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Epsilon Ursae Majoris
Ursa major star name.png
Alioth in Ursa Major.
Observation data
Epoch J2000      Equinox J2000
Constellation Ursa Major
Right ascension 12h 54m 01.6s
Declination +55° 57′ 35.4″
Apparent magnitude (V) 1.76
Spectral type A0pCr
U-B color index 0.02
B-V color index -0.02
Variable type alpha2-CVn
Radial velocity (Rv) -9.3 km/s
Proper motion (μ) RA: 111.74 mas/yr
Dec.: -8.99 mas/yr
Parallax (π) 40.3 ± 0.62 mas
Distance 81 ± 1 ly
(24.8 ± 0.4 pc)
Absolute magnitude (MV) -0.22
Mass ~3 M
Radius 3.7 R
Luminosity 108 L
Temperature 9,400 K
Metallicity ?
Rotation 38 km/s.
Age ? years
Other designations
Alioth, Allioth, Aliath, 77 Ursae Majoris, HR 4905, BD +56°1627, HD 112185, GCTP 2964.00, SAO 28553, FK5 483, CCDM 12540+5558, HIP 62956.
Book plate by Sydney Hall depicting Ursa Major's stars

Epsilon Ursae Majoris (ε UMa / ε Ursae Majoris) is the brightest star in the constellation Ursa Major (despite its Bayer designation being merely "epsilon"), and at magnitude 1.76 is the thirty-first brightest star in the sky. It has the traditional name Alioth (from the Arabic word alyat—fat tail of a sheep).

It is known as 北斗五 (the Fifth Star of the Northern Dipper) or 玉衡 (the Star of Jade Sighting-tube) in Chinese.

It is the star in the tail of the bear closest to its body, and thus the star in the handle of the Big Dipper closest to the bowl. It is also a member of the large and diffuse Ursa Major moving group. Historically, the star was frequently used in celestial navigation in the maritime trade, because it is listed as one of the 57 navigational stars.[1]


According to Hipparcos, Alioth is 81 light years (25 parsecs) from Earth. Its spectral type is A0p; the "p" stands for peculiar, as the spectrum of its light is characteristic of an Alpha2 Canum Venaticorum variable. Alioth, as a representative of this type, may harbor two interacting processes. First, the star's strong magnetic field separating different elements in the star's hydrogen 'fuel'. In addition, a rotation axis at an angle to the magnetic axis may be spinning different bands of magnetically-sorted elements into the line of sight between Alioth and the Earth. The intervening elements react differently at different frequencies of light as they whip in and out of view, causing Alioth to have very strange spectral lines that fluctuate over a period of 5.1 days.

With Alioth, the rotational and magnetic axes are at almost 90 degrees to one another. A map of the star below shows how darker (denser) regions of chromium form a band at right angles to the equator.

A recent study suggests Alioth's 5.1-day variation may be due to a substellar object of about 14.7 Jupiter masses in an eccentric orbit (e=0.5) with an average separation of 0.055 astronomical units.

Alioth has a relatively weak magnetic field, fifteen times weaker than α CVn, but it is still 100 times stronger than that of the Earth.

See also


  1. ^  This article incorporates content from the 1728 Cyclopaedia, a publication in the public domain.


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