1.
Astronomy
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Astronomy is a natural science that studies celestial objects and phenomena. It applies mathematics, physics, and chemistry, in an effort to explain the origin of those objects and phenomena and their evolution. Objects of interest include planets, moons, stars, galaxies, and comets, while the phenomena include supernovae explosions, gamma ray bursts, more generally, all astronomical phenomena that originate outside Earths atmosphere are within the purview of astronomy. A related but distinct subject, physical cosmology, is concerned with the study of the Universe as a whole, Astronomy is the oldest of the natural sciences. The early civilizations in recorded history, such as the Babylonians, Greeks, Indians, Egyptians, Nubians, Iranians, Chinese, during the 20th century, the field of professional astronomy split into observational and theoretical branches. Observational astronomy is focused on acquiring data from observations of astronomical objects, theoretical astronomy is oriented toward the development of computer or analytical models to describe astronomical objects and phenomena. The two fields complement each other, with theoretical astronomy seeking to explain the results and observations being used to confirm theoretical results. Astronomy is one of the few sciences where amateurs can play an active role, especially in the discovery. Amateur astronomers have made and contributed to many important astronomical discoveries, Astronomy means law of the stars. Astronomy should not be confused with astrology, the system which claims that human affairs are correlated with the positions of celestial objects. Although the two share a common origin, they are now entirely distinct. Generally, either the term astronomy or astrophysics may be used to refer to this subject, however, since most modern astronomical research deals with subjects related to physics, modern astronomy could actually be called astrophysics. Few fields, such as astrometry, are purely astronomy rather than also astrophysics, some titles of the leading scientific journals in this field includeThe Astronomical Journal, The Astrophysical Journal and Astronomy and Astrophysics. In early times, astronomy only comprised the observation and predictions of the motions of objects visible to the naked eye, in some locations, early cultures assembled massive artifacts that possibly had some astronomical purpose. Before tools such as the telescope were invented, early study of the stars was conducted using the naked eye, most of early astronomy actually consisted of mapping the positions of the stars and planets, a science now referred to as astrometry. From these observations, early ideas about the motions of the planets were formed, and the nature of the Sun, Moon, the Earth was believed to be the center of the Universe with the Sun, the Moon and the stars rotating around it. This is known as the model of the Universe, or the Ptolemaic system. The Babylonians discovered that lunar eclipses recurred in a cycle known as a saros
2.
Measurement
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Measurement is the assignment of a number to a characteristic of an object or event, which can be compared with other objects or events. The scope and application of a measurement is dependent on the context, however, in other fields such as statistics as well as the social and behavioral sciences, measurements can have multiple levels, which would include nominal, ordinal, interval, and ratio scales. Measurement is a cornerstone of trade, science, technology, historically, many measurement systems existed for the varied fields of human existence to facilitate comparisons in these fields. Often these were achieved by local agreements between trading partners or collaborators, since the 18th century, developments progressed towards unifying, widely accepted standards that resulted in the modern International System of Units. This system reduces all physical measurements to a combination of seven base units. The science of measurement is pursued in the field of metrology, the measurement of a property may be categorized by the following criteria, type, magnitude, unit, and uncertainty. They enable unambiguous comparisons between measurements, the type or level of measurement is a taxonomy for the methodological character of a comparison. For example, two states of a property may be compared by ratio, difference, or ordinal preference, the type is commonly not explicitly expressed, but implicit in the definition of a measurement procedure. The magnitude is the value of the characterization, usually obtained with a suitably chosen measuring instrument. A unit assigns a mathematical weighting factor to the magnitude that is derived as a ratio to the property of a used as standard or a natural physical quantity. An uncertainty represents the random and systemic errors of the measurement procedure, errors are evaluated by methodically repeating measurements and considering the accuracy and precision of the measuring instrument. Measurements most commonly use the International System of Units as a comparison framework, the system defines seven fundamental units, kilogram, metre, candela, second, ampere, kelvin, and mole. Instead, the measurement unit can only ever change through increased accuracy in determining the value of the constant it is tied to and this directly influenced the Michelson–Morley experiment, Michelson and Morley cite Peirce, and improve on his method. With the exception of a few fundamental quantum constants, units of measurement are derived from historical agreements, nothing inherent in nature dictates that an inch has to be a certain length, nor that a mile is a better measure of distance than a kilometre. Over the course of history, however, first for convenience and then for necessity. Laws regulating measurement were originally developed to prevent fraud in commerce.9144 metres, in the United States, the National Institute of Standards and Technology, a division of the United States Department of Commerce, regulates commercial measurements. Before SI units were adopted around the world, the British systems of English units and later imperial units were used in Britain, the Commonwealth. The system came to be known as U. S. customary units in the United States and is still in use there and in a few Caribbean countries. S
3.
Flux
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Flux is either of two separate simple and ubiquitous concepts throughout physics and applied mathematics. Within a discipline, the term is used consistently. Both concepts have mathematical rigor, enabling comparison of the underlying math when the terminology is unclear, for transport phenomena, flux is a vector quantity, describing the magnitude and direction of the flow of a substance or property. In electromagnetism, flux is a quantity, defined as the surface integral of the component of a vector field perpendicular to the surface at each point. As will be clear, the easiest way to relate the two concepts is that the surface integral of a flux according to the first definition is a flux according to the second definition. The word flux comes from Latin, fluxus means flow, as fluxion, this term was introduced into differential calculus by Isaac Newton. One could argue, based on the work of James Clerk Maxwell, the specific quote from Maxwell is, In the case of fluxes, we have to take the integral, over a surface, of the flux through every element of the surface. The result of operation is called the surface integral of the flux. It represents the quantity which passes through the surface, according to the first definition, flux may be a single vector, or flux may be a vector field / function of position. In the latter case flux can readily be integrated over a surface, by contrast, according to the second definition, flux is the integral over a surface, it makes no sense to integrate a second-definition flux for one would be integrating over a surface twice. Thus, Maxwells quote only makes sense if flux is being used according to the first definition and this is ironic because Maxwell was one of the major developers of what we now call electric flux and magnetic flux according to the second definition. This implies that Maxwell conceived as these fields as flows/fluxes of some sort, given a flux according to the second definition, the corresponding flux density, if that term is used, refers to its derivative along the surface that was integrated. By the Fundamental theorem of calculus, the flux density is a flux according to the first definition. Given a current such as electric current—charge per time, current density would also be a flux according to the first definition—charge per time per area. Concrete fluxes in the rest of this article will be used in accordance to their acceptance in the literature. In transport phenomena, flux is defined as the rate of flow of a property per unit area, the area is of the surface the property is flowing through or across. Here are 3 definitions in increasing order of complexity, each is a special case of the following. In all cases the frequent symbol j, is used for flux, q for the quantity that flows, t for time
4.
Astronomical object
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An astronomical object or celestial object is a naturally occurring physical entity, association, or structure that current astronomy has demonstrated to exist in the observable universe. In astronomy, the object and body are often used interchangeably. Examples for astronomical objects include planetary systems, star clusters, nebulae and galaxies, while asteroids, moons, planets, and stars are astronomical bodies. A comet may be identified as both body and object, It is a body when referring to the nucleus of ice and dust. The universe can be viewed as having a hierarchical structure, at the largest scales, the fundamental component of assembly is the galaxy. Galaxies are organized groups and clusters, often within larger superclusters. Disc galaxies encompass lenticular and spiral galaxies with features, such as spiral arms, at the core, most galaxies have a supermassive black hole, which may result in an active galactic nucleus. Galaxies can also have satellites in the form of dwarf galaxies, the constituents of a galaxy are formed out of gaseous matter that assembles through gravitational self-attraction in a hierarchical manner. At this level, the fundamental components are the stars. The great variety of forms are determined almost entirely by the mass, composition. Stars may be found in systems that orbit about each other in a hierarchical organization. A planetary system and various objects such as asteroids, comets and debris. The various distinctive types of stars are shown by the Hertzsprung–Russell diagram —a plot of stellar luminosity versus surface temperature. Each star follows a track across this diagram. If this track takes the star through a region containing a variable type. An example of this is the instability strip, a region of the H-R diagram that includes Delta Scuti, RR Lyrae, the table below lists the general categories of bodies and objects by their location or structure. International Astronomical Naming Commission List of light sources List of Solar System objects Lists of astronomical objects SkyChart, Sky & Telescope Monthly skymaps for every location on Earth
5.
Electromagnetic radiation
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In physics, electromagnetic radiation refers to the waves of the electromagnetic field, propagating through space carrying electromagnetic radiant energy. It includes radio waves, microwaves, infrared, light, ultraviolet, X-, classically, electromagnetic radiation consists of electromagnetic waves, which are synchronized oscillations of electric and magnetic fields that propagate at the speed of light through a vacuum. The oscillations of the two fields are perpendicular to other and perpendicular to the direction of energy and wave propagation. The wavefront of electromagnetic waves emitted from a point source is a sphere, the position of an electromagnetic wave within the electromagnetic spectrum can be characterized by either its frequency of oscillation or its wavelength. Electromagnetic waves are produced whenever charged particles are accelerated, and these waves can interact with other charged particles. EM waves carry energy, momentum and angular momentum away from their source particle, quanta of EM waves are called photons, whose rest mass is zero, but whose energy, or equivalent total mass, is not zero so they are still affected by gravity. Thus, EMR is sometimes referred to as the far field, in this language, the near field refers to EM fields near the charges and current that directly produced them, specifically, electromagnetic induction and electrostatic induction phenomena. In the quantum theory of electromagnetism, EMR consists of photons, quantum effects provide additional sources of EMR, such as the transition of electrons to lower energy levels in an atom and black-body radiation. The energy of a photon is quantized and is greater for photons of higher frequency. This relationship is given by Plancks equation E = hν, where E is the energy per photon, ν is the frequency of the photon, a single gamma ray photon, for example, might carry ~100,000 times the energy of a single photon of visible light. The effects of EMR upon chemical compounds and biological organisms depend both upon the power and its frequency. EMR of visible or lower frequencies is called non-ionizing radiation, because its photons do not individually have enough energy to ionize atoms or molecules, the effects of these radiations on chemical systems and living tissue are caused primarily by heating effects from the combined energy transfer of many photons. In contrast, high ultraviolet, X-rays and gamma rays are called ionizing radiation since individual photons of high frequency have enough energy to ionize molecules or break chemical bonds. These radiations have the ability to cause chemical reactions and damage living cells beyond that resulting from simple heating, Maxwell derived a wave form of the electric and magnetic equations, thus uncovering the wave-like nature of electric and magnetic fields and their symmetry. Because the speed of EM waves predicted by the wave equation coincided with the speed of light. Maxwell’s equations were confirmed by Heinrich Hertz through experiments with radio waves, according to Maxwells equations, a spatially varying electric field is always associated with a magnetic field that changes over time. Likewise, a varying magnetic field is associated with specific changes over time in the electric field. In an electromagnetic wave, the changes in the field are always accompanied by a wave in the magnetic field in one direction
6.
