  | |
| Observation data EpochJ2000.0 EquinoxJ2000.0 | |
|---|---|
| Constellation | Ursa Major |
| Right ascension | 11h 01m 50.47654s[1] |
| Declination | +56° 22′ 56.7339″[1] |
| Apparent magnitude (V) | +2.37[2] |
| Characteristics | |
| Spectral type | A1IVps[3] |
| U−Bcolor index | +0.00[2] |
| B−Vcolor index | -0.02[2] |
| Variable type | Suspected |
| Astrometry | |
| Radial velocity (Rv) | -12.0[4] km/s |
| Proper motion (μ) | RA: +81.43[1]mas/yr Dec.: +33.49[1]mas/yr |
| Parallax (π) | 40.90±0.16 mas[1] |
| Distance | 79.7 ± 0.3 ly (24.45 ± 0.10 pc) |
| Absolute magnitude (MV) | +0.61[5] |
| Details[6] | |
| Mass | 2.56±0.04 M☉ |
| Radius | 2.81±0.17 R☉ |
| Luminosity | 63.5 L☉ |
| Surface gravity (log g) | 3.93±0.06 cgs |
| Temperature | 9,700±300 K |
| Rotational velocity (v sin i) | 47±3 km/s |
| Age | 390±43 Myr |
| Other designations | |
| Merak, Mirak,[7] 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[8] | |
| Database references | |
| SIMBAD | data |
Merak/ˈmɪəræk/, also calledBeta Ursae Majoris (β Ursae Majoris, abbreviatedBeta UMa,β UMa),[9][10] is astar in the northernconstellation ofUrsa Major.
Theapparent visual magnitude of this star is +2.37,[2] which means it is readily visible to the naked eye. It is more familiar tonorthern hemisphere observers as one of the "pointer stars" in theBig Dipper, or the Plough (UK), which is a prominentasterism of seven stars that forms part of the larger constellation. Extending an imaginary straight line from this star through the nearbyAlpha Ursae Majoris (Dubhe) extends toPolaris, the north star.
In 1943, β Ursae Majoris was listed as a spectral standard for the class of A1 V.[11] When improved instruments made it possible to identify subgiantluminosity classes for early A-class stars, β Ursae Majoris was assigned that class A0 IV.[12] This was later revised to A1 IV.[3] It is considered to be a mildAm star, a type ofchemically peculiar star with unusually strong lines of certain metallic elements.[13]
Based uponparallax measurements, β Ursae Majoris is located at a distance of 79.7light-years (24.4parsecs) from theSun. It is asubgiant, a star that has exhausted the hydrogen in its core and is now cooling as it generates energy through thethermonuclear fusion of hydrogen in a shell outside the core. Theeffective temperature of the outer envelope is about 9,225 K,[14] giving it a white-hued glow that is typical forA-type stars.[15] It is larger than the Sun, with about 2.7 times themass and 2.84 times thesolar 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 theSun's luminosity.[14][16]
Observation of the star in theinfrared reveal anexcess emission that suggests the presence of a circumstellardebris disk of orbiting dust,[14] much like those discovered aroundFomalhaut andVega. The mean temperature of this disk is 120 K,[16] indicating that it is centered at a radius of 47 AU from the host star.[14] The dust has an estimated mass of about 0.27% the mass of the Earth.[16]
Beta Ursae Majoris is one of five stars in the Big Dipper that form a part of a looseopen cluster called theUrsa Major moving group, sharing the same region of space and not just the same patch of sky from Earth's 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.[17] Two stars are known to be located in relatively close proximity: 37 Ursae Majoris at 5.2 light-years (1.6 pc) andGamma Ursae Majoris at 11 light-years (3.4 pc); much closer to each other than these stars are to the Earth.[18]
β Ursae Majoris (Latinised toBeta Ursae Majoris) is the star'sBayer designation.
It bore the traditional nameMerak derived from theArabic المراقal-marāqq 'the loins' (of the bear).[7] In 2016, theInternational Astronomical Union organized aWorking Group on Star Names (WGSN)[19] 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 includedMerak for this star.[20]
TheHindus called the starPulaha, one of theSeven Rishis.[7]
InChinese,北斗 (Běi Dǒu), meaningNorthern Dipper, refers to an asterism equivalent to the Big Dipper. Consequently, theChinese name for Beta Ursae Majoris itself is北斗二 (Běi Dǒu èr, English:the Second Star of Northern Dipper) and天璇 (Tiān Xuán, English:Star of Celestial Rotating Jade).[21]
USSMerak (1918) andUSSMerak (AF-21) are bothUnited States navy ships.
