  Location of π2 Ursae Majoris (circled) near the center | |
| Observation data Epoch J2000.0 Equinox J2000.0 | |
|---|---|
| Constellation | Ursa Major |
| Right ascension | 08h 40m 12.8172s[1] |
| Declination | +64° 19′ 40.570″[1] |
| Apparent magnitude (V) | +4.620[2] |
| Characteristics | |
| Spectral type | K2 III[3] |
| U−Bcolor index | +1.193[2] |
| B−Vcolor index | +1.159[2] |
| Astrometry | |
| Radial velocity (Rv) | +14.62[4] km/s |
| Proper motion (μ) | RA: −59.861(80)mas/yr[1] Dec.: 26.544(88)mas/yr[1] |
| Parallax (π) | 13.2035±0.1164 mas[1] |
| Distance | 247 ± 2 ly (75.7 ± 0.7 pc) |
| Absolute magnitude (MV) | 0.150[3] |
| Details | |
| Mass | 1.234 ± 0.15[5] M☉ |
| Radius | 18.79±0.38[6] R☉ |
| Luminosity | 112.4±10.0[6] L☉ |
| Surface gravity (log g) | 1.8 ± 0.15[5] cgs |
| Temperature | 4336±99[6] K |
| Metallicity[Fe/H] | -0.25 ± 0.04[5] dex |
| Rotational velocity (v sin i) | 8[7] km/s |
| Age | 4.18 ± 1.95[3] Gyr |
| Other designations | |
| π2 Ursae Majoris, π2 UMa, Pi2 UMa, 4 Ursae Majoris,BD+64°698,FK5 2677,GC 11850,HD 73108,HIP 42527,HR 3403,PPM 16713,SAO 14616 | |
| Database references | |
| SIMBAD | data |
4 Ursae Majoris (sometimes abbreviated4 UMa) is theFlamsteed designation of astar in thenortherncircumpolar constellation ofUrsa Major. It also bears theBayer designation ofPi2 Ursae Majoris (Pi2 UMa,π2 Ursae Majoris,π2 UMa) and is traditionally namedMuscida. With anapparent visual magnitude of +4.6,[2] this star is visible from suburban or darker skies based upon theBortle Dark-Sky Scale. Fromparallax measurements made during theGaia mission, this star is at a distance of 247light-years (76parsecs) from Earth.[1] As of 2011[update], oneextrasolar planet has been confirmed to be orbiting the star.
This star has astellar classification of K2 III,[3] indicating that, at an estimated age of around four billion years,[3] it is anevolved star that has reached thegiant stage. It has a mass about 1.2 times largerthan the Sun, but has expanded to 18 times the Sun's girth.[5] Theeffective temperature of the star'souter atmosphere is4,415 K.[5] This heat gives it the cool, orange-hued glow of aK-type star.[8]
Pi2 Ursae Majoris is a member of theMilky Way galaxy'sthin disk population. It is following an orbit through the galaxy with aneccentricity of 0.10, which carries it as close to theGalactic Center as 27.7 kilolight-years (8.5 kiloparsecs) and as far as 34.1 kly (10.5 kpc). The inclination of this orbit lies close to thegalactic plane, so it departs this plane by no more than 260 ly (80 pc).[3]
Based upon observedradial velocity changes in the star, in 2007 the presence of a planetary companion was announced. The planet, designated 4 Ursae Majoris b, is at least seven times more massive thanJupiter. Its orbit iseccentric, orbiting 4 Ursae Majoris at 87% the distance fromSun toEarth. Compared to the Sun, this star has a lower abundance of elements other than hydrogen and helium, what astronomers term the star'smetallicity. This is curious, because mostmain-sequence stars with planets tend to have a higher abundance of metals.[5]
| Companion (in order from star) | Mass | Semimajor axis (AU) | Orbital period (days) | Eccentricity | Inclination | Radius |
|---|---|---|---|---|---|---|
| b | >7.1 ± 1.6 MJ | 0.87 ± 0.04 | 269.3 ± 1.96 | 0.432 ± 0.024 | — | — |
Withπ1,σ1,σ2,ρ,A andd, it composed the ArabicasterismAl Ṭhibā᾽, the Gazelle.[9] According to the catalogue of stars in theTechnical Memorandum 33-507 - A Reduced Star Catalog Containing 537 Named Stars,Al Ṭhibā were the title for seven stars:A asAlthiba I, π1 asAlthiba II, this star (π2) asAlthiba III, ρ asAlthiba IV, σ1 asAlthiba V, σ2 asAlthiba VI, and d asAlthiba VII.[10]