Wavelength
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In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the waves shape repeats, and thus the inverse of the spatial frequency. Wavelength is commonly designated by the Greek letter lambda, the concept can also be applied to periodic waves of non-sinusoidal shape. The term wavelength is also applied to modulated waves. Wavelength depends on the medium that a wave travels through, examples of wave-like phenomena are sound waves, light, 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, 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 waves over deep water a particle near the waters surface moves in a circle of the same diameter as the wave height. 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 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, 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 speed is the speed of light. Thus the wavelength of a 100 MHz electromagnetic wave is about, the wavelength of visible light ranges from deep red, roughly 700 nm, to violet, roughly 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 approximately 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 that stays in one place. A sinusoidal standing wave includes stationary points of no motion, called 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, the stationary wave can be viewed as the sum of two traveling sinusoidal waves of oppositely directed velocities. Consequently, wavelength, period, 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. In that case, the k, the magnitude of k, is still in the same relationship with wavelength as shown above
7.
Radiation
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In physics, radiation is the emission or transmission of energy in the form of waves or particles through space or through a material medium. Radiation is often categorized as either ionizing or non-ionizing depending on the energy of the radiated particles, Ionizing radiation carries more than 10 eV, which is enough to ionize atoms and molecules, and break chemical bonds. This is an important distinction due to the difference in harmfulness to living organisms. A common source of ionizing radiation is radioactive materials that emit α, β, or γ radiation, consisting of helium nuclei, electrons or positrons, gamma rays, X-rays and the higher energy range of ultraviolet light constitute the ionizing part of the electromagnetic spectrum. The lower-energy, longer-wavelength part of the spectrum including visible light, infrared light, microwaves and this type of radiation only damages cells if the intensity is high enough to cause excessive heating. Ultraviolet radiation has some features of both ionizing and non-ionizing radiation and these properties derive from ultraviolets power to alter chemical bonds, even without having quite enough energy to ionize atoms. The word radiation arises from the phenomenon of waves radiating from a source and this aspect leads to a system of measurements and physical units that are applicable to all types of radiation. This law does not apply close to a source of radiation or for focused beams. Radiation with sufficiently high energy can ionize atoms, that is to say it can knock electrons off atoms, ionization occurs when an electron is stripped from an electron shell of the atom, which leaves the atom with a net positive charge. Because living cells and, more importantly, the DNA in those cells can be damaged by this ionization, thus ionizing radiation is somewhat artificially separated from particle radiation and electromagnetic radiation, simply due to its great potential for biological damage. While an individual cell is made of trillions of atoms, only a fraction of those will be ionized at low to moderate radiation powers. If the source of the radiation is a radioactive material or a nuclear process such as fission or fusion. Particle radiation is subatomic particles accelerated to relativistic speeds by nuclear reactions, because of their momenta they are quite capable of knocking out electrons and ionizing materials, but since most have an electrical charge, they dont have the penetrating power of ionizing radiation. The exception is neutron particles, see below, there are several different kinds of these particles, but the majority are alpha particles, beta particles, neutrons, and protons. Roughly speaking, photons and particles with energies above about 10 electron volts are ionizing, particle radiation from radioactive material or cosmic rays almost invariably carries enough energy to be ionizing. The radiation is invisible and not directly detectable by human senses, as a result, in some cases, it may lead to secondary emission of visible light upon its interaction with matter, as in the case of Cherenkov radiation and radio-luminescence. Ionizing radiation has many uses in medicine, research and construction. Ultraviolet, of wavelengths from 10 nm to 125 nm, ionizes air molecules, causing it to be absorbed by air
8.
Spectrophotometry
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In chemistry, spectrophotometry is the quantitative measurement of the reflection or transmission properties of a material as a function of wavelength. Spectrophotometry uses photometers, known as spectrophotometers, that can measure a light intensity as a function of its color. A spectrophotometer is commonly used for the measurement of transmittance or reflectance of solutions, transparent or opaque solids, such as polished glass, or gases. However they can also be designed to measure the diffusivity on any of the listed light ranges that cover around 200 nm -2500 nm using different controls. Within these ranges of light, calibrations are needed on the machine using standards that vary in type depending on the wavelength of the photometric determination, an example of an experiment in which spectrophotometry is used is the determination of the equilibrium constant of a solution. A certain chemical reaction within a solution may occur in a forward, at some point, this chemical reaction will reach a point of balance called an equilibrium point. In order to determine the concentrations of reactants and products at this point. The amount of light passes through the solution is indicative of the concentration of certain chemicals that do not allow light to pass through. The use of spectrophotometers spans various scientific fields, such as physics, materials science, chemistry, biochemistry and they are widely used in many industries including semiconductors, laser and optical manufacturing, printing and forensic examination, as well in laboratories for the study of chemical substances. By 1940 several spectrophotometers were available on the market, but early models could not work in the ultraviolet, arnold O. Beckman developed an improved version at the National Technical Laboratories Company, later the Beckman Instrument Company and ultimately Beckman Coulter. Models A, B, and C were developed, then the model D, all the electronics were contained within the instrument case, and it had a new hydrogen lamp with ultraviolet continuum, and a better monochromator. This instrument was produced from 1941 until 1976 with essentially the same design, nobel chemistry laureate Bruce Merrifield said it was probably the most important instrument ever developed towards the advancement of bioscience. There are two classes of devices, single beam and double beam. A double beam spectrophotometer compares the intensity between two light paths, one path containing a reference sample and the other the test sample. A single-beam spectrophotometer measures the light intensity of the beam before. Although comparison measurements from double-beam instruments are easier and more stable, single-beam instruments can have a dynamic range and are optically simpler. Additionally, some specialized instruments, such as spectrophotometers built onto microscopes or telescopes, are single-beam instruments due to practicality, historically, spectrophotometers use a monochromator containing a diffraction grating to produce the analytical spectrum. The grating can either be movable or fixed, if a single detector, such as a photomultiplier tube or photodiode is used, the grating can be scanned stepwise so that the detector can measure the light intensity at each wavelength
9.
Ancient Greek
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Ancient Greek includes the forms of Greek used in ancient Greece and the ancient world from around the 9th century BC to the 6th century AD. It is often divided into the Archaic period, Classical period. It is antedated in the second millennium BC by Mycenaean Greek, the language of the Hellenistic phase is known as Koine. Koine is regarded as a historical stage of its own, although in its earliest form it closely resembled Attic Greek. Prior to the Koine period, Greek of the classic and earlier periods included several regional dialects, Ancient Greek was the language of Homer and of fifth-century Athenian historians, playwrights, and philosophers. It has contributed many words to English vocabulary and has been a subject of study in educational institutions of the Western world since the Renaissance. This article primarily contains information about the Epic and Classical phases of the language, Ancient Greek was a pluricentric language, divided into many dialects. The main dialect groups are Attic and Ionic, Aeolic, Arcadocypriot, some dialects are found in standardized literary forms used in literature, while others are attested only in inscriptions. There are also several historical forms, homeric Greek is a literary form of Archaic Greek used in the epic poems, the Iliad and Odyssey, and in later poems by other authors. Homeric Greek had significant differences in grammar and pronunciation from Classical Attic, the origins, early form and development of the Hellenic language family are not well understood because of a lack of contemporaneous evidence. Several theories exist about what Hellenic dialect groups may have existed between the divergence of early Greek-like speech from the common Proto-Indo-European language and the Classical period and they have the same general outline, but differ in some of the detail. The invasion would not be Dorian unless the invaders had some relationship to the historical Dorians. The invasion is known to have displaced population to the later Attic-Ionic regions, the Greeks of this period believed there were three major divisions of all Greek people—Dorians, Aeolians, and Ionians, each with their own defining and distinctive dialects. Often non-west is called East Greek, Arcadocypriot apparently descended more closely from the Mycenaean Greek of the Bronze Age. Boeotian had come under a strong Northwest Greek influence, and can in some respects be considered a transitional dialect, thessalian likewise had come under Northwest Greek influence, though to a lesser degree. Most of the dialect sub-groups listed above had further subdivisions, generally equivalent to a city-state and its surrounding territory, Doric notably had several intermediate divisions as well, into Island Doric, Southern Peloponnesus Doric, and Northern Peloponnesus Doric. The Lesbian dialect was Aeolic Greek and this dialect slowly replaced most of the older dialects, although Doric dialect has survived in the Tsakonian language, which is spoken in the region of modern Sparta. Doric has also passed down its aorist terminations into most verbs of Demotic Greek, by about the 6th century AD, the Koine had slowly metamorphosized into Medieval Greek
10.
Telescope
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A telescope is an optical instrument that aids in the observation of remote objects by collecting electromagnetic radiation. The first known practical telescopes were invented in the Netherlands at the beginning of the 1600s and they found use in both terrestrial applications and astronomy. Within a few decades, the telescope was invented, which used mirrors to collect. In the 20th century many new types of telescopes were invented, including radio telescopes in the 1930s, the word telescope now refers to a wide range of instruments capable of detecting different regions of the electromagnetic spectrum, and in some cases other types of detectors. The word telescope was coined in 1611 by the Greek mathematician Giovanni Demisiani for one of Galileo Galileis instruments presented at a banquet at the Accademia dei Lincei, in the Starry Messenger, Galileo had used the term perspicillum. The earliest recorded working telescopes were the telescopes that appeared in the Netherlands in 1608. Their development is credited to three individuals, Hans Lippershey and Zacharias Janssen, who were spectacle makers in Middelburg, Galileo heard about the Dutch telescope in June 1609, built his own within a month, and improved upon the design in the following year. Also in 1609, Thomas Harriot became the first person known to point a telescope skyward in order to make observations of a celestial object. The idea that the objective, or light-gathering element, could be a mirror instead of a lens was being investigated soon after the invention of the refracting telescope. The potential advantages of using parabolic mirrors—reduction of spherical aberration and no chromatic aberration—led to many proposed designs, in 1668, Isaac Newton built the first practical reflecting telescope, of a design which now bears his name, the Newtonian reflector. The invention of the lens in 1733 partially corrected color aberrations present in the simple lens and enabled the construction of shorter. The largest reflecting telescopes currently have objectives larger than 10 m, the 20th century also saw the development of telescopes that worked in a wide range of wavelengths from radio to gamma-rays. The first purpose built radio telescope went into operation in 1937, since then, a tremendous variety of complex astronomical instruments have been developed. The name telescope covers a range of instruments. Most detect electromagnetic radiation, but there are differences in how astronomers must go about collecting light in different frequency bands. The near-infrared can be collected much like light, however in the far-infrared and submillimetre range. For example, the James Clerk Maxwell Telescope observes from wavelengths from 3 μm to 2000 μm, on the other hand, the Spitzer Space Telescope, observing from about 3 μm to 180 μm uses a mirror. Also using reflecting optics, the Hubble Space Telescope with Wide Field Camera 3 can observe in the range from about 0.2 μm to 1.7 μm
11.
