translated from the French by John Howe.
- Wheelock, Frederic. Latin: An Introduction (Collins, 6th ed., 2005) ISBN 0060784237
External links

- [http://www.jambell.com/latin.html Latin Phrases for after dinner conversation (Thanks to Elaine Poole)]
- [http://www.ethnologue.com/show_language.asp?code=lat Ethnologue report for Latin]
- [http://forumromanum.org/literature/index.html Corpus Scriptorum Latinorum] is a comprehensive webography of Latin texts and their translations.
- [http://www.perseus.tufts.edu/ The Perseus Project] has many useful pages for the study of classical languages and literatures, including [http://www.perseus.tufts.edu/cgi-bin/resolveform?lang=Latin an interactive Latin dictionary].
- [http://lysy2.archives.nd.edu/cgi-bin/words.exe words by William whitaker] is a dictionary program online capable of looking up various word forms.
- [http://retiarius.org/ Retiarius.Org] includes a Latin text search engine.
- [http://www.nd.edu/~archives/latgramm.htm Latin-English dictionary and Latin grammar from U of Notre Dame]
- [http://latin-language.co.uk/ Latin language] History of Latin language, Latin texts with English translation and a collection of dictionaries.
- [http://augustinus.eresmas.net/scl/ Societas Circulorum Latinorum] gathers together Latin Circles all over the world.
- [http://www.learnlatin.tk LearnLatin.tk] - Free online course in Latin
- [http://www.latintests.net/ LatinTests.net] - Lets Latin learners test their grammar and vocabulary with self-checking quizzes.
- [http://thelatinlibrary.com/ The Latin Library] contains many Latin etexts
- [http://www.textkit.com/ Textkit] has Latin textbooks and etexts.
- [http://www.websters-online-dictionary.org/definition/Latin-english/ Latin–English Dictionary]: from Webster's Rosetta Edition.
- [http://www.language-reference.com/ Language reference] Cross-foreign-language lexicon powered by its own search engine. All cross combinations between Latin and French, German, Italian, Spanish.
- [http://comp.uark.edu/~mreynold/rhetor.html Rhetor by Gabriel Harvey] was originally published in 1577 and never again reprinted.
- [http://freewebs.com/omniamundamundis omniamundamundis] Latin hypertexts from fourteen ancient Roman authors.
- [http://www.saltspring.com/capewest/pron.htm Pronunciation of Biological Latin, Including Taxonomic Names of Plants and Animals]
- [http://www.yleradio1.fi/nuntii Nuntii Latini (News in Latin)], written and spoken (RealAudio) news in latin. Weekly review of world news in Classical Latin, the only international broadcast of its kind in the world, produced by YLE, the Finnish Broadcasting Company.
- [http://www.tranexp.com:2000/InterTran?url=http%3A%2F%2F&type=text&text=Replace%20Me&from=eng&to=ltt InterTran Latin], Translate from Latin to ENGLISH or vice versa.
- [http://www.latinvulgate.com Latin Vulgate] The Latin and English of the Old & New Testaments in parallel, along with the Complete Sayings of Jesus in parallel Latin and English. Category:Classical languages Category:Ancient languages Category:Fusional languages Category:Languages of Italy Category:Languages of Vatican City als:Latein zh-min-nan:Latin-gí ko:라틴어 ja:ラテン語 simple:Latin language th:ภาษาละติน

Atomic number

The atomic number (Z) is a term used in chemistry and physics to represent the number of protons found in the nucleus of an atom. In an atom of neutral charge, the number of electrons also equals the atomic number. The atomic number originally meant the number of an element's place in the periodic table. When Mendeleev arranged the known chemical elements grouped by their similarities in chemistry, it was noticeable that placing them in strict order of atomic mass resulted in some mismatches. Iodine and tellurium, if listed by atomic mass, appeared to be in the wrong order, and would fit better if their places in the table were swapped. Placing them in the order which fit chemical properties most closely, their number in the table was their atomic number. This number appeared to be approximately proportional to the mass of the atom, but, as the discrepancy showed, reflected some other property than mass. The anomalies in this sequence were finally explained after research by Henry Gwyn Jeffreys Moseley in 1913. Moseley discovered a strict relationship between the x-ray diffraction spectra of elements, and their correct location in the periodic table. It was later shown that the atomic number corresponds to the electric charge of the nucleus — in other words the number of protons. It is the charge which gives elements their chemical properties, rather than the atomic mass. The atomic number is closely related to the mass number (although they should not be confused) which is the number of protons and neutrons in the nucleus of an atom. The mass number often comes after the name of the element, e.g. carbon-14 (used in carbon dating).

