In astronomy, Bok globules are isolated and small dark nebulae, containing dense cosmic dust and gas from which star formation may take place. Bok globules are found within H II regions, have a mass of about 2 to 50 solar masses contained within a region about a light year or so across, they contain molecular hydrogen, carbon oxides and helium, around 1% silicate dust. Bok globules most result in the formation of double- or multiple-star systems. Bok globules were first observed by astronomer Bart Bok in the 1940s. In an article published in 1947, he and Edith Reilly hypothesized that these clouds were "similar to insect's cocoons" that were undergoing gravitational collapse to form new stars, from which stars and star clusters were born; this hypothesis was difficult to verify due to the observational difficulties of establishing what was happening inside a dense dark cloud that obscured all visible light emitted from within it. An analysis of near-infrared observations published in 1990 confirmed that stars were being born inside Bok globules.
Further observations have revealed that some Bok globules contain embedded warm sources, some contain Herbig–Haro objects, some show outflows of molecular gas. Millimeter-wave emission line studies have provided evidence for the infall of material onto an accreting protostar, it is now thought that a typical Bok globule contains about 10 solar masses of material in a region about a light-year or so across, that Bok globules most result in the formation of double- or multiple-star systems. Bok globules are still a subject of intense research. Known to be some of the coldest objects in the natural universe, their structure and density remains somewhat a mystery. Methods applied so far have relied on column density derived from near-infrared extinction and star counting in a bid to probe these objects further. Bok globules that are irradiated by ultraviolet light from hot nearby stars exhibit stripping of materials to produce a tail; these types are called "cometary globules". Molecular cloud Barnard 68 CG 4 NGC 281 IC 2944 A Star in the Making
In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats. It is thus the inverse of the spatial frequency. Wavelength is determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings and is a characteristic of both traveling waves and standing waves, as well as other spatial wave patterns. Wavelength is designated by the Greek letter lambda; the term wavelength is sometimes applied to modulated waves, to the sinusoidal envelopes of modulated waves or waves formed by interference of several sinusoids. Assuming a sinusoidal wave moving at a fixed wave speed, wavelength is inversely proportional to frequency of the wave: waves with higher frequencies have shorter wavelengths, lower frequencies have longer wavelengths. Wavelength depends on the medium. Examples of wave-like phenomena are sound waves, water waves and periodic electrical signals in a conductor.
A sound wave is a variation in air pressure, while in light and other electromagnetic radiation the strength of the electric and the magnetic field vary. Water waves are variations in the height of a body of water. In a crystal lattice vibration, atomic positions vary. Wavelength is a measure of the distance between repetitions of a shape feature such as peaks, valleys, or zero-crossings, not a measure of how far any given particle moves. For example, in sinusoidal waves over deep water a particle near the water's surface moves in a circle of the same diameter as the wave height, unrelated to wavelength; the range of wavelengths or frequencies for wave phenomena is called a spectrum. The name originated with the visible light spectrum but now can be applied to the entire electromagnetic spectrum as well as to a sound spectrum or vibration spectrum. In linear media, any wave pattern can be described in terms of the independent propagation of sinusoidal components; the wavelength λ of a sinusoidal waveform traveling at constant speed v is given by λ = v f, where v is called the phase speed of the wave and f is the wave's frequency.
In a dispersive medium, the phase speed itself depends upon the frequency of the wave, making the relationship between wavelength and frequency nonlinear. In the case of electromagnetic radiation—such as light—in free space, the phase speed is the speed of light, about 3×108 m/s, thus the wavelength of a 100 MHz electromagnetic wave is about: 3×108 m/s divided by 108 Hz = 3 metres. The wavelength of visible light ranges from deep red 700 nm, to violet 400 nm. For sound waves in air, the speed of sound is 343 m/s; the wavelengths of sound frequencies audible to the human ear are thus between 17 m and 17 mm, respectively. Note that the wavelengths in audible sound are much longer than those in visible light. A standing wave is an undulatory motion. A sinusoidal standing wave includes stationary points of no motion, called nodes, the wavelength is twice the distance between nodes; the upper figure shows three standing waves in a box. The walls of the box are considered to require the wave to have nodes at the walls of the box determining which wavelengths are allowed.
