0 Members and 1 Guest are viewing this topic.
Accretion disks can form around black holes, neutron stars, white dwarf stars and young stars.An accretion disk has two components:Some matter is robbed of its angular momentum, and it spirals into the central bodyThe angular momentum is transferred to some other matter, which spirals further from the central body, becoming less dense. This makes the outer edge a bit hard to define.Some matter remains in the disk, forming planets (if it is cool enough) or a warm gas/dust cloud (glowing in infra-red), or a hot plasma (glowing in X-Rays). The glowing edge forms a boundary which could define the accretion disk (but the location of this edge will differ depending on what sort of telescope you use to observe it, at which wavelengths). See: http://en.wikipedia.org/wiki/Accretion_disk
The original question does not have a precise answer because in the absence of any othe material the gravitational field of an object extends right out to infinity. You therefore have to define something else to allow you to set sime sort of size to the accretion zone.In the case of a black hole the smallest distance friom the centre is the event horizon which is about one mile for each soar mass that the black hole contains so a typical 10 stellar mass black hole is about ten miles across which is pretty small. The billion solar mass black holes at the centres of large elliptical galaxies are only about as big as the solar system. The event horizon is not really a useful limit though because that defines the smallest size that we could possibly identify. A more useful limit might be the point at which cold accreting gas would heat up due to the gained gravitational energy being turned into kinetic energy to glow like a star. For a stellar mas black hole this would be approximately the size of a normal star with the same mass and colour as the temperature chosen.This is OK for small black holes but there are no such thing as billiion solar mass stars around so there is no easy bench markfor sars of this size and the relationship is not a simple linear one like size of the event horizon. and this limit is notreally discussed in most texts however ther is one futher fact that could be used. That is the fact that if billion solar mass lump of gas was coollapsing it is possible for it to collapse directly into a black hole without ever becoming a star and synthesising new nucleii so the maximum size of te acretion zone of a black hole is probably about the size of the solar system which at the distance of galaxies is a pretty small thing.