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**天棓四 (天龍座γ)**
	天棓四的位置（紅圈）
觀測資料; 曆元 J2000
星座	天龍座
星官	天棓（紫微垣）
赤經	17h 56m 36.36988s
赤緯	+51° 29′ 20.0242″
視星等（V）	2.23
特性
光谱分类	K8 III
U−B 色指数	+1.87
B−V 色指数	+1.52
天体测定
徑向速度 (Rv)	–28.19 ± 0.36 km/s
自行 (μ)	赤经：–8.48 mas/yr ; 赤纬：–22.79 mas/yr
视差 (π)	21.14 ± 0.10 mas
距离	154.3 ± 0.7 ly ; (47.3 ± 0.2 pc)
绝对星等 (MV)	−1.93 ± 0.07
詳細資料
質量	2.14±0.16 M☉
半徑	51.8±0.26 R☉
表面重力 (log g)	1.33±0.04
亮度	590±12 L☉
溫度	3,990±42 K
金属量 [Fe/H]	+0.11±0.05 dex
自轉速度 (v sin i)	4.5±0.05 km/s
年齡	1.3±0.25 Gyr
其他命名
	Eltanin、天龍座33、BD +51°2282, FK5 676、HD 164058、HIP 87833, HR 6705、 SAO 30653
參考資料庫
SIMBAD	资料

天棓四（天龍座γ Draconis，縮寫Gamma Dra、γ Dra），國際天文學聯合會的正式名稱為Eltanin /ɛlˈteɪnɪn/^[8]^[9]，是位於天龍座北部的一顆恆星。它是天龍星中最亮的天體，其星等為2.2等，但它歷史上排名第三，因此在拜耳命名法中是γ星^[1]，事實上，它比β星亮近半個星等，比α星亮一個星等以上。

天棓四距離太陽154.3光年（47.3秒差距）^[1]，是由依巴谷天體量測衛星的視差量測所確定的^[10]^[11]。在1728年，英國天文學家詹姆士·布拉德雷（英語：James Bradley）在嘗試量測這顆恆星的視差失敗時，發現了地球相對運動導致的光行差。布拉德雷的發現顯然證實了哥白尼「地球圍繞太陽旋轉的理論」^[12]。它正以約-28公里/秒的徑向速度向太陽系靠近。

在過150萬年，天棓四將在距離地球28光年以內通過。如果它現時的絕對星等不變，屆時它將是夜空中最亮的恆星，幾乎和現時的天狼星一樣亮^[12]^[13]。在英國的倫敦附近，它是通過天頂最亮的恆星，這導致它在這些地方被吹捧為「天頂之星」^[14]。在東南偏南的這顆紅星附近，是天琴座中一顆明亮且著名的織女星。

性質

天棓四是一顆演化中的巨星，其恆星分類為K8pIII。自1943年以來，這顆恆星的光譜一直是其他恆星分類的穩定錨點之一^[15]。它的質量大約是太陽的兩倍^[3]，而它的大小已經擴張至太陽的50倍左右^[6]。它的輻射約是太陽的590倍^[7]，在3,990 K的有效溫度下，其光度與太陽的外層大氣相當^[3]。這比太陽冷，使這顆恆星呈現出K型恆星的橙色光芒^[16]。

在雙星目錄中，天棓四是6合星；所有這些都是由美國天文學家舍本·衛斯里·伯納姆發現的^[17]。最接近的可能是物理上相關的，並且相隔約7014149597870700000♠1,000 AU。這個天體的光度表明它是一顆紅矮星^[18]，其它的都是與天棓四無關的遙遠恆星^[19]。

命名法

天棓四，在拜耳命名法的名稱是「γDraconis」（天龍座γ，拉丁化是「Gamma Draconis」）。

它有著傳統的名字：Eltanin^[12]^[20]，來自阿拉伯語التنينAt-Tinnin，意思是「大蛇」。「Rastaban」這個名字以前用於天棓四，這兩個詞有一個阿拉伯詞根，意思是「蛇」或「龍」。在2016年，國際天文學聯合會組織了IAU恆星名稱工作組（WGSN）^[21]對恆星的專有名稱進行編目和標準化。WGSN於2016年8月21日核准了這顆恆星的名稱為「Eltanin」，現在它已被列入國際天文學聯合會的恆星名稱目錄^[9]。

天棓四與天棓三（天龍座β）、天棓增九（天龍座μ）、天龍座ν、和天棓一（天龍座ξ）合稱為「駱駝媽媽」，後來被稱為「五度駱駝」（英語：Quinque Dromedarii）^[14]。

在中國，天棓 (Tiān Bàng)的意思是「天翼」，是指由天龍座γ、天龍座ξ、天龍座ν、天龍座β星和武仙座ι組成的星官^[22]。因此，天龍座γ的中文名稱是天棓四 (Tiān Bàng sì）天棓的第四顆星（英語：the Fourth Star of Celestial Flail。）^[23]。

