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Interstellar

Interstellar

In astronomy, the interstellar medium (or ISM) is the matter and energy content that exists between the stars (or their immediate circumstellar environment) within a galaxy. The ISM plays a crucial role in astrophysics precisely because of its intermediate role between stellar and galactic scales. Stars themselves form within cold regions of the ISM, and replenish the ISM with matter and energy through stellar winds and supernovae. In turn, this interplay between stars and the ISM sets the rate at which a galaxy depletes its gaseous content and therefore determines its lifespan of active star formation. The ISM consists of an extremely dilute (by terrestrial standards) plasma, consisting of a mixture of atoms, molecules, dust, electromagnetic radiation, cosmic rays, and the magnetic field. The matter normally consists of about 99% gas particles and usually 1% dust. It fills interstellar space. This mixture is usually extremely tenuous, with typical gas densities ranging from a few single to a few hundred particles per cubic centimeter. As a result of primordial nucleosynthesis, the gas is roughly 90% hydrogen and 10% helium, with additional elements ("metals" in astronomical parlance) present in trace amounts. The medium is also responsible for extinction, namely the decreasing light intensity of a star as the light travels through the medium. This extinction is caused by refraction and absorption of photons in certain wavelengths. For example, a typical absorption wavelength of atomic hydrogen lies at about 121.5 nanometers, the Lyman-alpha transition. Therefore, it is nearly impossible to see light emitted at that wavelength from a star, because most of it is absorbed during the trip to Earth by Lyman-alpha absorption. The interstellar medium is usually divided into three phases, depending on the temperature of the gas: hot (millions of kelvins), warm (thousands of kelvins), and cold (tens of kelvins). This "three-phase" model of the ISM was initially developed by McKee and Ostriker in a 1977 paper, which has formed the basis for further study over the past quarter-century. The relative proportions of the phases are still a matter of considerable contention in scientific circles. Features prominent in the study of the interstellar medium include molecular clouds, interstellar clouds, supernova remnants, planetary nebulae, and similar diffuse structures.

History

Originally, astronomers thought that space was an empty vacuum. In 1913, Norwegian explorer and physicist Kristian Birkeland may have been the first to predict that space is not only a plasma, but also contains "dark matter". He wrote: "It seems to be a natural consequence of our points of view to assume that the whole of space is filled with electrons and flying electric ions of all kinds. We have assumed that each stellar system in evolutions throws off electric corpuscles into space. It does not seem unreasonable therefore to think that the greater part of the material masses in the universe is found, not in the solar systems or nebulae, but in "empty" space. (See "Polar Magnetic Phenomena and Terrella Experiments", in The Norwegian Aurora Polaris Expedition 1902-1903 (publ. 1913, p.720).

Matter

Matter is commonly referred to as the substance of which physical objects are composed. In physics, it is everything that is constituted of elementary fermions. Philosophically, matter constitutes the formless substratum of all things, which exists only potentially and from which reality is produced. In the sense of content, matter is also used in contrast to form.

Matter in physics

Matter occupies space and has mass. It is composed predominantly of atoms, which consist of protons, neutrons, and electrons. All gauge bosons (of which the photon is one), which mediate the four fundamental forces, are not considered matter, even though they certainly have energy and some also mass. Matter thus consists of quarks and leptons. There are six types of quarks (strange, charm, top, bottom, up, and down) which combine to form hadrons, primarily baryons and mesons, through the strong interaction and are actually thought to always be confined. Among the baryons are the proton and the neutron, which further combine to form the nuclei of all elements of the periodic table. Usually these nuclei are surrounded by a cloud of electrons. A nucleus with as many electrons as protons, which is thus electrically neutral, is called an atom, otherwise it is an ion. Chemistry is the science that studies how nuclei and electrons combine to form compounds. In bulk, matter can exist in several different phases, according to particle density and energy density or alternatively pressure and temperature. These phases include gases, plasmas, liquids, fluids, superfluids, solids, and Bose-Einstein condensate. As circumstances change, matter may change from one phase into another. These phenomena are called phase transitions, and their energetics are studied in the field of thermodynamics. In small quantities, matter can exhibit properties that are entirely different from those of bulk material. Homogeneous matter has a definite composition and properties and any amount of the matter has the same composition and properties. Homogenous matter may or may not be a mixture. Iron and brass would examples of each. Heterogeneous matter does not have a definite composition, for example, granite. Matter constitutes the observable Universe. It can be converted to energy (see annihilation), and vice versa - can be created out of energy (see matter creation) and undergo other formations and alterations.

Energy

Energy is a measure of being able to do work. This is a fundamental concept pertaining to the ability for action. In physics, it is a quantity that every physical system possesses. This quantity is not absolute but relative to a state of the system known as its reference state or reference level. The energy of a physical system is defined as the amount of mechanical work that the system can produce if it changes its state to its reference state; for example if a liter of water cools down to 0°C or if a car hits a tree and decelerates from 120 km/h to 0 km/h. Energy of an object can be in several forms, potential—due to the position of the object relative to other objects; kinetic—energy because of its motion; chemical—due to chemical bonds between atoms that make up the substance; electrical—due to its charge; thermal—due to its heat; and nuclear—due to the instability of the nuclei of its atoms. In the case where the "object" is an electromagnetic wave or light, then radiant energy can also be defined. One form of energy can be readily transformed into another; for instance, a battery converts chemical energy into electrical energy, which can be converted into thermal energy. Similarly, potential energy is converted into kinetic energy of moving water and turbine in a dam, which in turn transforms into electric energy by generator. The law of conservation of energy states that in a closed system the total amount of energy, corresponding to the sum of a system's constituent energy components, remains constant. This law follows from translational symmetry of time (that is, independence of any physical process on the moment it started). Some works (thus some forms of energy) are not easily measured by the unaided observer. The term 'energy' is also used in a spiritual or non-scientific way that cannot be quantified, to make certain prepositions look like they are more plausible, by imitating the scientific terminology. Usually this has something to do with mystical and/or healing type references such as acupuncture and reiki.

