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An O-type star is a hot, blue-white star of spectral type O in the Yerkes classification system employed by astronomers. They have temperatures in excess of 30,000 Kelvin (K) and so appear to the left on the Hertzsprung–Russell diagram. Stars of this type are identified by their dominant Helium II absorption lines, strong lines of other ionised elements, and Hydrogen and neutral He lines weaker than spectral type B. Stars of this type are particularly rare; only 0.00003% of the main sequence are O-type stars. However, because they are usually very bright, they can be seen when further away than dimmer stars, and two of the 90 brightest stars as seen from Earth are O type. Due to high temperature and luminosity, O-type stars end their lives rather quickly in violent supernova explosions, resulting in black holes or neutron stars. Most of these stars are young massive main sequence, giant, or supergiant stars, but the central stars of planetary nebulae, old low-mass stars near the end of their lives, also usually have O spectra. O-type stars are typically located in regions of active star formation, such as the spiral arms of a spiral galaxy. These stars illuminate any surrounding material and are largely responsible for the distinct coloration of a galaxy's arms. Furthermore, O-type stars are frequently in multiple star systems where their evolution is more difficult to predict due to mass transfer and the possibility of component stars going supernova at different times. ==Spectrum== ''O-type stars'' are defined and identified by the relative strength of certain spectral lines. The key lines are the prominent HeII lines at 454.1 nm and 420.0 nm, which vary from very weak at O9.5 to very strong in O2–O7, and the HeI lines at 447.1 nm and 402.6 nm, which vary from absent in O2/3 to prominent in O9.5. The O7 class is defined where the 454.1-nanometer HeII and 447.1-nanometer HeI lines have equal strength. The very hottest O-type stars have such weak neutral He lines that they are best separated on the relative strength of the NIII and NIV lines. The O spectral class subdivisions are complex, with definitions for O3.5, O6.5, O8.5, O9.5, and O9.7 indicating the development of spectroscopic analysis since the class was first defined for the emission line stars now assigned to WR. The only star given a spectral type of O in the original Henry Draper catalogue is HR 2583 (=WR 4), now classified as WN4.〔The Draper Catalogue of stellar spectra photographed with the 8-inch Bache telescope as a part of the Henry Draper memorial, Edward C. Pickering, ''Annals of Harvard College Observatory'' 27 (1890), 〕 The luminosity classes of O-type stars are assigned on the relative strengths of the HeII emission lines and certain ionised N and Si lines. These are indicated by the "f" suffix on the spectral type, with "f" alone indicating NIII and HeII emission, "(f)" meaning the He emission is weak or absent, "((f))" meaning the N emission is weak or absent, "f *" indicating the addition of very strong NIV emission, and "f+" the presence of SiIV emission. Luminosity class V, main-sequence stars, generally have weak or missing emission lines, with giants and supergiants showing increasing emission line strength. At O2–O4, the distinction between main sequence and supergiant stars is narrow and may not even represent true luminosity or evolutionary differences. At intermediate O5–O8 classes, the distinction between O((f)) main sequence, O(f) giants, and Of supergiants is well-defined and represents a definite increase in luminosity. The increasing strength of SiIV emission is also an indicator of increasing luminosity and this is the primary means of assigning luminosity classes to the late O-type stars. 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「O-type star」の詳細全文を読む スポンサード リンク
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