Period 4 element

A period 4 element is one of the chemical elements in the fourth row (or period) of the periodic table of the elements. These are:

Chemical elements in the fourth period
e^--conf.
Group	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18
# Name	19 K	20 Ca	21 Sc	22 Ti	23 V	24 Cr	25 Mn	26 Fe	27 Co	28 Ni	29 Cu	30 Zn	31 Ga	32 Ge	33 As	34 Se	35 Br	36 Kr

Alkali metals	Alkaline earths	Lanthanide	Actinides	Transition metals
Poor metals	Metalloids	Nonmetals	Halogens	Noble gases

Period 1 element - Period 2 element - Period 3 element - Period 4 element - Period 5 element - Period 6 element - Period 7 element - Period 8 element -
Category:Periodic table
ja:第4周期元素 th:ธาตุคาบ 4

Stellar nucleosynthesis

Stellar nucleosynthesis is the collective term for the nuclear reactions taking place in stars to build the nuclei of the heavier elements. (For other such processes, see nucleosynthesis.) The processes involved began to be understood early in the 20th century, when it was first realised that the energy released from nuclear reactions accounted for the longevity of the Sun as a source of heat and light. The prime energy producer in the sun is the fusion of hydrogen to helium, which occurs at a minimum temperature of 3 million kelvins.

History

In 1920, Arthur Eddington, on the basis of the precise measurements of atoms by F.W. Aston, was the first to suggest that stars obtained their energy from nuclear fusion of hydrogen to form helium. In 1928, George Gamow derived what is now called the Gamow factor, a quantum-mechanical formula that gave the probability of bringing two nuclei sufficiently close for the strong nuclear force to overcome the Coulomb barrier. The Gamow factor was used in the decade that followed by Atkinson and Houtermans and later by Gamow himself and Teller to derive the rate at which nuclear reactions would proceed at the high temperatures believed to exist in stellar interiors. In 1939, in a paper entitled "Energy Production in Stars", Hans Bethe analyzed the different possibilities for reactions by which hydrogen is fused into helium. He selected two processes that he believed to be the sources of energy in stars. The first one, the proton-proton chain, is the dominant energy source in stars with masses up to about the mass of the Sun. The second process, the carbon-nitrogen-oxygen cycle, which was also considered by Carl Friedrich von Weizsäcker in 1938, is most important in more massive stars. Later, many important details were added to Bethe's theory, like the publication of a celebrated paper in 1957 by Burbidge, Burbidge, Fowler and Hoyle. This latter work collected and refined earlier researches into a coherent picture that accounted for the observed relative abundances of the elements.

Key Reactions

The most important reactions in stellar nucleosynthesis are:
- Hydrogen burning:
- The proton-proton chain
- The carbon-nitrogen-oxygen cycle
- Helium burning:
- The triple-alpha process
- The alpha process
- Burning of heavier elements:
- Carbon burning process
- Neon burning process
- Oxygen burning process
- Silicon burning process
- Production of elements heavier than iron:
- Neutron capture:
- The R-process
- The S-process
- Proton capture:
- The P-process

