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Concept
K-type main-sequence star
Summary
A K-type main-sequence star, also referred to as a K-type dwarf red dwarf, or orange 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 ("red dwarfs") and yellow/white G-type main-sequence stars. They have masses between 0.6 and 0.9 times the mass of the Sun and surface temperatures between 3,900 and 5,300 K. These stars are of particular interest in the search for extraterrestrial life due to their stability and long lifespan. Well-known examples include Alpha Centauri B (K1 V) and Epsilon Indi (K5 V).
In modern usage, the names applied to K-type main sequence stars vary. When explicitly defined, late K dwarfs are typically grouped with early to mid-M-class stars as red dwarfs, but in other cases red dwarf is restricted just to M-class stars. In some cases all K stars are included as red dwarfs, and occasionally even earlier stars. The term orange dwarf is often applied to early-K stars, but in some cases it is used for all K-type main sequence 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 the years, are:
Sigma Draconis (K0 V)
Epsilon Eridani (K2 V)
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 the K8 and K9 classifications are rarely seen. A few examples such as HIP 111288 (K8V) and HIP 3261 (K9V) have been defined and used.
Official source
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