For example, for an electromagnetic wave, if the box has ideal metal walls, the condition for nodes at the walls results because the metal walls cannot support a tangential electric field, forcing the wave to have zero amplitude at the wall. The stationary wave can be viewed as the sum of two traveling sinusoidal waves of oppositely directed velocities. Wavelength and wave velocity are related just as for a traveling wave. For example, the speed of light can be determined from observation of standing waves in a metal box containing an ideal vacuum. Traveling sinusoidal waves are represented mathematically in terms of their velocity v, frequency f and wavelength λ as: y = A cos = A cos where y is the value of the wave at any position x and time t, A is the amplitude of the wave, they are commonly expressed in terms of wavenumber k and angular frequency ω as: y = A cos = A cos in which wavelength and wavenumber are related to velocity and frequency as: k = 2 π λ = 2 π f v = ω
William P. Bidelman
William Pendry Bidelman whose friends called him "Billy", was an American astronomer. Born in Los Angeles, raised in North Dakota, he was noted for classifying the spectra of stars, considered a pioneer in recognizing and classifying sub-groups of the peculiar stars. Bidelman's undergraduate degree was from Harvard College, his Ph. D. in astronomy was from the University of Chicago under advisor William Wilson Morgan. He was a physicist in the Army during World War II. A professional astronomer for over 50 years, Bidelman taught for ~41 years at The University of Chicago, The University of California,He co-discovered the class of barium stars with Philip Keenan, the phosphorus and the mercury stars, was the first to describe the hydrogen-deficient carbon stars. Born in Los Angeles, Bidelman was raised in North Dakota, where he met his future wife of 69 years, he was a father of a grandfather. As an Emeritus Professor William P. Bidelman continued working in astronomy after he retired from teaching, was 92 when he died in Murfreesboro, Tennessee.
As an undergraduate at Harvard College, Bidelman received an Honorary Harvard College Scholarship for academic excellence in 1939. He graduated in 1940. Bidelman entered the graduate program at the University of Chicago affiliated with Yerkes Observatory, his doctoral advisor was William W. Morgan, who discovered the first definite evidence that our Milky Way Galaxy is a spiral galaxy, with Philip Keenan, the Morgan-Keener system of stellar classification; as a graduate student, Bidelman assisted Morgan and Keenan by taking some of the spectrograms for their book, An Atlas of Stellar Spectra. For his 1943 dissertation, Bidelman reported the Double Cluster in the I Persei association is physically associated with neighborhood supergiant stars, is part of an association of O- and B-type stars, designated 47 stars as its members. Bidelman received his Ph. D. in 1943. The Yerkes astronomy graduate program directed by Otto Struve began issuing degrees in 1940, he was among their first ten graduates.
Bidelman served in the U. S. Army's Ballistic Research Laboratory at Aberdeen Proving Ground for over 2 years during World War II, he attended the 1942 American Astronomical Society's annual meeting despite a small assembly due to gasoline rationing during World War II. In 1945, when Bidelman left Aberdeen he was hired at Yerkes as an Instructor. Under Otto Struve Yerkes became the leading astrophysics center, when he directed it. In addition to Bidelman, by 1946 the Yerkes astronomy staff included Paul Ledoux, Arne Slettebak, Armin Deutsch, Marshall Wrubel, Arthur D. Code, Carlos Cesco, Víctor M. Blanco, W. W. Morgan, Otto Struve, Jesse L. Greenstein, Gerard P. Kuiper, George Van Biesbroeck, Louis G. Henyey Anne B. Underhill, Guido Münch, Nancy G. Roman, the two future Nobel Prize winners, Subrahmanyan Chandrasekhar and Gerhard Herzberg. Other astronomers at Yerkes when Bidelman was there were Kaj Strand, W. Albert Hiltner, Aden B. Meinel, visiting professors Bengt Strömgren from Denmark, Jan Oort, Hendrik C. van de Hulst and Adriaan Blaauw from the Netherlands.