References

- H. A. Bethe, Energy Production in Stars, Phys. Rev. 55 (1939) 103; [http://prola.aps.org/abstract/PR/v55/i1/p103_1?qid=45414e63da12f8b5&qseq=5&show=10 online edition (subscription needed)]
- H. A. Bethe, Energy Production in Stars, Phys. Rev. 55 (1939) 434-456; [http://prola.aps.org/abstract/PR/v55/i5/p434_1?qid=45414e63da12f8b5&qseq=3&show=10 online edition (subscription needed)]
- Alak K. Ray (2004) Stars as thermonuclear reactors: their fuels and ashes [http://arxiv.org/abs/astro-ph/0405568 (arxiv.org article)]

External links

- [http://nobelprize.org/physics/articles/fusion/index.html How the Sun Shines] by John N. Bacall
- http://helios.gsfc.nasa.gov/nucleo.html Category:Nucleosynthesis category:Stars

Supernova

:For other uses, see Supernova (disambiguation). Supernova (disambiguation).]] Supernovae refer to several types of stellar explosions that produce extremely bright objects made of plasma that decline to invisibility over weeks or months. There are two possible routes to this end: either a massive star may cease to generate fusion energy in its core and collapses inward under the force of its own gravity, or a white dwarf star may accumulate material from a companion star until it reaches its Chandrasekhar limit and undergoes a thermonuclear explosion. In either case, the resulting supernova explosion expels much or all of the stellar material with great force. The explosion drives a blast wave into the surrounding space, forming a supernova remnant. One famous example of this process is the remnant of SN 1604, shown at right. Supernova explosions are the main source of all the elements heavier than oxygen, and they are the only source of many important elements. For example, all the calcium in our bones and all the iron in our hemoglobin were synthesized in a supernova explosion, billions of years ago. Supernovae inject these heavy elements into the interstellar medium, thus enriching the molecular clouds that are the sites of stellar formation. This enrichment process is what determined the composition of the Solar System 4.5 billion years ago, and ultimately made possible the chemistry of life on Earth. Supernovae generate tremendous temperatures, and under the right conditions, the fusion reactions that take place during the peak moments of a supernova can produce some of the heaviest elements like californium. "Nova" (pl. novae) is Latin for "new", referring to what appears to be a very bright new star shining in the celestial sphere; the prefix "super" distinguishes this from an ordinary nova, which also involves a star increasing in brightness, though to a lesser extent and through a different mechanism. However, it is misleading to consider a supernova as a new star, because it really represents the death of a star (or at least its radical transformation into something else).

Classification

As part of the attempt to understand supernova explosions, astronomers have classified them according to the lines of different chemical elements that appear in their spectra. See "Optical Spectra of Supernovae" by Filippenko ([http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1997ARA%26A..35..309F&db_key=AST&high=3f6510b0d828671 Annual Review of Astronomy and Astrophysics, Volume 35, 1997, pp. 309-355]) for a good description of the classes. The first element for division is the presence or absence of a line from hydrogen. If a supernova's spectrum contains a hydrogen line, it is classified Type II, otherwise it is Type I. Among those groups, there are subdivisions according to the presence of other lines and the shape of its light curve.