Passband
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A passband is the range of frequencies or wavelengths that can pass through a filter. For example, a radio receiver contains a filter to select the frequency of the desired radio signal out of all the radio waves picked up by its antenna. The passband of a receiver is the range of frequencies it can receive, a bandpass-filtered signal, is known as a bandpass signal, in contrast to a baseband signal. In telecommunications, optics, and acoustics, a passband is the portion of the spectrum that is transmitted by some filtering device. In other words, it is a band of frequencies which passes through some filter or set of filters, the accompanying figure shows a schematic of a waveform being filtered by a bandpass filter consisting of a highpass and a lowpass filter. Radio receivers generally include a tunable band-pass filter with a passband that is enough to accommodate the bandwidth of the radio signal transmitted by a single station. There are two categories of digital communication transmission methods, baseband and passband. In baseband transmission, line coding is utilized, resulting in a train or pulse amplitude modulated signal. This is typically used over non-filtered wires such as fiber optical cables and short-range copper links, for example, V.29, V.35, IEEE802.3, SONET/SDH. In passband transmission, digital methods are employed so that only a limited frequency range is used in some bandpass filtered channel. Passband transmission is utilized in wireless communication and in bandpass filtered channels such as POTS lines. It also allows for frequency-division multiplexing, the digital bit stream is converted first into an equivalent baseband signal, and then to a RF signal. On the receiver side a demodulator is used to detect the signal, a combined equipment for modulation and demodulation is called a modem. In general, there is a relationship between the width of a filters passband and the time required for the filter to respond to new inputs. This is a consequence of the mathematics of Fourier analysis, the limiting frequencies of a passband are defined as those at which the relative intensity or power decreases to a specified fraction of the maximum intensity or power. This decrease in power is often specified to be the half-power points, the difference between the limiting frequencies is called the bandwidth, and is expressed in hertz. The related term bandpass is an adjective that describes a type of filter or filtering process, it is confused with passband. The two words are both words that follow the English rules of formation, the primary meaning is the latter part of the compound
12.
Infrared
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It extends from the nominal red edge of the visible spectrum at 700 nanometers, to 1000000 nm. Most of the radiation emitted by objects near room temperature is infrared. Like all EMR, IR carries radiant energy, and behaves both like a wave and like its quantum particle, the photon, slightly more than half of the total energy from the Sun was eventually found to arrive on Earth in the form of infrared. The balance between absorbed and emitted infrared radiation has an effect on Earths climate. Infrared radiation is emitted or absorbed by molecules when they change their rotational-vibrational movements and it excites vibrational modes in a molecule through a change in the dipole moment, making it a useful frequency range for study of these energy states for molecules of the proper symmetry. Infrared spectroscopy examines absorption and transmission of photons in the infrared range, Infrared radiation is used in industrial, scientific, and medical applications. Night-vision devices using active near-infrared illumination allow people or animals to be observed without the observer being detected, Infrared thermal-imaging cameras are used to detect heat loss in insulated systems, to observe changing blood flow in the skin, and to detect overheating of electrical apparatuses. Thermal-infrared imaging is used extensively for military and civilian purposes, military applications include target acquisition, surveillance, night vision, homing, and tracking. Humans at normal body temperature radiate chiefly at wavelengths around 10 μm, Infrared radiation extends from the nominal red edge of the visible spectrum at 700 nanometers to 1 mm. This range of wavelengths corresponds to a range of approximately 430 THz down to 300 GHz. Below infrared is the portion of the electromagnetic spectrum. Sunlight, at a temperature of 5,780 kelvins, is composed of near thermal-spectrum radiation that is slightly more than half infrared. At zenith, sunlight provides an irradiance of just over 1 kilowatt per square meter at sea level, of this energy,527 watts is infrared radiation,445 watts is visible light, and 32 watts is ultraviolet radiation. Nearly all the radiation in sunlight is near infrared, shorter than 4 micrometers. On the surface of Earth, at far lower temperatures than the surface of the Sun, almost all thermal radiation consists of infrared in mid-infrared region, much longer than in sunlight. Of these natural thermal radiation processes only lightning and natural fires are hot enough to produce much visible energy, thermal infrared radiation also has a maximum emission wavelength, which is inversely proportional to the absolute temperature of object, in accordance with Wiens displacement law. Therefore, the band is often subdivided into smaller sections. Due to the nature of the blackbody radiation curves, typical hot objects, such as exhaust pipes, the three regions are used for observation of different temperature ranges, and hence different environments in space
13.
Ultra-violet
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Ultraviolet is an electromagnetic radiation with a wavelength from 10 nm to 400 nm, shorter than that of visible light but longer than X-rays. UV radiation constitutes about 10% of the light output of the Sun. It is also produced by electric arcs and specialized lights, such as lamps, tanning lamps. Consequently, the effects of UV are greater than simple heating effects. Suntan, freckling and sunburn are familiar effects of over-exposure, along with risk of skin cancer. Living things on dry land would be damaged by ultraviolet radiation from the Sun if most of it were not filtered out by the Earths atmosphere. More-energetic, shorter-wavelength extreme UV below 121 nm ionizes air so strongly that it is absorbed before it reaches the ground, Ultraviolet is also responsible for the formation of bone-strengthening vitamin D in most land vertebrates, including humans. The UV spectrum thus has both beneficial and harmful to human health. Ultraviolet rays are invisible to most humans, the lens in a human eye ordinarily filters out UVB frequencies or higher, and humans lack color receptor adaptations for ultraviolet rays. Under some conditions, children and young adults can see ultraviolet down to wavelengths of about 310 nm, near-UV radiation is visible to some insects, mammals, and birds. Small birds have a fourth color receptor for ultraviolet rays, this gives birds true UV vision, reindeer use near-UV radiation to see polar bears, who are poorly visible in regular light because they blend in with the snow. UV also allows mammals to see urine trails, which is helpful for animals to find food in the wild. The males and females of some species look identical to the human eye. Ultraviolet means beyond violet, violet being the color of the highest frequencies of visible light, Ultraviolet has a higher frequency than violet light. He called them oxidizing rays to emphasize chemical reactivity and to them from heat rays. The terms chemical and heat rays were eventually dropped in favour of ultraviolet and infrared radiation, in 1878 the effect of short-wavelength light on sterilizing bacteria was discovered. By 1903 it was known the most effective wavelengths were around 250 nm, in 1960, the effect of ultraviolet radiation on DNA was established. The discovery of the ultraviolet radiation below 200 nm, named vacuum ultraviolet because it is absorbed by air, was made in 1893 by the German physicist Victor Schumann
14.
Charge-coupled device
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A charge-coupled device is a device for the movement of electrical charge, usually from within the device to an area where the charge can be manipulated, for example conversion into a digital value. This is achieved by shifting the signals between stages within the one at a time. CCDs move charge between capacitive bins in the device, with the shift allowing for the transfer of charge between bins, in recent years CCD has become a major technology for digital imaging. In a CCD image sensor, pixels are represented by p-doped metal-oxide-semiconductors capacitors, the charge-coupled device was invented in 1969 at AT&T Bell Labs by Willard Boyle and George E. Smith. The lab was working on bubble memory when Boyle and Smith conceived of the design of what they termed, in their notebook. The device could be used as a shift register, the essence of the design was the ability to transfer charge along the surface of a semiconductor from one storage capacitor to the next. The concept was similar in principle to the device, which was developed at Philips Research Labs during the late 1960s. The first patent on the application of CCDs to imaging was assigned to Michael Tompsett, the initial paper describing the concept listed possible uses as a memory, a delay line, and an imaging device. The first experimental device demonstrating the principle was a row of closely spaced metal squares on a silicon surface electrically accessed by wire bonds. The first working CCD made with integrated circuit technology was a simple 8-bit shift register and this device had input and output circuits and was used to demonstrate its use as a shift register and as a crude eight pixel linear imaging device. Development of the device progressed at a rapid rate, by 1971, Bell researchers led by Michael Tompsett were able to capture images with simple linear devices. Several companies, including Fairchild Semiconductor, RCA and Texas Instruments, picked up on the invention, fairchilds effort, led by ex-Bell researcher Gil Amelio, was the first with commercial devices, and by 1974 had a linear 500-element device and a 2-D100 x 100 pixel device. Steven Sasson, an engineer working for Kodak, invented the first digital still camera using a Fairchild 100 x 100 CCD in 1975. The first KH-11 KENNAN reconnaissance satellite equipped with charge-coupled device array technology for imaging was launched in December 1976, under the leadership of Kazuo Iwama, Sony also started a large development effort on CCDs involving a significant investment. Eventually, Sony managed to mass-produce CCDs for their camcorders, before this happened, Iwama died in August 1982, subsequently, a CCD chip was placed on his tombstone to acknowledge his contribution. He was also awarded the 2012 IEEE Edison Medal For pioneering contributions to imaging devices including CCD Imagers, cameras, in a CCD for capturing images, there is a photoactive region, and a transmission region made out of a shift register. An image is projected through a lens onto the capacitor array, once the array has been exposed to the image, a control circuit causes each capacitor to transfer its contents to its neighbor. The last capacitor in the array dumps its charge into a charge amplifier, by repeating this process, the controlling circuit converts the entire contents of the array in the semiconductor to a sequence of voltages
15.
Magnitude (astronomy)
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In astronomy, magnitude is a logarithmic measure of the brightness of an object, measured in a specific wavelength or passband, usually in the visible or near-infrared spectrum. An imprecise but systematic determination of the magnitude of objects was introduced in ancient times by Hipparchus, astronomers use two different definitions of magnitude, apparent magnitude and absolute magnitude. This distance is 10 parsecs for stars and 1 astronomical unit for planets, a minor planets size is typically estimated based on its absolute magnitude in combination with its presumed albedo. The brighter an object appears, the lower the value of its magnitude, with the brightest objects reaching negative values. The Sun has an apparent magnitude of −27, the full moon −13, the brightest planet Venus measures −5, and Sirius, an apparent magnitude can also be assigned to man-made objects in Earth orbit. The brightest satellite flares are ranked at −9, and the International Space Station, ISS, the scale is logarithmic, and defined such that each step of one magnitude changes the brightness by a factor of the fifth root of 100, or approximately 2.512. For example, a magnitude 1 star is exactly a hundred times brighter than a magnitude 6 star, the magnitude system dates back roughly 2000 years to the Greek astronomer Hipparchus who classified stars by their apparent brightness, which they saw as size. To the unaided eye, a prominent star such as Sirius or Arcturus appears larger than a less prominent star such as Mizar. For all the other Stars, which are seen by the Help of a Telescope. Note that the brighter the star, the smaller the magnitude, Bright first magnitude stars are 1st-class stars, the system was a simple delineation of stellar brightness into six distinct groups but made no allowance for the variations in brightness within a group. He concluded that first magnitude stars measured 2 arc minutes in apparent diameter, with second through sixth magnitude stars measuring 1 1⁄2′, 1 1⁄12′, 3⁄4′, 1⁄2′, the development of the telescope showed that these large sizes were illusory—stars appeared much smaller through the telescope. However, early telescopes produced a spurious disk-like image of a star that was larger for brighter stars, early photometric measurements demonstrated that first magnitude stars are about 100 times brighter than sixth magnitude stars. Thus in 1856 Norman Pogson of Oxford proposed that a scale of 5√100 ≈2.512 be adopted between magnitudes, so five magnitude steps corresponded precisely to a factor of 100 in brightness. Every interval of one magnitude equates to a variation in brightness of 5√100 or roughly 2.512 times. Consequently, a first magnitude star is about 2.5 times brighter than a second star,2.52 brighter than a third magnitude star,2.53 brighter than a fourth magnitude star. This is the modern system, which measures the brightness, not the apparent size. Using this logarithmic scale, it is possible for a star to be brighter than “first class”, so Arcturus or Vega are magnitude 0, and Sirius is magnitude −1.46. As mentioned above, the scale appears to work in reverse, the larger the negative value, the brighter
16.