George Herbig there, remembered it as an "exciting, stimulating place to work" and a "powerhouse in astronomy" while under Struve's direction. Bidelman spent long hours observing in remote west Texas at McDonald Observatory because he, like other Yerkes faculty, was an astronomer at the University of Texas. At the suggestion of Struve, the two universities had cooperated to create McDonald Observatory when the UT system had no astronomy department but W. J. McDonald gave them money in 1926 for an observatory, while in Wisconsin, the Yerkes astronomers needed a larger telescope but lacked the funds to obtain one. Otto Struve, who directed both Yerkes and McDonald, has been demanding, his managerial style included daily inspections of the faculty to see. Despite reports of tensions there was "close knit camaraderie" and "boisterous parties" evidenced by Yerkes "spontaneous party songs" including "The Billy Bidelman Song". Sung to the tune of "The Battle Hymn of the Republic", it consisted of repeating three times the line: "Struve, Hiltner, Chandrasekhar too," followed by: "And Billy Bidelman".
In 1946, W. W. Morgan and William P. Bidelman published a paper on interstellar reddening using the MK system of spectral classifications and photoelectric photometry. Morgan said this paper with Bidelman on interstellar reddening was "the principal paper along the way" to the UBV system, which he devised with Harold Johnson. In 1947, Bidelman became first to note the concentration of type M supergiant stars around χ Per, suggesting they were young Population I objects; this group, along with the Double Cluster, was named the Perseus OB1 Association. Based on its radial velocity, Bidelman became first to see that S Persei is part of the Per OB1 association, confirmed. Among the first stars that were studied at far-infrared wavelengths, M-type supergiants may be used to find the spiral arms of our galaxy. Bidelman found four red supergiant stars in 1947, bringing the total known to 13. How red supergiant stars evolved was considered an "astronomical puzzle", so the Double Cluster was used to test ideas about the evolution of red supergiant stars during the 1960s.
The M-type supergiants of h and χ Per became the prototypes of this class of stars, the major source of data for their properties. Unlike most the usual young star clusters including few supergiant stars, 18 were found in the Double Cluster of Perseus by 2007, which Robert F. Wing noted as the 60th anniversary of Bidelman's "important paper", saying Bidelman's 1947 two-dimensional classifications of the
In astrophysics, accretion is the accumulation of particles into a massive object by gravitationally attracting more matter gaseous matter, in an accretion disk. Most astronomical objects, such as galaxies and planets, are formed by accretion processes; the accretion model that Earth and the other terrestrial planets formed from meteoric material was proposed in 1944 by Otto Schmidt, followed by the protoplanet theory of William McCrea and the capture theory of Michael Woolfson. In 1978, Andrew Prentice resurrected the initial Laplacian ideas about planet formation and developed the modern Laplacian theory. None of these models proved successful, many of the proposed theories were descriptive; the 1944 accretion model by Otto Schmidt was further developed in a quantitative way in 1969 by Viktor Safronov. He calculated, in detail, the different stages of terrestrial planet formation. Since the model has been further developed using intensive numerical simulations to study planetesimal accumulation.
It is now accepted. Prior to collapse, this gas is in the form of molecular clouds, such as the Orion Nebula; as the cloud collapses, losing potential energy, it heats up, gaining kinetic energy, the conservation of angular momentum ensures that the cloud forms a flatted disk—the accretion disk. A few hundred thousand years after the Big Bang, the Universe cooled to the point where atoms could form; as the Universe continued to expand and cool, the atoms lost enough kinetic energy, dark matter coalesced sufficiently, to form protogalaxies. As further accretion occurred, galaxies formed. Indirect evidence is widespread. Galaxies grow through smooth gas accretion. Accretion occurs inside galaxies, forming stars. Stars are thought to form inside giant clouds of cold molecular hydrogen—giant molecular clouds of 300,000 M☉ and 65 light-years in diameter. Over millions of years, giant molecular clouds are prone to fragmentation; these fragments form small, dense cores, which in turn collapse into stars.