Summary

Type I: No hydrogen Balmer lines
Type II: Has hydrogen Balmer lines

Type Ia

Type Ia supernovae lack helium and present a silicon absorption line in their spectra near peak light. The most commonly accepted theory of these type of supernovae is that they are the result of a carbon-oxygen white dwarf accreting matter from a nearby companion star, typically a red giant, until it reaches the Chandrasekhar limit. The increase in pressure raises the temperature near the center, and a period of convection lasting ~100 years begins. At some point in this simmering phase, a deflagration flame front powered by fusion is born, although the details of the ignition---the location and number of points where the flame begins---is still unknown. This flame accelerates dramatically, through the Rayleigh-Taylor instability and interactions with turbulence. It is still a matter of considerable debate as to whether this flame transitions from a subsonic deflagration into a supersonic detonation. The energy release from the thermonuclear burning (~10⁴⁴ joules) causes the star to explode violently and to release a shock wave in which matter is typically ejected at speeds on the order of 10,000 km/s. The energy released in the explosion also causes an extreme increase in luminosity. The theory of these type of supernovae is similar to that of novae, in which a white dwarf accretes matter more slowly and does not reach the Chandrasekhar limit. In the case of a nova, the infalling matter causes a fusion reaction of material near its surface but does not cause the star to collapse. Type Ia supernovae have a characteristic light curve (graph of luminosity as a function of time after the explosion). Near the time of maximum luminosity, the spectrum contains lines of intermediate-mass elements from oxygen to calcium; these are the main constituents of the outer layers of the star. Months after the explosion, when the outer layers have expanded to the point of transparency, the spectrum is dominated by light emitted by material near the core of the star: heavy elements synthesized during the explosion, most prominently iron-group elements. The radioactive decay of Nickel-56 through Cobalt-56 to Iron-56 produces high-energy photons which dominate the energy output of the ejecta at intermediate to late times. Unlike the other types of supernove, Type Ia supernovae are generally found in all types of galaxies, including ellipticals. They show no preference for regions of current star formation. The similarity in the shapes of the luminosity profiles of all known Type Ia supernovae has led to their use as a standard candle in extragalactic astronomy. The cause of this similarity in the luminosity curve is still an open question. In 1998, observations of Type Ia supernovae indicated the unexpected result that the universe seems to undergo an accelerating expansion. A Type Ia supernova releases the highest amounts of energy amongst all known classifications of supernovae. The farthest single object ever detected in the universe (galaxies or globular clusters do not count) was a Type Ia supernova located billions of light-years (tens of yottameters) away.

Type Ib and Ic

The early spectra of Types Ib and Ic do not show lines of hydrogen, nor the strong silicon absorption feature near 615 nanometers. These events, like supernovae of Type II, are probably massive stars running out of fuel at their centers; however, the progenitors of Types Ib and Ic have lost most of their envelopes due to strong stellar winds or interaction with a companion. Type Ib supernovae are thought to be the result of a Wolf-Rayet star collapsing. There is some evidence that Type Ic supernovae may be the progenitors of gamma ray bursts, though it is also thought that any supernova may be a GRB dependent upon the geometry of the explosion.

