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{{Short description|In astronomy, a B-type star}}
{{Distinguish|B(e) star{{!}}B[e] star}}
{{Distinguish|B(e) star{{!}}B[e] star}}
[[File:Achernar.svg|thumb|right|upright=1.0|Depiction of [[Achernar]], the brightest Be star]]
[[File:Achernar.svg|thumb|right|upright=1.0|Depiction of [[Achernar]], the brightest Be star]]
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==Definition and classification==
==Definition and classification==
Many stars have B-type spectra and show hydrogen emission lines, including many [[supergiant]]s, [[Herbig Ae/Be star]]s, mass-transferring [[binary system]]s, and [[B(e) star|B[e] stars]]. It is preferred to restrict usage of the term Be star to non-supergiant stars showing one or more [[Balmer series]] lines in emission. These are sometimes referred to as classical Be stars. The emission lines may be present only at certain times.<ref name=porter>{{cite journal|bibcode= 2003PASP..115.1153P|doi=10.1086/378307|title=Classical Be Stars|journal=Publications of the Astronomical Society of the Pacific|volume=115|issue=812|pages=1153|year=2003|last1=Porter|first1=John M.|last2=Rivinius|first2=Thomas}}</ref>
Many stars have B-type spectra and show hydrogen emission lines, including many [[supergiant]]s, [[Herbig Ae/Be star]]s, mass-transferring [[binary system]]s, and [[B(e) star|B[e] stars]]. It is preferred to restrict usage of the term Be star to non-supergiant stars showing one or more [[Balmer series]] lines in emission. These are sometimes referred to as classical Be stars. The emission lines may be present only at certain times.<ref name=porter>{{cite journal|bibcode= 2003PASP..115.1153P|doi=10.1086/378307|title=Classical Be Stars|journal=Publications of the Astronomical Society of the Pacific|volume=115|issue=812|pages=1153|year=2003|last1=Porter|first1=John M.|last2=Rivinius|first2=Thomas|doi-access=free}}</ref>


Although the Be type spectrum is most strongly produced in class B stars, it is also detected in O and A [[shell star]]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]]s and [[giant star]]s are also included.<ref name="RiviniusCarciofi2013">{{cite journal|last1=Rivinius|first1=Thomas|last2=Carciofi|first2=Alex C.|last3=Martayan|first3=Christophe|title=Classical Be stars|journal=The Astronomy and Astrophysics Review|volume=21|issue=1|pages=69|year=2013|issn=0935-4956|doi=10.1007/s00159-013-0069-0|arxiv = 1310.3962 |bibcode = 2013A&ARv..21...69R }}</ref>
Although the Be type spectrum is most strongly produced in class B stars, it is also detected in O and A [[shell star]]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]]s and [[giant star]]s are also included.<ref name="RiviniusCarciofi2013">{{cite journal|last1=Rivinius|first1=Thomas|last2=Carciofi|first2=Alex C.|last3=Martayan|first3=Christophe|title=Classical Be stars|journal=The Astronomy and Astrophysics Review|volume=21|issue=1|pages=69|year=2013|issn=0935-4956|doi=10.1007/s00159-013-0069-0|arxiv = 1310.3962 |bibcode = 2013A&ARv..21...69R |s2cid=118652497}}</ref>