The cores range in mass from a fraction to several times that of the Sun and are called protostellar nebulae. They possess diameters of 2,000–20,000 astronomical units and a particle number density of 10,000 to 100,000/cm3. Compare it with the particle number density of the air at the sea level—2.8×1019/cm3. The initial collapse of a solar-mass protostellar nebula takes around 100,000 years; every nebula begins with a certain amount of angular momentum. Gas in the central part of the nebula, with low angular momentum, undergoes fast compression and forms a hot hydrostatic core containing a small fraction of the mass of the original nebula; this core forms the seed of. As the collapse continues, conservation of angular momentum dictates that the rotation of the infalling envelope accelerates, which forms a disk; as the infall of material from the disk continues, the envelope becomes thin and transparent and the young stellar object becomes observable in far-infrared light and in the visible. Around this time the protostar begins to fuse deuterium.
If the protostar is sufficiently massive, hydrogen fusion follows. Otherwise, if its mass is too low, the object becomes a brown dwarf; this birth of a new star occurs 100,000 years after the collapse begins. Objects at this stage are known as Class I protostars, which are called young T Tauri stars, evolved protostars, or young stellar objects. By this time, the forming star has accreted much of its mass. At the next stage, the envelope disappears, having been gathered up by the disk, the protostar becomes a classical T Tauri star; the latter have accretion disks and continue to accrete hot gas, which manifests itself by strong emission lines in their spectrum. The former do not possess accretion disks. Classical T Tauri stars evolve into weakly lined T Tauri stars; this happens after about 1 million years. The mass of the disk around a classical T Tauri star is about 1–3% of the stellar mass, it is accreted at a rate of 10−7 to 10−9 M☉ per year. A pair of bipolar jets is present as well; the accretion explains all peculiar properties of classical T Tauri stars: strong flux in the emission lines, magnetic activity, photometric variability and jets.
The emission lines form as the accreted gas hits the "surface" of the star, which happens around its magnetic poles. The jets are byproducts of accretion: they carry away excessive angular momentum; the classical T Tauri stage lasts about 10 million years. The disk disappears due to accretion onto the central star, planet formation, ejection by jets, photoevaporation by ultraviolet radiation from the central star and nearby stars; as a result, the young star becomes a weakly lined T Tauri star, over hundreds of millions of years, evolves into an ordinary Sun-like star, dependent on its initial mass. Self-accretion of cosmic dust accelerates the growth of the particles into boulder-sized planetesimals; the more massive planetesimals accrete some smaller ones. Accretion disks are common around smaller stars, or stellar remnants in a close binary, or black holes surrounded by material, such as those at the centers of galaxies; some dynamics in the disk, such as dynamical friction, are necessary to allow orbiting gas to lose angular momentum and fall onto the central mas
Convection is the heat transfer due to the bulk movement of molecules within fluids such as gases and liquids, including molten rock. Convection includes sub-mechanisms of advection, diffusion. Convection cannot take place in most solids because neither bulk current flows nor significant diffusion of matter can take place. Diffusion of heat takes place in rigid solids, but, called heat conduction. Convection, additionally may take place in soft solids or mixtures where solid particles can move past each other. Thermal convection can be demonstrated by placing a heat source at the side of a glass filled with a liquid, observing the changes in temperature in the glass caused by the warmer fluid circulating into cooler areas. Convective heat transfer is one of the major types of heat transfer, convection is a major mode of mass transfer in fluids. Convective heat and mass transfer takes place both by diffusion – the random Brownian motion of individual particles in the fluid – and by advection, in which matter or heat is transported by the larger-scale motion of currents in the fluid.
