K-type main-sequence star

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K-type main-sequence star
Sigma Draconis.jpg
Sigma (σ) Draconis, or Alsafi, is a K-type main-sequence star
Characteristics
TypeClass of medium-small main sequence star
Mass range0.6M☉ to 0.9M☉.
Temperature3900 K to 5300 K
Average luminosity Class V
External links
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Wikidata-logo.svg Q863936

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. [1] 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). [2]

Contents

Nomenclature

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, [3] but in other cases red dwarf is restricted just to M-class stars. [4] [5] In some cases all K stars are included as red dwarfs, [6] and occasionally even earlier stars. [7] The term orange dwarf is often applied to early-K stars, [8] but in some cases it is used for all K-type main sequence stars. [9]

Spectral standard stars

Properties of typical K-type main-sequence stars [1]
Spectral type Mass
(M☉)		 Radius
(R☉)		 Luminosity
(L☉)		 Effective temperature
(K)		 Color index
(B − V)
K0V0.880.8130.465,2700.82
K1V0.860.7970.415,1700.86
K2V0.820.7830.375,1000.88
K3V0.780.7550.284,8300.99
K4V0.730.7130.204,6001.09
K5V0.700.7010.174,4401.15
K6V0.690.6690.144,3001.24
K7V0.640.6300.104,1001.34
K8V0.620.6150.0873,9901.36
K9V0.590.6080.0793,9301.40

The revised Yerkes Atlas system (Johnson & Morgan 1953) [10] 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: [11]

Other primary MK standard stars include: [12]

Based on the example set in some references (e.g. Johnson & Morgan 1953, [13] Keenan & McNeil 1989 [12] ), 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. [14]

Planets

These stars are of particular interest in the search for extraterrestrial life [15] because they are stable on the main sequence for a very long time (17–70 billion years, compared to 10 billion for the Sun). [16] 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. [17] 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. [18]

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 hazard

61 Cygni, a binary K-type star system 61 Cygni Proper Motion.gif
61 Cygni, a binary K-type star system

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. [19] 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. [19] [20]

See also

Related Research Articles

<span class="mw-page-title-main">Stellar classification</span> Classification of stars based on spectral properties

In astronomy, stellar classification is the classification of stars based on their spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a prism or diffraction grating into a spectrum exhibiting the rainbow of colors interspersed with spectral lines. Each line indicates a particular chemical element or molecule, with the line strength indicating the abundance of that element. The strengths of the different spectral lines vary mainly due to the temperature of the photosphere, although in some cases there are true abundance differences. The spectral class of a star is a short code primarily summarizing the ionization state, giving an objective measure of the photosphere's temperature.

<span class="mw-page-title-main">White dwarf</span> Type of stellar remnant composed mostly of electron-degenerate matter

A white dwarf is a stellar core remnant composed mostly of electron-degenerate matter. A white dwarf is very dense: its mass is comparable to the Sun's, while its volume is comparable to Earth's. A white dwarf's low luminosity comes from the emission of residual thermal energy; no fusion takes place in a white dwarf. The nearest known white dwarf is Sirius B, at 8.6 light years, the smaller component of the Sirius binary star. There are currently thought to be eight white dwarfs among the hundred star systems nearest the Sun. The unusual faintness of white dwarfs was first recognized in 1910. The name white dwarf was coined by Willem Luyten in 1922.

<span class="mw-page-title-main">Red dwarf</span> Dim, low mass stars on the main sequence

A red dwarf is the smallest and coolest kind of star on the main sequence. Red dwarfs are by far the most common type of star in the Milky Way, at least in the neighborhood of the Sun. However, individual red dwarfs cannot be easily observed as a result of their low luminosity. From Earth, not one star that fits the stricter definitions of a red dwarf is visible to the naked eye. Proxima Centauri, the nearest star to the Sun, is a red dwarf, as are fifty of the sixty nearest stars. According to some estimates, red dwarfs make up three-quarters of the stars in the Milky Way.

<span class="mw-page-title-main">G-type main-sequence star</span> Stellar classification

A G-type main-sequence star, also often, and imprecisely called a yellow dwarf, or G star, is a main-sequence star of spectral type G. Such a star has about 0.9 to 1.1 solar masses and an effective temperature between about 5,300 and 6,000 K. Like other main-sequence stars, a G-type main-sequence star converts the element hydrogen to helium in its core by means of nuclear fusion, but can also fuse helium when hydrogen runs out. The Sun, the star in the center of the Solar System to which the Earth is gravitationally bound, is an example of a G-type main-sequence star. Each second, the Sun fuses approximately 600 million tons of hydrogen into helium in a process known as the proton–proton chain, converting about 4 million tons of matter to energy. Besides the Sun, other well-known examples of G-type main-sequence stars include Alpha Centauri, Tau Ceti, and 51 Pegasi.