==Discovery==
==Discovery==
The first star recognized as a Be star was [[Gamma Cassiopeiae]], observed 1866 by [[Angelo Secchi]], the first star ever observed with emission lines.<ref name=secchi>{{cite journal|doi=10.1002/asna.18670680405|title=Schreiben des Herrn Prof.Secchi, Dir. Der Sternwarte des Collegio Romano, an den Herausgeber|journal=Astronomische Nachrichten|volume=68|issue=4|pages=63–64|year=1867|last1=Secchi|first1=A.|bibcode = 1866AN.....68...63S |url=https://zenodo.org/record/1424669/files/article.pdf}}</ref> Many other bright stars were found to show similar spectra, although many of these are no longer considered to be classical Be stars.<ref name=merrill>{{cite journal|bibcode=1925ApJ....61..389M|title=Discovery and Observations of Stars of Class Be|journal=Astrophysical Journal|volume=61|pages=389|author1=Merrill|first1=P. W.|last2=Humason|first2=M. L.|last3=Burwell|first3=C. G.|year=1925|doi=10.1086/142899}}</ref> The brightest is [[Achernar]], although it was not recognised as a Be star until 1976.<ref name=snow>{{cite journal|bibcode=1976ApJ...203L..87S|title=Evidence for mass loss at moderate to high velocity in Be stars|journal=Astrophysical Journal|volume=203|pages=L87|author1=Snow|first1=T. P.|last2=Marlborough|first2=J. M.|year=1976|doi=10.1086/182025}}</ref><ref name=massa>{{cite journal|bibcode=1975PASP...87..777M|title=The influence of rotation and stellar winds upon the Be phenomenon|journal=Astronomical Society of the Pacific|volume=87|pages=777|author1=Massa|first1=D.|year=1975|doi=10.1086/129842}}</ref>
The first star recognized as a Be star was [[Gamma Cassiopeiae]], observed 1866 by [[Angelo Secchi]], the first star ever observed with emission lines.<ref name=secchi>{{cite journal|doi=10.1002/asna.18670680405|title=Schreiben des Herrn Prof.Secchi, Dir. Der Sternwarte des Collegio Romano, an den Herausgeber|journal=Astronomische Nachrichten|volume=68|issue=4|pages=63–64|year=1867|last1=Secchi|first1=A.|bibcode = 1866AN.....68...63S |url=https://zenodo.org/record/1424669}}</ref> Many other bright stars were found to show similar spectra, although many of these are no longer considered to be classical Be stars.<ref name=merrill>{{cite journal|bibcode=1925ApJ....61..389M|title=Discovery and Observations of Stars of Class Be|journal=Astrophysical Journal|volume=61|pages=389|last1=Merrill|first1=P. W.|last2=Humason|first2=M. L.|last3=Burwell|first3=C. G.|year=1925|doi=10.1086/142899|doi-access=free}}</ref> The brightest is [[Achernar]], although it was not recognised as a Be star until 1976.<ref name=snow>{{cite journal|bibcode=1976ApJ...203L..87S|title=Evidence for mass loss at moderate to high velocity in Be stars|journal=Astrophysical Journal|volume=203|pages=L87|last1=Snow|first1=T. P.|last2=Marlborough|first2=J. M.|year=1976|doi=10.1086/182025|doi-access=free}}</ref><ref name=massa>{{cite journal|bibcode=1975PASP...87..777M|title=The influence of rotation and stellar winds upon the Be phenomenon|journal=Astronomical Society of the Pacific|volume=87|pages=777|last1=Massa|first1=D.|year=1975|doi=10.1086/129842|doi-access=free}}</ref>


==Model==
==Model==
{{anchor|decretion disk}}
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.<ref name=struve>{{cite journal|bibcode=1931ApJ....73...94S|title=On the Origin of Bright Lines in Spectra of Stars of Class B|journal=Astrophysical Journal|volume=73|pages=94|author1=Struve|first1=Otto|year=1931|doi=10.1086/143298}}</ref> 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.<ref name="RiviniusCarciofi2013"/>
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.<ref name=struve>{{cite journal|bibcode=1931ApJ....73...94S|title=On the Origin of Bright Lines in Spectra of Stars of Class B|journal=Astrophysical Journal|volume=73|pages=94|last1=Struve|first1=Otto|year=1931|doi=10.1086/143298}}</ref> 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.<ref name="RiviniusCarciofi2013"/>


==Shell stars==
==Shell stars==
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== References ==
== References ==
{{reflist}}
{{reflist|25em}}


==Further reading==
==Further reading==
*{{cite book|isbn=978-94-010-1498-4|doi=10.1007/978-94-010-1498-4|title=Be and Shell Stars|year=1976|last1=Slettebak|first1=A.}}
*{{cite book|isbn=978-94-010-1498-4|doi=10.1007/978-94-010-1498-4|title=Be and Shell Stars|year=1976|last1=Slettebak|first1=A.|editor1-first=Arne|editor1-last=Slettebak}}