In the context of heat and mass transfer, the term "convection" is used to refer to the combined effects of advective and diffusive transfer. Sometimes the term "convection" is used to refer to "free heat convection" where bulk-flow in a fluid is due to temperature-induced differences in buoyancy, as opposed to "forced heat convection" where forces other than buoyancy move the fluid. However, in mechanics, the correct use of the word "convection" is the more general sense, different types of convection should be further qualified, for clarity. Convection can be qualified in terms of being natural, gravitational, granular, or thermomagnetic, it may be said to be due to combustion, capillary action, or Marangoni and Weissenberg effects. Heat transfer by natural convection plays a role in the structure of Earth's atmosphere, its oceans, its mantle. Discrete convective cells in the atmosphere can be seen as clouds, with stronger convection resulting in thunderstorms. Natural convection plays a role in stellar physics.
The convection mechanism is used in cooking, when using a convection oven, which uses fans to circulate hot air around food in order to cook the food faster than a conventional oven. The word convection may have different but related usages in different scientific or engineering contexts or applications; the broader sense is in fluid mechanics, where convection refers to the motion of fluid regardless of cause. However, in thermodynamics "convection" refers to heat transfer by convection. Convection occurs on a large scale in atmospheres, planetary mantles, it provides the mechanism of heat transfer for a large fraction of the outermost interiors of our sun and all stars. Fluid movement during convection may be invisibly slow, or it may be obvious and rapid, as in a hurricane. On astronomical scales, convection of gas and dust is thought to occur in the accretion disks of black holes, at speeds which may approach that of light. Convective heat transfer is a mechanism of heat transfer occurring because of bulk motion of fluids.
Heat is the entity of interest being advected, diffused. This can be contrasted with conductive heat transfer, the transfer of energy by vibrations at a molecular level through a solid or fluid, radiative heat transfer, the transfer of energy through electromagnetic waves. Heat is transferred by convection in numerous examples of occurring fluid flow, such as wind, oceanic currents, movements within the Earth's mantle. Convection is used in engineering practices of homes, industrial processes, cooling of equipment, etc; the rate of convective heat transfer may be improved by the use of a heat sink in conjunction with a fan. For instance, a typical computer CPU will have a purpose-made fan to ensure its operating temperature is kept within tolerable limits. A convection cell known as a Bénard cell is a characteristic fluid flow pattern in many convection systems. A rising body of fluid loses heat because it encounters a cold surface. In liquid, this occurs. In the example of the Earth's atmosphere, this occurs.
Because of this heat loss the fluid becomes denser than the fluid underneath it, still rising. Since it cannot descend through the rising fluid, it moves to one side. At some distance, its downward force overcomes the rising force beneath it, the fluid begins to descend; as it descends, it warms again and the cycle repeats itself. Atmospheric circulation is the large-scale movement of air, is a means by which thermal energy is distributed on the surface of the Earth, together with the much slower ocean circulation system; the large-scale structure of the atmospheric circulation varies from year to year, but the basic climatological structure remains constant. Latitudinal circulation occurs because incident solar radiation per unit area is highest at the heat equator, decreases as the latitude increases, reaching minima at the poles, it consists of two primary convection cells, the Hadley cell and the polar vortex, with the Hadley cell experiencing stronger convection due to the release of latent heat energy by condensation of water vapor at higher altitudes during cloud formation.
