Alioth

Alioth

Template:Location mark Location of Alioth (circled)
Observation data Epoch J2000      Equinox J2000
Constellation	Ursa Major
Right ascension	Template:RA
Declination	Template:DEC
Apparent magnitude (V)	1.77
Characteristics
Spectral type	A1III-IVp kB9
U−B Template:Engvar	+0.02
B−V Template:Engvar	−0.02
Variable type	α2 CVn
Astrometry
Radial velocity (Rv)	−9.3 km/s
Proper motion (μ)	RA: +111.91 mas/yr Dec.: −8.24 mas/yr
Parallax (π)	39.51±0.20 mas
Distance	Template:Rnd ± Template:Rnd ly (Template:Rnd ± Template:Rnd pc)
Absolute magnitude (MV)	−0.2
Details
Mass	2.91 Template:Solar mass
Radius	Template:Val Template:Solar radius
Luminosity	Template:Val Template:Solar luminosity
Surface gravity (log g)	3.59 cgs
Temperature	Template:Val K
Metallicity [Fe/H]	+0.00 dex
Rotation	Template:Val days
Rotational velocity (v sin i)	33 km/s
Age	300 Myr
Metallicity [Fe/H]	{{{metal_fe2}}} dex
Other designations
Template:Odlist
Database references
SIMBAD	data

Alioth Template:IPAc-en, also called Epsilon Ursae Majoris, is a star in the northern constellation of Ursa Major. The designation is Latinised from ε Ursae Majoris and abbreviated Epsilon UMa or ε UMa. Despite being designated "ε" (epsilon), it is the brightest star in the constellation and at magnitude 1.77 is the thirty-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 the Big Dipper (or 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]

Physical characteristics

File:Sidney Hall - Urania's Mirror - Ursa Major.jpg

Book plate by Sidney Hall depicting Ursa Major's stars

File:EpsUMaLightCurve.png

A light curve for Epsilon Ursae Majoris, plotted from TESS data

According to Hipparcos, Epsilon Ursae Majoris is Template:Convert from the Sun. Its spectral type is A1p; the "p" stands for peculiar, as its spectrum is characteristic of an α² Canum Venaticorum variable. Epsilon Ursae Majoris, as a representative of this type, may harbor two interacting processes: first, the star's strong magnetic field separating different elements in its hydrogen '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 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.

Epsilon Ursae Majoris's rotational and magnetic poles are at almost 90 degrees to one another. Darker (denser) regions of chromium form a band at right angles to the equator.

It has long been suspected that Epsilon Ursae Majoris is a spectroscopic binary, possibly with more than one companion. A 2008 study suggested 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. 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. Observations of Alioth with the Navy Precision Optical Interferometer also did not detected a companion.

Epsilon Ursae Majoris has a relatively weak magnetic field for a variable of this type, 15 times weaker than α Canum Venaticorum, but it is still 100 times stronger than that of the Earth.

Name and etymology

ε Ursae Majoris (Latinised to Epsilon Ursae Majoris) is the star's Bayer designation.

The traditional name Alioth comes from the Arabic alyat al-hamal ("the sheep's fat tail"). In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN) to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016 included a table of the first two batches of names approved by the WGSN; which included Alioth for this star.

This star was known to the Hindus as Añgiras, one of the Seven Rishis.

In Chinese, (), meaning Northern Dipper, refers to an asterism equivalent to the Big Dipper. Consequently, the Chinese name for Epsilon Ursae Majoris itself is (, Template:Langx) and (, Template:Langx).

Namesakes

The United States Navy's Crater class cargo ship Template:USS was named after the star.

See also

• List of brightest stars
• List of nearest bright stars
• Lists of stars
• Historical brightest stars
• List of nearest giant stars

References

1. ^{^} Template:1728