Type II

Stars far more massive than our sun evolve in far more complex fashions. In the core of our sun, 589 million tonnes of hydrogen fuse into 584 million tonnes of helium every second, the extra 4.3 millon tonnes of mass is converted into pure energy which then radiates outwards. The helium produced in the core accumulates there since temperatures in the core are not yet high enough to cause it to fuse. Eventually, as the hydrogen at the core is exhausted, having been either fused to helium or progressively diluted by the ongoing build-up of helium "ash", fusion begins to slow down and gravity begins to cause the core to contract. This contraction spikes the temperature high enough to initiate a shorter phase of helium fusion, which accounts for less than 10% of the star's total lifetime. In stars with less than about 10 solar masses, the carbon produced by helium fusion does not fuse, and the star gradually cools to being a white dwarf. White dwarf stars can become Type I supernovae as described above. A much larger star, however, has the kind of gravity needed to create temperatures and pressures sufficient to cause the carbon in the core to begin to fuse once the star contracts. The cores of these massive stars become layered like onions as progressively heavier atomic nuclei build up at the center, with an outermost layer of hydrogen gas, sinking down on a layer of hydrogen fusing into helium, with the helium sinking down into a layer of helium fusing into carbon, with the carbon sinking down to fuse into even heavier elements. These stars go through progressive stages where the core will shrink, built-up atomic nuclei which were previously unfusable begin to fuse, and the core springs back into equilibrium with gravity. This causes them to be irregular variables—as each new burst of fusion pushes elements out of the fusing core into what is called the "stellar envelope", and dims the star, causing gravity to pull mass back into the fusing core and begin the cycle over again. The limiting factor in this process is the amount of energy that is released through fusion, which is dependent on the binding energy of these atomic nuclei. Each additional step produces progressively heavier nuclei, which is also more and more tightly bound by the strong force, this means it releases less energy per fusion reaction than lighter elements fusing. Among most tightly bound of all nuclei is iron, chemical symbol Fe. It represents the "bottom of the hill" for lighter elements to fuse, and for heavier elements to fission. Lighter elements release energy when they fuse and heavier elements release energy when they fission. As iron "ash" begins to accumulate in the core of the star, gravity pulls more and more mass into the area of fusion, which, in turn, goes through all of the steps of fusion: Hydrogen to helium by the proton chain, helium to carbon by the triple-alpha process, carbon and helium combine into oxygen, oxygen fuses into neon, neon into magnesium, magnesium into silicon and silicon into iron. The iron (Fe) core is under huge gravitational pressure, and since there is no fusion and cannot be supported by ordinary gas pressure, it is supported by electron degeneracy pressure, the electrons pushing against other electrons. If it builds up to the Chandrasekhar limit at which electron degeneracy pressure cannot sustain it, the iron core begins to collapse. The collapsing core produces high energy gamma rays, which decompose some iron nuclei into 13 He plus 4 neutrons, a process known as photodissociation. However, no nuclear reaction of an iron nucleus can create energy; it can only absorb it. Thus, where reactions in the core have for millions of years been radiating energy outward, balancing the star against gravity, they suddenly begin sucking energy inwards, joining hands with gravity to cause the core, a massive structure the size of our sun, to collapse within a fraction of a second. As the density in the collapsing core skyrockets, electrons and protons are pushed together until their electrical attraction overcomes their inherent nuclear repulsion from each other. This combination, a process called "electron capture", creates a neutron and releases a neutrino. The neutrinos escape from the core, carrying away energy and further accelerating the collapse, which proceeds in milliseconds as the core detaches from the outer layers of the star and reaches the density of nuclear matter, where the neutrons press against each other and the entire core is the density of an atomic nucleus. This is the core collapse. At this point neutron degeneracy pressure is sufficient to balance gravity; however the core has actually overshot the equilibrium point and undergoes a slight bounce, creating a shock wave which slams into the collapsing outer layers of the star. A "proto-neutron star" begins to form at the core, though if it is massive enough, it will continue collapsing to form a black hole. The core collapse phase is known to be so dense and energetic that only neutrinos are able to escape the collapsing star. Most of gravitational potential energy of the collapse gets converted to a 10 second neutrino burst, releasing about 10⁴⁶ joules (100 foes). Of this energy, about 10⁴⁴ J (1 foe) is reabsorbed by the star producing an explosion. The energy per particle in a supernova is typically 1 to 150 picojoules (tens to hundreds of MeV). The neutrinos produced by a supernova have been actually observed in the case of Supernova 1987A leading astronomers to conclude that the core collapse picture is basically correct. Several currently operational neutrino detectors have established a Supernova Early Warning System, which will attempt to notify the astronomical community in the event of a supernova in our galaxy. This energy is small enough that the standard model of particle physics is likely to be basically correct, but the high densities may include corrections to the standard model. In particular, earth based accelerators can produce particle interactions which are of much higher energy than are found in supernova, but these experiments involve individual particles interacting with individual particles, and it is likely that the high densities within the supernova will produce novel effects. The interactions between neutrinos and the other particles in the supernova take place with the weak nuclear force which is believed to be well understood. However, the interactions between the protons and neutrons involve the strong nuclear force which is much less well understood. The major unsolved problem with type II supernova is that it is not understood how the burst of neutrinos transfers its energy to the rest of the star producing the shock wave which causes the star to explode. From the above discussion, only one percent of the energy needs to be transferred to produce an explosion, but getting that one percent of transfer has proven very difficult. In the 1990's, one model for doing this involved convective overturn, which suggests that convection, either from neutrinos from below, or infalling matter from above, completes the process of destroying the progenitor star. Heavier elements than iron are formed during this explosion by neutron capture, and from the pressure of the neutrinos pressing into the boundary of the "neutrinosphere", seeding the surrounding space with a cloud of gas and dust which is richer in heavy elements than the one the star originally formed from. Neutrino physics, which is modeled by the standard model, is crucial to the understanding of this process. The other crucial area of investigation is the hydrodynamics of the plasma that makes up the dying star, how it behaves during the core collapse determines when and how the "shock wave" forms and when and how it "stalls" and is re-energized. Computer models have been very successful at calculating the behavior of type II supernova once the shock has been created. By ignoring the first second of the explosion, and assuming that an explosion is started, astrophysicists have been able to make detailed predictions about the elements produced by the supernova and of the expected light curve from the supernova. The remaining core of the star may become a neutron star or a black hole, depending on its mass, although because the processes of supernova collapse are poorly understood, it is unknown what the cutoff mass is. Type II supernovae can be further classified based on the shape of their light curves into Type II-P and Type II-L. Type II-P reach a "plateau" in their light curve while II-Ls have a "linear" decrease in their light curve ("linear" in magnitude versus time, or exponential in luminosity versus time). This is believed to result from differences in the envelope of the stars. II-Ps have a large hydrogen envelope that traps energy released in the form of gamma rays and releases it slowly, while II-Ls are believed to have much smaller envelopes converting less of the gamma ray energy into visible light. One can also sub-divide supernovae of Type II based on their spectra. While most Type II supernova show very broad emission lines which indicate expansion velocities of many thousands of km/s, some have relatively narrow features which may be produced by the interaction of the ejecta with circumstellar material; these are called Type IIn, where the "n" stands for "narrow". A few supernovae, such as SN 1987K and 1993J, appear to change types: they show lines of hydrogen at early times, but, over a period of weeks to months, become dominated by lines of helium. The term "Type IIb" is used to describe the combination of features normally associated with Types II and Ib. These are likely massive stars which have lost most, but not all, of their hydrogen envelopes. As the ejecta expand, the hydrogen layer quickly becomes optically thin and reveals the deeper layers. There has been some speculation that some exceptionally large stars may instead produce a "hypernova" when they die. In the proposed hypernova mechanism, the core of a very massive star collapses directly into a black hole and two extremely energetic jets of plasma are emitted from its rotational poles at nearly light speed. These jets emit intense gamma rays, and are one of many candidate explanations for gamma ray bursts.

