Herbig Ae/Be star

A young, intermediate-mass star of spectral type A or B that has not reached the main sequence. It is commonly identified by emission lines and circumstellar gas or dust.

Pre-main-sequence star: A star that is still contracting toward the main sequence and has not begun sustained hydrogen fusion in its core. Herbig Ae/Be stars are the intermediate-mass members of this broader evolutionary class.
Pre-main-sequence evolution: The contraction and structural change a star undergoes before sustained hydrogen fusion begins in its core. It explains why Herbig Ae/Be stars are luminous while still contracting.
George Herbig: An American astronomer known for research on star formation and young stellar objects. His work established observational criteria for identifying young emission-line stars.
Star formation: The process by which dense regions of interstellar gas and dust collapse to form stars. Herbig Ae/Be stars trace the intermediate-mass pathway through this process.
Intermediate-mass star formation: The formation of stars with masses between those of low-mass and high-mass stars. How accretion proceeds as radiation pressure rises remains a central issue for this mass range.
Accretion disk: A rotating disk of gas and dust that spirals inward as material is accreted onto a central object. Disk material feeds many Herbig Ae/Be stars and produces infrared excess.
Spectral classification: The system of grouping stars by the features and temperatures of their spectra. The A or B spectral type is part of the class's defining criteria.
Merle Walker: An American astronomer who studied young stars and helped define the Herbig Ae/Be class. Walker independently identified luminous A-type emission-line stars associated with star-forming regions.
Planet formation: The assembly of planets from gas, dust, and solid bodies in disks around young stars. Their disks offer sites for investigating planet formation around stars more massive than the Sun.
Radiation pressure: The force exerted when electromagnetic radiation transfers momentum to matter. Strong stellar radiation may oppose the inflow needed to build massive Herbig Be stars.

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