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Bolometric Magnitude
ABSOLUTE MAGNITUDE is a measure of the luminosity of a celestial object , on a logarithmic astronomical magnitude scale. An object's absolute magnitude is defined to be equal to the apparent magnitude that the object would have if were viewed from a distance of exactly 10 parsecs (32.6 light years ), with no extinction (or dimming) of its light due to absorption by interstellar dust particles. By hypothetically placing all objects at a standard reference distance from the observer, their luminosities can be directly compared on a magnitude scale. As with all astronomical magnitudes , the absolute magnitude can be specified for different wavelength ranges corresponding to specified filter bands or passbands ; for stars a commonly quoted absolute magnitude is the ABSOLUTE VISUAL MAGNITUDE, which uses the visual (V) band of the spectrum (in the UBV photometric system )
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Absolute Magnitude (magazine)
ABSOLUTE MAGNITUDE is a discontinued, semi-professional science fiction magazine started in 1993 under the name Harsh Mistress. However, in 1994 after only two issues the name was changed to Absolute Magnitude. In 2002 the name was changed again to Absolute Magnitude "> Absolute Magnitude was nominated for the 2002 Hugo Award for Best Semiprozine with Lapine noted as the editor. CONTENTS * 1 Anthology * 2 Notable contributors * 3 See also * 4 References ANTHOLOGYAbsolute Magnitude is also a collection of sixteen stories taken from the magazine between 1993 and 1997. The anthology was published by Tor Books and was released on April 15, 1997. It has been issued in both hardcover and paperback editions. NOTABLE CONTRIBUTORS * Ben Bova * Terry Bisson
Terry Bisson
* Hal Clement
Hal Clement
* Alan Dean Foster * Barry B
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Luminosity
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 given 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
Sun
, L⊙, which has a total power output of 7026384600000000000♠3.846×1026 W . Luminosity
Luminosity
can also be given in terms of magnitude. The absolute bolometric magnitude (Mbol) of an object is a logarithmic measure of its total energy emission
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Celestial Object
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 terms "object" and "body" are often used interchangeably. However, an ASTRONOMICAL BODY or CELESTIAL BODY is a single, tightly bound contiguous entity, while an astronomical or celestial object is a complex, less cohesively bound structure, that may consist of multiple bodies or even other objects with substructures. 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 frozen nucleus of ice and dust, and an object when describing the entire comet with its diffuse coma and tail
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Logarithmic Scale
A LOGARITHMIC SCALE is a nonlinear scale used when there is a large range of quantities. Common uses include the earthquake strength , sound loudness , light intensity , and pH of solutions . It is based on orders of magnitude , rather than a standard linear scale , so the value represented by each equidistant mark on the scale is the value at the previous mark multiplied by a constant
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Magnitude (astronomy)
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
Hipparchus
. Astronomers use two different definitions of magnitude: apparent magnitude and absolute magnitude. The apparent magnitude (m, or vmag for the visible spectrum) is the brightness of an object as it appears in the night sky from Earth, while the absolute magnitude (Mv, V and H) describes the intrinsic brightness of an object as it would appear if it were placed at a certain distance from Earth. This distance is 10 parsecs for stars and 1 astronomical unit for planets and small Solar System
Solar System
bodies . A minor planet 's size is typically estimated based on its absolute magnitude in combination with its presumed albedo
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Apparent Magnitude
The APPARENT MAGNITUDE (M) 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 (i.e. inverse relation). 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 usually measured in a specific passband corresponding to some photometric system such as the UBV system
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Parsecs
The PARSEC (symbol: PC) is a unit of length used to measure large distances to astronomical objects outside the Solar System
Solar System
. A parsec is the distance at which one astronomical unit subtends an angle of one arcsecond . One parsec is equal to about 3.26 light-years (31 trillion km or 19 trillion miles) in length. The nearest star, Proxima Centauri , is about 1.3 parsecs (4.2 light-years) from the Sun
Sun
. Most of the stars visible to the unaided eye in the night sky are within 500 parsecs of the Sun. The parsec unit was likely first suggested in 1913 by the British astronomer Herbert Hall Turner . Named as a portmanteau of the PARallax of one arcSECond, it was defined so as to make calculations of astronomical distances quick and easy for astronomers from only their raw observational data
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Light Year
The LIGHT-YEAR is a unit of length used to express astronomical distances . It is about 9.5 quadrillion metres or 5.9 trillion miles . As defined by the International Astronomical Union
International Astronomical Union
(IAU), a light-year is the distance that light travels in vacuum in one Julian year (365.25 days). Because it includes the word "year ", the term light-year is sometimes misinterpreted as a unit of time. The light-year is most often used when expressing distances to stars and other distances on a galactic scale, especially in nonspecialist and popular science publications. The unit usually used in professional astrometry is the parsec (symbol: pc, about 3.26 light-years; the distance at which one astronomical unit subtends an angle of one second of arc )