Pixel
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The address of a pixel corresponds to its physical coordinates. LCD pixels are manufactured in a grid, and are often represented using dots or squares. Each pixel is a sample of an image, more samples typically provide more accurate representations of the original. The intensity of each pixel is variable, in color imaging systems, a color is typically represented by three or four component intensities such as red, green, and blue, or cyan, magenta, yellow, and black. The word pixel is based on a contraction of pix and el, the word pixel was first published in 1965 by Frederic C. Billingsley of JPL, to describe the elements of video images from space probes to the Moon. Billingsley had learned the word from Keith E. McFarland, at the Link Division of General Precision in Palo Alto, McFarland said simply it was in use at the time. The word is a combination of pix, for picture, the word pix appeared in Variety magazine headlines in 1932, as an abbreviation for the word pictures, in reference to movies. By 1938, pix was being used in reference to pictures by photojournalists. The concept of a picture element dates to the earliest days of television, some authors explain pixel as picture cell, as early as 1972. In graphics and in image and video processing, pel is often used instead of pixel, for example, IBM used it in their Technical Reference for the original PC. A pixel is generally thought of as the smallest single component of a digital image, however, the definition is highly context-sensitive. For example, there can be printed pixels in a page, or pixels carried by electronic signals, or represented by digital values, or pixels on a display device, or pixels in a digital camera. This list is not exhaustive and, depending on context, synonyms include pel, sample, byte, bit, dot, Pixels can be used as a unit of measure such as,2400 pixels per inch,640 pixels per line, or spaced 10 pixels apart. For example, a high-quality photographic image may be printed with 600 ppi on a 1200 dpi inkjet printer, even higher dpi numbers, such as the 4800 dpi quoted by printer manufacturers since 2002, do not mean much in terms of achievable resolution. The more pixels used to represent an image, the closer the result can resemble the original, the number of pixels in an image is sometimes called the resolution, though resolution has a more specific definition.3 megapixels. The pixels, or color samples, that form an image may or may not be in one-to-one correspondence with screen pixels. In computing, a composed of pixels is known as a bitmapped image or a raster image
17.
Point spread function
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The point spread function describes the response of an imaging system to a point source or point object. A more general term for the PSF is an impulse response. The PSF in many contexts can be thought of as the blob in an image that represents an unresolved object. In functional terms it is the spatial version of the transfer function of the imaging system. It is a concept in Fourier optics, astronomical imaging, medical imaging, electron microscopy and other imaging techniques such as 3D microscopy. The degree of spreading of the point object is a measure for the quality of an imaging system, in non-coherent imaging systems such as fluorescent microscopes, telescopes or optical microscopes, the image formation process is linear in power and described by linear system theory. This means that two objects A and B are imaged simultaneously, the result is equal to the sum of the independently imaged objects. In other words, the imaging of A is unaffected by the imaging of B and vice versa, the image of a complex object can then be seen as a convolution of the true object and the PSF. However, when the light is coherent, image formation is linear in the complex field. Recording the intensity image then can lead to cancellations or other non-linear effects, by virtue of the linearity property of optical imaging systems, i. e. This is known as the principle, valid for linear systems. When the object is divided into discrete point objects of varying intensity, as the PSF is typically determined entirely by the imaging system, the entire image can be described by knowing the optical properties of the system. This process is usually formulated by a convolution equation, in microscope image processing and astronomy, knowing the PSF of the measuring device is very important for restoring the image with deconvolution. For the case of beams, the PSF can be mathematically modeled using the concepts of Gaussian beams. For instance, as shown in the figure on the right, deconvolution of the mathematically modeled PSF, the point spread function may be independent of position in the object plane, in which case it is called shift invariant. In addition, if there is no distortion in the system, if the imaging system produces an inverted image, we may simply regard the image plane coordinate axes as being reversed from the object plane axes. With these two assumptions, i. e. that the PSF is shift-invariant and that there is no distortion, the 2D impulse function may be regarded as the limit of the square post function, shown in the figure below. We imagine the object plane as being decomposed into square areas such as this, with each having its own associated square post function
18.
Astronomical seeing
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The astronomical seeing conditions on a given night at a given location describe how much the Earths atmosphere perturbs the images of stars as seen through a telescope. The most common seeing measurement is the diameter of the intensity across the seeing disc. The size of the disc is determined by the astronomical seeing conditions at the time of the observation. The best conditions give a seeing disk diameter of ~0.4 arcseconds and are found at high-altitude observatories on small islands such as Mauna Kea or La Palma and this can easily mean a factor of 100 between the potential and practical resolution. Starting in the 1990s, new adaptive optics have been introduced that can correct for these effects. In viewing an object such as Mars, occasionally a still patch of air will come in front of the planet. Before the use of charge-coupled devices, there was no way of recording the image of the planet in the brief moment other than having the observer remember the image and draw it later. This had the effect of having the image of the planet be dependent on the observers memory, the effects of atmospheric seeing are qualitatively similar throughout the visible and near infra-red wavebands. At large telescopes the long exposure image resolution is slightly higher at longer wavelengths. However, when light enters the Earths atmosphere, the different temperature layers and different wind speeds distort the light waves, the effects of the atmosphere can be modeled as rotating cells of air moving turbulently. At most observatories, the turbulence is only significant on scales larger than r0, the distortion changes at a high rate, typically more frequently than 100 times a second. In a typical image of a star with an exposure time of seconds or even minutes. The diameter of the disk, most often defined as the full width at half maximum, is a measure of the astronomical seeing conditions. It follows from definition that seeing is always a variable quantity, different from place to place, from night to night. Astronomers often talk about good nights with a low average seeing disc diameter, the FWHM of the seeing disc is usually measured in arcseconds, abbreviated with the symbol. A1. 0″ seeing is a one for average astronomical sites. The seeing of an environment is usually much worse. Good seeing nights tend to be clear, cold nights without wind gusts, warm air rises, degrading the seeing, as do wind and clouds
19.
Point source
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A point source is a single identifiable localised source of something. A point source has negligible extent, distinguishing it from other source geometries, sources are called point sources because in mathematical modeling, these sources can usually be approximated as a mathematical point to simplify analysis. The actual source need not be small, if its size is negligible relative to other length scales in the problem. For example, in astronomy, stars are routinely treated as point sources, in mathematics, a point source is a singularity from which flux or flow is emanating. Although singularities such as this do not exist in the universe, mathematical point sources are often used as approximations to reality in physics. Generally a source of light can be considered a point source if the resolution of the instrument is too low to resolve its apparent size. There are two types and sources of light, a point source, and an extended source. Radio emissions generated by an electrical circuit are usually polarized. Gamma ray and X-ray sources may be treated as a point source if sufficiently small, radiological contamination and nuclear sources are often point sources. This has significance in physics and radiation protection. As the pressure oscillates up and down, a point source acts in turn as a fluid point source. They are usually a capsule and are most commonly used for gamma, x-ray. In vacuum, heat escapes as radiation isotropically, if the source remains stationary in a compressible fluid such as air, flow patterns can form around the source due to convection, leading to an anisotropic pattern of heat loss. The most common form of anisotropy is the formation of a plume above the heat source. Examples, Geological hotspots on the surface of the Earth which lie at the tops of thermal plumes rising from deep inside the Earth Plumes of heat studied in thermal pollution tracking, fluid point sources are commonly used in fluid dynamics and aerodynamics. A point source of fluid is the inverse of a point sink. If the fluid is moving a plume is generated from the point source
20.
Globular cluster
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A globular cluster is a spherical collection of stars that orbits a galactic core as a satellite. Globular clusters are very tightly bound by gravity, which gives them their spherical shapes, the name of this category of star cluster is derived from the Latin globulus—a small sphere. A globular cluster is known more simply as a globular. Globular clusters are found in the halo of a galaxy and contain considerably more stars and are older than the less dense open clusters. Globular clusters are common, there are about 150 to 158 currently known globular clusters in the Milky Way. These globular clusters orbit the Galaxy at radii of 40 kiloparsecs or more, larger galaxies can have more, Andromeda Galaxy, for instance, may have as many as 500. Some giant elliptical galaxies such as M87, have as many as 13,000 globular clusters, every galaxy of sufficient mass in the Local Group has an associated group of globular clusters, and almost every large galaxy surveyed has been found to possess a system of globular clusters. The Sagittarius Dwarf galaxy and the disputed Canis Major Dwarf galaxy appear to be in the process of donating their associated globular clusters to the Milky Way and this demonstrates how many of this galaxys globular clusters might have been acquired in the past. Although it appears that globular clusters contain some of the first stars to be produced in the galaxy, their origins, the first known globular cluster, now called M22, was discovered in 1665 by Abraham Ihle, a German amateur astronomer. However, given the small aperture of early telescopes, individual stars within a cluster were not resolved until Charles Messier observed M4 in 1764. The first eight globular clusters discovered are shown in the table, subsequently, Abbé Lacaille would list NGC104, NGC4833, M55, M69, and NGC6397 in his 1751–52 catalogue. The M before a number refers to Charles Messiers catalogue, while NGC is from the New General Catalogue by John Dreyer, when William Herschel began his comprehensive survey of the sky using large telescopes in 1782 there were 34 known globular clusters. Herschel discovered another 36 himself and was the first to virtually all of them into stars. He coined the term globular cluster in his Catalogue of a Second Thousand New Nebulae, the number of globular clusters discovered continued to increase, reaching 83 in 1915,93 in 1930 and 97 by 1947. A total of 152 globular clusters have now discovered in the Milky Way galaxy. These additional, undiscovered globular clusters are believed to be hidden behind the gas, beginning in 1914, Harlow Shapley began a series of studies of globular clusters, published in about 40 scientific papers. He examined the RR Lyrae variables in the clusters and would use their period–luminosity relationship for distance estimates, later, it was found that RR Lyrae variables are fainter than Cepheid variables, which caused Shapley to overestimate the distance to the clusters. Of the globular clusters within the Milky Way, the majority are found in a halo around the core
21.
