Polaris, also known as the North Star, Alpha Ursae Minoris or Star of Arcady, is the brightest star in Ursa Minor constellation. It is the closest bright star to the North Celestial Pole. The pole marks true north, which makes the North Star important in navigation, as the star’s elevation above the horizon closely matches the observer’s latitude.

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The North Star has a reputation for being bright, but it is not among the top 10 or even the top 40 brightest stars in the night sky. It is only the 48th brightest star, and owes its reputation to the fact that it is the nearest relatively bright (second magnitude) star to the North Celestial Pole.

Polaris is also the star that marks the end of the Little Dipper‘s handle. It is approximately 434 light years or 133 parsecs distant from Earth.

Polaris goes by many different names, including the Northern Star, Pole Star, Lodestar, Guiding Star, and Cynosūra, derived from the Greek κυνόσουρα for “the dog’s tail.” In ancient Greek times, Ursa Minor was taken to represent a dog, not a bear.


Polaris A, image: NASA

Polaris A is a classic Population I Cepheid variable. It is the brightest Cepheid variable in the sky. Cepheid variables are stars that astronomers use to measure distances to galaxies and clusters. The North Star’s brightness varies by 0.03 magnitudes over a period of 3.97 days. Its variability had been theorized since 1852, but was not confirmed until 1911, when the Danish astronomer Ejnar Hertzsprung confirmed the variation.

Polaris A’s amplitude and period of the variations have not remained the same since discovery. Before 1963, the amplitude was more than 0.1 magnitude and slowly decreasing. It decreased quite rapidly after 1966 until it reached 0.05 magnitude, and has been erratic but stayed neared that range since. The period of the variations increased more or less steadily until 1963, when it remained constant from 3 years, and then started increasing again. Currently, the measurements show a steady increase of 3.2 seconds per year.

Alpha Ursae Minoris Aa has two companions. The one closer to the supergiant, designated Alpha Ursae Minoris Ab, is a dwarf belonging to the spectral class F7. It lies at an average distance of 17 astronomical units and has an orbital period of about 29.6 years. The star has a highly eccentric orbit that takes it between 27 and 6.7 astronomical units from the primary component.

The star further away from the supergiant, designated Alpha Ursae Minoris B, lies 18 arc seconds away. It is a dwarf star with the stellar classification F3 1.4. It is at least 2,400 astronomical units away from Polaris A and has an orbital period of at least 42,000 years. Polaris B was discovered by William Herschel in 1780. It can be seen in a modest telescope.

Polaris also has two more distant companions, Alpha UMi C and Alpha UMi D.

Polaris is the brightest in a small semicircle of stars known as the “Engagement Ring.” The stars lie within a field which is six degrees wide.

The North Celestial Pole lies roughly halfway between Polaris and Lambda UMi.

The binary nature of Polaris A was confirmed in 1929 after an examination of the star’s spectrum. All three components in the star system were revealed in Hubble images in January 2006.

Polaris may be 2.5 times brighter today than when the Greek astronomer Ptolemy observed the star, according to a recent research led by Scott Engle of Villanova University in Pennsylvania.

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The research, based on historical records and presented in a poster session at the American Astronomical Society meeting in Washington, D.C., in January 2014, has revealed that the star has changed from third to second magnitude since Ptolemy’s times. Engle and his team used historic measurements of the star by Ptolemy in the year 137, the Persian astronomer Al Sufi in 964, the Danish astronomer Tycho Brahe in the 16th century, and others.

Polaris – Alpha Ursae Minoris

Constellation: Ursa Minor Distance: 325 – 425 light years (99 – 129.5 parsecs) Coordinates: Alpha Ursae Minoris A – 02h 31m 49.09s (right ascension), +89°15’50.8” (declination) Alpha Ursae Minoris B – 02h 30m 41.63s (right ascension), +89°15’38.1” (declination) Apparent magnitude: 1.98v (Alpha Ursae Minoris Aa), 9.2 (Alpha Ursae Minoris Ab), 8.7 (Alpha Ursae Minoris B) Absolute magnitude: -3.6 (Alpha UMi Aa), 3.6 (Alpha UMi Ab), 3.1 (Alpha UMi B) Spectral class: F7Ib (Alpha UMi Aa), F6V (Alpha UMi Ab), F3V (Alpha UMi B) Variable type: Classical Cepheid (Alpha UMi Aa) Mass: 4.5 solar masses (Alpha UMi Aa), 1.26 (Alpha UMi Ab), 1.39 (Alpha UMi B) Radius: 46±3 solar radii (Alpha UMi Aa), 1.04 (Alpha UMi Ab), 1.38 (Alpha UMi B) Luminosity: 2,500 solar luminosities (Alpha UMi Aa), 3 (Alpha UMi Ab), 3.9 (Alpha UMi B) Temperature: 6,015 K (Alpha Umi Aa), 6,900 (Alpha UMi B) Designations: Polaris, Alpha Ursae Minoris, North Star, 1 Ursae Minoris, Cynosura, Alruccabah, Star of Arcady. Navigatoria, Yilduz, Mismar, HR 424, BD +88°8, HD 8890, SAO 308, FK5 907, GC 2243, ADS 1477, CCDM 02319+8915, HIP 11767