Beta Ursae Majoris

β Ursae Majoris

Merak in Ursa Major.
Observation data Epoch J2000.0      Equinox J2000.0
Constellation	Ursa Major
Right ascension	11h 01m 50.47654s
Declination	+56° 22′ 56.7339″
Apparent magnitude (V)	+2.37
Characteristics
Spectral type	A1IVps
U−B color index	+0.00
B−V color index	-0.02
Variable type	Suspected
Astrometry
Radial velocity (Rv)	-12.0 km/s
Proper motion (μ)	RA: +81.43 mas/yr Dec.: +33.49 mas/yr
Parallax (π)	40.90 ± 0.16 mas
Distance	79.7 ± 0.3 ly (24.45 ± 0.10 pc)
Details
Mass	2.7 M☉
Radius	3.021 ± 0.038 R☉
Luminosity	63.015 ± 1.307 L☉
Surface gravity (log g)	3.83 cgs
Temperature	9377 ± 75 K
Rotational velocity (v sin i)	46 km/s
Age	500 ± 100 Myr
Other designations
Merak, Mirak, β Ursae Majoris, β UMa, Beta UMa, 48 Ursae Majoris, BD+57°1302, FK5 416, GC 15145, HD 95418, HIP 53910, HR 4295, PPM 32912, SAO 27876.
Database references
SIMBAD	data

Beta Ursae Majoris (Beta UMa, β Ursae Majoris, β UMa) is a star in the northern circumpolar constellation of Ursa Major. It has the traditional name Merak.

The apparent visual magnitude of this star is +2.37, which means it is readily visible to the naked eye. It is more familiar to northern hemisphere observers as one of the "pointer stars" in the Big Dipper, or "The Plough" in England, which is a prominent asterism of seven stars that forms part of the larger constellation. Extending an imaginary straight line from this star through the nearby Alpha Ursae Majoris (Dubhe) extends to Polaris, the north star.

Properties

Based upon parallax measurements of this star, it is located at a distance of 79.7 light-years (24.4 parsecs) from the Earth. The spectrum shows this to have a stellar classification of A1 V, making it a fairly typical main sequence star that is generating energy through the thermonuclear fusion of hydrogen at its core. The effective temperature of the outer envelope is about 9,225 K, giving it a white-hued glow that is typical for A-type stars. It is larger than the Sun, with about 2.7 times the mass and 2.84 times the Solar radius. If they were viewed from the same distance, Beta Ursae Majoris would appear much brighter than the Sun, as it is radiating 68 times the Sun's luminosity.

Observation of the star in the infrared reveal an excess emission that suggests the presence of a circumstellar debris disk of orbiting dust, much like those discovered around Fomalhaut and Vega The mean temperature of this disk is 120 K, indicating that it is centered at a radius of 47 Astronomical Units from the host star. The dust has an estimated mass of about 0.27% the mass of the Earth.

Beta Ursae Majoris is one of five stars in the Big Dipper that form a part of a loose open cluster called the Ursa Major moving group, sharing the same region of space and not just the same patch of sky by our perspective. This group has an estimated age of about (500 ± 100) million years. As the members of this group share a common origin and motion through space, this yields an estimate for the age of Beta Ursae Majoris. Two stars are known to be located in relatively close proximity: 37 Ursae Majoris at 5.2 light-years (1.6 pc) and Gamma Ursae Majoris at 11.0 light-years (3.4 pc); much closer to each other than these stars are to the Earth.

Name and etymology

• The name Merak is derived from the Arabic المراق al-maraqq "the loins" (of the bear).
• This star as Pulaha, one of the Seven Rishis.
• In Chinese, 北斗 (Běi Dǒu), meaning Northern Dipper, refers to an asterism consisting of β Ursae Majoris, α Ursae Majoris, γ Ursae Majoris, δ Ursae Majoris, ε Ursae Majoris, ζ Ursae Majoris and η Ursae Majoris. Consequently, β Ursae Majoris itself is known as 北斗二 (Běi Dǒu èr, English: the Second Star of Northern Dipper) and 天璇 (Tiān Xuán, English: Star of Celestial Rotating Jade).

In culture

USS Merak (1918) and USS Merak (AF-21), both of United States navy ship.

In the original Star Trek series episode The Cloud Minders, the planet "Merak II" is said to be suffering from a botanical plague that can only be treated by the use of the mineral zenite.

The video game Devil Survivor 2 features Merak, alongside the other stars of the Big Digger, as the Septentrione, strange beings that appear in Japan on each day of the game. Merak features on the 2nd day in Osaka, Japan.