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 of from 0.6 to 0.9 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. Orange dwarfs are about three to four times as abundant as sun-like stars, making planet searches easier.
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.
See also 
- Solar twin
- Red dwarf
- Stellar classification, Class K
- Star count, survey of stars
- Habitability of orange dwarf stars
- A Modern Mean Stellar Color and Effective Temperatures (Teff) # Sequence for O9V-Y0V Dwarf Stars, E. Mamajek, 2011, website
- 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.
- SIMBAD, entries for Alpha Centauri B and Epsilon Indi, accessed on line June 19, 2007.
- , retrieved on May 6, 2009.
- 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
- MK ANCHOR POINTS, Robert F. Garrison
- 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.
- 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