Accretion (science)
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Accretion is a process in which the size of something gradually increases by steady addition of smaller parts. This term is used with specific meanings in several scientific fields.
Accretion in Atmospheric Science
In atmospheric science for growth of a precipitation particle by the collision of an ice crystal or snowflake with a supercooled liquid droplet that freezes upon impact.
Accretion in Astrophysics
In astrophysics, the term accretion is used for at least two distinct processes.
The first and most common is the growth of a massive object by gravitationally attracting more matter, typically gaseous matter in an accretion disk. Accretion disks are common around smaller stars or stellar remnants in a close binary, or black holes in the centers of spiral galaxies. Some dynamics in the disk are necessary to allow orbiting gas to lose angular momentum and fall onto the central massive object.
The second process is somewhat analogous to the one in atmospheric science. In the nebular theory, accretion refers to the collision and sticking of cooled microscopic dust and ice particles electrostatically, in protoplanetary disks and jovian protoplanet systems, eventually leading to planetesimals which gravitationally accrete more small particles and other planetesimals.
Use of the term accretion disk for the protoplanetary disk thus leads to confusion over the planetary accretion process, although in many cases it may well be that both accretion processes are happening simultaneously! (See T Tauri star.)
If that weren't confusing enough, the jovian protoplanets probably have disks of their own, in close analogy to the solar system as a whole. The jovian protoplanet may be accreting gas from its surrounding disk in the first process, at the same time that dust and ice particles in the disk are accreting into moonlets and ring systems, in the second process.
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