Forms of Energy

Below is a list of different energy forms. Lotka (1956, p. 5) asked an interesting question about what defines an energy form. :"We are equipped with two separate and distinct senses, the one responding to electromagnetic waves ranging from about 4×10^-4 to 8×10^-4 mm., light waves; the other to somewhat longer waves otherwise of the same character, heat waves. Accordingly we have two separate terms in our language light and heat, to denote two phenomena which, objectively considered, are not separated by any line of division, but merge into one another by gradual transition. Here the question might be raised whether an electromagnetic wave of a length of 9×10^-4 mm. is a light wave or a heat wave." That is to ask, if all forms of energy are defined in terms of infinitesimal increments of the wave spectrum, what makes one form of energy different to another?
- Kinetic energy: the energy of moving objects
- Thermal energy: the energy associated with heat
- Sound energy: the energy of compression waves
- Electrical energy: the energy of moving charged particles
- Potential Energy: the energy that an object has due to position; also known as stored energy
- Chemical energy: the stored energy of chemical substances
- Nuclear energy: the stored energy of the atomic nucleus
- Radiant energy: the energy of electromagnetic waves, including light

Units

SI

The SI unit for both energy and work is the joule (J), named in honour of James Prescott Joule and his experiments on the mechanical equivalent of heat. In slightly more fundamental terms, 1 joule is equal to 1 newton-metre and, in terms of SI base units:

1\ \mathrm = 1\ \mathrm \left( \frac \right ) ^ 2 = 1\ \frac

An energy unit that is used in particle physics is the electronvolt (eV). One eV is equivalent to 1.60217653×10⁻¹⁹ J. In spectroscopy the unit cm^-1 = 0.0001239 eV is used to represent energy since energy is inversely proportional to wavelength from the equation

E = h \nu = h c/\lambda

. (Note that torque, which is typically expressed in newton-metres, has the same dimension and this is not a simple coincidence: a torque of 1 newton-metre applied on 1 radian requires exactly 1 newton-metre=joule of energy.)

Other units of energy

In cgs units, one erg is 1 g cm² s⁻², equal to 1.0×10⁻⁷ J. Another obsolete metric unit is the litre-atmosphere (101.325 J). The imperial/US units for both energy and work include the foot-pound force (1.3558 J), the British thermal unit (Btu) which has various values in the region of 1055 J, and the horsepower-hour (2.6845 MJ). The energy unit used for everyday electricity, particularly for utility bills, is the kilowatt-hour (kW h), and one kW h is equivalent to 3.6×10⁶ J (3600 kJ or 3.6 MJ; the metric units usually are self-consistent, and this particular one may seem arbitrary; it's not, the metric measurement for time is the second, and there are 3,600 seconds in an hour -- in other words, 1 kW second = 1 kJ, but the kW h is a more convenient unit for everyday use). The calorie is mainly used in nutrition and equals the amount of heat necessary to raise the temperature of one kilogram of water by 1 degree Celsius, at a pressure of 1 atm. This amount of heat depends somewhat on the initial temperature of the water, which results in various different units sharing the name of "calorie" but having slightly different energy values. It is equal to 4.1868 kJ. The calories used for food energy in nutrition are the large calories based on the kilogram rather than the gram, often identified as food calories. These are sometimes called kilocalories with that calorie being the small calorie based on the gram, and as a result the prefixes are generally avoided for the large calories (i.e., 1 kcal is 4.184 kJ, never 4.184 MJ, even if "calories" are also used for the other, larger unit in the same document or the same nutrition label). Food calories are sometimes noted as Calories (1000 calories) or simply abbreviated Cal with the capital C, but that convention is more often found in chemistry or physics textbooks—which do not use these large calories—than it is in real-world applications by those who do use these calories. (This convention is also, of course, useless when the word calorie appears in a location where it would ordinarily be capitalized, as at the beginning of a sentence or in the first column of a nutrition label as a substitute for the quantity being measured, which is energy, when all the other quantities such as "Iron" and "Sugars" are also capitalized.)

Transfer of energy

Work

Main article: mechanical work. Work is a defined as a path integral of force F over distance s: :

W = \int \mathbf \cdot \mathrm\mathbf

The equation above says that the work (

W

) is equal to the integral of the dot product of the force (

\mathbf

) on a body and the infinitesimal of the body's position (

\mathbf

Heat

Main article: Heat. Heat is the common name for thermal energy of an object that is due to the motion of the atoms and molecules that constitute the object. This motion can be translational (motion of molecules or atoms as a whole; vibrational - relative motion of atoms within molecules or rotational motion. It is the form of energy which is usually linked with a change in temperature or in a change in phase of matter. In chemistry, heat is the amount of energy which is absorbed or released when atoms are rearranged between various molecules by a chemical reaction. The relationship between heat and energy is similar to that between work and energy. Heat flows from areas of high temperature to areas of low temperature. All objects (matter) have a certain amount of internal energy that is related to the random motion of their atoms or molecules. This internal energy is directly proportional to the temperature of the object. When two bodies of different temperature come in to thermal contact, they will exchange internal energy until the temperature is equalised. The amount of energy transferred is the amount of heat exchanged. It is a common misconception to confuse heat with internal energy, but there is a difference: the change of the internal energy is the heat that flows from the surroundings into the system plus the work performed by the surroundings on the system. Heat Energy is transferred in three different ways: conduction, convection and/or radiation.

Conservation of energy

The first law of thermodynamics says that the total inflow of energy into a system must equal the total outflow of energy from the system, plus the change in the energy contained within the system. This law is used in all branches of physics, but frequently violated by quantum mechanics (see off shell). Noether's theorem relates the conservation of energy to the time invariance of physical laws. An example of the conversion and conservation of energy is a pendulum. At its highest points the kinetic energy is zero and the potential gravitational energy is at its maximum. At its lowest point the kinetic energy is at its maximum and is equal to the decrease of potential energy. If one unrealistically assumes that there is no friction, the energy will be conserved and the pendulum will continue swinging forever. (In practice, available energy is never perfectly conserved when a system changes state; otherwise, the creation of perpetual motion machines would be possible.) Another example is a chemical explosion in which potential chemical energy is converted to kinetic energy and heat in a very short time.

