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  • neutron star | astronomy | Britannica.com
    ray wavelengths The very short periods of for example the Crab NP 0532 and Vela pulsars 33 and 83 milliseconds respectively rule out the possibility that they might be white dwarfs The pulses result from electrodynamic phenomena generated by their rotation and their strong magnetic fields as in a dynamo In the case of radio pulsars neutrons at the surface of the star decay into protons and electrons As these charged particles are released from the surface they enter the intense magnetic field that surrounds the star and rotates along with it Accelerated to speeds approaching that of light the particles give off electromagnetic radiation by synchrotron emission This radiation is released as intense radio beams from the pulsar s magnetic poles Many binary X ray sources such as Hercules X 1 contain neutron stars Cosmic objects of this kind emit X rays by compression of material from companion stars accreted onto their surfaces Neutron stars are also seen as objects called rotating radio transients RRATs and as magnetars The RRATs are sources that emit single radio bursts but at irregular intervals ranging from four minutes to three hours The cause of the RRAT phenomenon is unknown Magnetars are highly magnetized neutron stars that have a magnetic field of between 10 14 and 10 15 gauss Most investigators believe that neutron stars are formed by supernova explosions in which the collapse of the central core of the supernova is halted by rising neutron pressure as the core density increases to about 10 15 grams per cubic cm If the collapsing core is more massive than about three solar masses however a neutron star cannot be formed and the core would presumably become a black hole Comments Share Email Print Cite Last Updated 4 20 2014 You may also be interested in universe star science The 20 Brightest Stars The 20 Nearest Stars variable star quasar supernova binary star nova giant star dwarf star Keep exploring Stars Fact or Fiction Stars Explosions in Space Constellations Fact or Fiction 7 Scary Surgical Instruments Then and Now Editor Picks My Favorite Frogs of Canada and the United States What made you want to look up neutron star To From Subject Comments Please limit to 900 characters Cancel Britannica Stories Behind The News Philosophy Religion Healing the Schism Pope Meets Patriarch Behind The News Science Gravitational Waves Observed Spotlight History Thomas Malthus s 250th Birthday See More Stories FEATURED QUIZZES Vocabulary Quiz True or False Spell It See More Quizzes About Us About Our Ads Contact Us Privacy Policy Terms of Use 2016 Encyclopædia Britannica Inc MLA style neutron star Encyclopædia Britannica Encyclopædia Britannica Online Encyclopædia Britannica Inc 2016 Web 12 Feb 2016 http www britannica com topic neutron star APA style neutron star 2016 In Encyclopædia Britannica Retrieved from http www britannica com topic neutron star Harvard style neutron star 2016 Encyclopædia Britannica Online Retrieved 12 February 2016 from http www britannica com topic neutron star Chicago Manual of Style Encyclopædia Britannica Online

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  • laser | instrument | Britannica.com
    particular wavelengths and amplifies that light typically producing a very narrow beam of radiation The emission generally covers an extremely limited range of visible infrared or ultraviolet wavelengths Many different types of lasers have been developed with highly varied characteristics Laser is an acronym for light amplification by the stimulated emission of radiation History The laser is an outgrowth of a suggestion made by Albert Einstein in 1916 that under

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  • proton | subatomic particle | Britannica.com
    ionized hydrogen are given high velocities in particle accelerators and are commonly used as projectiles to produce and study nuclear reactions Protons are the chief constituent of primary cosmic rays and are among the products of some types of artificial nuclear reactions Comments Share Email Print Cite Last Updated 4 4 2014 You may also be interested in matter science atom subatomic particle neutron hadron baryon nucleon quark antiproton antineutron physical science Keep exploring Space Objects Fact or Fiction Nature Tip of the Iceberg Quiz Science Quiz 11 Popular Or Just Plain Odd Presidential Pets 8 Birds That Can t Fly What made you want to look up proton To From Subject Comments Please limit to 900 characters Cancel Britannica Stories Behind The News Philosophy Religion Healing the Schism Pope Meets Patriarch Behind The News Science Gravitational Waves Observed Spotlight History Thomas Malthus s 250th Birthday See More Stories FEATURED QUIZZES Vocabulary Quiz True or False Spell It See More Quizzes About Us About Our Ads Contact Us Privacy Policy Terms of Use 2016 Encyclopædia Britannica Inc MLA style proton Encyclopædia Britannica Encyclopædia Britannica Online Encyclopædia Britannica Inc 2016 Web 12 Feb 2016 http www britannica com science proton subatomic particle APA style proton 2016 In Encyclopædia Britannica Retrieved from http www britannica com science proton subatomic particle Harvard style proton 2016 Encyclopædia Britannica Online Retrieved 12 February 2016 from http www britannica com science proton subatomic particle Chicago Manual of Style Encyclopædia Britannica Online s v proton accessed February 12 2016 http www britannica com science proton subatomic particle While every effort has been made to follow citation style rules there may be some discrepancies Please refer to the appropriate style manual or other sources if you have any questions Update Link Click anywhere inside the article to add

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  • Sun | astronomy | Britannica.com
    of the system constituting more than 99 percent of its entire mass The Sun is the source of an enormous amount of energy a portion of which provides Earth with the light and heat necessary to support life The Sun is classified as a G2 V star with G2 standing for the second hottest stars of the yellow G class of surface temperature about 5 800 kelvins K and the

