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A general term used to describe a ring of material or gas around a star or a stellar system. Though known as a 'disc', a structure like this does not always form a flat disc shape, and can also be found as a ring or torus of matter.
Typically, discs of this kind are associated with stellar formation. As a newly-forming protostar rotates and draws material inward through gravity, the need to preserve angular momentum causes that material to form into a flattened disc. Initially composed a gas and dust, material within this protoplanetary disc is drawn together by gravity to form larger and larger accumulations. Eventually this process causes the formation of small planetesimals, which can in turn give rise to fully-formed planets.
This gradual process can take tens of millions of years, but eventually most of the disc's material becomes concentrated into planetary bodies, and the original disc largely dissipates. This does not mean that the disc is entirely lost, and elements can survive even in the case of mature stars like the Sun. Within the Solar System, the Asteroid Belt represents a circumstellar disc in which the planetesimal bodies never formed a full planet. Much further out from the Sun (with an inner edge very approximately five thousand times more distant than the Asteroid Belt) lies the Hills Cloud, the inner toroidal element of the more extensive Oort Cloud.
Structures like this are often found around other stars, especially during the first tens of millions of years of their existence. Indeed, it is relatively common to find multiple disc structures within a stellar system, creating patterns of concentric rings extending away from the central star. In binary or multiple sustems, each of the individual stars in the system can potentially possess its own circumstellar disc or discs, and in some cases discs can form around the entire system.
An entirely different kind of ring structure can emerge around hot blue or white stars of the kind known as shell stars. Stars of this kind rotate on their axis extremely rapidly, to the extent that not only is the star's shape deformed, but a ring-shaped envelope of gas erupts from the star's equator to extend outward into its system.
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