Longitudinal circulation, on the other hand, comes about because the ocean has a higher specific heat capacity than land (and thermal conduct
A star is type of astronomical object consisting of a luminous spheroid of plasma held together by its own gravity. The nearest star to Earth is the Sun. Many other stars are visible to the naked eye from Earth during the night, appearing as a multitude of fixed luminous points in the sky due to their immense distance from Earth; the most prominent stars were grouped into constellations and asterisms, the brightest of which gained proper names. Astronomers have assembled star catalogues that identify the known stars and provide standardized stellar designations. However, most of the estimated 300 sextillion stars in the Universe are invisible to the naked eye from Earth, including all stars outside our galaxy, the Milky Way. For at least a portion of its life, a star shines due to thermonuclear fusion of hydrogen into helium in its core, releasing energy that traverses the star's interior and radiates into outer space. All occurring elements heavier than helium are created by stellar nucleosynthesis during the star's lifetime, for some stars by supernova nucleosynthesis when it explodes.
Near the end of its life, a star can contain degenerate matter. Astronomers can determine the mass, age and many other properties of a star by observing its motion through space, its luminosity, spectrum respectively; the total mass of a star is the main factor. Other characteristics of a star, including diameter and temperature, change over its life, while the star's environment affects its rotation and movement. A plot of the temperature of many stars against their luminosities produces a plot known as a Hertzsprung–Russell diagram. Plotting a particular star on that diagram allows the age and evolutionary state of that star to be determined. A star's life begins with the gravitational collapse of a gaseous nebula of material composed of hydrogen, along with helium and trace amounts of heavier elements; when the stellar core is sufficiently dense, hydrogen becomes converted into helium through nuclear fusion, releasing energy in the process. The remainder of the star's interior carries energy away from the core through a combination of radiative and convective heat transfer processes.
The star's internal pressure prevents it from collapsing further under its own gravity. A star with mass greater than 0.4 times the Sun's will expand to become a red giant when the hydrogen fuel in its core is exhausted. In some cases, it will fuse heavier elements in shells around the core; as the star expands it throws a part of its mass, enriched with those heavier elements, into the interstellar environment, to be recycled as new stars. Meanwhile, the core becomes a stellar remnant: a white dwarf, a neutron star, or if it is sufficiently massive a black hole. Binary and multi-star systems consist of two or more stars that are gravitationally bound and move around each other in stable orbits; when two such stars have a close orbit, their gravitational interaction can have a significant impact on their evolution. Stars can form part of a much larger gravitationally bound structure, such as a star cluster or a galaxy. Stars have been important to civilizations throughout the world, they have used for celestial navigation and orientation.
Many ancient astronomers believed that stars were permanently affixed to a heavenly sphere and that they were immutable. By convention, astronomers grouped stars into constellations and used them to track the motions of the planets and the inferred position of the Sun; the motion of the Sun against the background stars was used to create calendars, which could be used to regulate agricultural practices. The Gregorian calendar used nearly everywhere in the world, is a solar calendar based on the angle of the Earth's rotational axis relative to its local star, the Sun; the oldest dated star chart was the result of ancient Egyptian astronomy in 1534 BC. The earliest known star catalogues were compiled by the ancient Babylonian astronomers of Mesopotamia in the late 2nd millennium BC, during the Kassite Period; the first star catalogue in Greek astronomy was created by Aristillus in 300 BC, with the help of Timocharis. The star catalog of Hipparchus included 1020 stars, was used to assemble Ptolemy's star catalogue.
Hipparchus is known for the discovery of the first recorded nova. Many of the constellations and star names in use today derive from Greek astronomy. In spite of the apparent immutability of the heavens, Chinese astronomers were aware that new stars could appear. In 185 AD, they were the first to observe and write about a supernova, now known as the SN 185; the brightest stellar event in recorded history was the SN 1006 supernova, observed in 1006 and written about by the Egyptian astronomer Ali ibn Ridwan and several Chinese astronomers. The SN 1054 supernova, which gave birth to the Crab Nebula, was observed by Chinese and Islamic astronomers. Medieval Islamic astronomers gave Arabic names to many stars that are still used today and they invented numerous astronomical instruments that could compute the positions of the stars, they built the first large observatory research institutes for the purpose of producing Zij star catalogues. Among these, the Book of Fixed Stars was written by the Persian astronomer Abd al-Rahman al-Sufi, who observed a number of stars, star clusters and galaxies.
