A K-type main-sequence star, also referred to as a K-type dwarf or an 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).
(B − V)
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:
Other primary MK standard stars include:
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.
Main article: Habitability of K-type main-sequence star systems
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 (17-70 billion years, compared to 10 billion for the Sun).
Like M-type stars, they tend to have a very small mass, leading to their extremely long lifespan that offers plenty of time for life to develop on orbiting Earth-like, terrestrial planets. In addition, K-type stars emit less ultraviolet and other ionizing radiation (which can damage DNA and thus hamper the emergence of nucleic acid based life) than G-type stars like the Sun. In fact, many peak in the red.
However, there is growing evidence that K-type dwarf stars emit dangerously high energy radiation levels, in the form of X-rays and far ultraviolet (FUV) radiation, for considerably longer into their early main sequence phase than in comparison to G-type and early M-type dwarf stars. This prolonged radiation saturation period may be sterilising, destroying the atmospheres of, or at least delaying the emergence of life for Earth-like planets orbiting inside the habitable zones around K-type dwarf stars.
K-type main-sequence stars are about three to four times as abundant as G-type main-sequence stars, making planet searches easier. While M-type stars are the most abundant, they are more likely to have tidally locked planets in orbit and are more prone to producing solar flares and cold spots that would more easily strike nearby rocky planets, potentially making it much harder for life to develop. Due to their greater heat, the habitable zones of K-type stars are also much wider than those of M-type stars. For all of these reasons, they may be the most favorable stars to focus on in the search for exoplanets and extraterrestrial life.
Some of the nearest K-type stars known to have planets include Epsilon Eridani, HD 192310, Gliese 86, and 54 Piscium.