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 Alpha Ursae Minoris - Polaris

At magnitude 1.97, Alpha Ursae Minoris is the brightest star in the constellation Ursa Minor, the Smaller Bear. However, α UMi is only the 48th brightest star in the sky; it is famous not because of its brightness but due to its unique position near the celestial pole. Because α UMi lies nearly in a direct line along the Earth's axis of rotation "above" the North Pole, the star stands almost motionless in the sky, and all other stars appear to rotate around it. Hence, Polaris is more commonly known as the North Star, the Pole Star, or Polaris (and sometimes as the Lodestar).

Polaris is a relatively bright star, and is conspicuous because no other stars of similar brightness appear close to it. It is easy to find Polaris by following the line traced from Merak to Dubhe (β and α UMa) - also known as the Pointers, the two stars at the end of the bowl of the Big Dipper. One can also follow the central point of the "W" shape of Cassiopeia to Polaris. Polaris also marks the end of the handle of the Little Dipper, the prominent figure of Ursa Minor.

Polaris is not visible from the Southern Hemisphere. There is no real southern pole star; the only visible to the naked eye that is closest to the south celestial pole is σ Oct, sometimes called Polaris Australis. However, the bright constellation Crux (the Southern Cross) points fairly accurately toward the south celestial pole.

Today, Polaris is less than 42' away from the true pole, and its distance will diminish to a minimum of 27.5' in the year 2101 A.D.; subsequently, it will begin to move away. This change in its location is a consequence of precession, the slow wobble in the orientation of the Earth's axis (due to gravitational effects of the Sun and Moon) which makes the north celestial pole trace out a 47° circle on the celestial sphere every 25,800 years. Other stars along this circle were the pole star in the past, and will be again in the future. When the Pyramids were built some 4,600 years ago, the star Thuban (α Dra) marked the pole. Vega was the Pole Star 12,000 years ago.

History and Mythology

Today, Polaris makes an excellent reference point for celestial navigation. Its first such use is represented on the earliest known Assyrian tablets. To the Bedouin people of the Negev and Sinai, Polaris is known as "al-jadiyy", the "billy goat"; Polaris and Canopus (α Car) are the two principal stars used for nomadic wandering at night. Because it is always visible, Polaris became associated with a steadfast nature, as opposed to Canopus, which "flees" below the horizon. The Mayan god of the pole star is a monkey's head.

At this time of Christopher Columbus, Polaris was nearly 4° away from the pole, and Columbus did not need to use it for navigation because the compass had already been invented. But he did check the direction of his compass needle against the position of the star. After leaving the Canary Islands, he noticed that the compass needle pointed northwest, thus discovering a phenomenon called variance, where a secondary magnetic field is superimposed on the primary field. In more recent history, Polaris was listed as one of the navigational stars in Nathaniel Bowditch's 1802 book, "The American Practical Navigator".

Components and Properties

Polaris is a lovely double star with a yellowish primary, and a pale white magnitude 9.1 secondary lying 18.6" distant. The secondary, α UMi B, can be seen with even a modest telescope, and was first noticed by William Herschel in 1780. It must be at least 2400 AU away from α UMi A, and take at least 42,000 years to orbit. In 1929, another very close companion (α UMi Ab) was discovered by examining the spectrum of α UMi A. It takes 29.6 years to go around, orbiting at an average distance of 18.5 AU; a high eccentricity takes it between 6.7 and 27 AU.

Polaris is about 430 light-years from Earth. The primary, α UMi A, is an evolved F7 Ib-II supergiant 2500 times more luminous than our Sun, with a temperature of about 6000 K, a radius 45 times the Sun's, and a mass of six Suns. The visual secondary, α UMi B, is an F3 V main-sequence star with a luminosity of 28 suns, and contains about 1.4 solar masses. The much closer spectroscopic companion is an F7 V star of about 1.25 solar masses. Recent observations show that Polaris may be part of a loose open cluster of type A and F stars.

Variability

The primary, α UMi A, is a classic Cepheid variable - in fact, the brightest and closest Cepheid in the sky. Since Cepheids are important "standard candles" for determining distance, Polaris is heavily studied. Hydrogen fusion has stopped in the star's core, and it is now passing through a phase of instability wherein it pulsates. Polaris is particularly interesting as the pulsations have nearly ceased. Around 1900, the star's luminosity varied 8% from its average (0.15 magnitudes in total) with a 3.97 day period. However, the amplitude of its variation has been declining quickly since the middle of the 20th century. The variation reached a minimum of 1% in the mid 1990s and has remained there since. During the same time, the star has brightened by 15%, and its period has lengthened by about 8 seconds each year.

Just as a violin string has a "fundamental" tone that gives its pitch, it also vibrates in higher-frequency overtones. Comparison with other Cepheids shows that Polaris is pulsating not with its natural fundamental period, but in its first overtone. The star may be in the process of evolving into its fundamental period of 5.7 days with a greater variation. Recent research suggests that Polaris is 2.5 times brighter today than when Ptolemy observed it. This is a remarkable rate of change, 100 times larger than predicted by current theories of stellar evolution.