Accuracy and precision
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Precision is a description of random errors, a measure of statistical variability. The two concepts are independent of other, so a particular set of data can be said to be either accurate, or precise. In the fields of science, engineering and statistics, the accuracy of a measurement system is the degree of closeness of measurements of a quantity to that quantitys true value. The precision of a measurement system, related to reproducibility and repeatability, is the degree to which repeated measurements under unchanged conditions show the same results. Although the two words precision and accuracy can be synonymous in colloquial use, they are contrasted in the context of the scientific method. A measurement system can be accurate but not precise, precise but not accurate, neither, for example, if an experiment contains a systematic error, then increasing the sample size generally increases precision but does not improve accuracy. The result would be a consistent yet inaccurate string of results from the flawed experiment, eliminating the systematic error improves accuracy but does not change precision. A measurement system is considered if it is both accurate and precise. Related terms include bias and error, the terminology is also applied to indirect measurements—that is, values obtained by a computational procedure from observed data. Statistical literature prefers to use the terms bias and variability instead of accuracy and precision, bias is the amount of inaccuracy and variability is the amount of imprecision. In military terms, accuracy refers primarily to the accuracy of fire, ideally a measurement device is both accurate and precise, with measurements all close to and tightly clustered around the true value. The accuracy and precision of a measurement process is established by repeatedly measuring some traceable reference standard. Such standards are defined in the International System of Units and maintained by national organizations such as the National Institute of Standards. This also applies when measurements are repeated and averaged, further, the central limit theorem shows that the probability distribution of the averaged measurements will be closer to a normal distribution than that of individual measurements. With regard to accuracy we can distinguish, the difference between the mean of the measurements and the value, the bias. Establishing and correcting for bias is necessary for calibration, the combined effect of that and precision. A common convention in science and engineering is to express accuracy and/or precision implicitly by means of significant figures, here, when not explicitly stated, the margin of error is understood to be one-half the value of the last significant place. For instance, a recording of 843.6 m, or 843.0 m, or 800.0 m would imply a margin of 0.05 m, while a recording of 8,436 m would imply a margin of error of 0.5 m
22.
Apparent magnitude
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The apparent magnitude of a celestial object is a number that is a measure of its brightness as seen by an observer on Earth. The brighter an object appears, the lower its magnitude value, the Sun, at apparent magnitude of −27, is the brightest object in the sky. It is adjusted to the value it would have in the absence of the atmosphere, furthermore, the magnitude scale is logarithmic, a difference of one in magnitude corresponds to a change in brightness by a factor of 5√100, or about 2.512. The measurement of apparent magnitudes or brightnesses of celestial objects is known as photometry, apparent magnitudes are used to quantify the brightness of sources at ultraviolet, visible, and infrared wavelengths. An apparent magnitude is measured in a specific passband corresponding to some photometric system such as the UBV system. In standard astronomical notation, an apparent magnitude in the V filter band would be denoted either as mV or often simply as V, the scale used to indicate magnitude originates in the Hellenistic practice of dividing stars visible to the naked eye into six magnitudes. The brightest stars in the sky were said to be of first magnitude, whereas the faintest were of sixth magnitude. Each grade of magnitude was considered twice the brightness of the following grade and this rather crude scale for the brightness of stars was popularized by Ptolemy in his Almagest, and is generally believed to have originated with Hipparchus. This implies that a star of magnitude m is 2.512 times as bright as a star of magnitude m +1 and this figure, the fifth root of 100, became known as Pogsons Ratio. The zero point of Pogsons scale was defined by assigning Polaris a magnitude of exactly 2. However, with the advent of infrared astronomy it was revealed that Vegas radiation includes an Infrared excess presumably due to a disk consisting of dust at warm temperatures. At shorter wavelengths, there is negligible emission from dust at these temperatures, however, in order to properly extend the magnitude scale further into the infrared, this peculiarity of Vega should not affect the definition of the magnitude scale. Therefore, the scale was extrapolated to all wavelengths on the basis of the black body radiation curve for an ideal stellar surface at 11000 K uncontaminated by circumstellar radiation. On this basis the spectral irradiance for the zero magnitude point, with the modern magnitude systems, brightness over a very wide range is specified according to the logarithmic definition detailed below, using this zero reference. In practice such apparent magnitudes do not exceed 30, astronomers have developed other photometric zeropoint systems as alternatives to the Vega system. The AB magnitude zeropoint is defined such that an objects AB, the dimmer an object appears, the higher the numerical value given to its apparent magnitude, with a difference of 5 magnitudes corresponding to a brightness factor of exactly 100. Since an increase of 5 magnitudes corresponds to a decrease in brightness by a factor of exactly 100, each magnitude increase implies a decrease in brightness by the factor 5√100 ≈2.512. Inverting the above formula, a magnitude difference m1 − m2 = Δm implies a brightness factor of F2 F1 =100 Δ m 5 =100.4 Δ m ≈2.512 Δ m
23.
Air mass (astronomy)
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In astronomy, air mass is the optical path length through Earth’s atmosphere for light from a celestial source. As it passes through the atmosphere, light is attenuated by scattering and absorption, the atmosphere through which it passes. Consequently, celestial bodies at the horizon appear less bright than when at the zenith, the attenuation, known as atmospheric extinction, is described quantitatively by the Beer–Lambert–Bouguer law. “Air mass” normally indicates relative air mass, the path length relative to that at the zenith at sea level so, by definition, Air mass increases as the angle between the source and the zenith increases, reaching a value of approximately 38 at the horizon. The region above Earth’s atmosphere, where there is no attenuation of solar radiation, is considered to have “air mass zero”. Tables of air mass have been published by authors, including Bemporad, Allen. The angle of a body with the zenith is the zenith angle. A body’s angular position can also be given in terms of altitude, the angle above the horizon, the altitude h. Atmospheric refraction causes light to follow a circular path that is slightly longer than the geometric path. Most air mass formulas are based on the apparent zenith angle, when the zenith angle is small to moderate, a good approximation is given by assuming a homogeneous plane-parallel atmosphere. The air mass X then is simply the secant of the angle z, X = sec z. At a zenith angle of 60°, the air mass is approximately 2, the Earth is not flat, however, and, depending on accuracy requirements, this formula is usable for zenith angles up to about 60° to 75°. At greater zenith angles, the accuracy degrades rapidly, with X = sec z becoming infinite at the horizon, the horizon air mass in the more-realistic spherical atmosphere is usually less than 40. Many formulas have been developed to fit tabular values of air mass, one by Young and Irvine included a simple corrective term, X = sec z t and this gives usable results up to approximately 80°, but the accuracy degrades rapidly at greater zenith angles. The calculated air mass reaches a maximum of 11.13 at 86. 6°, becomes zero at 88°, the plot of this formula on the accompanying graph includes a correction for atmospheric refraction so that the calculated air mass is for apparent rather than true zenith angle. Hardie introduced a polynomial in sec z −1, X = sec z −0.0018167 −0.0028752 −0.00080833 which gives usable results for zenith angles of up to perhaps 85°. As with the formula, the calculated air mass reaches a maximum. Rozenberg suggested X = −1, which gives reasonable results for zenith angles
24.
Light curve
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In astronomy, a light curve is a graph of light intensity of a celestial object or region, as a function of time. The light is usually in a particular frequency interval or band, the study of the light curve, together with other observations, can yield considerable information about the physical process that produces it or constrain the physical theories about it. Light waves can also be used in botany to determine a plants reactions to light intensities, in astronomy, light curves from a supernova are used to determine what type of supernova it is. If the supernovas light curve has a maximum and slopes down gradually. If the supernovas light curve has a sharp maximum, slopes down quickly. In planetary science, a curve can be used to derive the rotation period of a minor planet, moon. Thus, astronomers measure the amount of produced by an object as a function of time. The time separation of peaks in the curve gives an estimate of the rotational period of the object. The difference between the maximum and minimum brightnesses can be due to the shape of the object, or to bright, for example, an asymmetrical asteroids light curve generally has more pronounced peaks, while a more spherical objects light curve will be flatter. The Asteroid Lightcurve Database of the Collaborative Asteroid Lightcurve Link uses a code to assess the quality of a period solution for minor planet light curves. Its quality code parameter U ranges from 0 to 3, U =0 → Result later proven incorrect U =1 → Result based on fragmentary light curve, U =2 → Result based on less than full coverage. Period may be wrong by 30 percent or ambiguous, U =3 → Secure result within the precision given. A trailing plus sign or minus sign is used to indicate a slightly better or worse quality than the unsigned value. In botany, a light curve shows the response of leaf tissue or algal communities to varying light intensities. Since photosynthesis is limited by ambient carbon dioxide levels, light curves are often repeated at several different constant carbon dioxide concentrations. The AAVSO online light curve generator can plot light curves for thousands of variable stars Lightcurves, An Introduction by NASAs Imagine the Universe
25.
Galaxy
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A galaxy is a gravitationally bound system of stars, stellar remnants, interstellar gas, dust, and dark matter. The word galaxy is derived from the Greek galaxias, literally milky, Galaxies range in size from dwarfs with just a few billion stars to giants with one hundred trillion stars, each orbiting its galaxys center of mass. Galaxies are categorized according to their morphology as elliptical, spiral. Many galaxies are thought to have holes at their active centers. The Milky Ways central black hole, known as Sagittarius A*, has a four million times greater than the Sun. Recent estimates of the number of galaxies in the observable universe range from 200 billion to 2 trillion or more, most of the galaxies are 1,000 to 100,000 parsecs in diameter and separated by distances on the order of millions of parsecs. The space between galaxies is filled with a gas having an average density of less than one atom per cubic meter. The majority of galaxies are organized into groups, clusters. At the largest scale, these associations are generally arranged into sheets and filaments surrounded by immense voids. In Greek mythology, Zeus places his son born by a mortal woman, the infant Heracles, on Heras breast while she is asleep so that the baby will drink her divine milk and will thus become immortal. Hera wakes up while breastfeeding and then realizes she is nursing a baby, she pushes the baby away, some of her milk spills and. In the astronomical literature, the capitalized word Galaxy is often used to refer to our galaxy, the Milky Way, to distinguish it from the other galaxies in our universe. The English term Milky Way can be traced back to a story by Chaucer c. 1380, See yonder, lo, the Galaxyë Which men clepeth the Milky Wey, For hit is whyt. However, the word Universe was later understood to mean the entirety of existence, so this expression fell into disuse and the objects instead became known as galaxies. Tens of thousands of galaxies have been catalogued, but only a few have well-established names, such as the Andromeda Galaxy, the Magellanic Clouds, the Whirlpool Galaxy, and the Sombrero Galaxy. Astronomers work with numbers from certain catalogues, such as the Messier catalogue, the NGC, the IC, the CGCG, all of the well-known galaxies appear in one or more of these catalogues but each time under a different number. For example, Messier 109 is a galaxy having the number 109 in the catalogue of Messier, but also codes NGC3992, UGC6937, CGCG 269-023, MCG +09-20-044. One of the arguments to do so is that these impressive objects deserve better than uninspired codes, for instance, Bodifee and Berger propose the informal, descriptive name Callimorphus Ursae Majoris for the well-formed barred galaxy Messier 109 in Ursa Major
26.
