| K-type main-sequence star | |
|---|---|
 Sigma Draconis, officially named Alsafi, is a K-type main-sequence star. | |
| Characteristics | |
| Type | Class of medium-smallmain sequence star |
| Mass range | 0.60–0.86M☉ |
| Temperature | 3,930–5,270K |
| Average luminosity | 0.079–0.47L☉ |
| External links | |
 Media category | |
 Q863936 | |
AK-type main-sequence star[a] is amain-sequence (corehydrogen-burning)star ofspectral type K. The spectralluminosity class isV.[1][2] These stars are intermediate in size betweenred dwarfs andyellow dwarfs, hence the termorange dwarfs often applied to this type.
K-type main-sequence stars have masses between 0.6 and 0.9 times themass of theSun andsurface temperatures between 3,900 and 5,300K.[3] These stars are of particular interest in the search forextraterrestrial life due to their stability and long lifespan. These stars stay on the main sequence for up to 70 billion years, a length of time much larger than the time theuniverse has existed (13.8 billion years); as such, none have had sufficient time to leave the main sequence.[4] Well-known examples includeAlpha Centauri B (K1 V),Epsilon Indi (K5 V) andEpsilon Eridani (K2 V).[5]
There aresubdwarf stars, that is stars of luminosity class VI, of spectral class K. These stars are fusing hydrogen in their cores like normal main-sequence stars, but due to their lowmetallicity they lie about twomagnitudes below the main sequence (ie. less luminous).[6]
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 asred dwarfs,[7] but in other casesred dwarf is restricted just to M-class stars.[8][9] In some cases all K stars are included as red dwarfs,[10] and occasionally even earlier stars.[11] The termorange dwarf is often applied to early-K stars,[12] but in some cases it is used for all K-type main sequence stars.[13]
| Spectral type | Mass (M☉) | Radius (R☉) | Luminosity (L☉) | Effective temperature (K) | Color index (B − V) |
|---|---|---|---|---|---|
| K0V | 0.86 | 0.813 | 0.47 | 5,290 | 0.82 |
| K1V | 0.85 | 0.797 | 0.40 | 5,170 | 0.86 |
| K2V | 0.82 | 0.783 | 0.37 | 5,100 | 0.88 |
| K3V | 0.76 | 0.755 | 0.27 | 4,870 | 0.99 |
| K4V | 0.72 | 0.710 | 0.20 | 4,600 | 1.09 |
| K5V | 0.70 | 0.701 | 0.17 | 4,440 | 1.15 |
| K6V | 0.67 | 0.669 | 0.14 | 4,300 | 1.24 |
| K7V | 0.64 | 0.630 | 0.10 | 4,090 | 1.34 |
| K8V | 0.61 | 0.615 | 0.086 | 3,990 | 1.36 |
| K9V | 0.60 | 0.608 | 0.079 | 3,930 | 1.40 |
The revised Yerkes Atlas system (Johnson & Morgan 1953)[14] 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 remained unchanged over the years, are:[15]
Other primary MK standard stars include:[16]
Based on the example set in some references (e.g. Johnson & Morgan 1953,[17] Keenan & McNeil 1989[16]), 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.[18]
These stars are of particular interest in the search forextraterrestrial life[19] because they are stable on the main sequence for a very long time (17–70 billion years, compared to 10 billion for the Sun).[4] 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.
Some of the nearest K-type stars known to have planets includeEpsilon Eridani,HD 192310,Gliese 86, and54 Piscium.
K-type main-sequence stars are about three to four times as abundant as G-type main-sequence stars, making planet searches easier.[20] K-type stars emit less totalultraviolet and otherionizing radiation than G-type stars like the Sun (which can damageDNA and thus hamper the emergence of nucleic acid based life). In fact, many peak in the red.[21]
While M-type stars are the most abundant, they are more likely to have tidally locked planets in habitable-zone orbits 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 forexoplanets and extraterrestrial life.
Despite K-stars' lower total UV output, in order for their planets to have habitable temperatures, they must orbit much nearer to their K-star hosts, offsetting or reversing any advantage of a lower total UV output. There is also growing evidence that K-type dwarf stars emit dangerously high levels of X-rays and far ultraviolet (FUV) radiation for considerably longer into their earlymain sequence phase than do either heavier G-type stars or lighter early M-type dwarf stars.[22] This prolonged radiation saturation period may sterilise, destroy the atmospheres of, or at least delay the emergence of life for Earth-like planets orbiting inside the habitable zones around K-type dwarf stars.[22][23]
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