名字

美國海軍埃塔明號 (AK-93)（英语：USS Etamin）是一艘以這顆恆星命名的美國海軍巨爵級貨船（英语：Crater class cargo ship）。

注釋和參考文獻

References

^ ^1.0 ^1.1 ^1.2 ^1.3 ^1.4 ^1.5 ^1.6 van Leeuwen, F. Hipparcos, the New Reduction. Astronomy and Astrophysics. November 2007, 474 (2): 653–664 [2010-11-21]. Bibcode:2007A&A...474..653V. S2CID 18759600. arXiv:0708.1752 . doi:10.1051/0004-6361:20078357.
^ ^2.0 ^2.1 Oja, T. UBV photometry of stars whose positions are accurately known. VI. Astronomy and Astrophysics Supplement Series. August 1991, 89 (2): 415–419. Bibcode:1991A&AS...89..415O.
^ ^3.0 ^3.1 ^3.2 ^3.3 Hatzes, A. P.; Endl, M.; Cochran, W. D.; MacQueen, P. J.; Han, I.; Lee, B.-C.; Kim, K.-M.; Mkrtichian, D.; Döllinger, M.; Hartmann, M.; Karjalainen, M.; Dreizler, S., The Radial Velocity Variability of the K-giant γ Draconis: Stellar Variability Masquerading as a Planet, The Astronomical Journal, 2018-02-16, 155 (3): 120, Bibcode:2018AJ....155..120H, ISSN 0004-6256, arXiv:1801.05239 , doi:10.3847/1538-3881/aaa8e1  （英语）
^ Massarotti, Alessandro; et al, Rotational and Radial Velocities for a Sample of 761 HIPPARCOS Giants and the Role of Binarity, The Astronomical Journal, January 2008, 135 (1): 209–231, Bibcode:2008AJ....135..209M, doi:10.1088/0004-6256/135/1/209 
^ Piau, L.; et al, Surface convection and red-giant radius measurements, Astronomy and Astrophysics, February 2011, 526: A100, Bibcode:2011A&A...526A.100P, S2CID 118533297, arXiv:1010.3649 , doi:10.1051/0004-6361/201014442
^ ^6.0 ^6.1 Baines, Ellyn K.; Thomas Armstrong, J.; Clark, James H.; Gorney, Jim; Hutter, Donald J.; Jorgensen, Anders M.; Kyte, Casey; Mozurkewich, David; Nisley, Ishara; Sanborn, Jason; Schmitt, Henrique R. Angular Diameters and Fundamental Parameters of Forty-four Stars from the Navy Precision Optical Interferometer. The Astronomical Journal. November 2021, 162 (5): 198. Bibcode:2021AJ....162..198B. ISSN 0004-6256. S2CID 238998021. arXiv:2211.09030 . doi:10.3847/1538-3881/ac2431 .
^ ^7.0 ^7.1 ^7.2 Soubiran, C.; Creevey, O. L.; Lagarde, N.; Brouillet, N.; Jofré, P.; Casamiquela, L.; Heiter, U.; Aguilera-Gómez, C.; Vitali, S.; Worley, C.; de Brito Silva, D., Gaia FGK benchmark stars: Fundamental Teff and log g of the third version, Astronomy and Astrophysics, 2024-02-01, 682: A145, Bibcode:2024A&A...682A.145S, ISSN 0004-6361, arXiv:2310.11302 , doi:10.1051/0004-6361/202347136 Gamma Draconis' database entry at VizieR.
^ Kunitzsch, Paul; Smart, Tim. A Dictionary of Modern star Names: A Short Guide to 254 Star Names and Their Derivations 2nd rev. Cambridge, Massachusetts: Sky Pub. 2006. ISBN 978-1-931559-44-7.
^ ^9.0 ^9.1 IAU Catalog of Star Names. [28 July 2016].
^ Perryman, M. A. C.; Lindegren, L.; Kovalevsky, J.; et al, The Hipparcos Catalogue, Astronomy and Astrophysics, July 1997, 323: L49–L52, Bibcode:1997A&A...323L..49P
^ Perryman, Michael, The Making of History's Greatest Star Map, Astronomers’ Universe, Heidelberg: Springer-Verlag, 2010, Bibcode:2010mhgs.book.....P, ISBN 978-3-642-11601-8, doi:10.1007/978-3-642-11602-5
^ ^12.0 ^12.1 ^12.2 Kaler, James B. ELTANIN (Gamma Draconis). Stars. University of Illinois. [2010-11-21].
^ Tomkin, Jocelyn. Once and Future Celestial Kings. Sky and Telescope. April 1998, 95 (4): 59–63. Bibcode:1998S&T....95d..59T. – based on computations from HIPPARCOS data. (The calculations exclude stars whose distance or proper motion is uncertain.) PDF^{[永久失效連結]}
^ ^14.0 ^14.1 Allen, R. H., Star Names: Their Lore and Meaning Reprint, New York, NY: Dover Publications Inc: 207, 1963 [2010-12-12], ISBN 0-486-21079-0
^ Garrison, R. F., Anchor Points for the MK System of Spectral Classification, Bulletin of the American Astronomical Society, December 1993, 25: 1319 [2012-02-04], Bibcode:1993AAS...183.1710G, （原始内容存档于2019-06-25）
^ The Colour of Stars, Australia Telescope, Outreach and Education (Commonwealth Scientific and Industrial Research Organisation), December 21, 2004 [2012-01-16], （原始内容存档于2012-03-18）
^ Mason, Brian D.; et al. The 2001 US Naval Observatory Double Star CD-ROM. I. The Washington Double Star Catalog. The Astronomical Journal. December 2001, 122 (6): 3466–3471. Bibcode:2001AJ....122.3466M. doi:10.1086/323920 .
^ Ayres, Thomas R.; Brown, Alexander; Harper, Graham M., The Coronae of γ Draconis, The Astrophysical Journal, November 2006, 651 (2): 1126–1129, Bibcode:2006ApJ...651.1126A, S2CID 120205574, doi:10.1086/507763 
^ Brown, A. G. A.; Vallenari, A.; Prusti, T.; de Bruijne, J. H. J.; et al. Gaia Data Release 2. Summary of the contents and survey properties. Astronomy & Astrophysics. 2018. Bibcode:2018A&A...616A...1G. arXiv:1804.09365 . doi:10.1051/0004-6361/201833051.
^ γ Dra (Eltanin). Wikisky.org. [2010-11-22].
^ IAU Working Group on Star Names (WGSN), International Astronomical Union, [22 May 2016].
^ （中文） 中國星座神話, written by 陳久金. Published by 台灣書房出版有限公司, 2005, ISBN 978-986-7332-25-7.
^ （中文）香港太空館 - 研究資源 - 亮星中英對照表互联网档案馆的存檔，存档日期January 30, 2011，., Hong Kong Space Museum. Accessed on line November 23, 2010.

