Be star
B-type stars with hydrogen emission lines from circumstellar disks.
Wikipedia / Wikimedia Commons
Be stars are a varied group of B-type stars that display emission lines in their spectra. A more specific category, often called classical Be stars, refers to a B star that is not a supergiant and whose spectrum currently shows, or has shown at some point, one or more Balmer emission lines.
Many B-type stars with hydrogen emission lines exist, including supergiants, Herbig Ae/Be stars, binary systems undergoing mass transfer, and B[e] stars. It is standard practice to reserve the term "Be star" for non-supergiant stars that exhibit one or more Balmer series lines in emission—these are the classical Be stars. The emission lines may appear only intermittently. While the Be spectral type is most common among B-class stars, it is also observed in O and A shell stars, which are sometimes grouped under the Be star label. Be stars are mostly main sequence objects, but some subgiants and giants are also included.
The first star identified as a Be star was Gamma Cassiopeiae, observed in 1866 by Angelo Secchi; it was also the first star ever seen with emission lines. Many other bright stars were later found to have similar spectra, though several are no longer classified as classical Be stars. The brightest Be star is Achernar, though it was not recognized as such until 1976.
By the early 20th century, it became clear that the emission lines in Be stars originate from circumstellar material ejected from the star, aided by its rapid rotation. All observed features of Be stars can now be explained by a gaseous disk formed from material expelled by the star. The infrared excess and polarization arise from starlight scattering off the disk, while the line emission results from the reprocessing of ultraviolet starlight within the gaseous disk.
Some Be stars show spectral features interpreted as a detached "shell" of gas—more accurately a disk or ring—surrounding the star. These shell features are thought to occur when the disk around a Be star is aligned edge-on to our line of sight, producing very narrow absorption lines in the spectrum.
Be stars are often variable both visually and spectroscopically. They are classified as Gamma Cassiopeiae variables when a transient or variable disk is observed. Be stars that vary without a clear cause are listed simply as BE in the General Catalogue of Variable Stars. Some of these are believed to be pulsating stars and are someti
- type
- Stellar classification
- spectral_type
- B (also O and A shell stars)
- defining_feature
- Balmer emission lines
- first_recognized
- Gamma Cassiopeiae (1866)
- brightest_example
- Achernar (recognized 1976)
- variability_classification
- Gamma Cassiopeiae variables or BE
Lore & Background
The first star recognized as a Be star was Gamma Cassiopeiae, observed in 1866 by Angelo Secchi, the first star ever observed with emission lines. Many other bright stars were found to show similar spectra, although many of these are no longer considered to be classical Be stars. The brightest is Achernar, although it was not recognized as a Be star until 1976.
With the understanding of the processes of emission line formation in the early 20th century it became clear that these lines in Be stars must come from circumstellar material ejected from the star helped by the rapid rotation of the star. All the observational characteristics of Be stars can now be explained with a gaseous disk that is formed of material ejected from the star. The infrared excess and the polarization result from the scattering of stellar light in the disk, while the line emission is formed by re-processing stellar ultraviolet light in the gaseous disc.
Some Be stars exhibit spectral features that are interpreted as a detached 'shell' of gas surrounding the star, or more accurately a disc or ring. These shell features are thought to be caused when the disc of gas that is present around many Be stars is aligned edge on to us so that it creates very narrow absorption lines in the spectrum.
Reader's Guide
Be stars are significant as a class of variable stars whose emission lines reveal the presence of circumstellar disks, providing a natural laboratory for studying stellar rotation, mass loss, and disk physics. Their variability, often classified as Gamma Cassiopeiae variables when a transient or variable disk is observed, links them to broader phenomena in stellar evolution. The inclusion of O and A shell stars under the Be star banner shows the spectral range of the phenomenon. The model of a gaseous disk formed from ejected material explains the infrared excess, polarization, and line emission, making Be stars key objects for understanding how rapidly rotating stars interact with their environment. Their legacy includes ongoing monitoring through databases such as the Be Star Spectra (BeSS) database, which continues to collect observations of these stars.
Did You Know?
- The first star recognized as a Be star was Gamma Cassiopeiae, observed in 1866 by Angelo Secchi.
- Achernar, the brightest Be star, was not recognized as a Be star until 1976.
- Be stars are often visually and spectroscopically variable, and some are classified as Gamma Cassiopeiae variables.
- Some Be stars exhibit shell features caused when their gas disc is aligned edge-on, creating narrow absorption lines.
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