Types of energy

All forms of energy: thermal, chemical, electrical, radiant, nuclear etc. can be in fact reduced to kinetic energy or potential energy. For example thermal energy is essentially kinetic energy of atoms and molecules; chemical energy can be visualized to be the potential energy of atoms within molecules; electrical energy can be visualized to be the potential and kinetic energy of electrons; similarly nuclear energy is the potential energy of nucleons in atomic nucleii.

Kinetic energy

Main article: Kinetic energy. Kinetic energy is the portion of energy related to the motion. :

E_k = \int \mathbf \cdot \mathrm\mathbf

The equation above says that the kinetic energy (

E_k

) is equal to the integral of the dot product of the velocity (

\mathbf

) of a body and the infinitesimal of the body's momentum (

\mathbf

). For non-relativistic velocities, that is velocities much smaller than the speed of light, we can use the Newtonian approximation :

E_k = \begin \frac \end mv^2

where E_k is kinetic energy m is mass of the body v is velocity of the body At near-light velocities, we use the correct relativistic formula: :

E_k = m c^2 (\gamma - 1) = \gamma m c^2 - m c^2 \;\!

\gamma = \frac

where v is the velocity of the body m is its rest mass c is the speed of light in a vacuum, which is approximately 300,000 kilometers per second

\gamma m c^2 \,

is the total energy of the body

m c^2 \,

is again the rest mass energy. See also, E=mc². In the form of a Taylor series, the relativistic formula for can be written as: :

E_k = \frac mv^2 - \frac \frac  + \cdots

Hence, the second and higher terms in the series correspond with the "inaccuracy" of the Newtonian approximation for kinetic energy in relation to the relativistic formula. However, the phrase "conservation of energy" is often confusing to a non scientist. This is so, because of the common usage of the terms "save energy" or conserve energy" used in campaigns for conservation of energy resources like electricity or fossil fuels.

Potential energy

Main article: Potential energy. In contrast to kinetic energy, which is the energy of a system due to its motion, or the internal motion of its particles, the potential energy of a system is the energy associated with the spatial configuration of its components and their interaction with each other. Any number of particles which exert forces on each other automatically constitute a system with potential energy. Such forces, for example, may arise from electrostatic interaction (see Coulomb's law), or gravity. In an isolated system consisting of two stationary objects that exert a force

f(x)

on each other and lay on the x-axis, their potential energy is most generally defined as :

E_p = -\int f(x) \, dx

where the force between the objects varies only with distance

x

and is integrated along the line connecting the two objects. To further illustrate the relationship between force and potential energy, consider the same system of two objects situated along the x-axis. If the potential energy due to one of the objects at any point

x

U(x)

, then the force on the that object

x

is :

f(x) = -\frac

This mathematical relationship demonstrates the direct connection between force and potential energy: the force between two objects is in the direction of decreasing potential energy, and the magnitude of the force is proportional to the extent to which potential energy decreases. A large force is associated with a large decrease in potential energy, while a small force is associated with a small decrease in potential energy. Notice how, in this case, the force on an object depends entirely on its potential energy. These two relationships – the definition of potential energy based on force, and the dependence of force on potential energy – show how the concepts of force and potential energy are intimately linked: if two objects do not exert forces on each other, there is no potential energy between them. If two objects do exert forces on each other, then potential energy naturally arises in the system as part of the system's total energy. Since potential energy arises from forces, any change in the system's spatial configuration will either increase or decrease the system's potential energy as the objects are repositioned. When a system moves to a lower potential energy state, energy is either released in some form or converted into another form of energy, such as kinetic energy. The potential energy can be "stored" as gravitational energy, elastic energy, chemical energy, rest mass energy or electrical energy, but arises in all cases from the spatial positioning and interaction of objects within a system. Unlike kinetic energy, which exists in any moving body, potential energy exists in any body which is interacting with another object. For example a mass released above the Earth initially has potential energy resulting from the gravitational attraction of the Earth, which is transferred to kinetic energy as the gravitational force acts on the object and its potential energy is decreased as it falls. Equation: :

E_p = mgh \;

where m is the mass, h is the height and g is the value of acceleration due to gravity at the Earth's surface (see gee).

Internal energy

Main article: Internal energy. Internal energy is the kinetic energy associated with the motion of molecules, and the potential energy associated with the rotational, vibrational and electric energy of atoms within molecules. Internal energy, like energy, is a quantifiable state function of a system.

History

In the past, energy was discussed in terms of easily observable effects it has on the properties of objects or changes in state of various systems. Basically, if something changed, some sort of energy was involved in that change. As it was realized that energy could be stored in objects, the concept of energy came to embrace the idea of the potential for change as well as change itself. Such effects (both potential and realized) come in many different forms; examples are the electrical energy stored in a battery, the chemical energy stored in a piece of food, the thermal energy of a water heater, or the kinetic energy of a moving train. To simply say energy is "change or the potential for change", however, misses many important examples of energy as it exists in the physical world. The concept of energy and work are relatively new additions to the physicist’s toolbox. Neither Galileo nor Newton made any contributions to the theoretical model of energy, and it was not until the middle of the 19th century that these concepts were introduced. The development of steam engines required engineers to develop concepts and formulas that would allow them to describe the mechanical and thermal efficiencies of their systems. Engineers such as Sadi Carnot and James Prescott Joule, mathematicians such as Émile Claperyon and Hermann von Helmholtz , and amateurs such as Julius Robert von Mayer all contibuted to the notions that the ability to perform certain tasks, called work, was somehow related to the amount of energy in the system. The nature of energy was elusive, however, and it was argued for some years whether energy was a substance (the caloric) or merely a physical quantity, such as momentum. William Thomson (Lord Kelvin) amalgamated all of these laws into his laws of thermodynamics, which aided in the rapid development of energetic descriptions of chemical processes by Rudolf Clausius, Josiah Willard Gibbs, Walther Nernst. In addition, this allowed Ludwig Boltzmann to describe entropy in mathematical terms, and to discuss, along with Jožef Stefan, the laws of radiant energy. For further information, see the Timeline of thermodynamics.

