K型主序星

橙矮星，也稱橙色主序星，即K型主序星（KV），是主序帶（以氫為燃料）上光譜類型為K、亮度分類為V的恆星。這些恆星的大小介於M型主序星和G型主序星之間，質量是太陽質量的0.5至0.9倍，表面溫度在3,900至5,300K。Empirical bolometric corrections for the main-sequence , G. M. H. J. Habets and J. R. W. Heintze, Astronomy and Astrophysics Supplement 46 (November 1981), pp. 193–237.K型主序星在宇宙中的占比约16%—18%左右。

此種恆星的例子有半人馬座α星B、印第安座 εSIMBAD, entries for Alpha Centauri B  and Epsilon Indi , accessed on line June 19, 2007.、奎宿增三、HD 13445、牛宿增十、天苑四等。

 宜居性和争议 
這種恆星是部分学者在尋找在地球之外的生命時最感興趣的，因為它們停留在主序星阶段上穩定的時間很長（早K型约150—300億年，高金属丰度的晚K型主序星能达到略多于700亿年K-type starI,Baraffe,G,et al.Evolutionary models for low-mass stars and brown dwarfs: Uncertainties and limits at very young ages[J].Astronomy & Astrophysics, 2002.Kippenhahn R , Weigert A .Stellar Structure and Evolution[J].北京大学出版社, 2013.DOI:10.1007/978-3-642-30304-3.，相比较之下太陽只有约105億年的寿命且常简化为100亿年Aumer M , Binney J J .Kinematics and history of the solar neighbourhood revisited[J].Oxford University Press, 2009(3).DOI:10.1111/J.1365-2966.2009.15053.X.），使環繞其運行的類地行星有更充足的機會產生具備適居性的環境Orange stars are just right for life , retrieved on May 6, 2009.。学术界和天文爱好者中的相当一部分流行观点都认为K型主序星是最为理想的选项，主要是基于其较长的寿命、极弱的紫外辐射以及不像红矮星有着严重的耀斑活动和潮汐锁定问题，相关的常用词汇则有“橙矮星优势（K Dwarf Advantage）”和“金发姑娘（Goldilocks）”，但21世纪以来的相关研究也存在较大争议。Richey-Yowell, T., Barman, T. S., Loyd, R. O. P., Meadows, V. S., Schneider, A., and Shkolnik, E. L., “The K Dwarf Advantage: Assessing the Habitability of Planets Orbiting K Stars”, HST Proposal, p. 15955, 2019.Richey-Yowell, T., “Evaluating the K Dwarf Advantage through the Ultraviolet and X-ray Evolution of K Stars”, vol. 54, no. 5, Art. no. 102.254, 2022.Richey-Yowell, T., “Do K Stars Really Have an Advantage? Assessing the Habitability of Planets around K Stars by their Ultraviolet Evolution”, in The Astrobiology Science Conference (AbSciCon) 2022, 2022, Art. no. 449-02.Arney, Giada N.. “The K Dwarf Advantage for Biosignatures on Directly Imaged Exoplanets.” The Astrophysical Journal Letters 873 (2019): n. pag.Richey-Yowell, Tyler et al. “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 (2022): n. pag.

 潜在问题 
21世纪的新研究表明，紫外辐射是一把双刃剑，过强会破坏生态系统，过弱则不能驱动复杂有机物等的产生与演化Oishi M , Kamaya H .A SIMPLE EVOLUTIONAL MODEL OF THE UV HABITABLE ZONE AND THE POSSIBILITY OF PERSISTENT LIFE EXISTENCE: THE EFFECTS OF MASS AND METALLICITY[J].Astrophysical Journal, 2016, 833(2):293.DOI:10.3847/1538-4357/833/2/293.Sparrman, V. (2022). Post-Main Sequence Habitability for Outer Solar System Moons (Dissertation). Retrieved from https://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-477353，只有0.8倍太阳质量至1.8倍太阳质量的恒星的紫外宜居带和可见光宜居带能够重合Guo J , Zhang F , Zhang X ,et al.Habitable Zones and UV Habitable Zones around Host Stars[J].Astrophysics & Space Science, 2010, 325(1):25.DOI:10.1007/s10509-009-0173-9.（考虑恒星自转快使得辐射向恒星两极集中而减轻恒星赤道面的辐射，则可以上延至太阳质量的2.2倍，但对天生紫外辐射驱动不足的恒星则无用Ahlers J P , Fromont E F , Kopparappu R ,et al.The Habitable Zones of Rapidly Rotating Main Sequence A/F Stars[J].The Astrophysical Journal, 2022, 928(1):35 (9pp).DOI:10.3847/1538-4357/ac5596.）。大多数K型主序星的质量都不到太阳质量的0.8倍，近紫外辐射过弱、不利于分子变异和借助近紫外合成某些有机物，反而成为其缺点。Lingam M , Loeb A .Is Life Most Likely Around Sun-like Stars?[J].Journal of Cosmology & Astroparticle Physics, 2017.DOI:10.1088/1475-7516/2018/05/020.仅一些特定情况的K型主序星，依靠适度的耀斑活动等方式额外增加紫外线辐射因素，使得紫外光宜居带外延至与可见光宜居带有所相交（理论上红矮星也可以依靠耀斑活动来外延紫外宜居带，只是耀斑过重则适得其反），或许可以提高孕育生命的可能性。

