Spectral Standard Stars
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 years, are Sigma Draconis (K0 V), Epsilon Eridani (K2 V), and 61 Cygni A (K5 V). 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) . 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, 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.
Read more about this topic: K-type Main-sequence Star
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