Energy and Economy

Main articles: energy development, energy policy The way in which humans use energy is one of the defining characteristics of an economy. The progression from animal power to steam power, then the internal combustion engine and electricity, are key elements in the development of modern civilization. Future energy development, for example of renewable energy, may be key to avoiding the effects of global warming.

Notes

This definition is one of the most common; e.g. [http://observe.arc.nasa.gov/nasa/space/stellardeath/stellardeath_6.html Glossary at the NASA homepage]

External links

- [http://www.unitconversion.org/unit_converter/energy.html Online Energy and Work Converter] - convert between various units of energy and work, such as joule, erg, gigawatt-hour, newton meter, calorie, Btu, and so on
- [http://www.unitconversion.org/unit_converter/energy-v.html Interactive Energy and Work Conversion Table] - convert selected unit to all other units of energy and work
- [http://jumk.de/calc/energy.shtml Conversions of energy units]
- [http://www.physicsweb.org/article/world/15/7/2 What does energy really mean? From Physics World]
- [http://www.energy.ca.gov/glossary/ Glossary of Energy Terms]
- [http://www.iea.org International Energy Agency IEA - OECD] Category:Introductory physics Category:Fundamental physics concepts Category:Physical quantity ko:에너지 ms:Tenaga ja:エネルギー simple:Energy th:พลังงาน

Galaxy

:This article is about celestial bodies. For alternate meanings, see galaxy (disambiguation). galaxy (disambiguation), is about 56,000 light years in diameter and approximately 60 million light years distant.]] A galaxy is a vast gravitationally bound system of stars, interstellar gas and dust, plasma, and (possibly) unseen dark matter. Typical galaxies contain 10 million to one trillion (10⁷ to 10¹²) stars, all orbiting a common center of gravity. In addition to single stars and a tenuous interstellar medium, most galaxies contain a large number of multiple star systems and star clusters as well as various types of nebulae. Most galaxies are several thousand to several hundred thousand light years in diameter and are usually separated from one another by distances on the order of millions of light years. Although so-called dark matter and dark energy appear to account for well over 90% of the mass of most galaxies, the nature of these unseen components is not well understood. There is some evidence that supermassive black holes may exist at the center of many, if not all, galaxies. Intergalactic space, the space between galaxies, is filled with a tenuous plasma with an average density less than one atom per cubic meter. There are probably more than 10¹¹ galaxies in the visible universe.

Types of galaxies

Galaxies come in three main types: ellipticals, spirals, and irregulars. A slightly more extensive description of galaxy types based on their appearance is given by the Hubble sequence. While the Hubble sequence does encompass all galaxies, it is entirely based upon visual morphological type. Hence, it may miss certain important characteristics of galaxies such as star formation rate. Our own galaxy, the Milky Way, sometimes simply called the Galaxy (with uppercase), is a large disk-shaped barred spiral galaxy about 30 kiloparsecs or 100,000 light years in diameter and 3,000 light years in thickness. It contains about 3×10¹¹ stars and has a total mass of about 6×10¹¹ times the mass of the Sun. In spiral galaxies, the spiral arms have the shape of approximate logarithmic spirals, a pattern that can be theoretically shown to result from a disturbance in a uniformly rotating mass of stars. Like the stars, the spiral arms also rotate around the center, but they do so with constant angular velocity. That means that stars pass in and out of spiral arms. The spiral arms are thought to be areas of high density or density waves. As stars move into an arm, they slow down, thus creating a higher density; this is akin to a "wave" of slowdowns moving along a highway full of moving cars. The arms are visible because the high density facilitates star formation and they therefore harbor many bright and young stars. A new set of galaxies, classified as Ultra Compact Dwarf Galaxies, were discovered in 2003 by Michael Drinkwater of the University of Queensland.

Larger scale structures

Only a few galaxies exist by themselves; these are known as field galaxies. Most galaxies are gravitationally bound to a number of other galaxies. Structures containing up to about 50 galaxies are called groups of galaxies, and larger structures containing many thousands of galaxies packed into an area a few megaparsecs across are called clusters. Clusters of galaxies are often dominated by a single giant elliptical galaxy, which over time tidally destroys its satellite galaxies and adds their mass to its own. Superclusters are giant collections containing tens of thousands of galaxies, found in clusters, groups and sometimes individually; at the supercluster scale, galaxies are arranged into sheets and filaments surrounding vast empty voids. Above this scale, the universe appears to be isotropic and homogeneous. Our galaxy is a member of the Local Group, which it dominates together with the Andromeda Galaxy; overall the Local Group contains about 30 galaxies in a space about one megaparsec across. The Local Group is part of the Virgo Supercluster, which is dominated by the Virgo Cluster (of which our Galaxy is not a member).