K型主序星的恒星风和耀斑活动对宜居带的破坏比F型主序星和G型主序星要严重（与一些刻板印象不同，其实在恒星活动方面最温和的反而是A型主序星，在A型主序星的基础上增大或减小质量和亮度都会逐渐增加不稳定性Pedersen, M. G., Antoci, V., Korhonen, H., White, T. R., Jessen-Hansen, J., Lehtinen, J., et al. (2017). Do A-type stars flare? Mon. Not. R. Astron. Soc. 466, 3060–3076. doi: 10.1093/mnras/stw3226C. Schröder, Schmitt J H M M .X-ray emission from A-type stars[J].Astronomy & Astrophysics, 2007, 475(2):677-684.DOI:10.1051/0004-6361:20077429.Wood, B. E., Müller, H.-R., Redfield, S., & Edelman, E. 2014, ApJ, 781, L33 [NASA ADS] [CrossRef] [Google Scholar]，但是A型主序星吃亏在寿命偏短/演化时间急促和紫外辐射偏强，K型主序星虽然长寿但是实际上仅比红矮星、早B、O和WR型恒星要温和）Wood B E , Mueller H R , Zank G P ,et al.Measured Mass Loss Rates of Solar-like Stars as a Function of Age and Activity[J].The Astrophysical Journal, 2002, 574(1):412-425.DOI:10.1086/340797.Johnstone C P , Bartel M ,M. Güdel.The active lives of stars: A complete description of the rotation and XUV evolution of F, G, K, and M dwarfs[J].Astronomy and Astrophysics, 2021.DOI:10.1051/0004-6361/202038407.Wood B E .Implications of Recent Stellar Wind Measurements[J].Journal of Physics Conference Series, 2018, 1100.DOI:10.1088/1742-6596/1100/1/012028.Kopparapu R K , Ramirez R , Kasting J F ,et al.HABITABLE ZONES AROUND MAIN-SEQUENCE STARS: NEW ESTIMATES (vol 765, pg 131, 2013)[J].The Astrophysical Journal, 2013, 770(1):82.DOI:10.1088/0004-637X/770/1/82.，而K型主序星的剧烈活动的减慢速度和行星系统的演化速度比更大质量的恒星要缓慢，在相同年龄、等距离下，K型主序星的极紫外辐射（不同于近紫外辐射和其他紫外辐射，极紫外辐射对生物圈是纯破坏性作用）是G型主序星的3—4倍（红矮星则是10—100倍，考虑到红矮星的宜居带距离很近则更糟糕），X射线是G型主序星的2—3倍，如果再考虑上K型主序星的宜居带更近，则破坏性更高Airapetian, Vladimir S. et al. “Impact of space weather on climate and habitability of terrestrial-type exoplanets.” International Journal of Astrobiology 19 (2019): 136 - 194.Richey-Yowell, T., Shkolnik, E.L., Schneider, A.C., Osby, E., Barman, T.S., & Meadows, V.S. (2019). HAZMAT. V. The Ultraviolet and X-Ray Evolution of K Stars. The Astrophysical Journal, 872.Variability and Habitability – UV/X-ray emissionsZhu, E., & Preibisch, T. (2025). X-ray activity of nearby G-, K-, and M-type stars and implications for planet habitability around M stars. Astronomy & Astrophysics.。尤其是靠近K/M边界的K型主序星，虽然其寿命大多已经超过500亿年，但是其不仅有紫外辐射驱动过少、宜居带的恒星风过于严重的问题，而且其耀斑活动的剧烈程度也开始接近于红矮星的水平James C , Wheatland M S , Kai Y .Superflare rate variability on M dwarfs[J].Monthly Notices of the Royal Astronomical Society, 2024(1):1.DOI:10.1093/mnras/stae818.Chen H , Zhan Z , Youngblood A ,et al.Persistence of Flare-Driven Atmospheric Chemistry on Rocky Habitable Zone Worlds[J].  2021.DOI:10.1038/s41550-020-01264-1.，以及极易引发对宜居带行星的潮汐锁定，环境不容乐观。

也有观点认为，随着宇宙年龄的不断增大，在比较长的时间范围内，K型主序星甚至红矮星将会逐渐占据压倒性优势，即使其演化速度再慢、剧烈活动的衰减速度再慢，到了主序星阶段的中后期都会越来越宜居（中年尤其晚年的K型主序星比G型主序星更温和，甚至红矮星到了主序星末期阶段将变为蓝矮星时也可能会十分宜居）；但是目前的宇宙还很年轻（仅138.2亿岁，年龄过小，多数恒星则只有数十亿岁），K型主序星和红矮星暂时还不充分具备相对于F/G型主序星（已经率先演化成熟）的优势。Loeb, Abraham et al. “Relative likelihood for life as a function of cosmic time.” Journal of Cosmology and Astroparticle Physics 2016 (2016): 040 - 040.

 參考資料 

 相關條目 
 太陽相似體
 紅矮星
 G型主序星

Category:主序星