== External links ==
== External links ==
* Philippe Stee's homepage: [http://www.oca.eu/stee/page1/page3/page3.html Hot and Active Stars Research]
* Philippe Stee's homepage: [http://www.oca.eu/stee/page1/page3/page3.html Hot and Active Stars Research]
* Article from Olivier Thizy: [https://www.webcitation.org/68Wayk4TQ?url=http://www.shelyak.com/dossier.php?id_dossier=24 Be Stars]
* Article from Olivier Thizy: [https://web.archive.org/web/20110927005933/http://shelyak.com/dossier.php?id_dossier=24 Be Stars]
* {{Cite web |title=Be Star Spectra (BeSS) Database |url=http://basebe.obspm.fr/basebe/ |website=basebe.obspm.fr}}


{{Star|state=collapsed}}
{{Star|state=collapsed}}

Latest revision as of 20:46, 4 December 2023

Not to be confused with B[e] star.
Depiction of Achernar, the brightest Be star

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

Definition and classification

[edit]

Many stars have B-type spectra and show hydrogen emission lines, including many supergiants, Herbig Ae/Be stars, mass-transferring binary systems, and B[e] stars. It is preferred to restrict usage of the term Be star to non-supergiant stars showing one or more Balmer series lines in emission. These are sometimes referred to as classical Be stars. The emission lines may be present only at certain times.[1]

Although the Be type spectrum is most strongly produced in class B stars, it is also detected in O and A shell stars, and these are sometimes included under the "Be star" banner. Be stars are primarily considered to be main sequence stars, but a number of subgiants and giant stars are also included.[2]

Discovery

[edit]

The first star recognized as a Be star was Gamma Cassiopeiae, observed 1866 by Angelo Secchi, the first star ever observed with emission lines.[3] Many other bright stars were found to show similar spectra, although many of these are no longer considered to be classical Be stars.[4] The brightest is Achernar, although it was not recognised as a Be star until 1976.[5][6]

Model

[edit]

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.[7] 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.[2]

Shell stars

[edit]
Main article: Shell star

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

[edit]
Main articles: Gamma Cassiopeiae variable and Lambda Eridani variable

Be stars are often visually and spectroscopically variable. Be stars can be classified as Gamma Cassiopeiae variables 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. Some of these are thought to be pulsating stars and are sometimes called Lambda Eridani variables.

References

[edit]
  1. ^ Porter, John M.; Rivinius, Thomas (2003). "Classical Be Stars". Publications of the Astronomical Society of the Pacific. 115 (812): 1153. Bibcode:2003PASP..115.1153P. doi:10.1086/378307.
  2. ^ a b Rivinius, Thomas; Carciofi, Alex C.; Martayan, Christophe (2013). "Classical Be stars". The Astronomy and Astrophysics Review. 21 (1): 69. arXiv:1310.3962. Bibcode:2013A&ARv..21...69R. doi:10.1007/s00159-013-0069-0. ISSN 0935-4956. S2CID 118652497.
  3. ^ Secchi, A. (1867). "Schreiben des Herrn Prof.Secchi, Dir. Der Sternwarte des Collegio Romano, an den Herausgeber". Astronomische Nachrichten. 68 (4): 63–64. Bibcode:1866AN.....68...63S. doi:10.1002/asna.18670680405.
  4. ^ Merrill, P. W.; Humason, M. L.; Burwell, C. G. (1925). "Discovery and Observations of Stars of Class Be". Astrophysical Journal. 61: 389. Bibcode:1925ApJ....61..389M. doi:10.1086/142899.
  5. ^ Snow, T. P.; Marlborough, J. M. (1976). "Evidence for mass loss at moderate to high velocity in Be stars". Astrophysical Journal. 203: L87. Bibcode:1976ApJ...203L..87S. doi:10.1086/182025.
  6. ^ Massa, D. (1975). "The influence of rotation and stellar winds upon the Be phenomenon". Astronomical Society of the Pacific. 87: 777. Bibcode:1975PASP...87..777M. doi:10.1086/129842.
  7. ^ Struve, Otto (1931). "On the Origin of Bright Lines in Spectra of Stars of Class B". Astrophysical Journal. 73: 94. Bibcode:1931ApJ....73...94S. doi:10.1086/143298.

Further reading

[edit]
[edit]
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