Alpha Ursae Majoris

Alpha Ursae Majoris

Location of α Ursae majoris (circled)
Observation data Epoch J2000      Equinox J2000
Constellation	Ursa Major
Right ascension	11h 03m 43.67152s
Declination	+61° 45′ 03.7249″
Apparent magnitude (V)	1.79
Characteristics
Spectral type	K0III + F0V
U−B color index	+0.93
B−V color index	+1.07
Variable type	Pulsating
Astrometry
Radial velocity (Rv)	–9 km/s
Proper motion (μ)	RA: –134.11 mas/yr Dec.: –34.70 mas/yr
Parallax (π)	26.54 ± 0.48 mas
Distance	123 ± 2 ly (37.7 ± 0.7 pc)
Absolute magnitude (MV)	–1.10 ± 0.04
Orbit
Companion	α UMa B
Period (P)	44.4 yr
Semi-major axis (a)	0.603"
Eccentricity (e)	0.4
Inclination (i)	180°
Longitude of the node (Ω)	2000°
Periastron epoch (T)	1958.000
Details
α UMa A
Mass	4.25 M☉
Luminosity	316 L☉
Surface gravity (log g)	2.46 cgs
Temperature	4,660 K
Metallicity [Fe/H]	–0.20 ± 0.07 dex
Rotational velocity (v sin i)	2.6 ± 1.0 km/s
α UMa B
Mass	~1.6 M☉
Other designations
Dubhe, Ak, α Ursae Majoris, α UMa, Alpha UMa, 50 UMa, BD+62°1161, CCDM J11037+6145AB, FK5 417, GC 15185, HD 95689, HIP 54061, HR 4301, PPM 17705, SAO 15384, WDS J11037+6145AB
Database references
SIMBAD	data

Alpha Ursae Majoris (Alpha UMa, α Ursae Majoris, α UMa) is the second-brightest star in the constellation of Ursa Major (despite its Bayer designation of "alpha"). It has the traditional name Dubhe, and a rarer name Ak.

Description

Alpha Ursae Majoris forms part of the Big Dipper (also known as the Plough or the Great Bear), and is the northern of the 'pointers' (or 'guards'), the two stars of Ursa Major which point towards Polaris, the North Star. Dubhe is about 123 light years away and is a giant star that has evolved away from the main sequence after consuming the hydrogen at its core. It is a spectroscopic binary with a main sequence companion α UMa B that has a stellar classification of F0V. The companion star orbits at a mean separation of about 23 astronomical units (AU) and completes an orbit every 44.4 years. The yellow giant primary star shows rapid tiny pulsations at many frequencies, its brightness changing by less than a hundredth of a magnitude.

There is another spectroscopic binary 8 arc minutes distant, a 7th magnitude pair showing an F8 spectral type. It is sometimes referred to as Alpha Ursae majoris C, but is separately catalogued as HD 95638.

Although it is part of the constellation of Ursa Major, it does not form part of the Ursa Major Moving Group of stars that have a common motion through space.

Name and etymology

• The traditional name Dubhe comes from the Arabic for "bear", dubb, from the phrase ظهر الدب الاكبر‎ żahr ad-dubb al-akbar "the back of the Greater Bear". The other traditional name Ak was meaning The Eye.
• This star as Kratu, one of the Seven Rishis.
• In Chinese, 北斗 Běi Dǒu, meaning Northern Dipper, refers to an asterism consisting of α Ursae Majoris, β Ursae Majoris, γ Ursae Majoris, δ Ursae Majoris, ε Ursae Majoris, ζ Ursae Majoris and η Ursae Majoris. Consequently, α Ursae Majoris itself is known as 北斗一 Běi Dǒu yī, (English: the First Star of Northern Dipper) and 天樞 Tiān Shū, (English: Star of Celestial Pivot).

In culture

Dubhe is the official star of the State of Utah. USS Dubhe (ID-2562) was a ship in the United States navy. The Danish National Home Guard Navy ship MHV 806 is named Dubhe.

Photos

Dubhe "A" and faint companion binaries