K-type main-sequence star

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 and 0.8 times the mass of the Sun and surface temperatures between 3,900 and 5,200 K.^{, Tables VII, VIII.} Better-known examples include Alpha Centauri B (K1 V) and Epsilon Indi. 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. K-type main-sequence stars are about three to four times as abundant as G-type main-sequence stars, making planet searches easier.

The revised Yerkes Atlas system (Johnson & Morgan 1953) 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). 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) . Based on the example set in some references (e.g. Johnson & Morgan 1953, 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.

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