Naming of supernovae

Supernova discoveries are reported to the International Astronomical Union's [http://cfa-www.harvard.edu/iau/cbat.html Central Bureau for Astronomical Telegrams], which sends out a circular with the name it assigns to it. The name is formed by the year of discovery, and a one- or two-letter designation. The first 26 supernovae of the year get a letter from A to Z. After Z, they start with aa, ab, and so on.

Notable supernovae

International Astronomical Union Be aware that the years listed are only the years in which the supernovae were first observed on Earth. The supernovae themselves are at distances hundreds or thousands of light years from Earth, varying how long it took for the light of each supernova to reach it.
- 1006 – SN 1006 – Extremely bright supernova; accounts found in Egypt, Iraq, Italy, Switzerland, China, Japan, and possibly France and Syria
- 1054 – SN 1054 – the formation of the Crab Nebula, recorded by Chinese astronomers and possibly by Native Americans
- 1181 – SN 1181 – Recorded by Chinese and Japanese astronomers, supernova in Cassiopeia most likely left as its remnant the strange star 3C 58.
- 1572 – SN 1572 – Supernova in Cassiopeia, observed by Tycho Brahe, whose book De Nova Stella on the subject gives us the word "nova"
- 1604 – SN 1604 – Supernova in Ophiuchus, observed by Johannes Kepler; latest supernova to be observed in the Milky Way
- 1885 – S Andromedae in the Andromeda Galaxy, discovered by Ernst Hartwig
- 1987 – Supernova 1987A in the Large Magellanic Cloud, observed within hours of its start, it was the first opportunity for modern theories of supernova formation to be tested against observations.
- – Cassiopeia A – Supernova in Cassiopeia, not observed on Earth, but estimated to be ~300 years old. Is the brightest remnant in the radio band. The 1604 supernova was used by Galileo as evidence against the Aristotelian dogma of his period, that the heavens never changed. Supernovae often leave behind supernova remnants; the study of these objects has helped to increase our knowledge of supernovae.

