K-type main-sequence star

Summary

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.[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]

Spectral standard starsEdit

Properties of typical K-type main-sequence stars[1]
Spectral type Mass
(M)
Radius
(R)
Luminosity
(L)
Effective temperature
(K)
Color index
(B − V)
K0V 0.88 0.813 0.46 5,270 0.82
K1V 0.86 0.797 0.41 5,170 0.86
K2V 0.82 0.783 0.37 5,100 0.88
K3V 0.78 0.755 0.28 4,830 0.99
K4V 0.73 0.713 0.20 4,600 1.09
K5V 0.70 0.701 0.17 4,440 1.15
K6V 0.69 0.669 0.14 4,300 1.24
K7V 0.64 0.630 0.10 4,100 1.34
K8V 0.62 0.615 0.087 3,990 1.36
K9V 0.59 0.608 0.079 3,930 1.40

The revised Yerkes Atlas system (Johnson & Morgan 1953)[3] 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:[4]

Other primary MK standard stars include:[5]

Based on the example set in some references (e.g. Johnson & Morgan 1953,[6] Keenan & McNeil 1989[5]), 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.[7]

PlanetsEdit

These stars are of particular interest in the search for extraterrestrial life[8] because they are stable on the main sequence for a very long time (17-70 billion years, compared to 10 billion for the Sun).[9]

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.[10]

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.[11] 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.[11][12]

K-type main-sequence stars are about three to four times as abundant as G-type main-sequence stars, making planet searches easier.[13] 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.

See alsoEdit

ReferencesEdit

  1. ^ a b 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. ^ 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.
  4. ^ Garrison, R. F. (1993). "Anchor Points for the MK System of Spectral Classification". American Astronomical Society Meeting Abstracts. 183. Bibcode:1993AAS...183.1710G.
  5. ^ a b 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.
  6. ^ 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.
  7. ^ 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.
  8. ^ David Shiga (May 6, 2009). "Orange stars are just right for life". New Scientist. Retrieved 2019-06-05.
  9. ^ Bill Steigerwald (Mar 10, 2019). ""Goldilocks" Stars May Be "Just Right" for Finding Habitable Worlds". NASA. Retrieved 2022-12-06.
  10. ^ 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.
  11. ^ a b Richey-Yowell, Tyler; Shkolnik, Evgenya L.; 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. doi:10.3847/1538-4357/ac5f48.
  12. ^ GEORGINA TORBET (April 22, 2022). "What UV Radiation From The 'Goldilocks' Stars Could Really Mean". Retrieved 2022-05-14.
  13. ^ "Orange stars are just right for life". May 6, 2009. Retrieved 2019-06-05.