<span class="mw-page-title-main">Groombridge 1830</span> Star in the constellation Ursa Major

Groombridge 1830 is a star in the constellation Ursa Major.

<span class="mw-page-title-main">Sigma Draconis</span> Star in the constellation Draco

Sigma Draconis is a single star in the northern constellation of Draco. It has the proper name Alsafi, while Sigma Draconis, which is latinised from σ Draconis and abbreviated Sig Dra or σ Dra, is the Bayer designation. It has an apparent visual magnitude of 4.7, which is bright enough to be faintly visible to the naked eye. Based on parallax measurements, this star is located at a distance of 18.8 light years from the Sun. It is receding from the Sun with a radial velocity of 26.6 km/s.

<span class="mw-page-title-main">F-type main-sequence star</span> Stellar classification

An F-type main-sequence star is a main-sequence, hydrogen-fusing star of spectral type F and luminosity class V. These stars have from 1.0 to 1.4 times the mass of the Sun and surface temperatures between 6,000 and 7,600 K.Tables VII and VIII. This temperature range gives the F-type stars a whitish hue when observed by the atmosphere. Because a main-sequence star is referred to as a dwarf star, this class of star may also be termed a yellow-white dwarf. Notable examples include Procyon A, Gamma Virginis A and B, and KIC 8462852.

<span class="mw-page-title-main">A-type main-sequence star</span> Stellar classification

An A-type main-sequence star or A dwarf star is a main-sequence star of spectral type A and luminosity class V (five). These stars have spectra defined by strong hydrogen Balmer absorption lines. They measure between 1.4 and 2.1 solar masses (M☉) and have surface temperatures between 7,600 and 10,000 K. Bright and nearby examples are Altair (A7), Sirius A (A1), and Vega (A0). A-type stars do not have convective zones and thus are not expected to harbor magnetic dynamos. As a consequence, because they do not have strong stellar winds, they lack a means to generate X-ray emissions.

<span class="mw-page-title-main">B-type main-sequence star</span> Stellar classification distinguished by bright blue luminosity

A B-type main-sequence star is a main-sequence (hydrogen-burning) star of spectral type B and luminosity class V. These stars have from 2 to 16 times the mass of the Sun and surface temperatures between 10,000 and 30,000 K. B-type stars are extremely luminous and blue. Their spectra have strong neutral helium absorption lines, which are most prominent at the B2 subclass, and moderately strong hydrogen lines. Examples include Regulus and Algol A.

Pi<sup>3</sup> Orionis Star in the constellation Orion

Pi3 Orionis (π3 Orionis, abbreviated Pi3 Ori, π3 Ori), also named Tabit, is a star in the equatorial constellation of Orion. At an apparent visual magnitude of 3.16, it is readily visible to the naked eye and is the brightest star in the lion's hide (or shield) that Orion is holding. As measured using the parallax technique, it is 26.32 light-years (8.07 parsecs) distant from the Sun.

<span class="mw-page-title-main">Beta Comae Berenices</span> Star in the constellation Coma Berenices

Beta Comae Berenices is a main sequence dwarf star in the northern constellation of Coma Berenices. It is located at a distance of about 29.95 light-years from Earth. The Greek letter beta (β) usually indicates that the star has the second highest visual magnitude in the constellation. However, with an apparent visual magnitude of 4.3, this star is actually slightly brighter than α Comae Berenices. It can be seen with the naked eye, but may be too dim to be viewed from a built-up urban area.

109 Piscium is a yellow hued G-type main-sequence star located about 108 light-years away in the zodiac constellation of Pisces. It is near the lower limit of visibility to the naked eye with an apparent visual magnitude of 6.27. The star is moving closer to the Earth with a heliocentric radial velocity of −45.5 km/s. It has one known exoplanet.

Kappa<sup>1</sup> Ceti Variable yellow dwarf star in the constellation Cetus

Kappa1 Ceti, Latinized from κ1 Ceti, is a variable yellow dwarf star approximately 30 light-years away in the equatorial constellation of Cetus.