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  • physics | science | Britannica.com
    universe as a whole considered as an isolated system this ultimate chaotic condition has been called the heat death The study of electricity and magnetism Although conceived of as distinct phenomena until the 19th century electricity and magnetism are now known to be components of the unified field of electromagnetism Particles with electric charge interact by an electric force while charged particles in motion produce and respond to magnetic forces as well Many subatomic particles including the electrically charged electron and proton and the electrically neutral neutron behave like elementary magnets On the other hand in spite of systematic searches undertaken no magnetic monopoles which would be the magnetic analogues of electric charges have ever been found The field concept plays a central role in the classical formulation of electromagnetism as well as in many other areas of classical and contemporary physics Einstein s gravitational field for example replaces Newton s concept of gravitational action at a distance The field describing the electric force between a pair of charged particles works in the following manner each particle creates an electric field in the space surrounding it and so also at the position occupied by the other particle each particle responds to the force exerted upon it by the electric field at its own position ultraviolet radiation types of electromagnetic radiation Encyclopædia Britannica Inc Classical electromagnetism is summarized by the laws of action of electric and magnetic fields upon electric charges and upon magnets and by four remarkable equations formulated in the latter part of the 19th century by the Scottish physicist James Clerk Maxwell The latter equations describe the manner in which electric charges and currents produce electric and magnetic fields as well as the manner in which changing magnetic fields produce electric fields and vice versa From these relations Maxwell inferred the existence of electromagnetic waves associated electric and magnetic fields in space detached from the charges that created them traveling at the speed of light and endowed with such mechanical properties as energy momentum and angular momentum The light to which the human eye is sensitive is but one small segment of an electromagnetic spectrum that extends from long wavelength radio waves to short wavelength gamma rays and includes X rays microwaves and infrared or heat radiation Optics diffraction grating Courtesy of Bausch Lomb Rochester N Y Because light consists of electromagnetic waves the propagation of light can be regarded as merely a branch of electromagnetism However it is usually dealt with as a separate subject called optics the part that deals with the tracing of light rays is known as geometrical optics while the part that treats the distinctive wave phenomena of light is called physical optics More recently there has developed a new and vital branch quantum optics which is concerned with the theory and application of the laser a device that produces an intense coherent beam of unidirectional radiation useful for many applications The formation of images by lenses microscopes telescopes and other optical devices is described by ray optics which assumes that the passage of light can be represented by straight lines that is rays The subtler effects attributable to the wave property of visible light however require the explanations of physical optics One basic wave effect is interference whereby two waves present in a region of space combine at certain points to yield an enhanced resultant effect e g the crests of the component waves adding together at the other extreme the two waves can annul each other the crests of one wave filling in the troughs of the other Another wave effect is diffraction which causes light to spread into regions of the geometric shadow and causes the image produced by any optical device to be fuzzy to a degree dependent on the wavelength of the light Optical instruments such as the interferometer and the diffraction grating can be used for measuring the wavelength of light precisely about 500 micrometres and for measuring distances to a small fraction of that length Atomic and chemical physics Millikan oil drop experiment Encyclopædia Britannica Inc One of the great achievements of the 20th century was the establishment of the validity of the atomic hypothesis first proposed in ancient times that matter is made up of relatively few kinds of small identical parts namely atoms However unlike the indivisible atom of Democritus and other ancients the atom as it is conceived today can be separated into constituent electrons and nucleus Atoms combine to form molecules whose structure is studied by chemistry and physical chemistry they also form other types of compounds such as crystals studied in the field of condensed matter physics Such disciplines study the most important attributes of matter not excluding biologic matter that are encountered in normal experience namely those that depend almost entirely on the outer parts of the electronic structure of atoms Only the mass of the atomic nucleus and its charge which is equal to the total charge of the electrons in the neutral atom affect the chemical and physical properties of matter Although there are some analogies between the solar system and the atom due to the fact that the strengths of gravitational and electrostatic forces both fall off as the inverse square of the distance the classical forms of electromagnetism and mechanics fail when applied to tiny rapidly moving atomic constituents Atomic structure is comprehensible only on the basis of quantum mechanics and its finer details require as well the use of quantum electrodynamics QED Atomic properties are inferred mostly by the use of indirect experiments Of greatest importance has been spectroscopy which is concerned with the measurement and interpretation of the electromagnetic radiations either emitted or absorbed by materials These radiations have a distinctive character which quantum mechanics relates quantitatively to the structures that produce and absorb them It is truly remarkable that these structures are in principle and often in