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Wolf–Rayet stars (often referred to as WR stars) are a heterogeneous set of stars with unusual spectra showing prominent broad emission lines of highly ionised helium and nitrogen or carbon. The spectra indicate very high surface temperatures of 30,000 K to around 200,000 K, surface enhancement of heavy elements, and strong stellar winds. Classic (or Population I) Wolf–Rayet stars are evolved, massive stars, O-type stars over 20 solar masses when they were on the main sequence, that have now completely lost their outer hydrogen and are fusing helium or heavier elements in the core. A subset of WR stars are the central stars of planetary nebulae (CSPNe), post Asymptotic Giant Branch stars that were similar to the Sun while on the main sequence, but have now ceased fusion and shed their atmospheres to show a bare carbon-oxygen core. Another group (type WNh) show hydrogen lines in their spectra and are young extremely massive stars still fusing hydrogen at the core, with nitrogen mixed to the surface and strong radiation-driven mass loss. They are all highly luminous due to their high temperatures, thousands of times the bolometric luminosity of the Sun () for the CSPNe, for the Population I WR stars, to for the WNh stars, although not exceptionally bright visually since most of their radiation output is in the ultraviolet.〔 The naked-eye stars Gamma Velorum and Theta Muscae, as well as the most massive known star, R136a1 in 30 Doradus, are all Wolf–Rayet stars. ==Observation history== In 1867, using the 40 cm Foucault telescope at the Paris Observatory, astronomers Charles Wolf and Georges Rayet discovered three stars in the constellation Cygnus (HD 191765, HD 192103 and HD 192641, now designated as WR 134, WR135, and WR137 respectively) that displayed broad emission bands on an otherwise continuous spectrum. Most stars only display absorption lines or bands in their spectra, as a result of overlying elements absorbing light energy at specific frequencies, so these were clearly unusual objects. The nature of the emission bands in the spectra of a Wolf–Rayet star remained a mystery for several decades. Edward C. Pickering theorized that the lines were caused by an unusual state of hydrogen, and it was found that this "Pickering series" of lines followed a pattern similar to the Balmer series, when half-integral quantum numbers were substituted. It was later shown that the lines resulted from the presence of helium; a gas that was discovered in 1868. Pickering noted similarities between Wolf–Rayet spectra and nebular spectra, and this similarity led to the conclusion that some or all Wolf Rayet stars were the central stars of planetary nebulae. By 1929, the width of the emission bands was being attributed to Doppler broadening, and hence that the gas surrounding these stars must be moving with velocities of 300–2400 km/s along the line of sight. The conclusion was that a Wolf–Rayet star is continually ejecting gas into space, producing an expanding envelope of nebulous gas. The force ejecting the gas at the high velocities observed is radiation pressure. It was well known that many stars with Wolf Rayet type spectra were the central stars of planetary nebulae, but also that many were not associated with an obvious planetary nebula or any visible nebulousity at all. In addition to helium, emission lines of carbon, oxygen and nitrogen were identified in the spectra of Wolf–Rayet stars. In 1938, the International Astronomical Union classified the spectra of Wolf–Rayet stars into types WN and WC, depending on whether the spectrum was dominated by lines of nitrogen or carbon-oxygen respectively. In 1969, several CSPNe with strong OVI emissions lines were grouped under a new "OVI sequence", or just OVI type. These were subsequently referred to as () stars.〔 Similar stars not associated with planetary nebulae were described shortly after and the WO classification was eventually also adopted for population I WR stars.〔 The understanding that certain late, and sometimes not-so-late, WN stars with hydrogen lines in their spectra are at a different stage of evolution from hydrogen-free WR stars has led to the introduction of the term ''WNh'' to distinguish these stars generally from other WN stars. They were previously referred to as WNL stars, although there are late-type WN stars without hydrogen as well as WR stars with hydrogen as early as WN5.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Wolf–Rayet star」の詳細全文を読む スポンサード リンク
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