History

This account of the history of the investigation of our own and other galaxies is largely taken from [1]. In 1610, Galileo Galilei used a telescope to study the bright band on the night sky known as the Milky Way and discovered that it was composed of a huge number of faint stars. In a treatise in 1755, Immanuel Kant, drawing on earlier work by Thomas Wright, speculated (correctly) that the galaxy might be a rotating body of a huge number of stars, held together by gravitational forces akin to the solar system but on much larger scales. The resulting disk of stars would be seen as a band on the sky from our perspective inside the disk. Kant also conjectured that some of the nebulae visible in the night sky might be separate galaxies. Towards the end of the 18th century, Charles Messier compiled a catalog containing the 109 brightest nebulae, later followed by a catalog of 5000 nebulae assembled by William Herschel. In 1845, Lord Rosse constructed a new telescope and was able to distinguish between elliptical and spiral nebulae. He also managed to make out individual point sources in some of these nebulae, lending credence to Kant's earlier conjecture. However, the nebulae were not universally accepted as distant separate galaxies until the matter was settled by Edwin Hubble in the early 1920s using a new telescope. He was able to resolve the outer parts of some spiral nebulae as collections of individual stars and identified some Cepheid variables, thus allowing him to estimate the distance to the nebulae: they were far too distant to be part of the Milky Way. In 1936, Hubble produced a classification system for galaxies that is used to this day, the Hubble sequence. The first attempt to describe the shape of the Milky Way and the position of the Sun within it was carried out by William Herschel in 1785 by carefully counting the number of stars in different regions of the sky. Using a refined approach, Kapteyn in 1920 arrived at the picture of a small (diameter ~15 kiloparsecs) ellipsoid galaxy with the Sun close to the center. A different method by Harlow Shapley based on the cataloging of globular clusters lead to a radically different picture: a flat disk with diameter ~70 kiloparsecs and the Sun far from the center. Both analyses failed to take into account the absorption of light by interstellar dust present in the galactic plane; once Robert Julius Trumpler had quantified this effect in 1930 by studying open clusters, the present picture of our galaxy as described above emerged. In 1944, Hendrik van de Hulst predicted microwave radiation at a wavelength of 21 cm, resulting from interstellar atomic hydrogen gas; this radiation was observed in 1951. This radiation allowed for much improved study of the Galaxy, since it is not affected by dust absorption and its Doppler shift can be used to map the motion of the gas in the Galaxy. These observations led to the postulation of a rotating bar structure in the center of the Galaxy. With improved radio telescopes, hydrogen gas could also be traced in other galaxies. In the 1970s it was realized that the total visible mass of galaxies (from stars and gas) does not properly account for the speed of the rotating gas, thus leading to the postulation of dark matter. dark matterBeginning in the 1990s, the Hubble Space Telescope yielded improved observations. Among other things, it established that the missing dark matter in our galaxy cannot solely consist of inherently faint and small stars. It photographed the Hubble Deep Field, providing evidence for hundreds of billions of galaxies in existence in the visible universe alone. Many scientists have tried to obtain a good estimate for the number of galaxies in the universe formally. The methods used to achieve such number varies, and therefore, the results are varying too. Also, as new and improved technology becomes available, astronomers can detect fainter objects that were not seen before. These objects that have come into view will in turn change the estimated number of galaxies. In 1999 the Hubble Space Telescope estimated that there were 125 billion galaxies in the universe, and recently with the new camera HST has observed 3000 visible galaxies, which is twice as much as they observed before with the old camera. The term "visible" is emphasized because observations with radio telescopes, infrared cameras, x-ray cameras, etc. would detect other galaxies that are not detected by Hubble. As observations keep on going and astronomers explore more of our universe, the number of galaxies detected will increase. In 2004, the galaxy Abell 1835 IR1916 became the most distant galaxy ever seen by humans.

Etymology

The word galaxy was derived from the Greek term for our own galaxy, kyklos galaktikos meaning "milky circle" for the system’s appearance in the sky. When astronomers speculated that certain objects previously classified as spiral nebulae were actually vast congeries of stars, this was called the "island universe theory"; but this was an obvious misnomer, since universe means everything there is. Consequently, this term fell into disuse, replaced by applying the term galaxy generically to all such bodies.

References

- James Binney: Galactic Astronomy, Princeton University Press, 1998
- Terence Dickinson: The Universe and Beyond (Fourth Edition), Firefly Books Ltd. 2004, 2004

External links

- [http://www.seds.org/messier/galaxy.html Galaxies, SEDS Messier pages]
- [http://www.anzwers.org/free/universe/ An Atlas of The Universe]
- [http://www.nightskyinfo.com/galaxies Galaxies - Information and amateur observations] Category:Astronomical objects Category:Large-scale structure of the cosmos ko:은하 ms:Galaksi ja:銀河 simple:Galaxy th:กาแล็กซี

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

Plasma (physics)

:This article is about plasma in the sense of an ionized gas. For other uses of the term, such as blood plasma, see plasma (disambiguation). plasma (disambiguation) In physics and chemistry, a plasma is an ionized gas, and is usually considered to be a distinct phase of matter. "Ionized" in this case means that at least one electron has been removed from a significant fraction of the molecules. The free electric charges make the plasma electrically conductive so that it couples strongly to electromagnetic fields. This fourth state of matter was first identified by Sir William Crookes in 1879 and dubbed "plasma" by Irving Langmuir in 1928, because it reminded him of a blood plasma [http://www.plasmacoalition.org/what.htm].

Common plasmas

blood plasma Plasmas are the most common phase of matter. The entire visible universe outside the Solar System is plasma, since all we can see are stars. Since the space between the stars is filled with a plasma, although a very sparse one (see interstellar- and intergalactic medium), essentially the entire volume of the universe is plasma. In the Solar System, the planet Jupiter accounts for most of the non-plasma, only about 0.1% of the mass and 10⁻¹⁵ of the volume within the orbit of Pluto. Alfvén also noted that due to their electric charge, very small grains also behave as ions and form part of a plasma (see dusty plasmas). Commonly encountered forms of plasma include:
- Artificially produced
- Inside fluorescent lamps (low energy lighting), neon signs
- Rocket exhaust
- The area in front of a spacecraft's heat shield during reentry into the atmosphere
- Fusion energy research
- The electric arc in an arc lamp or an arc welder
- Plasma ball (sometimes called a plasma sphere or plasma globe)
- Earth plasmas
- Flames (ie. fire)
- Lightning
- The ionosphere
- The polar aurorae
- Space and astrophysical
- The Sun and other stars (which are plasmas heated by nuclear fusion)
- The solar wind
- The Interplanetary medium (the space between the planets)
- The Interstellar medium (the space between star systems)
- The Intergalactic medium (the space between galaxies)
- The Io-Jupiter flux-tube
- Accretion disks
- Interstellar nebulae

Characteristics

The term plasma is generally reserved for a system of charged particles large enough to behave as one. Even a partially ionized gas in which as little as 1% of the particles are ionized can have the characteristics of a plasma (i.e. respond to magnetic fields and be highly electrically conductive). In technical terms, the typical characteristics of a plasma are: # Debye screening lengths that are short compared to the physical size of the plasma. # Large number of particles within a sphere with a radius of the Debye length. # Mean time between collisions usually is long when compared to the period of plasma oscillations.