practice amenable to precise calculation in terms of a few basic physical constants the mass and charge of the electron the speed of light and Planck s constant approximately 6 62606957 10 34 joule second the fundamental constant of the quantum theory named for the German physicist Max Planck Condensed matter physics transistor AT T Bell Labs Science Photo Library Photo Researchers Inc This field which treats the thermal elastic electrical magnetic and optical properties of solid and liquid substances grew at an explosive rate in the second half of the 20th century and scored numerous important scientific and technical achievements including the transistor Among solid materials the greatest theoretical advances have been in the study of crystalline materials whose simple repetitive geometric arrays of atoms are multiple particle systems that allow treatment by quantum mechanics Because the atoms in a solid are coordinated with each other over large distances the theory must go beyond that appropriate for atoms and molecules Thus conductors such as metals contain some so called free electrons or valence electrons which are responsible for the electrical and most of the thermal conductivity of the material and which belong collectively to the whole solid rather than to individual atoms Semiconductors and insulators either crystalline or amorphous are other materials studied in this field of physics Other aspects of condensed matter involve the properties of the ordinary liquid state of liquid crystals and at temperatures near absolute zero of the so called quantum liquids The latter exhibit a property known as superfluidity completely frictionless flow which is an example of macroscopic quantum phenomena Such phenomena are also exemplified by superconductivity completely resistance less flow of electricity a low temperature property of certain metallic and ceramic materials Besides their significance to technology macroscopic liquid and solid quantum states are important in astrophysical theories of stellar structure in for example neutron stars Nuclear physics particle physics particle tracks from collision of niobium nuclei Courtesy of the Department of Physics and Astronomy Michigan State University This branch of physics deals with the structure of the atomic nucleus and the radiation from unstable nuclei About 10 000 times smaller than the atom the constituent particles of the nucleus protons and neutrons attract one another so strongly by the nuclear forces that nuclear energies are approximately 1 000 000 times larger than typical atomic energies Quantum theory is needed for understanding nuclear structure Like excited atoms unstable radioactive nuclei either naturally occurring or artificially produced can emit electromagnetic radiation The energetic nuclear photons are called gamma rays Radioactive nuclei also emit other particles negative and positive electrons beta rays accompanied by neutrinos and helium nuclei alpha rays A principal research tool of nuclear physics involves the use of beams of particles e g protons or electrons directed as projectiles against nuclear targets Recoiling particles and any resultant nuclear fragments are detected and their directions and energies are analyzed to reveal details of nuclear structure and to learn more about the strong force A much weaker nuclear force the so called weak interaction is responsible for the emission of beta rays Nuclear collision experiments use beams of higher energy particles including those of unstable particles called mesons produced by primary nuclear collisions in accelerators dubbed meson factories Exchange of mesons between protons and neutrons is directly responsible for the strong force For the mechanism underlying mesons see below Fundamental forces and fields In radioactivity and in collisions leading to nuclear breakup the chemical identity of the nuclear target is altered whenever there is a change in the nuclear charge In fission and fusion nuclear reactions in which unstable nuclei are respectively split into smaller nuclei or amalgamated into larger ones the energy release far exceeds that of any chemical reaction Particle physics neutron depiction of protons neutrons pions and other hadrons One of the most significant branches of contemporary physics is the study of the fundamental subatomic constituents of matter the elementary particles This field also called high energy physics emerged in the 1930s out of the developing experimental areas of nuclear and cosmic ray physics Initially investigators studied cosmic rays the very high energy extraterrestrial radiations that fall upon the Earth and interact in the atmosphere see below The methodology of physics However after World War II scientists gradually began using high energy particle accelerators to provide subatomic particles for study Quantum field theory a generalization of QED to other types of force fields is essential for the analysis of high energy physics Subatomic particles cannot be visualized as tiny analogues of ordinary material objects such as billiard balls for they have properties that appear contradictory from the classical viewpoint That is to say while they possess charge spin mass magnetism and other complex characteristics they are nonetheless regarded as pointlike During the latter half of the 20th century a coherent picture evolved of the underlying strata of matter involving two types of subatomic particles fermions baryons and leptons which have odd half integral angular momentum spin 1 2 3 2 and make up ordinary matter and bosons gluons mesons and photons which have integral spins and mediate the fundamental forces of physics Leptons e g electrons muons taus gluons and photons are believed to be truly fundamental particles Baryons e g neutrons protons and mesons e g pions kaons collectively known as hadrons are believed to be formed from indivisible elements known as quarks which have never been isolated Quarks come in six types or flavours and have matching antiparticles known as antiquarks Quarks have charges that are either positive two thirds or negative one third of the electron s charge while antiquarks have the opposite charges Like quarks each lepton has an antiparticle with properties that mirror those of its