K-type main-sequence star
| K-type main-sequence star | |
|---|---|
 Sigma Draconis, officially named Alsafi, is a K-type main-sequence star. | |
| Characteristics | |
| Type | Class of medium-small main sequence star |
| Mass range | 0.60–0.86 M☉ |
| Temperature | 3,930–5,290 K |
| Average luminosity | 0.079–0.47 L☉ |
| External links | |
 Media category | |
 Q863936 | |
A K-type main-sequence star, also called a K-type dwarf or orange dwarf, is a star that is burning hydrogen in its core. It is a medium-sized star, bigger than a red dwarf but smaller than a yellow dwarf, which is why it is often called an orange dwarf. It is a type of main sequence star.
Description
[change | change source]These stars have a mass between 0.6 and 0.9 times the mass of the Sun. Their surface temperature is between 3,900 and 5,300 degrees Kelvin.[1] Scientists are very interested in these stars when looking for life outside Earth because they are stable and live a very long time. They can stay in this stage for up to 70 billion years, which is much longer than the age of the universe (13.8 billion years). This means none of these stars have changed from this stage yet.[2] Some well-known K-type stars are Alpha Centauri B (K1 V), Epsilon Indi (K5 V) and Epsilon Eridani (K2 V).[3]
Subdwarfs
[change | change source]Subdwarfs are a type of star called luminosity class VI, and some of them belong to the K spectral class. These stars, like normal main-sequence stars, are burning hydrogen in their cores. However, because they have very low amounts of metals (elements heavier than hydrogen and helium), they are less bright than normal stars. They are about two magnitudes dimmer than regular main-sequence stars.[4]
Names
[change | change source]Names for K-type main-sequence stars can be different depending on how people use them today. Sometimes, late K-type stars are grouped with early to middle M-type stars and called red dwarfs.[5] In other cases red dwarf is restricted just to M-class stars.[6][7] But other times, the name red dwarf is only used for M-type stars. In some cases, all K-type stars are called red dwarfs,[8] and sometimes even stars that are earlier than K-type are included.[9] The name orange dwarf is often used for early K-type stars,[10] but sometimes it is used for all K-type main-sequence stars.[11]
Spectral standard stars
[change | change source]| 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 from 1953[12] listed 12 K-type dwarf stars as standard examples for their group. However, not all of these stars are still used as standards today. The most important and reliable standard stars for K-type main-sequence dwarfs, which have stayed the same over the years, are called the "anchor points" of the MK classification system. These stars serve as the main examples for this type of star:[13]
- Sigma Draconis (K0 V)
- Epsilon Eridani (K2 V)
- 61 Cygni A (K5 V)
Other primary MK standard stars include:[14]
- 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)
Many authors follow examples from some references like Johnson & Morgan (1953)[15] and Keenan & McNeil (1989)[14] by treating the step between K7 V and M0 V stars as just one subdivision. Because of this, the K8 and K9 star classifications are not often used. However, a few stars have been classified as K8 or K9, such as HIP 111288 (K8V) and HIP 3261 (K9V). These examples show that these classifications do exist but are rare.[16]
Planets
[change | change source]K-type main-sequence stars are very interesting for the search for life outside Earth[17] because they stay stable for a very long time—between 17 and 70 billion years, much longer than the Sun's 10 billion years.[2] Like M-type stars, K-type stars have small masses, which helps them live so long and gives plenty of time for life to develop on planets around them.
Some nearby K-type stars known to have planets are Epsilon Eridani, HD 192310, Gliese 86, and 54 Piscium.
K-type stars are about three to four times more common than G-type stars like the Sun, which makes it easier to find planets around them.[18] They give off less harmful ultraviolet and other radiation compared to G-type stars, which is good because too much radiation can damage DNA and make it harder for life to start. Many K-type stars shine mostly in red light.[19]
M-type stars are the most common, but they often have planets that are tidally locked (always showing the same face to the star), and they can produce strong solar flares and cold spots. These conditions might make it harder for life to develop on planets around M-type stars. K-type stars have warmer and wider habitable zones (the area where life could exist) than M-type stars. Because of all these reasons, K-type stars might be the best stars to study when looking for planets and life beyond Earth.
Radiation hazard
[change | change source]
Even though K-type stars give off less total ultraviolet (UV) light, planets need to orbit closer to these stars to have temperatures suitable for life.[20] This closer orbit can reduce or cancel out the benefit of the star’s lower UV output.
Also, new research shows that K-type dwarf stars release high levels of X-rays and far ultraviolet (FUV) radiation for a longer time early in their life compared to bigger G-type stars or smaller early M-type stars. This long period of strong radiation might harm or even destroy the atmospheres of Earth-like planets near these stars. It could also stop or delay life from starting on those planets.[20][21]
See also
[change | change source]References
[change | change source]- 1 2 E. Mamajek (2022-04-16). "A Modern Mean Dwarf Stellar Color and Effective Temperature Sequence". Retrieved 2022-05-14.
- 1 2 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.
- ↑ "Alpha Centauri B". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2019-06-05.
- ↑ Kaler, James B. (1997-03-27). Stars and Their Spectra: An Introduction to the Spectral Sequence. Cambridge University Press. ISBN 978-0-521-58570-5. Retrieved 2025-12-15.
- ↑ 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.
- ↑ 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.
- ↑ 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.
- ↑ 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.
- ↑ 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.
- ↑ 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.
- ↑ 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.
- ↑ 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.
- ↑ Garrison, R. F. (1993). "Anchor Points for the MK System of Spectral Classification". American Astronomical Society Meeting Abstracts. 183. Bibcode:1993AAS...183.1710G.
- 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.
- ↑ 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.
- ↑ 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.
- ↑ Shiga, David (6 May 2009). "Orange stars are just right for life". New Scientist. Retrieved 2019-06-05.
- ↑ "Orange stars are just right for life". New Scientist. 6 May 2009. Retrieved 2019-06-05.
- ↑ 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.
- 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. 929 (2). American Astronomical Society: 169. arXiv:2203.15237. Bibcode:2022ApJ...929..169R. doi:10.3847/1538-4357/ac5f48.
- ↑ Toubet, Georgina (22 April 2022). "What UV radiation from the 'Goldilocks' stars could really mean". slashgear.com. Retrieved 2022-05-14.
