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A circumstellar disk (in context, just disk) is matter, typically in a ring, collected around a star. It can be composed of gas, dust, planetesimals, or a combination, in orbit around the star. The term protoplanetary disk is often used for the typical case: such a disk around a young star, which constitutes the raw material of future planets. The main asteroid belt around the Sun represents a kind of sparse circumstellar disk.
Excess infrared from a star is evidence of a disk, indicating thermal radiation for something large and cooler than the star. When the disk can be resolved, the material making up the disk and how it varies by radius can be determined from this EMR, which is affected by the dust grain size: the emission of grains is inefficient for wavelengths long in comparison to the grain's diameter.
The term circumbinary disk refers to a circumstellar disk that surrounds both members of a binary star. Under some cases (e.g., some configurations with a third companion), the disk can be tilted substantially from the axis of the stars and the orbit of the binary companion. The terms polar alignment and polar disk refer to a disk that is perpendicular to the orbital plane of the binary companions.
While circumstellar disks are ubiquitous for a star's early life, they do sometimes occur during later phases, e.g., a debris disk.