Be star

A rapidly rotating, non-supergiant B-type star whose spectrum has shown hydrogen emission lines at least once. The emission usually comes from a gaseous disk around the star.

Viscous decretion disk: A gaseous disk fed by material moving outward from a rapidly rotating star and spreading through viscous transport. It is the leading model for how Be-star gas forms and evolves around the star.
Be star shell phase: An observational phase in which absorption features from an equatorial disk appear superimposed on a Be star's spectrum. It occurs when a disk is viewed nearly edge-on and helps constrain its orientation.
B-type star: A hot, blue star classified by its spectrum, with neutral helium and ionized metals among its characteristic features. Be stars belong to this spectral class but are distinguished by emission-line behavior.
Stellar angular momentum: The rotational angular momentum carried by a star, determined by its mass distribution and spin. Explaining Be-star rotation requires identifying how stars acquire and retain unusually high angular momentum.
Stellar rotation: The rotation of a star about its axis, which affects its shape, internal mixing, and surrounding environment. Rapid rotation helps Be stars approach the speeds needed to place gas into orbit.
Spectral line profile: The measured intensity of a spectral line as a function of wavelength or frequency. Its changing shape traces the velocity and structure of gas around Be stars.
Classical Be star: A non-supergiant B-type star whose emission lines arise from a gaseous disk formed from material ejected by the star. This narrower category distinguishes ordinary Be stars from other objects sometimes called Be stars.
Binary mass transfer: The transfer of matter between stars in a gravitationally bound binary system. Past mass transfer may spin up some Be stars, offering an alternative to birth with rapid rotation.
Hydrogen-alpha line: The red spectral line produced when hydrogen atoms transition from the third to the second energy level. Its emission profile is a prominent diagnostic of gas in a Be-star disk.
Interferometry: A technique that combines waves from multiple telescopes to achieve fine angular resolution. It can resolve the angular structure and flattening of nearby Be-star disks.

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