Solid angle
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In geometry, a solid angle is the two-dimensional angle in three-dimensional space that an object subtends at a point. It is a measure of how large the object appears to an observer looking from that point, in the International System of Units, a solid angle is expressed in a dimensionless unit called a steradian. A small object nearby may subtend the same angle as a larger object farther away. For example, although the Moon is much smaller than the Sun, indeed, as viewed from any point on Earth, both objects have approximately the same solid angle as well as apparent size. This is evident during a solar eclipse, an objects solid angle in steradians is equal to the area of the segment of a unit sphere, centered at the angles vertex, that the object covers. A solid angle in steradians equals the area of a segment of a sphere in the same way a planar angle in radians equals the length of an arc of a unit circle. Solid angles are used in physics, in particular astrophysics. The solid angle of an object that is far away is roughly proportional to the ratio of area to squared distance. Here area means the area of the object when projected along the viewing direction. The solid angle of a sphere measured from any point in its interior is 4π sr, Solid angles can also be measured in square degrees, in square minutes and square seconds, or in fractions of the sphere, also known as spat. In spherical coordinates there is a formula for the differential, d Ω = sin θ d θ d φ where θ is the colatitude, at the equator you see all of the celestial sphere, at either pole only one half. Let OABC be the vertices of a tetrahedron with an origin at O subtended by the triangular face ABC where a →, b →, c → are the positions of the vertices A, B and C. Define the vertex angle θa to be the angle BOC and define θb, let φab be the dihedral angle between the planes that contain the tetrahedral faces OAC and OBC and define φac, φbc correspondingly. When implementing the above equation care must be taken with the function to avoid negative or incorrect solid angles. One source of errors is that the scalar triple product can be negative if a, b, c have the wrong winding. Computing abs is a sufficient solution since no other portion of the equation depends on the winding, the other pitfall arises when the scalar triple product is positive but the divisor is negative. Indices are cycled, s0 = sn and s1 = sn +1, the solid angle of a latitude-longitude rectangle on a globe is s r, where φN and φS are north and south lines of latitude, and θE and θW are east and west lines of longitude. Mathematically, this represents an arc of angle φN − φS swept around a sphere by θE − θW radians, when longitude spans 2π radians and latitude spans π radians, the solid angle is that of a sphere
27.
Minute and second of arc
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A minute of arc, arcminute, arc minute, or minute arc is a unit of angular measurement equal to 1/60 of one degree. Since one degree is 1/360 of a turn, one minute of arc is 1/21600 of a turn, a second of arc, arcsecond, or arc second is 1/60 of an arcminute, 1/3600 of a degree, 1/1296000 of a turn, and π/648000 of a radian. To express even smaller angles, standard SI prefixes can be employed, the number of square arcminutes in a complete sphere is 4 π2 =466560000 π ≈148510660 square arcminutes. The standard symbol for marking the arcminute is the prime, though a single quote is used where only ASCII characters are permitted. One arcminute is thus written 1′ and it is also abbreviated as arcmin or amin or, less commonly, the prime with a circumflex over it. The standard symbol for the arcsecond is the prime, though a double quote is commonly used where only ASCII characters are permitted. One arcsecond is thus written 1″ and it is also abbreviated as arcsec or asec. In celestial navigation, seconds of arc are used in calculations. This notation has been carried over into marine GPS receivers, which normally display latitude and longitude in the format by default. An arcsecond is approximately the angle subtended by a U. S. dime coin at a distance of 4 kilometres, a milliarcsecond is about the size of a dime atop the Eiffel Tower as seen from New York City. A microarcsecond is about the size of a period at the end of a sentence in the Apollo mission manuals left on the Moon as seen from Earth, since antiquity the arcminute and arcsecond have been used in astronomy. The principal exception is Right ascension in equatorial coordinates, which is measured in units of hours, minutes. These small angles may also be written in milliarcseconds, or thousandths of an arcsecond, the unit of distance, the parsec, named from the parallax of one arcsecond, was developed for such parallax measurements. It is the distance at which the radius of the Earths orbit would subtend an angle of one arcsecond. The ESA astrometric space probe Gaia is hoped to measure star positions to 20 microarcseconds when it begins producing catalog positions sometime after 2016, there are about 1.3 trillion µas in a turn. Currently the best catalog positions of stars actually measured are in terms of milliarcseconds, apart from the Sun, the star with the largest angular diameter from Earth is R Doradus, a red supergiant with a diameter of 0.05 arcsecond. The dwarf planet Pluto has proven difficult to resolve because its angular diameter is about 0.1 arcsecond, space telescopes are not affected by the Earths atmosphere but are diffraction limited. For example, the Hubble space telescope can reach a size of stars down to about 0. 1″
28.
Inverse-square law
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The fundamental cause for this can be understood as geometric dilution corresponding to point-source radiation into three-dimensional space. Newtons law of gravitation follows an inverse-square law, as do the effects of electric, magnetic, light, sound. The inverse-square law generally applies when some force, energy, or other conserved quantity is evenly radiated outward from a point source in three-dimensional space, hence, the intensity of radiation passing through any unit area is inversely proportional to the square of the distance from the point source. Gausss law is applicable, and can be used with any physical quantity that acts in accord to the inverse-square relationship. Gravitation is the attraction of two objects with mass, the force is always attractive and acts along the line joining them. If the distribution of matter in each body is spherically symmetric, then the objects can be treated as point masses without approximation, as shown in the shell theorem. Otherwise, if we want to calculate the attraction between bodies, we need to add all the point-point attraction forces vectorially and the net attraction might not be exact inverse square. As the law of gravitation, this law was suggested in 1645 by Ismael Bullialdus, but Bullialdus did not accept Kepler’s second and third laws, nor did he appreciate Christiaan Huygens’s solution for circular motion. Indeed, Bullialdus maintained the suns force was attractive at aphelion, robert Hooke and Giovanni Alfonso Borelli both expounded gravitation in 1666 as an attractive force. By 1679, Hooke thought gravitation had inverse square dependence and communicated this in a letter to Isaac Newton, the deviation of the exponent from 2 is less than one part in 1015. More generally, the irradiance, i. e. the intensity, for non-isotropic radiators such as parabolic antennas, headlights, and lasers, the effective origin is located far behind the beam aperture. If you are close to the origin, you dont have to go far to double the radius, so the signal drops quickly. When you are far from the origin and still have a signal, like with a laser, you have to travel very far to double the radius. This means you have a signal or have antenna gain in the direction of the narrow beam relative to a wide beam in all directions of an isotropic antenna. In photography and stage lighting, the law is used to determine the fall off or the difference in illumination on a subject as it moves closer to or further from the light source. The fractional reduction in electromagnetic fluence for indirectly ionizing radiation with increasing distance from a point source can be calculated using the inverse-square law, since emissions from a point source have radial directions, they intercept at a perpendicular incidence. The area of such a shell is 4πr 2 where r is the distance from the center. At large distances from the source, this power is distributed over larger and larger spherical surfaces as the distance from the source increases
29.
Luminosity
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In astronomy, luminosity is the total amount of energy emitted by a star, galaxy, or other astronomical object per unit time. It is related to the brightness, which is the luminosity of an object in a spectral region. In SI units luminosity is measured in joules per second or watts, values for luminosity are often given in the terms of the luminosity of the Sun, which has a total power output of 7026384600000000000♠3. 846×1026 W. The symbol for solar luminosity is L⊙. Luminosity can also be given in terms of magnitude, the absolute bolometric magnitude of an object is a logarithmic measure of its total energy emission. In astronomy, luminosity is the amount of energy a body radiates per unit of time. It is most frequently measured in two forms, visual and bolometric, although luminosities at other wavelengths are increasingly being used as instruments become available to measure them, a bolometer is the instrument used to measure radiant energy over a wide band by absorption and measurement of heating. When not qualified, the term luminosity means bolometric luminosity, which is measured either in the SI units, watts, a star also radiates neutrinos, which carry off some energy, contributing to the stars total luminosity. In practice bolometric magnitudes are measured by taking measurements at certain wavelengths, a stars luminosity can be determined from two stellar characteristics, size and effective temperature. The former is represented in terms of solar radii, R⊙, while the latter is represented in kelvins. To determine a stars radius, two metrics are needed, the stars angular diameter and its distance from Earth, often calculated using parallax. However, for most stars the angular diameter or parallax, or both, are far below our ability to measure with any certainty, an alternate way to measure stellar luminosity is to measure the stars apparent brightness and distance. Because luminosity is proportional to temperature to the power, the large variation in stellar temperatures produces an even vaster variation in stellar luminosity. Because the luminosity depends on a power of the stellar mass. The most luminous stars are young stars, no more than a few million years for the most extreme. In the Hertzsprung–Russell diagram, the x-axis represents temperature or spectral type while the y-axis represents luminosity or magnitude. The vast majority of stars are found along the sequence with blue Class 0 stars found at the top left of the chart while red Class M stars fall to the bottom right. Certain stars like Deneb and Betelgeuse are found above and to the right of the main sequence, blue and white supergiants are high luminosity stars somewhat cooler than the most luminous main sequence stars. A star like Deneb, for example, has a luminosity around 200,000 L⊙, a type of A2
30.
