Be stars are a heterogeneous set of stars with B spectral types and emission lines. A narrower definition, sometimes referred to as ''classical Be stars'', is a non-

supergiant Supergiants are among the most massive and most luminous stars. Supergiant stars occupy the top region of the Hertzsprung–Russell diagram with absolute visual magnitudes between about −3 and −8. The temperature range of supergiant stars spa ...

B star whose spectrum has, or had at some time, one or more Balmer emission lines.

Definition and classification

Many stars have B-type spectra and show hydrogen emission lines, including many

Herbig Ae/Be star A Herbig Ae/Be star (HAeBe) is a pre-main-sequence star – a young () star of spectral types A or B. These stars are still embedded in gas-dust envelopes and are sometimes accompanied by circumstellar disks. Hydrogen and calcium emission lines ...

s, mass-transferring

binary system A binary system is a system of two astronomical bodies which are close enough that their gravitational attraction causes them to orbit each other around a barycenter ''(also see animated examples)''. More restrictive definitions require that th ...

s, and B stars. It is preferred to restrict usage of the term Be star to non-supergiant stars showing one or more

Balmer series The Balmer series, or Balmer lines in atomic physics, is one of a set of six named series describing the spectral line emissions of the hydrogen atom. The Balmer series is calculated using the Balmer formula, an empirical equation discovered b ...

lines in emission. These are sometimes referred to as classical Be stars. The emission lines may be present only at certain times. Although the Be type spectrum is most strongly produced in class B stars, it is also detected in O and A

shell star A shell star is a star having a spectrum that shows extremely broad absorption lines, plus some very narrow absorption lines. They typically also show some emission lines, usually from the Balmer series but occasionally of other lines. The broa ...

s, and these are sometimes included under the "Be star" banner. Be stars are primarily considered to be main sequence stars, but a number of

subgiant A subgiant is a star that is brighter than a normal main-sequence star of the same spectral class, but not as bright as giant stars. The term subgiant is applied both to a particular spectral luminosity class and to a stage in the evolution ...

s and

giant star A giant star is a star with substantially larger radius and luminosity than a main-sequence (or ''dwarf'') star of the same surface temperature.Giant star, entry in ''Astronomy Encyclopedia'', ed. Patrick Moore, New York: Oxford University Press ...

s are also included.

Discovery

The first star recognized as a Be star was

Gamma Cassiopeiae Gamma Cassiopeiae, Latinized from γ Cassiopeiae, is a bright star at the center of the distinctive "W" asterism in the northern circumpolar constellation of Cassiopeia. Although it is a fairly bright star with an apparent visual mag ...

, observed 1866 by

Angelo Secchi Angelo Secchi (; 28 June 1818 – 26 February 1878) was an Italian Catholic priest, astronomer from the Italian region of Emilia. He was director of the observatory at the Pontifical Gregorian University (then called the Roman College) for ...

, 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 recognised as a Be star until 1976.

Model

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.

Shell stars

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.

Variability

Be stars are often visually and spectroscopically variable. Be stars can be classified as

Gamma Cassiopeiae variable A Gamma Cassiopeiae variable (γ Cassiopeiae variable) is a type of variable star, named for its prototype γ Cassiopeiae. Variability γ Cassiopeiae variables show irregular changes in brightness on a timescale of decades. These typically hav ...

s when a transient or variable disk is observed. Be stars that show variability without clear indication of the mechanism are listed simply as BE in the

General Catalogue of Variable Stars The General Catalogue of Variable Stars (GCVS) is a list of variable stars. Its first edition, containing 10,820 stars, was published in 1948 by the Academy of Sciences of the USSR and edited by B. V. Kukarkin and P. P. Parenago. Second and thi ...

. Some of these are thought to be pulsating stars and are sometimes called Lambda Eridani variables.

References

External links

* Philippe Stee's homepage
Hot and Active Stars Research
* Article from Olivier Thizy
Be Stars
{{DEFAULTSORT:Be Star Star types 1866 in science