A K-type main-sequence star (K V), also referred to as a orange dwarf or K dwarf, are main-sequence (hydrogen-burning) stars 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 of from 0.6 to 0.9 times the mass of the Sun[1] and surface temperatures between 3,900 and 5,200 K.[2], Tables VII, VIII. Famous examples include Alpha Centauri B (K1 V) and Epsilon Indi.[3]. 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. Orange dwarfs are about three to four times as abundant as sun-like stars, making planet searches easier.[4]
The revised Yerkes Atlas system (Johnson & Morgan 1953)[5] 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 years, are Sigma Draconis (K0 V), Epsilon Eridani (K2 V), and 61 Cygni A (K5 V)[6]. Other primary MK standard stars include 107 Piscium (K1 V), HD 219134 (K3 V), TW Piscis Austrini (K4 V), HD 120467 (K6 V), 61 Cygni B (K7 V) [7]. There are not yet any generally agreed upon K8 or K9 dwarf standard stars. Based on the example set in some references (e.g. Johnson & Morgan 1953[8], Keenan & McNeil 1989[7]), 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.
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