Distance
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Distance is a numerical description of how far apart objects are. In physics or everyday usage, distance may refer to a physical length, in most cases, distance from A to B is interchangeable with distance from B to A. In mathematics, a function or metric is a generalization of the concept of physical distance. A metric is a function that behaves according to a set of rules. The circumference of the wheel is 2π × radius, and assuming the radius to be 1, in engineering ω = 2πƒ is often used, where ƒ is the frequency. Chessboard distance, formalized as Chebyshev distance, is the number of moves a king must make on a chessboard to travel between two squares. Distance measures in cosmology are complicated by the expansion of the universe, the term distance is also used by analogy to measure non-physical entities in certain ways. In computer science, there is the notion of the distance between two strings. For example, the dog and dot, which vary by only one letter, are closer than dog and cat. In this way, many different types of distances can be calculated, such as for traversal of graphs, comparison of distributions and curves, distance cannot be negative, and distance travelled never decreases. Distance is a quantity or a magnitude, whereas displacement is a vector quantity with both magnitude and direction. Directed distance is a positive, zero, or negative scalar quantity, the distance covered by a vehicle, person, animal, or object along a curved path from a point A to a point B should be distinguished from the straight-line distance from A to B. For example, whatever the distance covered during a trip from A to B and back to A. In general the straight-line distance does not equal distance travelled, except for journeys in a straight line, directed distances are distances with a directional sense. They can be determined along straight lines and along curved lines, for instance, just labelling the two endpoints as A and B can indicate the sense, if the ordered sequence is assumed, which implies that A is the starting point. A displacement is a kind of directed distance defined in mechanics. A directed distance is called displacement when it is the distance along a line from A and B. This implies motion of the particle, the distance traveled by a particle must always be greater than or equal to its displacement, with equality occurring only when the particle moves along a straight path
31.
Temperature
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A temperature is an objective comparative measurement of hot or cold. It is measured by a thermometer, several scales and units exist for measuring temperature, the most common being Celsius, Fahrenheit, and, especially in science, Kelvin. Absolute zero is denoted as 0 K on the Kelvin scale, −273.15 °C on the Celsius scale, the kinetic theory offers a valuable but limited account of the behavior of the materials of macroscopic bodies, especially of fluids. Temperature is important in all fields of science including physics, geology, chemistry, atmospheric sciences, medicine. The Celsius scale is used for temperature measurements in most of the world. Because of the 100 degree interval, it is called a centigrade scale.15, the United States commonly uses the Fahrenheit scale, on which water freezes at 32°F and boils at 212°F at sea-level atmospheric pressure. Many scientific measurements use the Kelvin temperature scale, named in honor of the Scottish physicist who first defined it and it is a thermodynamic or absolute temperature scale. Its zero point, 0K, is defined to coincide with the coldest physically-possible temperature and its degrees are defined through thermodynamics. The temperature of zero occurs at 0K = −273. 15°C. For historical reasons, the triple point temperature of water is fixed at 273.16 units of the measurement increment, Temperature is one of the principal quantities in the study of thermodynamics. There is a variety of kinds of temperature scale and it may be convenient to classify them as empirically and theoretically based. Empirical temperature scales are historically older, while theoretically based scales arose in the middle of the nineteenth century, empirically based temperature scales rely directly on measurements of simple physical properties of materials. For example, the length of a column of mercury, confined in a capillary tube, is dependent largely on temperature. Such scales are only within convenient ranges of temperature. For example, above the point of mercury, a mercury-in-glass thermometer is impracticable. A material is of no use as a thermometer near one of its phase-change temperatures, in spite of these restrictions, most generally used practical thermometers are of the empirically based kind. Especially, it was used for calorimetry, which contributed greatly to the discovery of thermodynamics, nevertheless, empirical thermometry has serious drawbacks when judged as a basis for theoretical physics. Theoretically based temperature scales are based directly on theoretical arguments, especially those of thermodynamics, kinetic theory and they rely on theoretical properties of idealized devices and materials
32.
Variable star
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A variable star is a star whose brightness as seen from Earth fluctuates. Many, possibly most, stars have at least some variation in luminosity, an ancient Egyptian calendar of lucky and unlucky days composed some 3,200 years ago may be the oldest preserved historical document of the discovery of a variable star, the eclipsing binary Algol. This discovery, combined with supernovae observed in 1572 and 1604, proved that the sky was not eternally invariable as Aristotle. In this way, the discovery of variable stars contributed to the revolution of the sixteenth. The second variable star to be described was the eclipsing variable Algol, by Geminiano Montanari in 1669, chi Cygni was identified in 1686 by G. Kirch, then R Hydrae in 1704 by G. D. Maraldi. By 1786 ten variable stars were known, John Goodricke himself discovered Delta Cephei and Beta Lyrae. Since 1850 the number of variable stars has increased rapidly, especially after 1890 when it became possible to identify variable stars by means of photography. The latest edition of the General Catalogue of Variable Stars lists more than 46,000 variable stars in the Milky Way, as well as 10,000 in other galaxies, and over 10,000 suspected variables. The most common kinds of variability involve changes in brightness, but other types of variability also occur, by combining light curve data with observed spectral changes, astronomers are often able to explain why a particular star is variable. Variable stars are generally analysed using photometry, spectrophotometry and spectroscopy, measurements of their changes in brightness can be plotted to produce light curves. Peak brightnesses in the curve are known as maxima, while troughs are known as minima. By estimating the magnitude and noting the time of observation a visual lightcurve can be constructed. The American Association of Variable Star Observers collects such observations from participants around the world, from the light curve the following data are derived, are the brightness variations periodical, semiperiodical, irregular, or unique. What is the period of the brightness fluctuations, what is the shape of the light curve. From the spectrum the following data are derived, what kind of star is it, what is its temperature, is it a single star, or a binary. does the spectrum change with time. In very few cases it is possible to make pictures of a stellar disk and these may show darker spots on its surface. Combining light curves with spectral data often gives a clue as to the changes occur in a variable star. For example, evidence for a star is found in its shifting spectrum because its surface periodically moves toward and away from us
33.
Minor planet
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A minor planet is an astronomical object in direct orbit around the Sun that is neither a planet nor exclusively classified as a comet. Minor planets can be dwarf planets, asteroids, trojans, centaurs, Kuiper belt objects, as of 2016, the orbits of 709,706 minor planets were archived at the Minor Planet Center,469,275 of which had received permanent numbers. The first minor planet to be discovered was Ceres in 1801, the term minor planet has been used since the 19th century to describe these objects. The term planetoid has also used, especially for larger objects such as those the International Astronomical Union has called dwarf planets since 2006. Historically, the asteroid, minor planet, and planetoid have been more or less synonymous. This terminology has become complicated by the discovery of numerous minor planets beyond the orbit of Jupiter. A Minor planet seen releasing gas may be classified as a comet. Before 2006, the IAU had officially used the term minor planet, during its 2006 meeting, the IAU reclassified minor planets and comets into dwarf planets and small Solar System bodies. Objects are called dwarf planets if their self-gravity is sufficient to achieve hydrostatic equilibrium, all other minor planets and comets are called small Solar System bodies. The IAU stated that the minor planet may still be used. However, for purposes of numbering and naming, the distinction between minor planet and comet is still used. Hundreds of thousands of planets have been discovered within the Solar System. The Minor Planet Center has documented over 167 million observations and 729,626 minor planets, of these,20,570 have official names. As of March 2017, the lowest-numbered unnamed minor planet is 1974 FV1, as of March 2017, the highest-numbered named minor planet is 458063 Gustavomuler. There are various broad minor-planet populations, Asteroids, traditionally, most have been bodies in the inner Solar System. Near-Earth asteroids, those whose orbits take them inside the orbit of Mars. Further subclassification of these, based on distance, is used, Apohele asteroids orbit inside of Earths perihelion distance. Aten asteroids, those that have semi-major axes of less than Earths, Apollo asteroids are those asteroids with a semimajor axis greater than Earths, while having a perihelion distance of 1.017 AU or less. Like Aten asteroids, Apollo asteroids are Earth-crossers, amor asteroids are those near-Earth asteroids that approach the orbit of Earth from beyond, but do not cross it
34.
Active galactic nucleus
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Such excess non-stellar emission has been observed in the radio, microwaves, infrared, optical, ultra-violet, X-ray and gamma ray wavebands. A galaxy hosting an AGN is called an active galaxy, the radiation from an AGN is believed to be a result of accretion of matter by a supermassive black hole at the center of its host galaxy. Numerous subclasses of AGN have been defined based on their observed characteristics, early photographic observations of nearby galaxies detected some characteristic signatures of AGN emission, although there was not yet a physical understanding of the nature of the AGN phenomenon. Further spectroscopic studies by astronomers including Vesto Slipher, Milton Humason, in 1943, Carl Seyfert published a paper in which he described observations of nearby galaxies having bright nuclei that were sources of unusually broad emission lines. Galaxies observed as part of study included NGC1068, NGC4151, NGC3516. Active galaxies such as these are known as Seyfert galaxies in honor of Seyferts pioneering work, the development of radio astronomy was a major catalyst to understanding AGN. Some of the earliest detected radio sources are nearby active elliptical galaxies such as Messier 87, the 3C radio survey led to further progress in discovery of new radio sources as well as identifying the visible-light sources associated with the radio emission. In photographic images, some of these objects were nearly point-like or quasi-stellar in appearance, a major breakthrough was the measurement of the redshift of the quasar 3C273 by Maarten Schmidt, published in 1963. Shortly afterward, optical spectra were used to measured the redshifts of a number of quasars including 3C48. The enormous luminosities of these quasars as well as their unusual spectral properties indicated that their power source could not be ordinary stars, Accretion of gas onto a supermassive black hole was suggested as the source of quasars power in papers by Edwin Salpeter and Yakov ZelDovich in 1964. Today, AGN are a topic of astrophysical research, both observational and theoretical. For a long time it has argued that an AGN must be powered by accretion of mass onto massive black holes. AGN are both compact and persistently extremely luminous, thus AGN-like characteristics are expected whenever a supply of material for accretion comes within the sphere of influence of the central black hole. In the standard model of AGN, cold material close to a black hole forms an accretion disc, dissipative processes in the accretion disc transport matter inwards and angular momentum outwards, while causing the accretion disc to heat up. The radiation from the accretion disc excites cold atomic material close to the black hole, a large fraction of the AGNs radiation may be obscured by interstellar gas and dust close to the accretion disc, but this will be re-radiated at some other waveband, most likely the infrared. Some accretion discs produce jets of twin, highly collimated, the direction of the jet ejection is determined either by the angular momentum axis of the accretion disc or the spin axis of the black hole. The jet production mechanism and indeed the jet composition on very small scales are not understood at present due to the resolution of astronomical instruments being too low, there exists a class of radiatively inefficient solutions to the equations that govern accretion. The most widely known of these is the Advection Dominated Accretion Flow, radiatively inefficient AGN would be expected to lack many of the characteristic features of standard AGN with an accretion disc
35.
