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Epsilon Ursae Majoris

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Title: Epsilon Ursae Majoris  
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Epsilon Ursae Majoris

Epsilon Ursae Majoris
Epsilon Ursae Majoris is located in 100x100
Epsilon Ursae Majoris

Alioth in Ursa Major (circled).
Observation data
Epoch J2000      Equinox J2000
Constellation Ursa Major
Right ascension 12h 54m 01.74959s[1]
Declination +55° 57′ 35.3627″[1]
Apparent magnitude (V) 1.77[2]
Spectral type A1III-IVp kB9
U−B color index +0.02[2]
B−V color index -0.02[2]
Variable type α2-CVn
Radial velocity (Rv) -9.3[3] km/s
Proper motion (μ) RA: +111.91[1] mas/yr
Dec.: -8.24[1] mas/yr
Parallax (π) 39.51 ± 0.20[1] mas
Distance 82.6 ± 0.4 ly
(25.3 ± 0.1 pc)
Absolute magnitude (MV) –0.2[4]
Mass 2.91[5] M
Radius 4.2 ± 0.2[6] R
Luminosity 108 L
Surface gravity (log g) 3.5[4] cgs
Temperature 10,800[4] K
Metallicity [Fe/H] +0.00[4] dex
Rotational velocity (v sin i) 33[7] km/s
Other designations
Alioth, Allioth, Aliath, ε Ursae Majoris, ε UMa, Epsilon UMa, 77 Ursae Majoris, BD+56°1627, FK5 483, GC 17518, HD 112185, HIP 62956, HR 4905, PPM 33769, SAO 28553.
Database references

Epsilon Ursae Majoris (Epsilon UMa, ε Ursae Majoris, ε UMa) 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. 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 (Plough) 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]


Book plate by Sydney Hall depicting Ursa Major's stars

According to Hipparcos, Alioth is 81 light years (25 parsecs) from Earth. Its spectral type is A1p; 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. The kB9 suffix to the spectral type indicates that the calcium K line is present and representative of a B9 spectral type even though the rest of the spectrum indicates A1.

With Alioth, the rotational and magnetic axes are at almost 90 degrees to one another. 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, 15 times weaker than α CVn, but it is still 100 times stronger than that of the Earth.

Name and etymology


USS Allioth (AK-109) was a United States Navy Crater class cargo ship named after the star.

See also


  1. ^ a b c d e
  2. ^ a b c
  3. ^
  4. ^ a b c d
  5. ^
  6. ^
  7. ^
  8. ^
  9. ^ (Chinese) AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 6 月 15 日
  1. ^  This article incorporates text from a publication now in the public domain

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