Polaris
Polaris is a star in the northern circumpolar constellation of Ursa Minor. It is designated α Ursae Minoris (Latinized to Alpha Ursae Minoris) and is commonly called the North Star or Pole Star. With an apparent magnitude that fluctuates around 1.98, it is the brightest star in the constellation and is readily visible to the naked eye at night. The position of the star lies less than 1° away from the north celestial pole, making it the current northern pole star. The stable position of the star in the Northern Sky makes it useful for navigation.
| Observation data Epoch J2000 Equinox | |
|---|---|
| Constellation | Ursa Minor |
| Pronunciation | /pəˈlɛərɪs, -ˈlær-/; UK: /pəˈlɑːrɪs/ |
| α UMi A | |
| Right ascension | 02h 31m 49.09s |
| Declination | +89° 15′ 50.8″ |
| Apparent magnitude (V) | 1.98 (1.86 – 2.13) |
| α UMi B | |
| Right ascension | 02h 30m 41.63s |
| Declination | +89° 15′ 38.1″ |
| Apparent magnitude (V) | 8.7 |
| Characteristics | |
| α UMi A | |
| Spectral type | F7Ib |
| U−B color index | 0.38 |
| B−V color index | 0.60 |
| Variable type | Classical Cepheid |
| α UMi B | |
| Spectral type | F3V |
| U−B color index | 0.01 |
| B−V color index | 0.42 |
| Variable type | suspected |
| Astrometry | |
| Radial velocity (Rv) | −17 km/s |
| Proper motion (μ) | RA: 198.8±0.20 mas/yr Dec.: −15±0.30 mas/yr |
| Parallax (π) | 7.54 ± 0.11 mas |
| Distance | 323–433 ly (99–133 pc) |
| Absolute magnitude (MV) | −3.6 (α UMi Aa) 3.6 (α UMi Ab) 3.1 (α UMi B) |
| Position (relative to α UMi Aa) | |
| Component | α UMi Ab |
| Epoch of observation | 2005.5880 |
| Angular distance | 0.172″ |
| Position angle | 231.4° |
| Position (relative to α UMi Aa) | |
| Component | α UMi B |
| Epoch of observation | 2005.5880 |
| Angular distance | 18.217″ |
| Position angle | 230.540° |
| Orbit | |
| Primary | α UMi Aa |
| Companion | α UMi Ab |
| Period (P) | 29.59±0.02 yr |
| Semi-major axis (a) | 0.1204±0.0059" (≥2.90±0.03 AU) |
| Eccentricity (e) | 0.608±0.005 |
| Inclination (i) | 146.2±10.9° |
| Longitude of the node (Ω) | 191.4±4.9° |
| Periastron epoch (T) | 1987.66±0.13 |
| Argument of periastron (ω) (secondary) | 123.01±0.75° |
| Semi-amplitude (K1) (primary) | 3.72±0.03 km/s |
| Details | |
| α UMi Aa | |
| Mass | 5.4 M☉ |
| Radius | 37.5 R☉ |
| Luminosity (bolometric) | 1,260 L☉ |
| Surface gravity (log g) | 2.2 cgs |
| Temperature | 6015 K |
| Metallicity | 112% solar |
| Rotation | 119 days |
| Rotational velocity (v sin i) | 14 km/s |
| Age | 45 - 67 Myr |
| α UMi Ab | |
| Mass | 1.26 M☉ |
| Radius | 1.04 R☉ |
| Luminosity (bolometric) | 3 L☉ |
| Age | >500 Myr |
| α UMi B | |
| Mass | 1.39 M☉ |
| Radius | 1.38 R☉ |
| Luminosity (bolometric) | 3.9 L☉ |
| Surface gravity (log g) | 4.3 cgs |
| Temperature | 6900 K |
| Rotational velocity (v sin i) | 110 km/s |
| Age | 1.5 Gyr |
| Other designations | |
| α UMi A: 1 Ursae Minoris, BD+88°8, FK5 907, GC 2243, HD 8890, HIP 11767, HR 424, SAO 308 | |
| α UMi B: NSV 631, BD+88°7, GC 2226, SAO 305 | |
| Database references | |
| SIMBAD | α UMi A |
| α UMi B | |
As the closest Cepheid variable its distance is used as part of the cosmic distance ladder. The revised Hipparcos stellar parallax gives a distance to Polaris of about 433 light-years (133 parsecs), while the successor mission Gaia gives a distance of about 448 light-years (137 parsecs). Calculations by other methods vary widely.
Although appearing to the naked eye as a single point of light, Polaris is a triple star system, composed of the primary, a yellow supergiant designated Polaris Aa, in orbit with a smaller companion, Polaris Ab; the pair is in a wider orbit with Polaris B. The outer pair AB were discovered in August 1779 by William Herschel, where the 'A' refers to what is now known to be the Aa/Ab pair.