K-type main-sequence star
Template:Short description Template:Infobox astronomical formation A K-type main-sequence star, also referred to as a K-type 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.<ref name=mamajek>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> These stars are of particular interest in the search for extraterrestrial 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 the universe has existed (13.8 billion years), as such none have had sufficient time to leave the main sequence.<ref name="NASA Mar 2019" /> Well-known examples include Alpha Centauri B (K1 V), Epsilon Indi (K5 V) and Epsilon Eridani (K2 V).<ref>Template:Cite simbad</ref> Template:TOCLIMIT
NomenclatureEdit
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,<ref name=engle>Template:Cite journal</ref> but in other cases red dwarf is restricted just to M-class stars.<ref name=heath>Template:Cite journal</ref><ref name=farihi>Template:Cite journal</ref> In some cases all K stars are included as red dwarfs,<ref name=petterson>Template:Cite journal</ref> and occasionally even earlier stars.<ref name=alekseev>Template:Cite journal</ref> The term orange dwarf is often applied to early-K stars,<ref>Template:Cite journal</ref> but in some cases it is used for all K-type main sequence stars.<ref>Template:Cite book</ref>
Spectral standard starsEdit
| Spectral type | Mass (Template:Solar mass) |
Radius (Template:Solar radius) |
Luminosity (Template:Solar luminosity) |
Effective temperature (K) |
Color index Template:Nowrap |
|---|---|---|---|---|---|
| K0V | 0.88 | 0.813 | 0.46 | style="background-color:#Template:Color temperature"|5,270 | 0.82 |
| K1V | 0.86 | 0.797 | 0.41 | style="background-color:#Template:Color temperature"|5,170 | 0.86 |
| K2V | 0.82 | 0.783 | 0.37 | style="background-color:#Template:Color temperature"|5,100 | 0.88 |
| K3V | 0.78 | 0.755 | 0.28 | style="background-color:#Template:Color temperature"|4,830 | 0.99 |
| K4V | 0.73 | 0.713 | 0.20 | style="background-color:#Template:Color temperature"|4,600 | 1.09 |
| K5V | 0.70 | 0.701 | 0.17 | style="background-color:#Template:Color temperature"|4,440 | 1.15 |
| K6V | 0.69 | 0.669 | 0.14 | style="background-color:#Template:Color temperature"|4,300 | 1.24 |
| K7V | 0.64 | 0.630 | 0.10 | style="background-color:#Template:Color temperature"|4,100 | 1.34 |
| K8V | 0.62 | 0.615 | 0.087 | style="background-color:#Template:Color temperature"|3,990 | 1.36 |
| K9V | 0.59 | 0.608 | 0.079 | style="background-color:#Template:Color temperature"|3,930 | 1.40 |
The revised Yerkes Atlas system (Johnson & Morgan 1953)<ref>Template:Cite journal</ref> 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:<ref name="Garrison">Template:Cite journal</ref>
- Sigma Draconis (K0 V)
- Epsilon Eridani (K2 V)
- 61 Cygni A (K5 V)
Other primary MK standard stars include:<ref name="Keenan89">Template:Cite journal</ref>
- 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,<ref>Template:Cite journal</ref> Keenan & McNeil 1989<ref name="Keenan89"/>), 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.<ref name="Pecaut13">Template:Cite journal</ref>
PlanetsEdit
{{#invoke:Labelled list hatnote|labelledList|Main article|Main articles|Main page|Main pages}} These stars are of particular interest in the search for extraterrestrial life<ref name="New Scientist May 2009">Template:Cite magazine</ref> because they are stable on the main sequence for a very long time (17–70 billion years, compared to 10 billion for the Sun).<ref name="NASA Mar 2019">Template:Cite press release</ref> 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 include Epsilon Eridani, HD 192310, Gliese 86, and 54 Piscium.
K-type main-sequence stars are about three to four times as abundant as G-type main-sequence stars, making planet searches easier.<ref>Template:Cite magazine</ref> K-type stars emit less total ultraviolet and other ionizing radiation than G-type stars like the Sun (which can damage DNA and thus hamper the emergence of nucleic acid based life). In fact, many peak in the red.<ref>Template:Cite journal</ref>
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 for exoplanets and extraterrestrial life.
Radiation hazardEdit
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 early main sequence phase than do either heavier G-type stars or lighter early M-type dwarf stars.<ref name=Richey-YowellShkolnik2022>Template:Cite journal</ref> 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.<ref name=Richey-YowellShkolnik2022/><ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
See alsoEdit
- Solar analog
- G-type main-sequence star
- Star count, survey of stars