61 Ursae Majoris, abbreviated 61 UMa, is a single star in the northern circumpolar constellation of Ursa Major. It has a yellow-orange hue and is dimly visible to the naked eye with an apparent visual magnitude of 5.35. The distance to this star is 31.2 light years based on parallax, and it is drifting closer with a radial velocity of −5.2 km/s. The star has a relatively high proper motion traversing the sky at the rate of 0.381″ yr−1.

Gliese 832 is a red dwarf of spectral type M2V in the southern constellation Grus. The apparent visual magnitude of 8.66 means that it is too faint to be seen with the naked eye. It is located relatively close to the Sun, at a distance of 16.2 light years and has a high proper motion of 818.16 milliarcseconds per year. Gliese 832 has just under half the mass and radius of the Sun. Its estimated rotation period is a relatively leisurely 46 days. The star is roughly 6 billion years old.

<span class="mw-page-title-main">O-type main-sequence star</span> Main-sequence star of spectral type O

An O-type main-sequence star is a main-sequence star of spectral type O and luminosity class V. These stars have between 15 and 90 times the mass of the Sun and surface temperatures between 30,000 and 50,000 K. They are between 40,000 and 1,000,000 times as luminous as the Sun.

<span class="mw-page-title-main">Red giant</span> Type of large cool star that has exhausted its core hydrogen

A red giant is a luminous giant star of low or intermediate mass in a late phase of stellar evolution. The outer atmosphere is inflated and tenuous, making the radius large and the surface temperature around 5,000 K or lower. The appearance of the red giant is from yellow-white to reddish-orange, including the spectral types K and M, sometimes G, but also class S stars and most carbon stars.

<span class="mw-page-title-main">Habitability of K-type main-sequence star systems</span> Overview of the habitability of K-type main-sequence star systems

K-type main-sequence stars, also known as orange dwarfs, may be candidates for supporting extraterrestrial life. These stars are known as "Goldilocks stars" as they emit enough radiation in the non-UV ray spectrum to provide a temperature that allows liquid water to exist on the surface of a planet; they also remain stable in the main sequence longer than the Sun by burning their hydrogen slower, allowing more time for life to form on a planet around a K-type main-sequence star. The planet's habitable zone, ranging from 0.1–0.4 to 0.3–1.3 astronomical units (AU), depending on the size of the star, is often far enough from the star so as not to be tidally locked to the star, and to have a sufficiently low solar flare activity not to be lethal to life. In comparison, red dwarf stars have too much solar activity and quickly tidally lock the planets in their habitable zones, making them less suitable for life. The odds of complex life arising may be better on planets around K-type main-sequence stars than around Sun-like stars, given the suitable temperature and extra time available for it to evolve. Some planets around K-type main-sequence stars are potential candidates for extraterrestrial life.

<span class="mw-page-title-main">Habitability of red dwarf systems</span> Possible factors for life around red dwarf stars

The theorized habitability of red dwarf systems is determined by a large number of factors. Modern evidence indicates that planets in red dwarf systems are unlikely to be habitable, due to their low stellar flux, high probability of tidal locking and thus likely lack of magnetospheres and atmospheres, small circumstellar habitable zones and the high stellar variation experienced by planets of red dwarf stars, impeding their planetary habitability. However, the ubiquity and longevity of red dwarfs could provide ample opportunity to realize any small possibility of habitability.