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Extinction (astronomy)
In astronomy , EXTINCTION is the absorption and scattering of electromagnetic radiation by dust and gas between an emitting astronomical object and the observer . Interstellar extinction was first documented as such in 1930 by Robert Julius Trumpler
Robert Julius Trumpler
. However, its effects had been noted in 1847 by Friedrich Georg Wilhelm von Struve , and its effect on the colors of stars had been observed by a number of individuals who did not connect it with the general presence of galactic dust. For stars that lie near the plane of the Milky Way and are within a few thousand parsecs of the Earth, extinction in the visual band of frequencies (photometric system ) is on the order of 1.8 magnitudes per kiloparsec. For Earth
Earth
-bound observers, extinction arises both from the interstellar medium (ISM) and the Earth\'s atmosphere ; it may also arise from circumstellar dust around an observed object
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Cosmic Dust
COSMIC DUST, or EXTRATERRESTRIAL DUST, is dust which exists in outer space , as well as all over planet Earth . Most cosmic dust particles are between a few molecules to 0.1 µm in size. A smaller fraction of all dust in space consists of larger refractory minerals that condensed as matter left the stars. It is called "stardust" and is included in a separate section below. The dust density falling to Earth is approximately 10−6/m3 with each grain having a mass between 10−16kg (0.1 pg) and 10−4 kg (100 mg). Cosmic dust can be further distinguished by its astronomical location: intergalactic dust , interstellar dust, interplanetary dust (such as in the zodiacal cloud ) and circumplanetary dust (such as in a planetary ring ). In the Solar System , interplanetary dust causes the zodiacal light
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Wavelength
In physics , the WAVELENGTH of a sinusoidal wave is the SPATIAL PERIOD of the wave—the distance over which the wave's shape repeats, and thus the inverse of the spatial frequency . It is usually determined by considering the distance between consecutive corresponding points of the same phase , such as crests, troughs, or zero crossings and is a characteristic of both traveling waves and standing waves , as well as other spatial wave patterns. Wavelength is commonly designated by the Greek letter _lambda _ (λ). The concept can also be applied to periodic waves of non-sinusoidal shape. The term _wavelength_ is also sometimes applied to modulated waves, and to the sinusoidal envelopes of modulated waves or waves formed by interference of several sinusoids
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Filter (optics)
Optical
Optical
filters are devices that selectively transmit light of different wavelengths, usually implemented as plane glass or plastic devices in the optical path which are either dyed in the bulk or have interference coatings. The optical properties of filters are completely described by their frequency response, which specifies how the magnitude and phase of each frequency component of an incoming signal is modified by the filter. Filters mostly belong to one of two categories. The simplest, physically, is the absorptive filter; then there are interference or dichroic filters . Optical
Optical
filters selectively transmit light in a particular range of wavelengths , that is, colours , while absorbing the remainder. They can usually pass long wavelengths only (longpass), short wavelengths only (shortpass), or a band of wavelengths, blocking both longer and shorter wavelengths (bandpass)
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Passband
A PASSBAND is the range of frequencies or wavelengths that can pass through a filter . For example, a radio receiver contains a bandpass 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 (that is, a signal with energy only in a passband), is known as a BANDPASS SIGNAL, in contrast to a baseband signal . CONTENTS * 1 Filters * 2 Digital transmission * 3 Details * 4 See also * 5 References FILTERS _ Unrestricted signal (upper diagram). Bandpass filter applied to signal (middle diagram). Resulting passband signal (bottom diagram). A(f)_ is the frequency function of the signal or filter in arbitrary units
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UBV Photometric System
The UBV PHOTOMETRIC SYSTEM (Ultraviolet, Blue, Visual), also called the Johnson system (or Johnson-Morgan system), is a wide band photometric system for classifying stars according to their colors. It is the first known standardized photoelectric photometric system . The letters U, B, and V stand for ultraviolet , blue, and visual magnitudes, which are measured for a star in order to classify it in the UBV system. The choice of colors on the blue end of the spectrum is because of the bias that photographic film has for those colors. It was introduced in the 1950s by American astronomers Harold Lester Johnson and William Wilson Morgan . A 13" telescope and the 82" telescope at McDonald Observatory were used to define the system. The filters are selected so that the mean wavelengths of response functions are 364 nm for U, 442 nm for B, 540 nm for V
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Sun
The SUN is the star at the center of the Solar System
Solar System
. It is a nearly perfect sphere of hot plasma , with internal convective motion that generates a magnetic field via a dynamo process . It is by far the most important source of energy for life on Earth
Earth
. Its diameter is about 109 times that of Earth, and its mass is about 330,000 times that of Earth, accounting for about 99.86% of the total mass of the Solar System. About three quarters of the Sun's mass consists of hydrogen (~73%); the rest is mostly helium (~25%), with much smaller quantities of heavier elements, including oxygen , carbon , neon , and iron . The Sun
Sun
is a G-type main-sequence star (G2V) based on its spectral class . As such, it is informally referred to as a yellow dwarf. It formed approximately 4.6 billion years ago from the gravitational collapse of matter within a region of a large molecular clou