polaris

Polaris (α UMi / α Ursae Minoris / Alpha Ursae Minoris) is the brightest star in the constellation Ursa Minor. It is very close to the north celestial pole (42′ away as of 2006), making it the current northern pole star.

Name

"Polaris" comes from Stella Polaris, the Latin form of its common name "Pole Star". The rarely used Greek name Cynosura (Κυνόσουρα) means "tail of the dog" and is the source of the English word cynosure. Other names include the "North Star", the "Lode Star", or sometimes "Polaris Borealis".

Physical properties

Polaris is a trinary system, consisting of a large yellow Cepheid variable (α UMi A), distantly (about 2700 AU or 380 billion km) orbited by a bright yellow dwarf (α UMi B). Polaris B can be seen with even a modest telescope and was first noticed by William Herschel in 1780. In 1929, it was discovered by examining the spectrum of Polaris A that it had another very close dwarf companion (variously α UMi P, α UMi a or α UMi Ab). In January 2006, NASA released images from the Hubble telescope, directly showing all three members of the Polaris trinary system. The nearer dwarf star is in an orbit of only 20 AU (3 billion km) from Polaris A, explaining why its light is swamped by its close and much brighter companion.^[1]

Polaris is 431 light years (132 pc) from Earth, according to astrometric measurements of the Hipparcos satellite. Concerning the detailed physics, α UMi A is an F7 supergiant (Ib) or bright giant (II). The two smaller companions are: α UMi B an F3V main sequence star, orbiting in 2700 AU distance, and C a very close dwarf on a 20 AU orbit. Recent observations show that Polaris may be part of a loose open cluster of type A and F stars.

The giant star of Polaris is a classical Population I Cepheid variable (although it was once thought to be Population II due to its high galactic latitude). Since Cepheids are an important standard candle for determining distance, Polaris (as the closest such star) is heavily studied. Around 1900, the star 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 quickly declining since the middle of the 20th century. The variation reached a minimum of 1% in the mid 1990s and has remained at a low level. Over the same period, the star has brightened by 15% (on average), and the period has lengthened by about 8 seconds each year.

Recent research reported in Science suggests that Polaris is 2.5 times brighter today than when Ptolemy observed it (now 2mag, antiquity 3mag). The astronomer Edward Guinan considers this to be a remarkable rate of change and is on record as saying that "If they are real, these changes are 100 times larger than [those] predicted by current theories of stellar evolution."

Pole Star

Because α UMi lies nearly in a direct line with the axis of the Earth's rotation "above" the North Pole — the north celestial pole — Polaris stands almost motionless on the sky, and all the stars of the Northern sky appear to rotate around it. Therefore, it makes an excellent fixed point from which to draw measurements for celestial navigation and for astrometry. The antiquity of its use is attested by the fact that it is found represented on the earliest known Assyrian tablets. At present, Polaris is 0.7° away from the pole of rotation (1.4 times the Moon disc) and hence revolves around the pole in a small circle 1½° in diameter. Only twice during every sidereal day does Polaris accurately define the true north azimuth; the rest of the time it is only an approximation and must be corrected using tables or a rough rule of thumb.

Due to the precession of the equinoxes, Polaris will not always be the pole star. Over tens of thousands of years, perturbations to the Earth's axis of rotation will cause it to point to other regions of the sky, tracing out a circle. Other stars along this circle were the pole star in the past and will be again in the future, including Thuban and Vega. In the near future, Polaris is due to become an even more accurate pole star; the distance between Polaris and the pole will reach a minimum (of just under ½ degree) in 2100.

In the Northern Hemisphere, it is easy to find Polaris by following the line traced from Merak to Dubhe (β and α Ursae Majoris, 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. Polaris is not visible from the Southern Hemisphere except from an elevated position near the equator.

Polaris's fame as the North Star has given rise to a persistent misconception that it is the brightest star in the sky. Although Polaris is a relatively bright star and is conspicuous since no other stars of similar brightness are close to it, it is nowhere near the brightest; it is actually the 48th brightest star in the night sky. The brightest star in the sky (besides the Sun) is Sirius (see Winter sky and List of brightest stars).

There is no real southern pole star. The only star visible to the naked eye that is close to the south celestial pole is the dim Sigma Octantis, sometimes called Polaris Australis. However, the bright Southern Cross (Crux) points fairly accurately towards the south celestial pole.

In culture

• In astrology Polaris is one of the Behenian fixed stars and has the symbol .

• The constellation Ursa Major is inscribed on the Latter-day Saint temple in Salt Lake City, Utah, United States of America. This is symbolic of the use of the constellation to find Polaris in the sky, which in turn points north. The temple similarly provides direction for members of the church.

See also

• Star systems in fiction

References

External links

• Info on Polaris
• Polaris: astrometric orbit, position, and proper motion
• Polaris Ab imaged by Hubble