References

  1. 1 2 E. Mamajek (2022-04-16). "A Modern Mean Dwarf Stellar Color and Effective Temperature Sequence" . Retrieved 2022-05-14.
  2. ↑ "Alpha Centauri B". SIMBAD . Centre de données astronomiques de Strasbourg . Retrieved 2019-06-05.
  3. ↑ Engle, S. G.; Guinan, E. F. (2011). "Red Dwarf Stars: Ages, Rotation, Magnetic Dynamo Activity and the Habitability of Hosted Planets". 9th Pacific Rim Conference on Stellar Astrophysics. Proceedings of a Conference Held at Lijiang. 451: 285. arXiv: 1111.2872 . Bibcode:2011ASPC..451..285E.
  4. ↑ Heath, Martin J.; Doyle, Laurance R.; Joshi, Manoj M.; Haberle, Robert M. (1999). "Habitability of planets around red dwarf stars". Origins of Life and Evolution of the Biosphere. 29 (4): 405–24. Bibcode:1999OLEB...29..405H. doi: 10.1023/A:1006596718708 . PMID   10472629. S2CID   12329736.
  5. ↑ Farihi, J.; Hoard, D. W.; Wachter, S. (2006). "White Dwarf-Red Dwarf Systems Resolved with the Hubble Space Telescope. I. First Results". The Astrophysical Journal. 646 (1): 480–492. arXiv: astro-ph/0603747 . Bibcode:2006ApJ...646..480F. doi:10.1086/504683. S2CID   16750158.
  6. ↑ Pettersen, B. R.; Hawley, S. L. (1989). "A spectroscopic survey of red dwarf flare stars". Astronomy and Astrophysics. 217: 187. Bibcode:1989A&A...217..187P.
  7. ↑ Alekseev, I. Yu.; Kozlova, O. V. (2002). "Starspots and active regions on the emission red dwarf star LQ Hydrae". Astronomy and Astrophysics. 396: 203–211. Bibcode:2002A&A...396..203A. doi: 10.1051/0004-6361:20021424 .
  8. ↑ Cuntz, M.; Guinan, E. F. (2016). "About Exobiology: The Case for Dwarf K Stars". The Astrophysical Journal. 827 (1): 79. arXiv: 1606.09580 . Bibcode:2016ApJ...827...79C. doi: 10.3847/0004-637X/827/1/79 . S2CID   119268294.
  9. ↑ Stevenson, David S. (2013). "Stellar Evolution Near the Bottom of the Main Sequence". Under a Crimson Sun. Astronomers' Universe. pp. 63–103. doi:10.1007/978-1-4614-8133-1_3. ISBN   978-1-4614-8132-4.
  10. ↑ Johnson, H. L.; Morgan, W. W. (1953). "Fundamental stellar photometry for standards of spectral type on the Revised System of the Yerkes Spectral Atlas". The Astrophysical Journal. 117: 313. Bibcode:1953ApJ...117..313J. doi:10.1086/145697.
  11. ↑ Garrison, R. F. (1993). "Anchor Points for the MK System of Spectral Classification". American Astronomical Society Meeting Abstracts. 183. Bibcode:1993AAS...183.1710G.
  12. 1 2 Keenan, Philip C.; McNeil, Raymond C. (1989). "The Perkins Catalog of Revised MK Types for the Cooler Stars". The Astrophysical Journal Supplement Series. 71: 245. Bibcode:1989ApJS...71..245K. doi:10.1086/191373.
  13. ↑ Johnson, H. L.; Morgan, W. W. (1953). "Fundamental stellar photometry for standards of spectral type on the Revised System of the Yerkes Spectral Atlas". The Astrophysical Journal. 117: 313. Bibcode:1953ApJ...117..313J. doi:10.1086/145697.
  14. ↑ Pecaut, Mark J.; Mamajek, Eric E. (2013). "Intrinsic Colors, Temperatures, and Bolometric Corrections of Pre-main-sequence Stars". The Astrophysical Journal Supplement Series. 208 (1): 9. arXiv: 1307.2657 . Bibcode:2013ApJS..208....9P. doi:10.1088/0067-0049/208/1/9. S2CID   119308564.
  15. ↑ Shiga, David (6 May 2009). "Orange stars are just right for life". New Scientist . Retrieved 2019-06-05.
  16. ↑ Steigerwald, Bill (10 March 2019). ""Goldilocks" stars may be "just right" for finding habitable worlds". nasa.gov (Press release). NASA Goddard SFC . Retrieved 2022-12-06.
  17. ↑ "Orange stars are just right for life". New Scientist . 6 May 2009. Retrieved 2019-06-05.
  18. ↑ Heller, René; Armstrong, John (2014). "Superhabitable worlds". Astrobiology . 14 (1): 50–66. arXiv: 1401.2392 . Bibcode:2014AsBio..14...50H. doi:10.1089/ast.2013.1088. PMID   24380533. S2CID   1824897.
  19. 1 2 Richey-Yowell, Tyler; Shkolnik, Evgenya L.; Loyd, R.O. Parke; et al. (2022-04-26). "HAZMAT. VIII. A spectroscopic analysis of the ultraviolet evolution of K stars: Additional evidence for K dwarf rotational stalling in the first gigayear". The Astrophysical Journal . American Astronomical Society. 929 (2): 169. arXiv: 2203.15237 . Bibcode:2022ApJ...929..169R. doi: 10.3847/1538-4357/ac5f48 .
  20. ↑ Toubet, Georgina (22 April 2022). "What UV radiation from the 'Goldilocks' stars could really mean". slashgear.com. Retrieved 2022-05-14.
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