Polaris

(poe-LAHR-is)

Alpha Ursae Minoris

Polaris

Distance (Light Years) 431 ± 27
Apparent Magnitude 1.97
Color (B-V)0.6

Names For This Star

Our current name for the star, Polaris, is Latin and abbreviation for Stella Polaris, "The Pole Star." The name reflects the position of the star near the north celestial pole. In fact, in English Polaris is sometimes simply referred to as The Pole Star.

The star is also referred to as The Lodestar. The name associates the star with a naturally occurring magnetic rock a lodestone that can be used to make a magnetic compass. The name "Loadestar" then reflects the position of the star above the north horizon point. Like the compass, the star points the direction of north. A similar idea is contained in another Latin name for the star, Navigatoria, that is "The Navigator's Star." Other names for the star are Alruccabah, Cynosura, Phoenice, Tramontana, Angel Stern, The Star of Arcady, Yilduz, or Mismar. For the origin and significance of these names, see Allen.

Description of the Star

Polaris A is a hot, blue F7:Ib-II bright giant or supergiant star having a luminosity about 2400 times that of the sun.

The Polaris System

Polaris A has at least two companions. The brighter of the two, Polaris B, is a yellowish F3V main sequence star of Apparent Magnitude 8.20 having a luminosity about 8 times that of the sun. According to Burnham this star appears about 18.5 arc sec from the A star, corresponding to a projected separation of 2000 AU.

The A star has an additional close companion star orbiting with a 30.5 year period at 5 AU from the primary.

A Cepheid Variable Star

Polaris A is a pulsating variable star of a type known as a Cepheid variable, after prototypical star of this type, Delta Cephei. Cepheid variables are bright, giant stars that show periodic variations in luminosity. In general, the longer the period, the brighter the star. This correlation allows the luminosity (absolute magnitude) to be inferred from the period.

Polaris pulsates with 3.97 day period. The luminosity varies by 0.15 magnitude. Polaris is classified as a Population II Cepheid. Stars of this type are about 1.5 magnitudes smaller than Population I Cepheids.

The Distance Scale in the Universe

Cepheid variables have been very important in establishing a scale of distance in the universe, since the luminosity of a Cepheid variable star can be established by measuring the period of its brightness variation. Knowing the actual luminosity of the star, the distance to the star can be established from a photometric determination of its Apparent Magnitude, that is, of its apparent brightness.

Other Designations For This Star


Flamsteed

1 Ursae Minoris

Hipparcos Identifier (HIP Number)

11767

Harvard Revised (HR Number)

424

Henry Draper Catalog (HD Number)

8890

Bonner Durchmusterung (BD Number)

BD+88 8
Smithsonian Astrophysical
Observatory compendium (SAO Number)
308

Fundamental Katalog (FK5 Number)

907
 

 


Copyright © 1998 - 2010 by Arnold V. Lesikar,
Professor Emeritus
Dept. of Physics, Astronomy, and Engineering Science,
St. Cloud State University,St. Cloud, MN 56301-4498

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