partner the antiparticle of the negatively charged electron is the positive electron or positron that of the neutrino is the antineutrino In addition to their electric and magnetic properties quarks participate in both the strong force which binds them together and the weak force which underlies certain forms of radioactivity while leptons take part in only the weak force Baryons such as neutrons and protons are formed by combining three quarks thus baryons have a charge of 1 0 or 1 Mesons which are the particles that mediate the strong force inside the atomic nucleus are composed of one quark and one antiquark all known mesons have a charge of 2 1 0 1 or 2 Most of the possible quark combinations or hadrons have very short lifetimes and many of them have never been seen though additional ones have been observed with each new generation of more powerful particle accelerators The quantum fields through which quarks and leptons interact with each other and with themselves consist of particle like objects called quanta from which quantum mechanics derives its name The first known quanta were those of the electromagnetic field they are also called photons because light consists of them A modern unified theory of weak and electromagnetic interactions known as the electroweak theory proposes that the weak force involves the exchange of particles about 100 times as massive as protons These massive quanta have been observed namely two charged particles W and W and a neutral one W 0 In the theory of the strong force known as quantum chromodynamics QCD eight quanta called gluons bind quarks to form baryons and also bind quarks to antiquarks to form mesons the force itself being dubbed the colour force This unusual use of the term colour is a somewhat forced analogue of ordinary colour mixing Quarks are said to come in three colours red blue and green The opposites of these imaginary colours minus red minus blue and minus green are ascribed to antiquarks Only certain colour combinations namely colour neutral or white i e equal mixtures of the above colours cancel out one another resulting in no net colour are conjectured to exist in nature in an observable form The gluons and quarks themselves being coloured are permanently confined deeply bound within the particles of which they are a part while the colour neutral composites such as protons can be directly observed One consequence of colour confinement is that the observable particles are either electrically neutral or have charges that are integral multiples of the charge of the electron A number of specific predictions of QCD have been experimentally tested and found correct Quantum mechanics Although the various branches of physics differ in their experimental methods and theoretical approaches certain general principles apply to all of them The forefront of contemporary advances in physics lies in the submicroscopic regime whether it be in atomic nuclear condensed matter plasma or particle physics or in quantum optics or even in the study of stellar structure All are based upon quantum theory i e quantum mechanics and quantum field theory and relativity which together form the theoretical foundations of modern physics Many physical quantities whose classical counterparts vary continuously over a range of possible values are in quantum theory constrained to have discontinuous or discrete values Furthermore the intrinsically deterministic character of values in classical physics is replaced in quantum theory by intrinsic uncertainty According to quantum theory electromagnetic radiation does not always consist of continuous waves instead it must be viewed under some circumstances as a collection of particle like photons the energy and momentum of each being directly proportional to its frequency or inversely proportional to its wavelength the photons still possessing some wavelike characteristics Conversely electrons and other objects that appear as particles in classical physics are endowed by quantum theory with wavelike properties as well such a particle s quantum wavelength being inversely proportional to its momentum In both instances the proportionality constant is the characteristic quantum of action action being defined as energy time that is to say Planck s constant divided by 2π or ℏ atom Bohr model Encyclopædia Britannica Inc In principle all of atomic and molecular physics including the structure of atoms and their dynamics the periodic table of elements and their chemical behaviour as well as the spectroscopic electrical and other physical properties of atoms molecules and condensed matter can be accounted for by quantum mechanics Roughly speaking the electrons in the atom must fit around the nucleus as some sort of standing wave as given by the Schrödinger equation analogous to the waves on a plucked violin or guitar string As the fit determines the wavelength of the quantum wave it necessarily determines its energy state Consequently atomic systems are restricted to certain discrete or quantized energies When an atom undergoes a discontinuous transition or quantum jump its energy changes abruptly by a sharply defined amount and a photon of that energy is emitted when the energy of the atom decreases or is absorbed in the opposite case Although atomic energies can be sharply defined the positions of the electrons within the atom cannot be quantum mechanics giving only the probability for the electrons to have certain locations This is a consequence of the feature that distinguishes quantum theory from all other approaches to physics the uncertainty principle of the German physicist Werner Heisenberg This principle holds that measuring a particle s position with increasing precision necessarily increases the uncertainty as to the particle s momentum and conversely The ultimate degree of uncertainty is controlled by the magnitude of Planck s constant which is so small as to have no apparent effects except in the world of microstructures In the latter case however because both a particle s position and its velocity or momentum must be known precisely at some instant in order to predict its future history quantum theory precludes such certain prediction and thus escapes determinism Compton effect Encyclopædia Britannica Inc The complementary wave and particle aspects or wave particle duality of electromagnetic radiation and of material particles furnish another illustration of the uncertainty principle When an electron exhibits wavelike behaviour as in the phenomenon of electron diffraction this excludes its exhibiting particle like behaviour in the same observation Similarly when electromagnetic radiation in the form of