Plasma scaling

Plasmas and their characteristics exist over a wide range of scales (ie. they are scaleable over many orders of magnitude). The following chart deals only with conventional atomic plasmas and not other exotic phenomena, such as, quark gluon plasmas:

	Typical plasma scaling ranges: orders of magnitude (OOM)
Characteristic	Terrestrial plasmas	Cosmic plasmas
Size in metres (m)	10⁻⁶ m (lab plasmas) to: 10² m (lightning) (~8 OOM)	10⁻⁶ m (spacecraft sheath) to 10²⁵ m (intergalactic nebula) (~31 OOM)
Lifetime in seconds (s)	10⁻¹² s (laser-produced plasma) to: 10⁷ s (fluorescent lights) (~19 OOM)	10¹ s (solar flares) to: 10¹⁷ s (intergalactic plasma) (~17 OOM)
Density in particles per cubic metre	10⁷ to: 10²¹ (inertial confinement plasma)	10³⁰ (stellar core) to: 10⁰ (i.e., 1) (intergalactic medium)
Temperature in kelvins (K)	~0 K (Crystalline non-neutral plasma[http://sdphca.ucsd.edu/]) to: 10⁸ K (magnetic fusion plasma)	10² K (aurora) to: 10⁷ K (Solar core)
Magnetic fields in teslas (T)	10⁻⁴ T (Lab plasma) to: 10³ T (pulsed-power plasma)	10⁻¹² T (intergalactic medium) to: 10⁷ T (Solar core)

Temperatures

plasma scaling characteristic of the gas being excited.]] The defining characteristic of a plasma is ionization. Although ionization can be caused by UV radiation, energetic particles, or strong electric fields, (processes that tend to result in a non-Maxwellian electron distribution function), it is more commonly caused by heating the electrons in such a way that they are close to thermal equilibrium so the electron temperature is relatively well-defined. Because the large mass of the ions relative to the electrons hinders energy transfer, it is possible for the ion temperature to be very different from (usually lower than) the electron temperature. The degree of ionization is determined by the electron temperature relative to the ionization energy (and more weakly by the density) in accordance with the Saha equation. If only a small fraction of the gas molecules are ionized (for example 1%), then the plasma is said to be a cold plasma, even though the electron temperature is typically several thousand degrees. The ion temperature in a cold plasma is often near the ambient temperature. Because the plasmas utilized in plasma technology are typically cold, they are sometimes called technological plasmas. They are often created by using a very high electric field to accelerate electrons, which then ionize the atoms. The electric field is either capacitively or inductively coupled into the gas by means of a plasma source, e.g. microwaves. Common applications of cold plasmas include plasma-enhanced chemical vapor deposition, plasma ion doping, and reactive ion etching. A hot plasma, on the other hand, is nearly fully ionized. This is what would commonly be known as the "fourth-state of matter". The Sun is an example of a hot plasma. The electrons and ions are more likely to have equal temperatures in a hot plasma, but there can still be significant differences.

Densities

Next to the temperature, which is of fundamental importance for the very existence of a plasma, the most important property is the density. The word "plasma density" by itself usually refers to the electron density, that is, the number of free electrons per unit volume. The ion density is related to this by the average charge state

\langle Z\rangle

of the ions through

n_e=\langle Z\rangle n_i

. (See quasineutrality below.) The third important quantity is the density of neutrals

n_0

. In a hot plasma this is small, but may still determine important physics. The degree of ionization is

n_i/(n_0+n_i)

Potentials

reactive ion etching Since plasmas are very good conductors, electric potentials play an important role. The potential as it exists on average in the space between charged particles, independent of the question of how it can be measured, is called the plasma potential or the space potential. If an electrode is inserted into a plasma, its potential will generally lie considerably below the plasma potential due to the development of a Debye sheath. Due to the good electrical conductivity, the electric fields in plasmas tend to be very small, although where double layers are formed, the potential drop can be large enough to accelerate ions to relativistic velocities and produce synchrotron radiation such as x-rays and gamma rays. This results in the important concept of quasineutrality, which says that, on the one hand, it is a very good approximation to assume that the density of negative charges is equal to the density of positive charges (

n_e=\langle Z\rangle n_i

), but that, on the other hand, electric fields can be assumed to exist as needed for the physics at hand. The magnitude of the potentials and electric fields must be determined by means other than simply finding the net charge density. A common example is to assume that the electrons satisfy the Boltzmann relation,

n_e \propto e^

. Differentiating this relation provides a means to calculate the electric field from the density:

\vec = (k_BT_e/e)(\nabla n_e/n_e)

. It is, of course, possible to produce a plasma that is not quasineutral. An electron beam, for example, has only negative charges. The density of a non-neutral plasma must generally be very low, or it must be very small, otherwise it will be dissipated by the repulsive electrostatic force. In astrophysical plasmas, Debye screening prevents electric fields from directly affecting the plasma over large distances (ie. greater than the Debye length). But the existence of charged particles causes the plasma to generate and be affected by magnetic fields. This can and does cause extremely complex behavior, such as the generation of plasma double layers, an object that separates charge over a few tens of Debye lengths. The dynamics of plasmas interacting with external and self-generated magnetic fields are studied in the academic discipline of magnetohydrodynamics.