註釋

[LEEUWEN-1] 1.0 ^1.1 ^1.2 ^1.3 ^1.4 ^1.5 ^1.6 van Leeuwen, F. Hipparcos, the New Reduction. Astronomy and Astrophysics. November 2007, 474 (2): 653–664 [2010-11-21]. Bibcode:2007A&A...474..653V. S2CID 18759600. arXiv:0708.1752 . doi:10.1051/0004-6361:20078357.

[aaass89_2_415-2] 2.0 ^2.1 Oja, T. UBV photometry of stars whose positions are accurately known. VI. Astronomy and Astrophysics Supplement Series. August 1991, 89 (2): 415–419. Bibcode:1991A&AS...89..415O.

[hatzes-3] 3.0 ^3.1 ^3.2 ^3.3 Hatzes, A. P.; Endl, M.; Cochran, W. D.; MacQueen, P. J.; Han, I.; Lee, B.-C.; Kim, K.-M.; Mkrtichian, D.; Döllinger, M.; Hartmann, M.; Karjalainen, M.; Dreizler, S., The Radial Velocity Variability of the K-giant γ Draconis: Stellar Variability Masquerading as a Planet, The Astronomical Journal, 2018-02-16, 155 (3): 120, Bibcode:2018AJ....155..120H, ISSN 0004-6256, arXiv:1801.05239 , doi:10.3847/1538-3881/aaa8e1  （英语）

[aj135_1_209-4] Massarotti, Alessandro; et al, Rotational and Radial Velocities for a Sample of 761 HIPPARCOS Giants and the Role of Binarity, The Astronomical Journal, January 2008, 135 (1): 209–231, Bibcode:2008AJ....135..209M, doi:10.1088/0004-6256/135/1/209 

[aaa526_A100-5] Piau, L.; et al, Surface convection and red-giant radius measurements, Astronomy and Astrophysics, February 2011, 526: A100, Bibcode:2011A&A...526A.100P, S2CID 118533297, arXiv:1010.3649 , doi:10.1051/0004-6361/201014442

[baines2021-6] 6.0 ^6.1 Baines, Ellyn K.; Thomas Armstrong, J.; Clark, James H.; Gorney, Jim; Hutter, Donald J.; Jorgensen, Anders M.; Kyte, Casey; Mozurkewich, David; Nisley, Ishara; Sanborn, Jason; Schmitt, Henrique R. Angular Diameters and Fundamental Parameters of Forty-four Stars from the Navy Precision Optical Interferometer. The Astronomical Journal. November 2021, 162 (5): 198. Bibcode:2021AJ....162..198B. ISSN 0004-6256. S2CID 238998021. arXiv:2211.09030 . doi:10.3847/1538-3881/ac2431 .

