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Mizar
Observation data
Epoch J2000
Constellation Ursa Major
Right ascension 13h 23m 55.5s
Declination +54° 55′ 31″
Apparent magnitude (V) 2.23
Characteristics
Spectral type A2 V/A2 V/A1 V
B-V color index 0.13
U-B color index 0.09
Variable type  ?
Astrometry <tr valign=top><td>Radial velocity (Rv)</td><td>-9 km/s</td></tr><tr valign=top><td>Proper motion (μ)</td><td> RA: 121.23 mas/yr
Dec.: -22.01 mas/yr </td></tr><tr valign=top><td>Parallax (π)</td><td>41.73 ± 0.61 mas</td></tr><tr valign=top><td>Distance</td><td>Expression error: Unrecognised punctuation character "[". ± Expression error: Unrecognised punctuation character "[". ly
(Expression error: Unrecognised punctuation character "[". ± Expression error: Unrecognised punctuation character "[". pc)</td></tr><tr valign=top><td>Absolute magnitude (MV)</td><td>0.33</td></tr>
Other designations
Mizat, Mirza, Mitsar, 79 Ursae Majoris, HR 5054, BD +55 1598A, HD 116656, GCTP 3062.00, SAO 28737, FK5 497, GC 18133, ADS 8891, CCDM J13240+5456, HIP 65378.

Mizar (ζ UMa) is a star in the constellation Ursa Major, lying just at the corner of the Big Dipper's handle. The name comes from the Arabic ميزر mīzar, meaning a waistband or girdle. Mizar has apparent magnitude 2.40 and spectral class A1 V.

With good eyesight one can make out a faint companion just to the east, called Alcor or 80 Ursae Majoris. Alcor has magnitude 3.99 and spectral class A5 V. The two are often called the horse and rider, and the ability to see the second is a traditional test of eyesight. The two stars lie more than a quarter of a light year apart and although proper motions show they move together, it is still not known whether they form a true binary star system, and not an optical binary as currently thought.

More components were discovered with the advents of the telescope and spectroscopy. Mizar was the first telescopic binary discovered, most probably by Benedetto Castelli who in 1617 asked Galileo Galilei to observe it. Galileo then produced a detailed record of the double star. Later, around 1650, Riccioli wrote of Mizar appearing as a double. The secondary star, Mizar B, has magnitude 4.0 and spectral class A7, and comes within 380 AU of the primary; the two take thousands of years to revolve around each other. Mizar A then became the first spectroscopic binary to be discovered, by Pickering in 1889. The two components are both about 35 times as bright as the sun, and revolve around each other in about 20 days. Mizar B (but not Alcor!) was later found to be a spectroscopic binary as well. In 1996 the components of the Mizar A binary system were imaged in extremely high resolution using the Navy Prototype Optical Interferometer.

Alcor
Observation data
Epoch J2000.0
Constellation Ursa Major
Right ascension 13h 25m 13.5s
Declination +54° 59' 17"
Apparent magnitude (V) +3.99 <tr valign=top><td>Distance</td><td>81.2 ± 1.2 ly
(24.9 ± 0.4 pc)</td></tr><tr valign=top><td>Spectral type</td><td>A5V</td></tr>
Other designations
Saidak, 80 Ursae Majoris, HR 5062, HD 116842, BD +55 1603, HIP 65477, SAO 28751, GC 18155, ADS 8891, CCDM J13240+5456

The whole five-star system lies about 78 light-years away from us. The components are all members of the Ursa Major moving group, a mostly dispersed group of stars sharing a common birth, as determined by proper motion. The other stars of the Big Dipper, except Dubhe and Alkaid, belong to this group as well.

External links

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