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Glossary/Print.Php?Id=322668&Mode=Lett http://learn.open.ac.uk/mod/glossary/print.php?id=322668&mode=lett... Tuesday, 3 August 2010, 17:04 Site: OU online Course: Astrophysics (S382-10B) Glossary: Glossary 1 1 micron minimum: A local minimum in the continuum spectrum of most AGN occurring around a wavelength of 1 micron (1 µm). It is thought to represent the minimum between a hot thermal spectrum (the big blue bump) possibly due to emission from an accretion disc, and a cool thermal spectrum due to emission in the infrared by warm dust grains. 2 2dF: The 2 degree field multi-object spectrometer on the Anglo Australian Telescope. Used to conduct the 2dF galaxy redshift survey and the 2dF quasar survey. 2MASS: The 2 Micron All Sky Survey. A survey of the complete sky using two telescopes (one at Mount Hopkins, Arizona and one at Cerro-Tololo, Chile) in three infrared wavebands: J-band (1.25 µm), H-band (1.65 µm) and K-band (2.17 µm). 3 3C273: One of the closest quasars to us (redshift = 0.158), the optically brightest (magnitude = 12.9) and the first quasar to be identified, in the 3C radio source catalogue. 4 4000 angstrom break: A break, or absorption feature, at a wavelength of 4000Å in the spectra of galaxies caused by Balmer continuum absorption in the atmospheres of stars. A aberration of light: The phenomenon whereby the observed direction of a star changes with time, due to the combined effect of the motion of the observer and the finite speed of light. If the relative speed of the observer and star is small compared to the speed of light, the aberration is small: tan α = (v/c)sin θ, where α is the aberration angle, v is the speed of motion, c is the speed of light and θ is the angle between the direction of motion and the direction of the star. The aberration due to the Earth’s motion around the Sun is no more than 20.5 arc seconds. If the relative speeds are a significant fraction of the speed of light, the effects of special relativity become significant and the Lorentz transformations must be used to calculate the aberration angle. ablation: The process whereby material is lost from a body as a result of gas or fluid flow past it. The gas carries away (ablates) material from the body’s surface. absolute magnitude: A numerical quantity used to describe the luminosity of a body. The relationship between absolute magnitude and luminosity is given by: M = -2.5 log10 L + K where K is a constant that depends on the units of L and the waveband being considered. The absolute magnitude of an astronomical body is equal to the apparent magnitude the body would have if it were located at a distance of 10 parsecs from the observer, ignoring any effects due to obscuration. absolute temperature: Any temperature measured on the thermodynamic temperature scale. absolute temperature scale: See thermodynamic temperature scale. absolute visual magnitude: An absolute magnitude, measured in the visual (V) band (at wavelengths around 550 nm). absorption (and emission) of radiation: General processes whereby energy carried by electromagnetic radiation may be added to, or taken from, the total energy of the system responsible for the emission or absorption. A particular case is that in which an electron makes a radiative transition between two energy levels in an atom. When an electron makes a transition from a lower energy level E1 to a higher energy level E2, the atom increases its total energy by an amount E2 - E1. In the case of a radiative transition, the atom obtains this energy by absorbing a single photon of energy E2 - E1. Similarly, an atom can emit a photon of energy E2 - E1 in a radiative transition when one of its electrons makes a transition from a higher energy level E2 to a lower energy level E1. 1 of 142 Copyright © 2010 The Open University 03/08/20101 17:07 http://learn.open.ac.uk/mod/glossary/print.php?id=322668&mode=lett... absorption distance: The quantity X in the expression If absorbers have a constant comoving space density and constant proper sizes and cross sections, then the number of absorbers per unit absorption distance is constant. absorption edge: A jump in absorption cross-section at a particular wavelength or energy corresponding to an ionisation state of the absorbing atom or ion. absorption line: See absorption line spectrum. absorption line spectrum: A spectrum of the kind observed when continuum radiation (e.g. from a black body) passes through matter. Radiation is absorbed at specific energies (wavelengths) corresponding to atomic or molecular transitions in the absorbing material, giving rise to absorption lines in the spectrum (see, for example, hydrogen spectrum). The absorption spectrum gives information on the atomic or molecular species