[Soubiran2024-7] 7.0 ^7.1 ^7.2 Soubiran, C.; Creevey, O. L.; Lagarde, N.; Brouillet, N.; Jofré, P.; Casamiquela, L.; Heiter, U.; Aguilera-Gómez, C.; Vitali, S.; Worley, C.; de Brito Silva, D., Gaia FGK benchmark stars: Fundamental Teff and log g of the third version, Astronomy and Astrophysics, 2024-02-01, 682: A145, Bibcode:2024A&A...682A.145S, ISSN 0004-6361, arXiv:2310.11302 , doi:10.1051/0004-6361/202347136 Gamma Draconis' database entry at VizieR.

[Kunitzsch-8] Kunitzsch, Paul; Smart, Tim. A Dictionary of Modern star Names: A Short Guide to 254 Star Names and Their Derivations 2nd rev. Cambridge, Massachusetts: Sky Pub. 2006. ISBN 978-1-931559-44-7.

[IAU-CSN-9] 9.0 ^9.1 IAU Catalog of Star Names. [28 July 2016].

[aaa323_L49-10] Perryman, M. A. C.; Lindegren, L.; Kovalevsky, J.; et al, The Hipparcos Catalogue, Astronomy and Astrophysics, July 1997, 323: L49–L52, Bibcode:1997A&A...323L..49P

[GSM-11] Perryman, Michael, The Making of History's Greatest Star Map, Astronomers’ Universe, Heidelberg: Springer-Verlag, 2010, Bibcode:2010mhgs.book.....P, ISBN 978-3-642-11601-8, doi:10.1007/978-3-642-11602-5

[KALER-12] 12.0 ^12.1 ^12.2 Kaler, James B. ELTANIN (Gamma Draconis). Stars. University of Illinois. [2010-11-21].

[tomkin1998-13] Tomkin, Jocelyn. Once and Future Celestial Kings. Sky and Telescope. April 1998, 95 (4): 59–63. Bibcode:1998S&T....95d..59T. – based on computations from HIPPARCOS data. (The calculations exclude stars whose distance or proper motion is uncertain.) PDF^{[永久失效連結]}

[allen1963-14] 14.0 ^14.1 Allen, R. H., Star Names: Their Lore and Meaning Reprint, New York, NY: Dover Publications Inc: 207, 1963 [2010-12-12], ISBN 0-486-21079-0

[baas25_1319-15] Garrison, R. F., Anchor Points for the MK System of Spectral Classification, Bulletin of the American Astronomical Society, December 1993, 25: 1319 [2012-02-04], Bibcode:1993AAS...183.1710G, （原始内容存档于2019-06-25）

[csiro-16] The Colour of Stars, Australia Telescope, Outreach and Education (Commonwealth Scientific and Industrial Research Organisation), December 21, 2004 [2012-01-16], （原始内容存档于2012-03-18）

[wds-17] Mason, Brian D.; et al. The 2001 US Naval Observatory Double Star CD-ROM. I. The Washington Double Star Catalog. The Astronomical Journal. December 2001, 122 (6): 3466–3471. Bibcode:2001AJ....122.3466M. doi:10.1086/323920 .

[apj651_2_1126-18] Ayres, Thomas R.; Brown, Alexander; Harper, Graham M., The Coronae of γ Draconis, The Astrophysical Journal, November 2006, 651 (2): 1126–1129, Bibcode:2006ApJ...651.1126A, S2CID 120205574, doi:10.1086/507763 

[dr2-19] Brown, A. G. A.; Vallenari, A.; Prusti, T.; de Bruijne, J. H. J.; et al. Gaia Data Release 2. Summary of the contents and survey properties. Astronomy & Astrophysics. 2018. Bibcode:2018A&A...616A...1G. arXiv:1804.09365 . doi:10.1051/0004-6361/201833051.

[WIKISKY-20] γ Dra (Eltanin). Wikisky.org. [2010-11-22].

[WGSN-21] IAU Working Group on Star Names (WGSN), International Astronomical Union, [22 May 2016].

[22] （中文） 中國星座神話, written by 陳久金. Published by 台灣書房出版有限公司, 2005, ISBN 978-986-7332-25-7.

[23] （中文）香港太空館 - 研究資源 - 亮星中英對照表互联网档案馆的存檔，存档日期January 30, 2011，., Hong Kong Space Museum. Accessed on line November 23, 2010.

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天棓四

性質

命名法

名字

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注釋和參考文獻

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