A celestial body consisting of a large, self-luminous mass of hot gas held together by its own gravity; the sun is a typical star.

For a member S of a family of sets, the collection of all sets in the family that contain S as a subset.

A star-shaped group of tracks made by ionizing particles originating at a common point in a nuclear emulsion, cloud chamber, or bubble chamber; some stars are produced by successive disintegrations of an atom in a radioactive series, and others by nuclear reactions of the spallation type, such as those due to cosmic-ray particles. Also known as nuclear star.

A light source that subtends a very small angle at the entrance pupil of an optical instrument and is used to test the instrument.