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  • astronomy | Britannica.com
    the orbit of Halley s Comet the Eta Aquarid meteor shower occurs Micrometeorites interplanetary dust particle s the smallest meteoroidal particles can be detected from Earth orbiting satellites or collected by specially equipped aircraft flying in the stratosphere and returned for laboratory inspection Since the late 1960s numerous meteorites have been found in the Antarctic on the surface of stranded ice flows see Antarctic meteorite s Detailed analyses have shown that some of these meteorites have come from the Moon and others from Mars Yet others contain microscopic crystals whose isotopic proportions are unique and appear to be dust grains that formed in the atmospheres of different stars Determinations of age and chemical composition The age of the solar system taken to be close to 4 6 billion years has been derived from measurements of radioactivity in meteorites lunar samples and Earth s crust Abundances of isotopes of uranium thorium and rubidium and their decay products lead and strontium are the measured quantities Assessment of the chemical composition of the solar system is based on data from Earth the Moon and meteorites as well as on the spectral analysis of light from the Sun and planets In broad outline the solar system abundances of the chemical elements decrease with increasing atomic weight Hydrogen atoms are by far the most abundant constituting 91 percent helium is next with 8 9 percent and all other types of atoms together amount to only 0 1 percent Theories of origin The origin of Earth the Moon and the solar system as a whole is a problem that has not yet been settled in detail The Sun probably formed by condensation of the central region of a large cloud of gas and dust with the planets and other bodies of the solar system forming soon after their composition strongly influenced by the temperature and pressure gradients in the evolving solar nebula Less volatile materials could condense into solids relatively close to the Sun to form the terrestrial planets The abundant volatile lighter elements could condense only at much greater distances to form the giant gas planets After the early 1990s astronomers confirmed that stars other than the Sun have one or more planetlike objects revolving around them Studies of the properties of these solar systems have both supported and challenged astronomers theoretical models of how Earth s solar system formed See also solar system Origin of the solar system The origin of the planetary satellites is not entirely settled As to the origin of the Moon the opinion of astronomers had long oscillated between theories that saw its origin and condensation simultaneous with formation of Earth and those that posited a separate origin for the Moon and its later capture by Earth s gravitational field Similarities and differences in abundances of the chemical elements and their isotopes on Earth and Moon had challenged each group of theories Finally in the 1980s a model emerged that has gained the support of most lunar scientists that of a large impact on Earth with the expulsion of material that subsequently formed the Moon See Moon Origin and evolution For the outer planets with their multiple satellites many very small and quite unlike one another the picture is less clear Some of these moons have relatively smooth icy surfaces whereas others are heavily cratered at least one Jupiter s Io is volcanic Some of the moons may have formed along with their parent planets and others may have formed elsewhere and been captured Study of the stars Measuring observable stellar properties Pleiades Courtesy of Palomar Observatory California Institute of Technology The measurable quantities in stellar astrophysics include the externally observable features of the star s distance temperature radiation spectrum and luminosity composition of the outer layers diameter mass and variability in any of these Theoretical astrophysicists use these observations to model the structure of stars and to devise theories for their formation and evolution Positional information can be used for dynamical analysis which yields estimates of stellar masses In a system dating back at least to the Greek astronomer mathematician Hipparchus in the 2nd century bce apparent stellar brightness m is measured in magnitude s Magnitudes are now defined such that a first magnitude star is 100 times brighter than a star of sixth magnitude The human eye cannot see stars fainter than about sixth magnitude but modern instruments used with large telescopes can record stars as faint as about 30th magnitude By convention the absolute magnitude M is defined as the magnitude that a star would appear to have if it were located at a standard distance of 10 parsecs These quantities are related through the expression m M 5 log 10 r 5 in which r is the star s distance in parsecs The magnitude scale is anchored on a group of standard stars An absolute measure of radiant power is luminosity usually expressed in ergs per second ergs sec Sometimes the luminosity is stated in terms of the solar luminosity 3 86 10 33 ergs sec Luminosity can be calculated when m and r are known Correction might be necessary for the interstellar absorption of starlight There are several methods for measuring a star s diameter From the brightness and distance the luminosity L can be calculated and from observations of the brightness at different wavelengths the temperature T can be calculated Because the radiation from many stars can be well approximated by a Planck blackbody spectrum see Planck s radiation law these measured quantities can be related through the expression L 4π R 2 σ T 4 thus providing a means of calculating R the star s radius In this expression σ is the Stefan Boltzmann constant 5 67 10 5 ergs cm 2 K 4 sec in which