In contrast to the gas phase

Plasma is often called the fourth state of matter. It is distinct from the three lower-energy phases of matter; solid, liquid, and gas, although it is closely related to the gas phase in that it also has no definite form or volume. There is still some disagreement as to whether a plasma is a distinct state of matter or simply a type of gas. Most physicists consider a plasma to be more than a gas because of a number of distinct properties including the following:

Property	Gas	Plasma
Electrical Conductivity	Very low	Very high For many purposes the electric field in a plasma may be treated as zero, although when current flows the voltage drop, though small, is finite, and density gradients are usually associated with an electric field according to the Boltzmann relation. The possibility of currents couples the plasma strongly to magnetic fields, which are responsible for a large variety of structures such as filaments, sheets, and jets. Collective phenomena are common because the electric and magnetic forces are both long-range and potentially many orders of magnitude stronger than gravitational forces.
Independently acting species	One	Two or three Electrons, ions, and neutrals can be distinguished by the sign of their charge so that they behave independently in many circumstances, having different velocities or even different temperatures, leading to new types of waves and instabilities, among other things
Velocity distribution	Maxwellian	May be non-Maxwellian Whereas collisional interactions always lead to a Maxwellian velocity distribution, electric fields influence the particle velocities differently. The velocity dependence of the Coulomb collision cross section can amplify these differences, resulting in phenomena like two-temperature distributions and run-away electrons.
Interactions	Binary Two-particle collisions are the rule, three-body collisions extremely rare.	Collective Each particle interacts simultaneously with many others. These collective interactions are about ten times more important than binary collisions.

Complex plasma phenomena

Boltzmann relation. Langmuir coined the name plasma because of its similarity to blood plasma, and Hannes Alfvén noted its cellular nature. Note also the filamentary blue outer shell of X-ray emitting high-speed electrons]] Plasma may exhibit complex behaviour. And just as plasma properties scale over many orders of magnitude (see table above), so do these complex features. Many of these features were first studied in the laboratory, and in more recent years, have been applied to, and recognised throughout the universe. Some of these features include:
- Filamentation, the striations or "stringy things" seen in a "plasma ball", the aurora, lightning, electric arcs, and nebulae. They are caused by larger current densities, and are also called magnetic ropes or plasma cables.
- Double layers, localised charge separation regions that have a large potential difference across the layer, and a vanishing electric field on either side. Double layers are found between adjacent plasmas regions with different physical characteristics, and can accelerate ions and produce synchrotron radiation (such as x-rays and gamma rays).
- Birkeland currents, a magnetic-field-aligned electric current, first observed in the Earth's aurora, and also found in plasma filaments.
- Circuits. Birkeland currents imply electric circuits, that follow Kirchhoff's circuit laws. Circuits have a resistance and inductance, and the behaviour of the plasma depends on the entire circuit. Such circuits also store inductive energy, and should the circuit be disrupted, for example, by a plasma instability, the inductive energy will be released in the plasma.
- Cellular structure. Plasma double layers may separate regions with different properties such as magnetization, density, and temperature, resulting in cell-like regions. Examples include the magnetosphere, heliosphere, and heliospheric current sheet.
- Critical ionization velocity in which the relative velocity between an ionized plasma and a neutral gas, may cause further ionization of the gas, resulting in a greater influence of electomagnetic forces.

Ultracold plasmas

It is also possible to create ultracold plasmas, by using lasers to trap and cool neutral atoms to temperatures of 1 mK or lower. Another laser then ionizes the atoms by giving each of the outermost electrons just enough energy to escape the electrical attraction of its parent ion. The key point about ultracold plasmas is that by manipulating the atoms with lasers, the kinetic energy of the liberated electrons can be controlled. Using standard pulsed lasers, the electron energy can be made to correspond to a temperature of as low as 0.1 K a limit set by the frequency bandwidth of the laser pulse. The ions, however, retain the millikelvin temperatures of the neutral atoms. This type of non-equilibrium ultracold plasma evolves rapidly, and many fundamental questions about its behaviour remain unanswered. Experiments conducted so far have revealed surprising dynamics and recombination behaviour that are pushing the limits of our knowledge of plasma physics.

Mathematical descriptions

Plasmas may be usefully described with various levels of detail. However the plasma itself is described, if electric or magnetic fields are present, then Maxwell's equations will be needed to describe them. The coupling of the description of a conductive fluid to electromagnetic fields is known generally as magnetohydrodynamics, or simply MHD.

Fluid

The simplest possibility is to treat the plasma as a single fluid governed by the Navier Stokes Equations. A more general description is the two-fluid picture, where the ions and electrons are considered to be distinct.

Kinetic

For some cases the fluid description is not sufficient. Kinetic models include information on distortions of the velocity distribution functions with respect to a Maxwell-Boltzmann distribution. This may be important when currents flow, when waves are involved, or when gradients are very steep.

Particle-in-cell

Particle-in-cell (PIC) models include kinetic information by following the trajectories of a large number of individual particles. Charge and current densities are determined by summing the particles in cells which are small compared to the problem at hand but still contain many particles. The electric and magnetic fields are found from the charge and current densities with appropriate boundary conditions. PIC codes for plasma applications were developed at Los Alamos National Laboratory in the 1950's. Although often more calculationally intensive than alternative models, they are relatively easy to understand and program and can be very general.

Fundamental plasma parameters

Los Alamos National Laboratory All quantities are in Gaussian cgs units except temperature expressed in eV and ion mass expressed in units of the proton mass

\mu = m_i/m_p

; Z is charge state; k is Boltzmann's constant; K is wavelength; γ is the adiabatic index; ln Λ is the Coulomb logarithm.

