Polaris, Alpha Ursae Minoris

Multiple star containing a Cepheid variable (Alpha UMi, 1 UMi) in Ursa Minor

Polaris, The Pole Star, The North Star

Right Ascension 02 : 31 : 50.5 (h:m:s)
Declination +89 : 15 : 51 (deg:m:s)
Distance 390 (ly)
Visual brightness 2.02v (mag)
Spectral type F7 Ib-IIv

Polaris, or the North or Pole Star, is probably the most famous and wellknown star in the sky, as it is currently located close to the North Celestial Pole (NCP), i.e., the Earth's rotation axis is currently pointing into a direction closer than 1 degree from that to this star. The NCP will come even closer to Polaris until about 2100 to 2115 (sources vary; R. Burnham, Jr. gives 2102, supported by the precession period of 25,725 years given by Schmeidler (1994) and used here, while the National Audubon Society Field Guide to the Night Sky has 2105, e.g.), and then again leave the neighborhood of Alpha Ursae Minoris, because of the Precession of the Equinoxes. This phenomenon is caused by he gravitational pull of Sun and Moon on Earth's equatorial bulge, and results in a motion of Earth's rotational axis (the NCP) around the pole of the ecliptic (situated in constellation Draco at RA=18:00.0, Dec=66.5 deg) with a period of 25,725 years. About 4,600 year ago, Thuban (Alpha Draconis) was nearest to the North Celestial Pole, while 12,000 years ago it was, and in 14,000 year again will be bright Vega (Alpha Lyrae).

Karkoschka gives the coordinates for Polaris for recent and upcoming years as follows:

Epoch   RA      Dec

1900.0 1:22.6 +88.77 1950.0 1:48.8 +89.03 1990.0 2:21.3 +89.22 2000.0 2:31.8 +89.26 2010.0 2:43.6 +89.31 2050.0 3:47.5 +89.46
Situated in this position, people on the Northern hemisphere can find their North direction on Earth by locating this star.

The star Polaris has a number of other names also, e.g. the Lodestar or the Cynosure (according to the National Audubon Society's Field Guide to the Night Sky).

[Polaris by J. Schmidling] Astronomically, this is also a very remarkable star because of several reasons: First, it is a binary star with components A, a supergiant of mag 2.0, spectral type F7, and B, a main sequence star of mag 9 and spectral type F3 V, separated by about 18 arc seconds at position angle 217 deg, and separable with good 2-inch refractors and less under good viewing conditions, easy with a 3-inch. The companion was first seen by William Herschel in 1780. According to Burnham's Celestial Handbook, the stars actually form a physical pair with common proper motion. The orbital period must be several thousand years. Their projected distance is over 2000 AU or 300 billion km, a distance the light needs 12 days to travel. Four more faint optical companions, probably physically unrelated to Polaris, are listed in the Sky Catalogue 2000.0 and the ADS. At their distance of about 390 light years, the main star is of absolute magnitude -3.4 or about 1800 Solar luminosities, the companion of M=+3 or 4 times brighter than our Sun (Karkoschka gives another value for the distance: 800 light years).

The image at right was taken by Jack Schmidling with a 10" Newton at f/6. It was exposed 60 seconds on Tmax 400 film.

The supergiant is a Delta Cephei variable star with period 3.969778 days, varying between mag 1.92 and 2.07, according to the Sky Catalogue 2000.0. The amplitude of its variation has declined in the past years so that it was suspected that the variability of this star might have ceased, but this is probably not the case (at least not yet). Burnham classifies it as a type II Cepheid (W Virginis star). It was recently noted that Polaris pulsates with an overtone, at a rate 40 per cent faster than expected for a Cepheid of its size and luminosity.

In addition, the Cepheid shows variations in radial velocity, which indicate the presence of a further "astrometric" companion; the period of variations (and thus orbital period of this star) is 30.5 years. According to calculations of J.H. Moore (1929) and E. Roemer (1955) referred to by Burnham, one periastron occurred in 1928, the semi-major axis of the orbit is 290 million miles, and the excentricity amounts 0.64. The mass of this unseen companion star makes Polaris swing around the center of mass on an orbit of roughly 3 AU semi-major axis. As it does not notably contribute light to the spectrum of the system, it must be at least 5 magnitudes fainter than the main star.

Binocular observers may note a circlet or ringlet of 7th to 8th magnitude stars just south of Polaris; this asterism is sometimes called the "engagement ring" of Polaris.



[Constellations] | [SEDS] | [MAA]

Hartmut Frommert
Christine Kronberg