  | |
| 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] |
| Characteristics | |
| Spectral type | A1III-IVp kB9 |
| U−Bcolor index | +0.02[2] |
| B−Vcolor index | −0.02[2] |
| Variable type | α2 CVn |
| Astrometry | |
| Radial velocity (Rv) | −9.3[3] km/s |
| Proper motion (μ) | RA: +111.91mas/yr[1] Dec.: −8.24mas/yr[1] |
| Parallax (π) | 39.51±0.20 mas[1] |
| Distance | 82.6 ± 0.4 ly (25.3 ± 0.1 pc) |
| Absolute magnitude (MV) | −0.2[4] |
| Details | |
| Mass | 2.91[5] M☉ |
| Radius | 4.29+0.19 −0.21[6] R☉ |
| Luminosity | 104.4±9.3[6] L☉ |
| Surface gravity (log g) | 3.59[6] cgs |
| Temperature | 8,908±24[6] K |
| Metallicity [Fe/H] | +0.00[4] dex |
| Rotational velocity (v sin i) | 33[7] km/s |
| Age | 300[8] Myr |
| Other designations | |
| Alioth, Allioth, Aliath,ε UMa,77 Ursae Majoris,BD+56°1627,FK5 483,GC 17518,HD 112185,HIP 62956,HR 4905,SAO 28553,PPM 33769 | |
| Database references | |
| SIMBAD | data |
Alioth/ˈæliɒθ/, also calledEpsilon Ursae Majoris, is astar in the northernconstellation ofUrsa Major. The designation isLatinised fromε Ursae Majoris and abbreviatedEpsilon UMa orε UMa.[9][10] Despite being designated "ε" (epsilon), it is the brightest star in the constellation and atmagnitude 1.77 is thethirty-third brightest star in the sky.
It is the star in the tail of the bear closest to its body, and thus the star in the handle of theBig Dipper (or Plough) closest to the bowl. It is also a member of the large and diffuseUrsa Major moving group. Historically, the star was frequently used incelestial navigation in the maritime trade, because it is listed as one of the 57navigational stars.[1]
According toHipparcos, Epsilon Ursae Majoris is 81light-years (25parsecs) from theSun. Itsspectral type isA1p; the "p" stands forpeculiar, as itsspectrum is characteristic of anα2 Canum Venaticorum variable. Epsilon Ursae Majoris, as a representative of this type, may harbor two interacting processes: first, the star's strongmagnetic field separating different elements in the star'shydrogen 'fuel'; second, 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 Epsilon Ursae Majoris and the Earth. The intervening elements react differently at different frequencies of light as they whip in and out of view, causing Epsilon Ursae Majoris to have very strangespectral lines that fluctuate over a period of 5.1 days. ThekB9 suffix to the spectral type indicates that thecalcium K line is present and representative of a B9 spectral type even though the rest of the spectrum indicates A1.
Epsilon Ursae Majoris's rotational and magnetic poles are at almost 90 degrees to one another. Darker (denser) regions ofchromium form a band at right angles to the equator.
It has long been suspected that Epsilon Ursae Majoris is aspectroscopic binary, possibly with more than one companion.[12] A more recent study suggests Epsilon Ursae Majoris's 5.1-day variation may be due to a substellar object of about 14.7Jupiter masses in an eccentric orbit (e=0.5) with an average separation of 0.055astronomical units.[13] It is now thought that the 5.1-day period is the rotation period of the star, and no companions have been detected using the most modern equipment.[14] Observations of Alioth with theNavy Precision Optical Interferometer also did not detect a companion.[6]
Epsilon Ursae Majoris has a relatively weak magnetic field for a variable of this type, 15 times weaker thanα Canum Venaticorum, but it is still 100 times stronger thanthat of the Earth.[15]
ε Ursae Majoris (Latinised toEpsilon Ursae Majoris) is the star'sBayer designation.
The traditional nameAlioth comes from theArabicalyat al-hamal ("the sheep's fat tail"). In 2016, theInternational Astronomical Union organized aWorking Group on Star Names (WGSN)[16] to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016[17] included a table of the first two batches of names approved by the WGSN; which includedAlioth for this star.
This star was known to theHindus asAñgiras, one of theSeven Rishis.[18]
InChinese,北斗 (Běi Dǒu), meaningNorthern Dipper, refers to anasterism equivalent to the Big Dipper. Consequently, theChinese name for Epsilon Ursae Majoris itself is北斗五 (Běi Dǒu wu, English:the Fifth Star of Northern Dipper) and玉衡 (Yù Héng, English:Star of Jade Sighting-Tube).[19]
TheUnited States Navy'sCrater class cargo shipUSS Allioth (AK-109) was named after the star.
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This article incorporates text from a publication now in thepublic domain: Chambers, Ephraim, ed. (1728). "Allioth".Cyclopædia, or an Universal Dictionary of Arts and Sciences (1st ed.). James and John Knapton, et al.
