Accretion, means any growth or increase in size by a gradual external addition or inclusion.
The term accretion is used in the following areas:
In finance, accretion is predictable changes in the price of certain securities.
In geography, accretion means an increase of land by the deposit of waterborne sediment.
In science, accretion is increase in size by gradual addition of smaller parts.
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Viscosity within the disc generates heat and saps orbital momentum, causing material in the disc to spiral inward until it impacts in an accretion shock on the central body if the body is a star, or slips toward the event horizon if the central body is a fl hole.
The most spectacular accretion discs found in nature are those of active galactic nuclei and quasars, which are believed to be massive fl holes at the center of galaxies.
The huge luminosity of quasars is believed to be a result of friction caused by gas and dust falling into the accretion discs of supermassivefl holes, which can convert about half of the mass of an object into energy as compared to a few percent for nuclear fusion processes.
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