離宮四

離宮四（飛馬座η）
Eta Pegasi
	離宮四在飛馬座的位置
觀測資料; 曆元 J2000.0
星座	飛馬座
星官	室宿離宮
赤經	22h 43m 00.13743s
赤緯	+30° 13′ 16.4822″
視星等（V）	+2.95
特性
光譜分類	G2 II + F0 V
U−B 色指數	+0.57
B−V 色指數	+0.86
天體測定
徑向速度 (Rv)	+4.3 km/s
自行 (μ)	赤經：–100.06 mas/yr ; 赤緯：+15.46 mas/yr
視差 (π)	19.51 ± 0.18 mas
距離	167 ± 2 ly ; (51.3 ± 0.5 pc)
絕對星等 (MV)	–1.18
	軌道
繞行週期 (P)	813 天
偏心率 (e)	0.183
近心點曆元 (T)	2452025 HJD
近心點幅角 (ω); (secondary)	344.7°
半振幅 (K1); (primary)	14.37 km/s
詳細資料
η Peg A
質量	3.82 ± 0.52 M☉
半徑	18 R☉
表面重力 (log g)	2.40
亮度	247 L☉
溫度	5,450 K
金屬量 [Fe/H]	+0.39 dex
自轉	818
自轉速度 (v sin i)	1.4 km/s
其他命名
	Matar, 44 Peg, FK5 857, HD 215182, HIP 112158, HR 8650, SAO 90734.

離宮四，即飛馬座η（η Peg，η Pegasi）是一個位於飛馬座的雙星系統，距離地球約167光年。

特徵

離宮四的視星等為+2.95，^[2] 是飛馬座第五亮的恆星。根據視差測量，它距離地球約167光年（51秒差距）。^[1]

離宮四由一對聯星組成，兩顆恆星相互旋轉的周期為813天，軌道離心率為0.183。^[6] 其中較亮的主星是一顆亮巨星，恆星光譜為G2 II，^[3] 質量是太陽質量的4倍。^[5] 通過干涉測量並矯正周邊昏暗後測量出這顆恆星的角直徑為3.26 ± 0.07角分，^[11] 這意味著在距離地球167光年的實際位置上，它的半徑約為太陽半徑的18倍。^[8] 它的亮度是太陽亮度的247倍，^[7] 表面溫度為5,450K。^[7] 這顆恆星的自轉比較慢，視向自轉速度為1.7 km·s^–1，自轉周期約為818天。^[5] 離宮四的伴星是一顆F-型主序星，恆星光譜為F0 V.^[3]。在天球投影上，這對聯星附近還有兩顆G型恆星，目前尚不知道是否和這對聯星有物理聯繫。

離宮四和危宿一（寶瓶座α）、虛宿一（寶瓶座β）三顆中等質量的恆星組成一個恆星群落，它們在星際運動的方向垂直於銀河面。^[12]

命名

離宮四在西方的傳統名字為Matar，來自於阿拉伯語Al Saʽd al Maṭar（سعد المطر‎），意思為「幸運雨之星」。在中國古代星官系統中，離宮四屬於北方玄武七宿中室宿離宮（表示皇帝的行宮）第四星，這也是離宮四名字的來由。^[13]