K is the temperature in kelvins The radius R refers to the star s photosphere the region where the star becomes effectively opaque to outside observation Stellar angular diameters can be measured through interference effects Alternatively the intensity of the starlight can be monitored during occultation by the Moon which produces diffraction fringes whose pattern depends on the angular diameter of the star Stellar angular diameters of several milliarcseconds can be measured but so far only for relatively bright and close stars Many stars occur in binary systems see binary star with the two partners in orbits around their mutual centre of mass Such a system provides the best measurement of stellar masses The period P of a binary system is related to the masses of the two stars m 1 and m 2 and the orbital semimajor axis mean radius a via Kepler s third law P 2 4π 2 a 3 G m 1 m 2 G is the universal gravitational constant From diameters and masses average values of the stellar density can be calculated and thence the central pressure With the assumption of an equation of state the central temperature can then be calculated For example in the Sun the central density is 158 grams per cubic cm the pressure is calculated to be more than one billion times the pressure of Earth s atmosphere at sea level and the temperature around 15 million K 27 million F At this temperature all atoms are ionized and so the solar interior consists of a plasma an ionized gas with hydrogen nuclei i e protons helium nuclei and electrons as major constituents A small fraction of the hydrogen nuclei possess sufficiently high speeds that on colliding their electrostatic repulsion is overcome resulting in the formation by means of a set of fusion reactions of helium nuclei and a release of energy see proton proton cycle Some of this energy is carried away by neutrino s but most of it is carried by photon s to the surface of the Sun to maintain its luminosity Other stars both more and less massive than the Sun have broadly similar structures but the size central pressure and temperature and fusion rate are functions of the star s mass and composition The stars and their internal fusion and resulting luminosity are held stable against collapse through a delicate balance between the inward pressure produced by gravitational attraction and the outward pressure supplied by the photons produced in the fusion reactions Stars that are in this condition of hydrostatic equilibrium are termed main sequence stars and they occupy a well defined band on the Hertzsprung Russell H R diagram in which luminosity is plotted against colour index or temperature Spectral classification based initially on the colour index includes the major spectral types O B A F G K and M each subdivided into 10 parts see star Stellar spectra Temperature is deduced from broadband spectral measurements in several standard wavelength intervals Measurement of apparent magnitudes in two spectral regions the B and V bands centred on 4350 and 5550 angstroms respectively permits calculation of the colour index CI m B m V from which the temperature can be calculated For a given temperature there are stars that are much more luminous than main sequence stars Given the dependence of luminosity on the square of the radius and the fourth power of the temperature R 2 T 4 of the luminosity expression above greater luminosity implies larger radius and such stars are termed giant star s or supergiant star s Conversely stars with luminosities much less than those of main sequence stars of the same temperature must be smaller and are termed white dwarf star s Surface temperatures of white dwarfs typically range from 10 000 to 12 000 K 18 000 to 21 000 F and they appear visually as white or blue white The strength of spectral lines of the more abundant elements in a star s atmosphere allows additional subdivisions within a class Thus the Sun a main sequence star is classified as G2 V in which the V denotes main sequence Betelgeuse a red giant with a surface temperature about half that of the Sun but with a luminosity of about 10 000 solar units is classified as M2 Iab In this classification the spectral type is M2 and the Iab indicates a giant well above the main sequence on the H R diagram Star formation and evolution Horsehead Nebula Anglo Australian Observatory The range of physically allowable masses for stars is very narrow If the star s mass is too small the central temperature will be too low to sustain fusion reactions The theoretical minimum stellar mass is about 0 08 solar mass An upper theoretical limit of approximately 100 solar masses has been suggested but this value is not firmly defined Stars as massive as this will have luminosities about one million times greater than that of the Sun stellar evolution Encyclopædia Britannica Inc A general model of star formation and evolution has been developed and the major features seem to be established A large cloud of gas and dust can contract under its own gravitational attraction if its temperature is sufficiently low As gravitational energy is released the contracting central material heats up until a point is reached at which the outward radiation pressure balances the inward gravitational pressure and contraction ceases Fusion reactions take over as the star s primary source of energy and the star is then on the main sequence The time to pass through these formative stages and onto the main sequence is less than 100 million years for a star with as much mass as the Sun It takes longer for less massive stars and a much shorter time for those much more massive Once a star has reached its main sequence stage it evolves relatively slowly fusing hydrogen nuclei in its core to form helium nuclei Continued fusion not only releases the energy that is radiated but also results in nucleosynthesis the production of heavier nuclei Stellar evolution has of necessity been followed through computer modeling because the timescales for most stages are generally too extended for measurable changes to be observed even over a period of many years One exception is the supernova the violently explosive finale of certain stars Different types of supernovas can be distinguished by their spectral lines and by changes in luminosity