present and their physical state. accelerating model: A model of the universe in which the rate of expansion increases. The acceleration is a result of the changing densities of matter, radiation and dark energy with time. acceleration: The (instantaneous) rate of change of velocity of a body. For a particle moving in one-dimension along the x-axis, the acceleration ax at any time is the instantaneous rate of change of the particle’s velocity vx, and is given by the gradient of the particle’s velocity–time graph at the relevant time. This gradient is equal to the derivative of the velocity with respect to time at the relevant time, so the acceleration at time t may be written Acceleration is a vector quantity, characterized by a direction as well as a magnitude. In one dimension the sign of ax suffices to indicate the direction, but in two or three dimensions some other method must be used to indicate direction. This is often achieved by expressing the acceleration vector in terms of its (Cartesian) components, as in a = (ax, ay, az), where and v(t) is the velocity vector. acceleration due to gravity: The acceleration towards the centre of a massive body (such as a star or planet) of a freely falling object. The magnitude of the acceleration for a spherically symmetrical body is GM/r2, where M is the mass of the body inside the radius r, r is the distance from the centre of mass and G is the universal gravitational constant. The acceleration is independent of the mass of the falling object, and so is the same for all bodies in the same gravitational field. At the surface of the Earth the magnitude of the acceleration due to the Earth’s gravity is about 9.81 m s-2. This quantity varies slightly from place to place, but is generally represented by the symbol g. accretion: The process whereby the mass of an astronomical body grows as a result of the accumulation of matter. Accretion is most important in systems where matter is falling in under the influence of gravity; it allows a massive body to gain more mass from other matter in its surroundings. If the infalling matter has significant angular momentum, an accretion disc is formed. The luminosity of various types of astronomical object, including X-ray binaries and active galactic nuclei, is ultimately the result of accretion. accretion column: A form of accretion flow where the width in any direction perpendicular to the direction of the flow is small compared to the geometrical extent along the flow direction. The magnetically controlled accretion in polars is thought to occur via an accretion column. accretion curtain: A form of accretion flow between the inner edge of a magnetically disrupted accretion disc and the surface of the accretor. The magnetically controlled accretion in intermediate polars is thought to occur via accretion curtains. accretion disc: A flow of matter that is largely confined to a plane and that is spiralling in towards a central object. The azimuthal component of the flow velocity is close to the Keplerian value, while the radial component is much smaller. See also mass transfer. accretion disc luminosity: 2 of 142 Copyright © 2010 The Open University 03/08/20102 17:07 http://learn.open.ac.uk/mod/glossary/print.php?id=322668&mode=lett... The energy an accretion disc radiates per unit time. For a steady-state, geometrically thin, optically thick, infinite disc with a non-rotating central accretor with mass M and radius R* and accretion rate , the accretion disc luminosity is (G is the gravitational constant). This is just half the accretion luminosity. accretion disc spectrum: The continuum spectrum of a geometrically thin, optically thick, Keplerian, steady state accretion disc is a ‘stretched-out’ black body spectrum, with a characteristic flat region 1/3 where Fν ∝ ν accretion disc structure: See accretion disc. accretion efficiency: The accretion luminosity, expressed in units of the rate of accreted mass energy. If Lacc denotes the accretion luminosity, the mass accretion rate, and c the speed of light, then the accretion efficiency is . accretion luminosity: The gravitational energy liberated by accretion per unit time. If an object with mass M and radius R* accretes matter at a rate the accretion luminosity is (G is the gravitational constant). accretion powered compact binaries: See X-ray binary, cataclysmic variable. accretion powered compact binary: See X-ray binary; cataclysmic variable. accretion powered pulsar: A pulsar whose emission is powered by accretion from a companion star. See X-ray pulsar. accretion rate: The mass accumulated per unit time through accretion. Usually denoted by and –1 –1 –1 measured in kg s or g s or M0 yr (sometimes also called mass accretion rate).
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