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- Extreme_helium_star abstract "An extreme helium star (abbreviated EHe) is a low-mass supergiant that is almost devoid of hydrogen, the most common chemical element of the Universe. Since there are no conditions where stars devoid of hydrogen can be formed from molecular clouds, it is theorized that they are the product of the mergers of helium-core and carbon-oxygen core white dwarfs.Extreme helium stars form a sub-group within the broader category of hydrogen-deficient stars. The latter includes cool carbon stars like R Coronae Borealis, helium-rich spectral class O or B stars, population I Wolf-Rayet stars, AM CVn stars, white dwarfs of spectral type WC, and transition stars like PG 1159.The first known extreme helium star, HD 124448, was discovered in 1942 by Daniel M. Popper at the McDonald Observatory in Austin, USA. This star displayed no lines of hydrogen in its spectrum, but strong helium lines as well as the presence of carbon and oxygen. The second, PV Telescopii, was discovered in 1952, and by 1996 a total of 25 candidates had been found. (This list was narrowed to 21 by 2006.) A common characteristic of these stars is that the abundance ratio of carbon to helium is always in the range of 0.3 to 1%. This is despite wide variation of other abundance ratios in EHe stars.The known extreme helium stars are supergiants where hydrogen is underabundant by a factor of 10,000 or more. The surface temperatures of these stars range from 9,000–35,000 K. They are primarily composed of helium, with the second most abundant element, carbon, forming about one atom per 100 atoms of helium. The chemical composition of these stars implies that they have undergone both hydrogen and helium burning at some stage of their evolution.Two possible scenarios were proposed to explain the composition of extreme helium stars. The double-degenerate (DD) model explained the stars as forming in a binary system consisting of a smaller helium white dwarf and a more massive carbon-oxygen white dwarf. Both stars had ceased to produce energy through nuclear fusion and were now compact objects. The emission of gravitational radiation caused their orbit to decay until they merged. If the combined mass does not exceed the Chandrasekhar limit, the helium will accrete onto the C-O dwarf and ignite to form a supergiant. Later this will become an EHe star before cooling to become a white dwarf. The final-flash (FF) model suggested that an EHe star could form as a late evolutionary stage of a star after if had left the asymptotic giant branch. As the star is cooling to form a white dwarf, helium ignites in a shell around the core, causing the outer layers to expand rapidly. If the hydrogen in this envelope is consumed, the star becomes hydrogen deficient and it contracts to form an EHe.Examination of element abundances from seven EHe stars agreed with those predicted by the DD model.".
- Extreme_helium_star wikiPageExternalLink 511.
- Extreme_helium_star wikiPageExternalLink ehe_review.
- Extreme_helium_star wikiPageExternalLink Astronomers_Find_Origin_Of_Extreme_Helium_Stars.html.
- Extreme_helium_star wikiPageID "20709740".
- Extreme_helium_star wikiPageLength "5390".
- Extreme_helium_star wikiPageOutDegree "24".
- Extreme_helium_star wikiPageRevisionID "688090787".
- Extreme_helium_star wikiPageWikiLink AM_Canum_Venaticorum_star.
- Extreme_helium_star wikiPageWikiLink Asymptotic_giant_branch.
- Extreme_helium_star wikiPageWikiLink Austin,_Texas.
- Extreme_helium_star wikiPageWikiLink Category:Stars_by_spectral_type.
- Extreme_helium_star wikiPageWikiLink Chandrasekhar_limit.
- Extreme_helium_star wikiPageWikiLink Chemical_element.
- Extreme_helium_star wikiPageWikiLink Compact_star.
- Extreme_helium_star wikiPageWikiLink Daniel_M._Popper.
- Extreme_helium_star wikiPageWikiLink Gravitational_wave.
- Extreme_helium_star wikiPageWikiLink HD_124448.
- Extreme_helium_star wikiPageWikiLink Hydrogen.
- Extreme_helium_star wikiPageWikiLink Hydrogen-deficient_star.
- Extreme_helium_star wikiPageWikiLink McDonald_Observatory.
- Extreme_helium_star wikiPageWikiLink Molecular_cloud.
- Extreme_helium_star wikiPageWikiLink PG_1159_star.
- Extreme_helium_star wikiPageWikiLink PV_Telescopii.
- Extreme_helium_star wikiPageWikiLink R_Coronae_Borealis.
- Extreme_helium_star wikiPageWikiLink Stellar_classification.
- Extreme_helium_star wikiPageWikiLink Supergiant.
- Extreme_helium_star wikiPageWikiLink United_States.
- Extreme_helium_star wikiPageWikiLink Universe.
- Extreme_helium_star wikiPageWikiLink White_dwarf.
- Extreme_helium_star wikiPageWikiLink Wolf–Rayet_star.
- Extreme_helium_star wikiPageWikiLinkText "Extreme helium star".
- Extreme_helium_star wikiPageWikiLinkText "extreme helium star".
- Extreme_helium_star wikiPageUsesTemplate Template:Reflist.
- Extreme_helium_star wikiPageUsesTemplate Template:Star.
- Extreme_helium_star subject Category:Stars_by_spectral_type.
- Extreme_helium_star hypernym Supergiant.
- Extreme_helium_star type Star.
- Extreme_helium_star comment "An extreme helium star (abbreviated EHe) is a low-mass supergiant that is almost devoid of hydrogen, the most common chemical element of the Universe. Since there are no conditions where stars devoid of hydrogen can be formed from molecular clouds, it is theorized that they are the product of the mergers of helium-core and carbon-oxygen core white dwarfs.Extreme helium stars form a sub-group within the broader category of hydrogen-deficient stars.".
- Extreme_helium_star label "Extreme helium star".
- Extreme_helium_star sameAs Q2586037.
- Extreme_helium_star sameAs Estrella_extrema_dheli.
- Extreme_helium_star sameAs Extremer_Heliumstern.
- Extreme_helium_star sameAs Ege_heliuma_stelo.
- Extreme_helium_star sameAs Estrella_extrema_de_helio.
- Extreme_helium_star sameAs Étoile_à_hélium_extrême.
- Extreme_helium_star sameAs Stella_allelio_estrema.
- Extreme_helium_star sameAs 強ヘリウム星.
- Extreme_helium_star sameAs Estrela_de_hélio_extrema.
- Extreme_helium_star sameAs m.0521gp2.
- Extreme_helium_star sameAs Q2586037.
- Extreme_helium_star sameAs 極端氦星.
- Extreme_helium_star wasDerivedFrom Extreme_helium_star?oldid=688090787.
- Extreme_helium_star isPrimaryTopicOf Extreme_helium_star.