According to A. Zahoor, in the 11th century, the Persian polymath scholar Abu Rayhan Biruni described the Milky
International Standard Serial Number
An International Standard Serial Number is an eight-digit serial number used to uniquely identify a serial publication, such as a magazine. The ISSN is helpful in distinguishing between serials with the same title. ISSN are used in ordering, interlibrary loans, other practices in connection with serial literature; the ISSN system was first drafted as an International Organization for Standardization international standard in 1971 and published as ISO 3297 in 1975. ISO subcommittee TC 46/SC 9 is responsible for maintaining the standard; when a serial with the same content is published in more than one media type, a different ISSN is assigned to each media type. For example, many serials are published both in electronic media; the ISSN system refers to these types as electronic ISSN, respectively. Conversely, as defined in ISO 3297:2007, every serial in the ISSN system is assigned a linking ISSN the same as the ISSN assigned to the serial in its first published medium, which links together all ISSNs assigned to the serial in every medium.
The format of the ISSN is an eight digit code, divided by a hyphen into two four-digit numbers. As an integer number, it can be represented by the first seven digits; the last code digit, which may be 0-9 or an X, is a check digit. Formally, the general form of the ISSN code can be expressed as follows: NNNN-NNNC where N is in the set, a digit character, C is in; the ISSN of the journal Hearing Research, for example, is 0378-5955, where the final 5 is the check digit, C=5. To calculate the check digit, the following algorithm may be used: Calculate the sum of the first seven digits of the ISSN multiplied by its position in the number, counting from the right—that is, 8, 7, 6, 5, 4, 3, 2, respectively: 0 ⋅ 8 + 3 ⋅ 7 + 7 ⋅ 6 + 8 ⋅ 5 + 5 ⋅ 4 + 9 ⋅ 3 + 5 ⋅ 2 = 0 + 21 + 42 + 40 + 20 + 27 + 10 = 160 The modulus 11 of this sum is calculated. For calculations, an upper case X in the check digit position indicates a check digit of 10. To confirm the check digit, calculate the sum of all eight digits of the ISSN multiplied by its position in the number, counting from the right.
The modulus 11 of the sum must be 0. There is an online ISSN checker. ISSN codes are assigned by a network of ISSN National Centres located at national libraries and coordinated by the ISSN International Centre based in Paris; the International Centre is an intergovernmental organization created in 1974 through an agreement between UNESCO and the French government. The International Centre maintains a database of all ISSNs assigned worldwide, the ISDS Register otherwise known as the ISSN Register. At the end of 2016, the ISSN Register contained records for 1,943,572 items. ISSN and ISBN codes are similar in concept. An ISBN might be assigned for particular issues of a serial, in addition to the ISSN code for the serial as a whole. An ISSN, unlike the ISBN code, is an anonymous identifier associated with a serial title, containing no information as to the publisher or its location. For this reason a new ISSN is assigned to a serial each time it undergoes a major title change. Since the ISSN applies to an entire serial a new identifier, the Serial Item and Contribution Identifier, was built on top of it to allow references to specific volumes, articles, or other identifiable components.
Separate ISSNs are needed for serials in different media. Thus, the print and electronic media versions of a serial need separate ISSNs. A CD-ROM version and a web version of a serial require different ISSNs since two different media are involved. However, the same ISSN can be used for different file formats of the same online serial; this "media-oriented identification" of serials made sense in the 1970s. In the 1990s and onward, with personal computers, better screens, the Web, it makes sense to consider only content, independent of media; this "content-oriented identification" of serials was a repressed demand during a decade, but no ISSN update or initiative occurred. A natural extension for ISSN, the unique-identification of the articles in the serials, was the main demand application. An alternative serials' contents model arrived with the indecs Content Model and its application, the digital object identifier, as ISSN-independent initiative, consolidated in the 2000s. Only in 2007, ISSN-L was defined in the