大眾文化

美國海軍有一艘貨船「USS Matar (AK-119)」是以離宮四的英文名Maṭar命名。

參考資料

^ ^1.0 ^1.1 ^1.2 ^1.3 ^1.4 ^1.5 van Leeuwen, F. Validation of the new Hipparcos reduction. Astronomy and Astrophysics. November 2007, 474 (2): 653–664. Bibcode:2007A&A...474..653V. doi:10.1051/0004-6361:20078357.
^ ^2.0 ^2.1 ^2.2 ^2.3 Johnson, H. L.; et al. UBVRIJKL photometry of the bright stars. Communications of the Lunar and Planetary Laboratory. 1966, 4 (99). Bibcode:1966CoLPL...4...99J.
^ ^3.0 ^3.1 ^3.2 Parsons, Sidney B.; Ake, Thomas B., Ultraviolet and Optical Studies of Binaries with Luminous Cool Primaries and Hot Companions. V. The Entire IUE Sample, The Astrophysical Journal Supplement Series, November 1998, 119 (1): 83–104, Bibcode:1998ApJS..119...83P, doi:10.1086/313152
^ Wilson, Ralph Elmer. General Catalogue of Stellar Radial Velocities. Washington: Carnegie Institution of Washington. 1953. Bibcode:1953QB901.W495......
^ ^5.0 ^5.1 ^5.2 ^5.3 Pizzolato, N.; Maggio, A.; Sciortino, S., Evolution of X-ray activity of 1-3 Msun late-type stars in early post-main-sequence phases, Astronomy and Astrophysics, September 2000, 361: 614–628, Bibcode:2000A&A...361..614P
^ ^6.0 ^6.1 ^6.2 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
^ ^7.0 ^7.1 ^7.2 ^7.3 ^7.4 Hohle, M. M.; Neuhäuser, R.; Schutz, B. F., Masses and luminosities of O- and B-type stars and red supergiants, Astronomische Nachrichten, April 2010, 331 (4): 349, Bibcode:2010AN....331..349H, doi:10.1002/asna.200911355
^ ^8.0 ^8.1 Lang, Kenneth R., Astrophysical formulae, Astronomy and astrophysics library 1 3, Birkhäuser, 2006, ISBN 3540296921 . The radius (R_*) is given by:
${\begin{aligned}2\cdot R_{*}&={\frac {((51.3\cdot 3.26\cdot 10^{-3})\ {\text{AU}}}{0.0046491\ {\text{AU}}/R_{\bigodot }}}\\&\approx 36\cdot R_{\bigodot }\end{aligned}}$
^ ^9.0 ^9.1 Luck, R. Earle; Wepfer, Gordon G., Chemical Abundances for F and G Luminosity Class II Stars, Astronomical Journal, November 1995, 110: 2425, Bibcode:1995AJ....110.2425L, doi:10.1086/117702
^ MATAR -- Star in double system, SIMBAD Astronomical Object Database (Centre de Données astronomiques de Strasbourg), [2012-03-03], （原始內容存檔於2020-01-11）
^ Richichi, A.; Percheron, I.; Khristoforova, M., CHARM2: An updated Catalog of High Angular Resolution Measurements, Astronomy and Astrophysics, February 2005, 431: 773–777, Bibcode:2005A&A...431..773R, doi:10.1051/0004-6361:20042039
^ Ayres, Thomas R.; Brown, Alexander; Harper, Graham M. Chandra Observations of Coronal Emission from the Early G Supergiants α and β Aquarii. The Astrophysical Journal. July 2005, 627 (1): L53–L56. Bibcode:2005ApJ...627L..53A. doi:10.1086/431977.
^ 香港太空館 - 研究資源 - 亮星中英對照表網際網路檔案館的存檔，存檔日期2010-08-18.

外部連結

Matar（離宮四）

[aaa474_2_653-1] 1.0 ^1.1 ^1.2 ^1.3 ^1.4 ^1.5 van Leeuwen, F. Validation of the new Hipparcos reduction. Astronomy and Astrophysics. November 2007, 474 (2): 653–664. Bibcode:2007A&A...474..653V. doi:10.1051/0004-6361:20078357.

[clpl4_99-2] 2.0 ^2.1 ^2.2 ^2.3 Johnson, H. L.; et al. UBVRIJKL photometry of the bright stars. Communications of the Lunar and Planetary Laboratory. 1966, 4 (99). Bibcode:1966CoLPL...4...99J.

[apjss119_1_83-3] 3.0 ^3.1 ^3.2 Parsons, Sidney B.; Ake, Thomas B., Ultraviolet and Optical Studies of Binaries with Luminous Cool Primaries and Hot Companions. V. The Entire IUE Sample, The Astrophysical Journal Supplement Series, November 1998, 119 (1): 83–104, Bibcode:1998ApJS..119...83P, doi:10.1086/313152

[gcsrv-4] Wilson, Ralph Elmer. General Catalogue of Stellar Radial Velocities. Washington: Carnegie Institution of Washington. 1953. Bibcode:1953QB901.W495......

