|
發表於 2010-9-3 01:26:49
|
顯示全部樓層
Mizar (star)
From Wikipedia, the free encyclopediaJump to: navigation, search
This is an article about the star, for other uses please see Mizar (disambiguation)
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) 2.23
Characteristics
Spectral type A2 V/A2 V/A1 V
U−B color index 0.09
B−V color index 0.13
Astrometry
Radial velocity (Rv) −9 km/s
Proper motion (μ) RA: 121.23 mas/yr
Dec.: −22.01 mas/yr
Parallax (π) 41.73 ± 0.61 mas
Distance 78 ± 1 ly
(24 ± 0.4 pc)
Absolute magnitude (MV) 0.33
Other designations
Mizat, Mirza, Mitsar, Vasistha, 79 Ursae Majoris, HR 5054, BD +55 1598A, HD 116656, SAO 28737, FK5 497, HIP 65378.
Alcor Observation data
Epoch J2000.0 Equinox J2000.0
Constellation Ursa Major
Right ascension 13h 25m 13.5s
Declination +54° 59' 17"
Apparent magnitude (V) +3.99
Absolute magnitude (V) +2.01
Distance 81.2 ± 1.2 ly
(24.9 ± 0.4 pc)
Spectral type A5V
Other designations
Saidak, Suha, Arundhati, g Ursae Majoris, 80 Ursae Majoris, HR 5062, HD 116842, BD +55 1603, HIP 65477, SAO 28751
The Mizar-Alcor stellar sextuple system consists of the quadruplet system Mizar and the binary system Alcor.
Mizar (ζ UMa / ζ Ursae Majoris) is a quadruplet system of two binary stars in the constellation Ursa Major and is the second star from the end of the Big Dipper's handle. Its apparent magnitude is 2.23 and its spectral class is A1V. Mizar's name comes from the Arabic مئزر mīzar, meaning a waistband or girdle.)
With normal eyesight one can make out a faint companion just to the east, named Alcor or 80 Ursae Majoris. Alcor is of magnitude 3.99 and spectral class A5V. Mizar and Alcor together are sometimes called the "Horse and Rider," and the ability to resolve the two stars with the naked eye is often quoted as a test of eyesight, although even people with quite poor eyesight can see the two stars. Arabic literature says that only those with the sharpest eyesight can see the companion of Mizar. Astronomer Sir Patrick Moore has suggested that this in fact refers to another star which lies visually between Mizar and Alcor. Mizar and Alcor lie three light-years apart, and though their proper motions show they move together (they are both members of the Ursa Major Moving Group), it was long believed they do not form a true binary star system, but simply a double star. However, in 2009, it was reported by astronomer Eric Mamajek and collaborators that Alcor actually is itself a binary, consisting of Alcor A and Alcor B, and that this binary system is most likely gravitationally bound to Mizar, bringing the full count of stars in this complex system to six.[1] Their study also demonstrated that the Alcor binary and Mizar quadruple are much closer together than previously thought - approximately 74,000 ± 39,000 Astronomical Units.[2]
The whole four-star system lies about 78 light-years away from Earth. 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. |
|