Role of supernovae in stellar evolution

Supernovae tend to enrich the surrounding interstellar medium with metals (for astronomers, metals are all the elements after helium). Thus, each stellar generation has a slightly different composition, going from an almost pure mixture of hydrogen and helium to a more metal-rich composition. The different chemical abundances have important influences on the star's life, and may decisively influence the possibility of having planets orbiting it.

Possible threats to Earth

Several prominent stars within a few hundred light years from the Sun are candidates for becoming supernovas in a relatively short time, perhaps as little as 1000 years. Speculations as to the effects of a nearby supernova often focus on these large stars, such as Betelgeuse, a red supergiant at a distance of about 400 light years. This [http://www.tass-survey.org/richmond/answers/snrisks.txt document] contains estimates of the effects from the emissions of the different types of supernovas. Of interest is the conclusion that Type Ia supernovas are the most potentially dangerous, if they occur close enough to us. Since these supernovas are the result of accretion onto relatively dim, common, white dwarf stars, it is likely that a supernova that could affect the Earth will occur unpredictably, and take place in a star system that is not well studied. The predictable supernovas, such as Betelgeuse, while spectacular, will have little effect on Earth. The above referenced document estimates that a Type Ia supernova would have to be closer than 1000 parsec (3300 light years) to affect the Earth. There are likely to be many Type Ia candidates within this distance. However the typical rate for Type Ia supernovas in a galaxy is about 1 per 1000 years[http://snfactory.lbl.gov/snf-about.html], and therefore the probability of one occurring within 1000 parsecs of Earth, given that the Milky Way is about 30,000 parsecs in diameter and 1000 parsecs thick, is probably less than 1 per 1 million years. The probability of a Type Ia within 100 parsecs is about 1 per billion years or less. Thus it is likely that a nearby (100 to 1000 parsecs) Type Ia has occurred several times within the history of life on Earth (about 500 million years) but is unlikely to occur anytime within the lifespan of our species. A recent [http://xxx.lanl.gov/abs/astro-ph/0211361 article] estimates that a Type II supernova would have to be closer than 8 parsecs (26 light years) to destroy half of the Earth's protective ozone layer. The article was mostly concerned with atmospheric modelling and considered only the known radiation flux from SN 1987A, a Type II supernova in the Large Magellanic Cloud.

External links

- [http://news.bbc.co.uk/2/hi/science/nature/3981619.stm Supernova produces cosmic rays]
- [http://cfa-www.harvard.edu/iau/lists/Supernovae.html List of recent supernovae]
- [http://www.rochesterastronomy.org/snimages/ List of bright supernovae, including finding charts]
- The [http://snews.bnl.gov SNEWS] project (SuperNova Early Warning System) uses neutrino detectors to build a network that will (hopefully) provide advance notice of a supernova explosion
- A [http://stacks.iop.org/1367-2630/6/114 review article] on SNEWS
- A technical [http://xxx.lanl.gov/abs/astro-ph/0006305 review] article on Type Ia supernovae
- A [http://www.arxiv.org/abs/astro-ph/0212054 Science] article on a mechanism of explosion of Type Ia supernovae
- Another good [http://arxiv.org/PS_cache/hep-ph/pdf/0306/0306056.pdf review] of supernova events.
- An [http://arxiv.org/abs/hep-ph/9901300 article] on the connection between Supernovae and neutrinos.
- A mpeg [http://anon.nasa-global.speedera.net/anon.nasa-global/kepler_snr/supernova.mpg animation] of a supernova explosion.
- The Nearby Supernova Factory [http://snfactory.lbl.gov] attempts to find and catalog Type Ia supernovas in nearby galaxies to better understand the phenomenon, which is of critical importance in understanding the age of the Universe, the distances to other galaxies, and the exact nature of the expansion of the Universe. Obviously this project is likely to be the first to detect a Type Ia in our own galaxy. Category:Astronomical events Category:Astrophysics Category:Stellar phenomena Category:Stellar evolution Category:Space plasmas ko:초신성 ja:超新星 th:ซูเปอร์โนวา