Alcor (star)

Mizar

Mizar and Alcor in constellation Ursa Major
Observation data Epoch J2000      Equinox J2000
Constellation	Ursa Major
Right ascension	13h 23m 55.5s
Declination	+54° 55′ 31″
Apparent magnitude (V)	2.27
Characteristics
Spectral type	A2Vp/A2Vp/A1m/A5V
B−V color index	0.02
Astrometry
Radial velocity (Rv)	−6.31 km/s
Proper motion (μ)	RA: 119.01 mas/yr Dec.: −25.97 mas/yr
Parallax (π)	38.01 ± 1.71mas
Distance	86 ± 4 ly (26 ± 1 pc)
Absolute magnitude (MV)	0.33
Absolute bolometric magnitude (Mbol)	0.91 ± 0.07 / 0.91 ± 0.07
Orbit
Primary	Mizar Aa
Companion	Mizar Ab
Period (P)	20.53835 ± 0.00005 d
Semi-major axis (a)	9.83 ± 0.03 mas
Eccentricity (e)	0.5354 ± 0.0025
Inclination (i)	60.5 ± 0.3°
Longitude of the node (Ω)	106.0 ± 0.4°
Periastron epoch (T)	JD 2447636.07 ± 0.02
Argument of periastron (ω) (secondary)	104.3 ± 0.3°
Semi-amplitude (K1) (primary)	69.1 km/s
Semi-amplitude (K2) (secondary)	67.2 km/s
Details
Mizar Aa
Mass	2.43 ± 0.07 M☉
Radius	2.4 ± 0.1 R☉
Luminosity	33.3 ± 2.1 L☉
Temperature	9000 ± 200 K
Mizar Ab
Mass	2.50 ± 0.07 M☉
Radius	2.4 ± 0.1 R☉
Luminosity	33.3 ± 2.1 L☉
Temperature	9000 ± 200 K
Other designations
Mizat, Mirza, Mitsar, Vasistha, ζ Ursae Majoris, ζ UMa, Zeta UMa, 79 Ursae Majoris, BD+55 1598A, CCDM J13240+5456AB, FK5 497, GC 18133, HD 116656, HIP 65378, HR 5054, PPM 34007, SAO 28737, WDS J13239+5456Aa,Ab.
Database references
SIMBAD	data

Mizar and Alcor are two stars forming a naked eye double star in the handle of the Big Dipper (or Plough) asterism in the constellation of Ursa Major. Mizar is the second star from the end of the Big Dipper's handle, and Alcor its faint companion.

Mizar, also designated Zeta Ursae Majoris (ζ Ursae Majoris, abbreviated Zeta UMa, ζ UMa), is itself a quadruple system and Alcor, also designated 80 Ursae Majoris (80 UMa), is a binary, the pair together forming a sextuple system. The whole system lies about 83 light-years away from the Sun, as measured by the Hipparcos astrometry satellite.

ζ Ursae Majoris (Latinised to Zeta Ursae Majoris) is Mizar's Bayer designation. 80 Ursae Majoris is Alcor's Flamsteed designation.

The traditional name Mizar derives from the Arabic المئزر miʼzar meaning 'apron; wrapper, covering, cover'.Alcor was also originally Arabic سها suhā, meaning either the ‘forgotten’ or ‘neglected’ one.; notable as a faintly perceptible companion of Mizar.

In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN) to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016 included a table of the first two batches of names approved by the WGSN; which included Mizar for Zeta UMa and Alcor for 80 UMa. They are now so entered in the IAU Catalog of Star Names.