Frequencies

- electron gyrofrequency, the angular frequency of the circular motion of an electron in the plane perpendicular to the magnetic field: :

\omega_ = eB/m_ec = 1.76 \times 10^7 B \mbox

- ion gyrofrequency, the angular frequency of the circular motion of an ion in the plane perpendicular to the magnetic field: :

\omega_ = eB/m_ic = 9.58 \times 10^3 Z \mu^ B \mbox

- electron plasma frequency, the frequency with which electrons oscillate when their charge density is not equal to the ion charge density (plasma oscillation): :

\omega_ = (4\pi n_ee^2/m_e)^ = 5.64 \times 10^4 n_e^ \mbox

- ion plasma frequency: :

\omega_ = (4\pi n_iZ^2e^2/m_i)^ = 1.32 \times 10^3 Z \mu^ n_i^ \mbox

- electron trapping rate :

\nu_ = (eKE/m_e)^ = 7.26 \times 10^8 K^ E^ \mbox^

- ion trapping rate :

\nu_ = (ZeKE/m_i)^ = 1.69 \times 10^7 Z^ K^ E^ \mu^ \mbox^

- electron collision rate :

\nu_e = 2.91 \times 10^ n_e\,\ln\Lambda\,T_e^ \mbox^

- ion collision rate :

\nu_i = 4.80 \times 10^ Z^4 \mu^ n_i\,\ln\Lambda\,T_i^ \mbox^

Lengths

plasma oscillation http://history.nasa.gov/SP-345/ch15.htm#250 Ref]]
- Electron thermal de Broglie wavelength, approximate average de Broglie wavelength of electrons in a plasma: :

\Lambda_e= \sqrt= 6.919\times 10^\,T_e^\,\mbox

- classical distance of closest approach, the closest that two particles with the elementary charge come to each other if they approach head-on and each have a velocity typical of the temperature, ignoring quantum-mechanical effects: :

e^2/kT=1.44\times10^\,T^\,\mbox

- electron gyroradius, the radius of the circular motion of an electron in the plane perpendicular to the magnetic field: :

r_e = v_/\omega_ = 2.38\,T_e^B^\,\mbox

- ion gyroradius, the radius of the circular motion of an ion in the plane perpendicular to the magnetic field: :

r_i = v_/\omega_ = 1.02\times10^2\,\mu^Z^T_i^B^\,\mbox

- plasma skin depth, the depth in a plasma to which electromagnetic radiation can penetrate: :

c/\omega_ = 5.31\times10^5\,n_e^\,\mbox

- Debye length, the scale over which electric fields are screened out by a redistribution of the electrons: :

\lambda_D = (kT/4\pi ne^2)^ = 7.43\times10^2\,T^n^\,\mbox

Velocities

- electron thermal velocity, typical velocity of an electron in a Maxwell-Boltzmann distribution: :

v_ = (kT_e/m_e)^ = 4.19\times10^7\,T_e^\,\mbox

- ion thermal velocity, typical velocity of an ion in a Maxwell-Boltzmann distribution: :

v_ = (kT_i/m_i)^ = 9.79\times10^5\,\mu^T_i^\,\mbox

- ion sound velocity, the speed of the longitudinal waves resulting from the mass of the ions and the pressure of the electrons: :

c_s = (\gamma ZkT_e/m_i)^ = 9.79\times10^5\,(\gamma ZT_e/\mu)^\,\mbox

- Alfven velocity, the speed of the waves resulting from the mass of the ions and the restoring force of the magnetic field: :

v_A = B/(4\pi n_im_i)^ = 2.18\times10^\,\mu^n_i^B\,\mbox

Dimensionless

waves meeting the heliopause]]
- square root of electron/proton mass ratio :

(m_e/m_p)^ = 2.33\times10^ = 1/42.9

- number of particles in a Debye sphere :

(4\pi/3)n\lambda_D^3 = 1.72\times10^9\,T^n^

- Alven velocity/speed of light :

v_A/c = 7.28\,\mu^n_i^B

- electron plasma/gyrofrequency ratio :

\omega_/\omega_ = 3.21\times10^\,n_e^B^

- ion plasma/gyrofrequency ratio :

\omega_/\omega_ = 0.137\,\mu^n_i^B^

- thermal/magnetic energy ratio :

\beta = 8\pi nkT/B^2 = 4.03\times10^\,nTB^

- magnetic/ion rest energy ratio :

B^2/8\pi n_im_ic^2 = 26.5\,\mu^n_i^B^2

Miscellaneous

- Bohm diffusion coefficient :

D_B = (ckT/16eB) = 6.25\times10^6\,TB^\,\mbox^2/\mbox

- transverse Spitzer resistivity :

\eta_\perp = 1.15\times10^\,Z\,\ln\Lambda\,T^\,\mbox = 1.03\times10^\,Z\,\ln\Lambda\,T^\,\Omega\,\mbox

Fields of active research

Bohm diffusion is so effective at accelerating ions, that electric fields are used in ion drives]] This is just a partial list of topics. A more complete and organised list can be found on the Web site for Plasma science and technology [http://www.plasmas.com/topics.htm].
- Plasma theory
- Plasma equilibria and stability
- Plasma interactions with waves and beams
- Guiding center
- adiabatic invariant
- Debye sheath
- Coulomb collision
- Plasmas in nature
- The Earth's ionosphere
- Space plasmas, e.g. Earth's plasmasphere (an inner portion of the magnetosphere dense with plasma)
- plasma cosmology
- Plasma sources
- Dusty Plasmas
- Plasma diagnostics
- Thomson scattering
- Langmuir probe
- Spectroscopy
- Interferometry
- Ionospheric heating
- Incoherent scatter radar
- Plasma applications
- Fusion power
- Magnetic fusion energy (MFE) -- tokamak, stellarator, reversed field pinch, magnetic mirror, dense plasma focus
- Inertial fusion energy (IFE) (also Inertial confinement fusion — ICF)
- Plasma-based weaponry
- Industrial plasmas
- plasma chemistry
- plasma processing
- plasma display

External links

- [http://fusedweb.pppl.gov/CPEP/Chart_Pages/5.Plasma4StateMatter.html Plasmas: the Fourth State of Matter]
- [http://www.plasmas.org/ Plasma Science and Technology]
- [http://plasma-gate.weizmann.ac.il/PlasmaI.html Plasma on the Internet] comprehensi

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