during and after the outburst In Type Ia a white dwarf star attracts matter from its nearby companion when the white dwarf s mass exceeds about 1 4 solar masses the star implodes and is completely destroyed Type II supernovas are not as luminous as Type Ia and are the final evolutionary stage of stars more massive than about eight solar masses The nature of the final products of stellar evolution depend on stellar mass Some stars pass through an unstable stage in which their dimensions temperature and luminosity change cyclically over periods of hours or days These so called Cepheid variables serve as standard candles for distance measurements see above Determining astronomical distances Some stars blow off their outer layers to produce planetary nebulas The expanding material can be seen glowing in a thin shell as it disperses into the interstellar medium while the remnant core initially with a surface temperature as high as 100 000 K 180 000 F cools to become a white dwarf The maximum stellar mass that can exist as a white dwarf is about 1 4 solar masses and is known as the Chandrasekhar limit More massive stars may end up as either neutron star s or black hole s The average density of a white dwarf is calculated to exceed one million grams per cubic cm Further compression is limited by a quantum condition called degeneracy see degenerate gas in which only certain energies are allowed for the electrons in the star s interior Under sufficiently great pressure the electrons are forced to combine with protons to form neutrons The resulting neutron star will have a density in the range of 10 14 10 15 grams per cubic cm comparable to the density within atomic nuclei The behaviour of large masses having nuclear densities is not yet sufficiently understood to be able to set a limit on the maximum size of a neutron star but it is thought to be in the region of three solar masses Still more massive remnants of stellar evolution would have smaller dimensions and would be even denser that neutron stars Such remnants are conceived to be black holes objects so compact that no radiation can escape from within a characteristic distance called the Schwarzschild radius see gravitational radius This critical dimension is defined by R s 2 G M c 2 R s is the Schwarzschild radius G is the gravitational constant M is the object s mass and c is the speed of light For an object of three solar masses the Schwarzschild radius would be about three kilometres Radiation emitted from beyond the Schwarzschild radius can still escape and be detected Although no light can be detected coming from within a black hole the presence of a black hole may be manifested through the effects of its gravitational field as for example in a binary star system If a black hole is paired with a normal visible star it may pull matter from its companion toward itself This matter is accelerated as it approaches the black hole and becomes so intensely heated that it radiates large amounts of X rays from the periphery of the black hole before reaching the Schwarzschild radius A few candidates for stellar black holes have been found e g the X ray source Cygnus X 1 Each of them has an estimated mass clearly exceeding that allowable for a neutron star a factor crucial in the identification of possible black holes Supermassive black holes that do not originate as individual stars are thought to exist at the centres of active galaxies see below Study of other galaxies and related phenomena Whereas the existence of stellar black holes has been strongly indicated the existence of neutron stars was confirmed in 1968 when they were identified with the then newly discovered pulsar s objects characterized by the emission of radiation at short and extremely regular intervals generally between 1 and 1 000 pulses per second and stable to better than a part per billion Pulsars are considered to be rotating neutron stars remnants of some supernovas Study of the Milky Way Galaxy Milky Way Galaxy Dirk Hoppe Stars are not distributed randomly throughout space Many stars are in systems consisting of two or three members separated by less than 1 000 AU On a larger scale star cluster s may contain many thousands of stars Galaxies are much larger systems of stars and usually include clouds of gas and dust The solar system is located within the Milky Way Galaxy close to its equatorial plane and about 7 9 kiloparsecs from the galactic centre The galactic diameter is about 30 kiloparsecs as indicated by luminous matter There is evidence however for nonluminous matter so called dark matter extending out nearly twice this distance The entire system is rotating such that at the position of the Sun the orbital speed is about 220 km per second almost 500 000 miles per hour and a complete circuit takes roughly 240 million years Application of Kepler s third law leads to an estimate for the galactic mass of about 100 billion solar masses The rotational velocity can be measured from the Doppler shifts see Doppler effect observed in the 21 cm emission line of neutral hydrogen and the lines of millimetre wavelengths from various molecules especially carbon monoxide At great distances from the galactic centre the rotational velocity does not drop off as expected but rather increases slightly This behaviour appears to require a much larger galactic mass than can be accounted for by the known luminous matter Additional evidence for the presence of dark matter comes from a variety of other observations The nature and extent of the dark matter or missing mass constitutes one

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  • Nobel Prize | award | Britannica.com
