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A K-type main-sequence star (K V), also referred to as an orange dwarf or K dwarf, is a main-sequence (hydrogen-burning) star of spectral type K and luminosity class V. These stars are intermediate in size between red M-type main-sequence stars and yellow G-type main-sequence stars. They have masses between 0.45 to 0.8 times the mass of the Sun〔(A Modern Mean Stellar Color and Effective Temperatures (Teff) # Sequence for O9V-Y0V Dwarf Stars ), E. Mamajek, 2011, website〕 and surface temperatures between 3,900 and 5,200 K.〔(Empirical bolometric corrections for the main-sequence ), G. M. H. J. Habets and J. R. W. Heintze, ''Astronomy and Astrophysics Supplement'' 46 (November 1981), pp. 193–237.〕, Tables VII, VIII. Better-known examples include Alpha Centauri B (K1 V) and Epsilon Indi.〔SIMBAD, entries for (Alpha Centauri B ) and (Epsilon Indi ), accessed on line June 19, 2007.〕 These stars are of particular interest in the search for extraterrestrial life because they are stable on the main sequence for a very long time (15 to 30 billion years, compared to 10 billion for the Sun). This may create an opportunity for life to evolve on terrestrial planets orbiting such stars. K-type stars also emit less ultraviolet radiation (which can damage DNA and thus hamper the emergence of life) than G-type stars like the Sun.〔(), retrieved on Feb 8, 2015.〕 K-type main-sequence stars are about three to four times as abundant as G-type main-sequence stars, making planet searches easier.〔(), retrieved on May 6, 2009.〕 ==Spectral standard stars== The revised Yerkes Atlas system (Johnson & Morgan 1953)〔(Fundamental stellar photometry for standards of spectral type on the revised system of the Yerkes spectral atlas ) H.L. Johnson & W.W. Morgan, 1953, Astrophysical Journal, 117, 313〕 listed 12 K-type dwarf spectral standard stars, however not all of these have survived to this day as standards. The "anchor points" of the MK classification system among the K-type main-sequence dwarf stars, i.e. those standard stars that have remain unchanged over the years, are Epsilon Eridani (K2 V), and 61 Cygni A (K5 V).〔(MK ANCHOR POINTS ), Robert F. Garrison〕 Other primary MK standard stars include 70 Ophiuchi A (K0 V), 107 Piscium (K1 V), HD 219134 (K3 V), TW Piscis Austrini (K4 V), HD 120467 (K6 V), 61 Cygni B (K7 V) .〔(The Perkins Catalog of Revised MK Types for the Cooler Stars ), P.C. Keenan & R.C McNeil, "Astrophysical Journal Supplement Series" 71 (October 1989), pp. 245–266.〕 Based on the example set in some references (e.g. Johnson & Morgan 1953,〔(Fundamental stellar photometry for standards of spectral type on the revised system of the Yerkes spectral atlas ), H.L. Johnson & W.W. Morgan, 1953, Astrophysical Journal, 117, 313〕 Keenan & McNeil 1989〔), many authors consider the step between K7 V and M0 V to be a single subdivision, and one rarely encounters K8 or K9 classifications in the literature. Recent, however, HIP 111288 (K8V) and HIP 3261 (K9V) have been proposed as spectral standard stars.〔(Intrinsic Colors, Temperatures, and Bolometric Corrections of Pre-main-sequence Stars ), M.J. Pecaut & E.E. Mamajek, 2013, "Astrophysical Journal Supplement", 208, 9〕 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「K-type main-sequence star」の詳細全文を読む スポンサード リンク
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