Alpha Comae Berenices
Coma Berenices constellation
Epoch J2000.0 Equinox J2000.0 (ICRS)
|Right ascension||13h 09m 59.285s|
|Declination||+17° 31′ 46.04″|
|Apparent magnitude (V)||4.29 to 4.35
A: 4.85 / B: 5.53
|Spectral type||A: F5V / B: F5V
|U−B color index||−0.06|
|B−V color index||0.45|
|V−R color index||0.2|
|R−I color index||0.2|
|Radial velocity (Rv)||±0.9−17.7 km/s|
|Proper motion (μ)||RA: ±0.70−433.13 mas/yr
Dec.: ±0.51141.24 mas/yr
|Parallax (π)||56.10 ± 0.89 mas|
|Distance||58.1 ± 0.9 ly
(17.8 ± 0.3 pc)
|Absolute magnitude (MV)||3.82|
|Period (P)||±0.008219 25.8696yr|
|Semi-major axis (a)||44±0.00033" 0.671|
|Inclination (i)||±0.0062° 90.0501|
|Longitude of the node (Ω)||±0.0098° 12.2272|
|Periastron epoch (T)||056.84±0.3657|
|Argument of periastron (ω)
Alpha Comae Berenices (Alpha Com, α Com, α Comae Berenices), also named Diadem, is a star in the constellation Coma Berenices (Berenice's Hair). Although it has the Bayer designation "alpha", at magnitude 4.32 it is actually fainter than Beta Comae Berenices. It is said to represent the crown worn by Queen Berenice.
It bore the traditional names Diadem and Al Dafirah, the latter derived from the Arabic الضفيرة ađ̧-đ̧afīrah "the braid". In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN) to catalogue and standardize proper names for stars. The WGSN approved the name Diadem for this star on February 1st, 2017 and it is now so entered in the IAU Catalog of Star Names.
In Chinese, 太微左垣 (Tài Wēi Zuǒ Yuán), meaning Left Wall of Supreme Palace Enclosure, refers to an asterism consisting of α Comae Berenices, η Virginis, γ Virginis, δ Virginis and ε Virginis. Consequently, α Comae Berenices itself is known as 太微左垣五 (Tài Wēi Zuǒ Yuán wǔ, English: the Fifth Star of Left Wall of Supreme Palace Enclosure.), representing 東上將 (Dōngshǎngjiāng), meaning The First Eastern General. 東上將 (Dōngshǎngjiāng), westernized into Shang Tseang, but the name Shang Tseang was designated for "v Comae Berenices" by R.H. Allen and the meaning is "a Higher General" 
It is a binary star, with almost equal components of magnitudes 5.05 m and 5.08 m orbiting each other with a period of 25.87 years. The system, estimated to be 58 light years distant, appears so nearly "edge-on" from the Earth that the two stars appear to move back-and-forth in a straight line with a maximum separation of only 0.7 arcsec. Eclipses are predicted to occur between the two components however they have not been successfully observed due to miscalculations of the time of eclipse.
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- NSV 6116, database entry, New Catalogue of Suspected Variable Stars, the improved version, Sternberg Astronomical Institute, Moscow, Russia. Accessed on line September 3, 2008.
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