Supernova
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This causes the sudden appearance of a new bright star, before slowly fading from sight over several weeks or months. Supernovae are more energetic than novae, in Latin, nova means new, referring astronomically to what appears to be a temporary new bright star. Adding the prefix super- distinguishes supernovae from ordinary novae, which are far less luminous, the word supernova was coined by Walter Baade and Fritz Zwicky in 1931. It is pronounced /ˌsuːpərnoʊvə/ with the plural supernovae /ˌsuːpərnoʊviː/ or supernovas, only three Milky Way naked-eye supernova events have been observed during the last thousand years, though many have been seen in other galaxies using telescopes. The most recent directly observed supernova in the Milky Way was Keplers Supernova in 1604, Supernovae may expel much, if not all, of the material away from a star, at velocities up to 30,000 km/s or 10% of the speed of light. This drives an expanding and fast-moving shock wave into the interstellar medium, and in turn, sweeping up an expanding shell of gas and dust. Supernovae create, fuse and eject the bulk of the chemical elements produced by nucleosynthesis, Supernovae play a significant role in enriching the interstellar medium with the heavier atomic mass chemical elements. Furthermore, the shock waves from supernovae can trigger the formation of new stars. Supernova remnants are expected to accelerate a large fraction of primary cosmic rays. They are also potentially strong galactic sources of gravitational waves, in the second case, the core of a massive star may undergo sudden gravitational collapse, releasing gravitational potential energy as a supernova. While some observed supernovae are more complex than these two simplified theories, the astrophysical collapse mechanics have been established and accepted by most astronomers for some time, in Latin, Nova means new, referring astronomically to what appears to be a temporary new bright star. Adding the prefix super- distinguishes supernovae from ordinary novae, which are far less luminous, the word supernova was coined by Walter Baade and Fritz Zwicky in 1931. It is pronounced /ˌsuːpərnoʊvə/ with the plural supernovae /ˌsuːpərnoʊviː/ or supernovas, the earliest recorded supernova, SN185, was viewed by Chinese astronomers in 185 AD. The brightest recorded supernova was SN1006, which occurred in 1006 AD and was described in detail by Chinese, the widely observed supernova SN1054 produced the Crab Nebula. Johannes Kepler began observing SN1604 at its peak on October 17,1604 and it was the second supernova to be observed in a generation. There is some evidence that the youngest galactic supernova, G1. 9+0.3, occurred in the late 19th century, neither supernova was noted at the time. In the case of G1. 9+0.3, high extinction along the plane of the galaxy could have dimmed the event sufficiently to go unnoticed, the situation for Cassiopeia A is less clear. Infrared light echos have been detected showing that it was a type IIb supernova and was not in a region of especially high extinction, before the development of the telescope, there have only been five supernovae seen in the last millennium
36.
Methods of detecting exoplanets
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Any planet is an extremely faint light source compared to its parent star. For example, a star like the Sun is about a billion times as bright as the light from any of the planets orbiting it. In addition to the difficulty of detecting such a faint light source. For those reasons, very few of the extrasolar planets reported as of April 2014 have been observed directly, instead, astronomers have generally had to resort to indirect methods to detect extrasolar planets. As of 2016, several different indirect methods have yielded success and this leads to variations in the speed with which the star moves toward or away from Earth, i. e. the variations are in the radial velocity of the star with respect to Earth. The radial velocity can be deduced from the displacement in the parent stars spectral lines due to the Doppler effect, the radial-velocity method measures these variations in order to confirm the presence of the planet using the binary mass function. The speed of the star around the center of mass is much smaller than that of the planet. An especially simple and inexpensive method for measuring radial velocity is externally dispersed interferometry, until 2014, the radial-velocity method was by far the most productive technique used by planet hunters. It is also known as Doppler spectroscopy and it is also not possible to simultaneously observe many target stars at a time with a single telescope. Planets of Jovian mass can be detectable around stars up to a few light years away. This method easily finds massive planets that are close to stars, modern spectrographs can also easily detect Jupiter-mass planets orbiting 10 astronomical units away from the parent star, but detection of those planets requires many years of observation. It is easier to detect planets around stars, for two reasons, First, these stars are more affected by gravitational tug from planets. The second reason is that low-mass main-sequence stars generally rotate relatively slowly, fast rotation makes spectral-line data less clear because half of the star quickly rotates away from observers viewpoint while the other half approaches. Detecting planets around more massive stars is easier if the star has left the main sequence, sometimes Doppler spectrography produces false signals, especially in multi-planet and multi-star systems. Magnetic fields and certain types of activity can also give false signals. When the host star has multiple planets, false signals can also arise from having insufficient data, so that solutions can fit the data. Planets with orbits highly inclined to the line of sight from Earth produce smaller visible wobbles, one of the advantages of the radial velocity method is that eccentricity of the planets orbit can be measured directly. One of the disadvantages of the radial-velocity method is that it can only estimate a planets minimum mass
37.
Binary star
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A binary star is a star system consisting of two stars orbiting around their common barycenter. Systems of two or more stars are called multiple star systems and these systems, especially when more distant, often appear to the unaided eye as a single point of light, and are then revealed as multiple by other means. Research over the last two centuries suggests that half or more of visible stars are part of star systems. The term double star is used synonymously with binary star, however. Optical doubles are so called because the two stars close together in the sky as seen from the Earth, they are almost on the same line of sight. Nevertheless, their doubleness depends only on this effect, the stars themselves are distant from one another. A double star can be revealed as optical by means of differences in their measurements, proper motions. Most known double stars have not been studied closely to determine whether they are optical doubles or they are doubles physically bound through gravitation into a multiple star system. This also determines an empirical mass-luminosity relationship from which the masses of stars can be estimated. Binary stars are often detected optically, in case they are called visual binaries. Many visual binaries have long periods of several centuries or millennia. They may also be detected by indirect techniques, such as spectroscopy or astrometry, if components in binary star systems are close enough they can gravitationally distort their mutual outer stellar atmospheres. In some cases, these close binary systems can exchange mass, examples of binaries are Sirius, and Cygnus X-1. Binary stars are common as the nuclei of many planetary nebulae. This should be called a double star, and any two stars that are thus mutually connected, form the binary sidereal system which we are now to consider. By the modern definition, the binary star is generally restricted to pairs of stars which revolve around a common center of mass. Binary stars which can be resolved with a telescope or interferometric methods are known as visual binaries, for most of the known visual binary stars one whole revolution has not been observed yet, they are observed to have travelled along a curved path or a partial arc. The more general term double star is used for pairs of stars which are seen to be together in the sky
38.
Albedo
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Albedo is a measure for reflectance or optical brightness. It is dimensionless and measured on a scale from zero to one, surface albedo is defined as the ratio of radiation reflected to the radiation incident on a surface. The proportion reflected is not only determined by properties of the surface itself and these factors vary with atmospheric composition, geographic location and time. While bi-hemispherical reflectance is calculated for an angle of incidence. The temporal resolution may range from seconds to daily, seasonal or annual averages, unless given for a specific wavelength, albedo refers to the entire spectrum of solar radiation. Due to measurement constraints, it is given for the spectrum in which most solar energy reaches the surface. This spectrum includes visible light, which explains why surfaces with a low albedo appear dark, albedo is an important concept in climatology, astronomy, and environmental management. The term albedo was introduced into optics by Johann Heinrich Lambert in his 1760 work Photometria, any albedo in visible light falls within a range of about 0.9 for fresh snow to about 0.04 for charcoal, one of the darkest substances. Deeply shadowed cavities can achieve an effective albedo approaching the zero of a black body, when seen from a distance, the ocean surface has a low albedo, as do most forests, whereas desert areas have some of the highest albedos among landforms. Most land areas are in a range of 0.1 to 0.4. The average albedo of Earth is about 0.3 and this is far higher than for the ocean primarily because of the contribution of clouds. Earths surface albedo is regularly estimated via Earth observation satellite sensors such as NASAs MODIS instruments on board the Terra, thereby, the BRDF allows to translate observations of reflectance into albedo. Earths average surface temperature due to its albedo and the effect is currently about 15 °C. If Earth were frozen entirely, the temperature of the planet would drop below −40 °C. If only the land masses became covered by glaciers, the mean temperature of the planet would drop to about 0 °C. In contrast, if the entire Earth was covered by water — a so-called aquaplanet — the average temperature on the planet would rise to almost 27 °C, hence, the actual albedo α can then be given as, α = α ¯ + D α ¯ ¯. Directional-hemispherical reflectance is sometimes referred to as black-sky albedo and bi-hemispherical reflectance as white-sky albedo and these terms are important because they allow the albedo to be calculated for any given illumination conditions from a knowledge of the intrinsic properties of the surface. The albedos of planets, satellites and asteroids can be used to infer much about their properties, the study of albedos, their dependence on wavelength, lighting angle, and variation in time comprises a major part of the astronomical field of photometry
39.
Bidirectional reflectance distribution function
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The bidirectional reflectance distribution function is a function of four real variables that defines how light is reflected at an opaque surface. It is employed both in the optics of real-world light, in computer algorithms, and in computer vision algorithms. Each direction ω is itself parameterized by azimuth angle ϕ and zenith angle θ, the BRDF has units sr−1, with steradians being a unit of solid angle. The BRDF was first defined by Fred Nicodemus around 1965, the index i indicates incident light, whereas the index r indicates reflected light. The Spatially Varying Bidirectional Reflectance Distribution Function is a 6-dimensional function, f r, the functions defined by the BTF at each point on the surface are thus called Apparent BRDFs. In all these cases, the dependence on the wavelength of light has been ignored and binned into RGB channels, in reality, the BRDF is wavelength dependent, and to account for effects such as iridescence or luminescence the dependence on wavelength must be made explicit, f r. BRDF has also used for modeling light trapping in solar cells or low concentration solar photovoltaic systems. In the context of satellite remote sensing, NASA uses a BRDF model to characterise surface anisotropy, for a given land area, the BRDF is established based on selected multiangular observations of surface reflectance. While single observations depend on view geometry and solar angle, the MODIS BRDF/Albedo product describes intrinsic surface properties in several spectral bands, the BRDF/Albedo product can be used to model surface albedo depending on atmospheric scattering. W. Matusik et al. found that interpolating between measured samples produced realistic results and was easy to understand, lambertian model, representing perfectly diffuse surfaces by a constant BRDF. Phong reflectance model, a phenomenological model akin to plastic-like specularity, blinn–Phong model, resembling Phong, but allowing for certain quantities to be interpolated, reducing computational overhead. Torrance–Sparrow model, a model representing surfaces as distributions of perfectly specular microfacets. Cook–Torrance model, a specular-microfacet model accounting for wavelength and thus color shifting, ward model, a specular-microfacet model with an elliptical-Gaussian distribution function dependent on surface tangent orientation. Oren–Nayar model, a directed-diffuse microfacet model, with perfectly diffuse microfacets, ashikhmin-Shirley model, allowing for anisotropic reflectance, along with a diffuse substrate under a specular surface. HTSG, a physically based model. Fitted Lafortune model, a generalization of Phong with multiple specular lobes, lebedev model for analytical-grid BRDF approximation. Traditionally, BRDF measurements were taken for one specific lighting and viewing direction at a time using gonioreflectometers, unfortunately, using such a device to densely measure the BRDF is very time consuming. One of the first improvements on techniques used a half-silvered mirror
, Bibcode:2014MNRAS.444..392C, doi:10.1093/mnras/stu1476