[aaa361_614-5] 5.0 ^5.1 ^5.2 ^5.3 Pizzolato, N.; Maggio, A.; Sciortino, S., Evolution of X-ray activity of 1-3 Msun late-type stars in early post-main-sequence phases, Astronomy and Astrophysics, September 2000, 361: 614–628, Bibcode:2000A&A...361..614P

[aj135_1_209-6] 6.0 ^6.1 ^6.2 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

[an331_4_349-7] 7.0 ^7.1 ^7.2 ^7.3 ^7.4 Hohle, M. M.; Neuhäuser, R.; Schutz, B. F., Masses and luminosities of O- and B-type stars and red supergiants, Astronomische Nachrichten, April 2010, 331 (4): 349, Bibcode:2010AN....331..349H, doi:10.1002/asna.200911355

[lang2006-8] 8.0 ^8.1 Lang, Kenneth R., Astrophysical formulae, Astronomy and astrophysics library 1 3, Birkhäuser, 2006, ISBN 3540296921 . The radius (R_*) is given by:
${\begin{aligned}2\cdot R_{*}&={\frac {((51.3\cdot 3.26\cdot 10^{-3})\ {\text{AU}}}{0.0046491\ {\text{AU}}/R_{\bigodot }}}\\&\approx 36\cdot R_{\bigodot }\end{aligned}}$

[aj110_2425-9] 9.0 ^9.1 Luck, R. Earle; Wepfer, Gordon G., Chemical Abundances for F and G Luminosity Class II Stars, Astronomical Journal, November 1995, 110: 2425, Bibcode:1995AJ....110.2425L, doi:10.1086/117702

[SIMBAD-10] MATAR -- Star in double system, SIMBAD Astronomical Object Database (Centre de Données astronomiques de Strasbourg), [2012-03-03], （原始內容存檔於2020-01-11）

[aaa431_773-11] Richichi, A.; Percheron, I.; Khristoforova, M., CHARM2: An updated Catalog of High Angular Resolution Measurements, Astronomy and Astrophysics, February 2005, 431: 773–777, Bibcode:2005A&A...431..773R, doi:10.1051/0004-6361:20042039

[apj627_1_L53-12] Ayres, Thomas R.; Brown, Alexander; Harper, Graham M. Chandra Observations of Coronal Emission from the Early G Supergiants α and β Aquarii. The Astrophysical Journal. July 2005, 627 (1): L53–L56. Bibcode:2005ApJ...627L..53A. doi:10.1086/431977.

[13] 香港太空館 - 研究資源 - 亮星中英對照表網際網路檔案館的存檔，存檔日期2010-08-18.

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觀測資料曆元 J2000.0
離宮四在飛馬座的位置
星座	飛馬座
星官	室宿離宮
赤經	22^h 43^m 00.13743^s^[1]
赤緯	+30° 13′ 16.4822″^[1]
視星等（V）	+2.95^[2]
特性
光譜分類	G2 II + F0 V^[3]
U−B 色指數	+0.57^[2]
B−V 色指數	+0.86^[2]
天體測定

徑向速度 (R_v)	+4.3^[4] km/s
自行 (μ)	赤經：–100.06^[1] mas/yr 赤緯：+15.46^[1] mas/yr
視差 (π)	19.51 ± 0.18^[1] mas
距離	167 ± 2 ly (51.3 ± 0.5 pc)
絕對星等 (M_V)	–1.18^[5]

軌道^[6]
繞行週期 (P)	813 天
偏心率 (e)	0.183
近心點曆元 (T)	2452025 HJD
近心點幅角 (ω) (secondary)	344.7°
半振幅 (K₁) (primary)	14.37 km/s
詳細資料
η Peg A
質量	3.82 ± 0.52^[7] M_☉
半徑	18^[8] R_☉
表面重力 (log g)	2.40^[9]
亮度	247^[7] L_☉
溫度	5,450^[7] K
金屬量 [Fe/H]	+0.39^[9] dex
自轉	818^[5]
自轉速度 (v sin i)	1.4^[6] km/s

其他命名
Matar, 44 Peg, FK5 857, HD 215182, HIP 112158, HR 8650, SAO 90734.^[10]