    begins in the early autumn of the preceding year when the prize awarding institutions invite more than 6 000 individuals to propose or nominate candidates for the prizes Some 1 000 people submit nominations for each prize and the number of nominees usually ranges from 100 to about 250 Among those nominating are Nobel laureates members of the prize awarding institutions themselves scholars active in the fields of physics chemistry economics and physiology or medicine and officials and members of diverse universities and learned academies The respondents must supply a written proposal that details their candidates worthiness Self nomination automatically disqualifies the nominee Prize proposals must be submitted to the Nobel Committees on or before January 31 of the award year Nobel Prizes selection process Encyclopædia Britannica Inc On February 1 the six Nobel Committees one for each prize category start their work on the nominations received Outside experts are frequently consulted during the process in order to help the committees determine the originality and significance of each nominee s contribution During September and early October the Nobel Committees have accomplished their work and submit their recommendations to the Royal Swedish Academy of Sciences and the other prize awarding institutions A committee s recommendation is usually but not invariably followed The deliberations and the voting within these institutions are secret at all stages The final decision by the awarders must be made by November 15 Prizes may be given only to individuals except the Peace Prize which may also be conferred upon an institution An individual may not be nominated posthumously but a winner who dies before receiving the prize may be awarded it posthumously as with Dag Hammarskjöld for peace 1961 Erik Axel Karlfeldt for literature 1931 and Ralph M Steinman for physiology or medicine 2011 Steinman was named a winner several days after his death which was unbeknownst to the Nobel Assembly It was decided that he would remain a Nobel laureate since the purpose of the posthumous rule was to prevent prizes being deliberately awarded to deceased individuals The awards may not be appealed Official support whether diplomatic or political for a certain candidate has no bearing on the award process because the prize awarders as such are independent of the state Images Interactive quizzes Lists 1 2 3 4 5 6 7 8 Next Page Comments Share Email Print Cite Last Updated 10 12 2015 You may also be interested in Milton Friedman European Union EU Pablo Neruda Linus Pauling Yasser Arafat Irving Langmuir International Committee of the Red Cross Gustav Stresemann Ernest Rutherford Baron Rutherford of Nelson International Atomic Energy Agency IAEA Henri Dunant Institute of International Law Keep exploring World Religions Traditions All About Einstein Abraham Lincoln 9 U S Presidents with the Most Vetoes What made you want to look up Nobel Prize To From Subject Comments Please limit to 900 characters Cancel Britannica Stories Behind The News Philosophy Religion Healing the Schism Pope Meets Patriarch Behind The News Science Gravitational Waves Observed

    Original URL path: http://www.britannica.com/topic/Nobel-Prize (2016-02-13)
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  • Laser Interferometer Gravitational-Wave Observatory (LIGO) | astronomical observatory, Hanford, Washington and Livingston, Louisiana, United States | Britannica.com
    feet in diameter When a gravitational wave passes through the interferometer it will make one arm of the interferometer shorter and the other longer and these changes in distance will appear as a change in the interference fringes between the two beams LIGO is an extremely sensitive instrument it can detect a change in distance of 10 17 cm over the length of the arm Because it is so sensitive a spurious gravitational wave signal can be produced by many sources thermal noise minute fluctuations in electrical current and even small seismic disturbances caused by wind Thus two installations are required to make a solid detection gravitational wave black hole merger SXS LIGO The Advanced LIGO project was designed to make LIGO 10 times more sensitive and began observations in 2015 On September 14 the two detectors made the first observation of gravitational waves Two black holes about 1 3 billion light years away spiralled into each other The black holes were 36 and 29 times the mass of the Sun and formed a new black hole 62 times the mass of the Sun In the merger three solar masses were converted to energy in gravitational waves the amount of power radiated was 50 times more than all the stars shining in the universe in that moment Erik Gregersen Comments Share Email Print Cite Last Updated 2 11 2016 You may also be interested in North Cascades National Park Olympic National Park Mount Rainier Mount Rainier National Park Tacoma Narrows Bridge Palomar Observatory Mount Wilson Observatory Keck Observatory Olympic Mountains Lick Observatory San Juan Islands Yerkes Observatory Keep exploring USA Facts Physics and Natural Law Nature Tip of the Iceberg Quiz Cold Stones 9 Gems That Will Make You Feel Like a Peasant 10 Inventions That Changed Your World What made you want to look up Laser Interferometer Gravitational Wave Observatory LIGO To From Subject Comments Please limit to 900 characters Cancel Britannica Stories Behind The News Philosophy Religion Healing the Schism Pope Meets Patriarch Behind The News Science Gravitational Waves Observed Spotlight History Thomas Malthus s 250th Birthday See More Stories FEATURED QUIZZES Vocabulary Quiz True or False Spell It See More Quizzes About Us About Our Ads Contact Us Privacy Policy Terms of Use 2016 Encyclopædia Britannica Inc MLA style Laser Interferometer Gravitational Wave Observatory LIGO Encyclopædia Britannica Encyclopædia Britannica Online Encyclopædia Britannica Inc 2016 Web 12 Feb 2016 http www britannica com topic Laser Interferometer Gravitational wave Observatory APA style Laser Interferometer Gravitational Wave Observatory LIGO 2016 In Encyclopædia Britannica Retrieved from http www britannica com topic Laser Interferometer Gravitational wave Observatory Harvard style Laser Interferometer Gravitational Wave Observatory LIGO 2016 Encyclopædia Britannica Online Retrieved 12 February 2016 from http www britannica com topic Laser Interferometer Gravitational wave Observatory Chicago Manual of Style Encyclopædia Britannica Online s v Laser Interferometer Gravitational Wave Observatory LIGO accessed February 12 2016 http www britannica com topic Laser Interferometer Gravitational wave Observatory While every effort has been made to follow citation

    Original URL path: http://www.britannica.com/topic/Laser-Interferometer-Gravitational-wave-Observatory (2016-02-13)
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