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Created page with ''''Polaris''', also known as '''Alpha Ursae Minoris''' is the brightest star in the constellation Ursa Minor. Polaris is best known for being the current north or pole star,...'
'''Polaris''', also known as '''Alpha Ursae Minoris''' is the brightest star in the constellation [[Ursa Minor]]. Polaris is best known for being the current north or pole star, as it is very close to the north celestial pole. Polaris is actually a triple star system, although only Polaris A is visible in the night sky to the unaided eye, with an apparent magnitude of 1.97. The system is approximately 430 light years away.
==Polaris in Culture==
The traditional name Polaris is one of many known names of the star throughout history. The [[ancient Greeks]] called the star Φοινίκη or ''Phoenice''. Another Greek related word was κυνόσουρα or Cynosūra, meaning "the dog's tail", a designation that became popular in reference to the star in 17th and 18th century scientific works. The [[Bedouin]] of the [[Levant]] referred to the star as الجديّ ''al-jadiyy'', meaning "the billy goat". [[Arab]] astronomers however named the star ''Al Kaukab al Shamāliyy'', or "the star of the north". For [[Hinduism|Hindu mythology]], Polaris was known first as Grahadhāra, then as Dhruva (a devotee of [[Vishnu]]). <ref>http://penelope.uchicago.edu/Thayer/E/Gazetteer/Topics/astronomy/_Texts/secondary/ALLSTA/Ursa_Minor*.html</ref>
==The System==
Polaris is actually a ternary system of three known stars. '''Polaris A''', which is the north star visible from [[Earth]], is a yellow-white giant or [[supergiant]] of spectral class F7 II-Ib. The star is around five times the mass of our [[Sun]] with a radius 30 times as great.<ref name="mass">http://arxiv.org/abs/astro-ph/0609759</ref> Because of the enormous size, Polaris is actually some 2200-2500 times as visually luminous as our own Sun, with an average surface temperature of 6600 K. The star's estimated [[metallicity]] (abundance of elements heavier then [[hydrogen]] and [[helium]]), is 112 percent that of our Sun, based on the abundance of [[iron]] measured.<ref>http://adsabs.harvard.edu/abs/2001A%26A...373..159C</ref>
Polaris A is also a [[Cepheid variable]] star, which pulsates in luminosity with precise regularity. Such stars are used as standard candles for determining distance, especially on an extragalactic scale. As such, Polaris as studied extensively by astronomers. The variation in Polaris is only by 0.03 magnitudes, which occur over a period of 3.97 days.
'''Polaris B''' was first discovered by [[William Herschel]] in 1780. As an eighth magnitude star, it can be spotted using a small [[telescope]]. The star is a yellow-white dwarf of spectral type F3 V. Polaris B orbits its primary with at an average distance of 2400 AU, taking some 42,000 years to complete one orbit. The mass of the star is estimated to be 1.5 times that of our Sun.<ref>http://arxiv.org/abs/astro-ph/0002406</ref>
In 1929 it was discovered that the Polaris system had a third star, now named '''Polaris Ab''', which is very close to the primary star Polaris A. Polaris Ab is a yellow-white dwarf of spectral class F7 V. The mass is estimated to be around 1.4 times that of the Sun.<ref name="mass">http://arxiv.org/abs/astro-ph/0609759</ref> The star orbits the primary at an average distance of 18.5 AU, or about the distance of [[Uranus]] from the Sun. However the orbit is highly eccentric, running between 27 and 6.7 AU.<ref>http://hubblesite.org/newscenter/archive/releases/2006/02/fastfacts/</ref>
==Polaris as a Pole Star==
At present, Polaris is the closest visible star to the north celestial pole, only 0.7 degrees away. As such, the star is almost in a direct line with the Earth's [[axis]] of rotation. From an observer on Earth, the star appears virtually motionless as the night goes on, with all the other celestial objects in the northern sky seemingly orbiting it. At present, due to the [[precession of the equinoxes]], the celestial pole is actually moving closer to Polaris, and will reach it's closest in the year 2105 before pulling away.<ref>http://stars.astro.illinois.edu/sow/polaris.html</ref>
Because of the 26,000 year axial precession of the Earth, Polaris was not always the pole star, nor will it be in the future. Past pole stars include [[Beta Ursae Minoris]] from 1500 BC until 500 AD, Alpha Draconis ([[Thuban]]) in around 3000 BC, and [[Vega]] around 12,000 BC. In the future, [[Deneb]] will become the north star around 8000 AD and Vega in around 14,000 AD. Eventually in 27,800 AD Polaris will be the north star once again, but it will not be as close to the celestial pole this next time around, due to the star's proper motion.<ref>Norton, Arthur P. (1973). Norton's Star Atlas. Edinburgh: Sky Publishing. p. 10. ISBN 0-85248-900-5.</ref>
[[Category:Astronomy]]
==References==
{{reflist}}
==Polaris in Culture==
The traditional name Polaris is one of many known names of the star throughout history. The [[ancient Greeks]] called the star Φοινίκη or ''Phoenice''. Another Greek related word was κυνόσουρα or Cynosūra, meaning "the dog's tail", a designation that became popular in reference to the star in 17th and 18th century scientific works. The [[Bedouin]] of the [[Levant]] referred to the star as الجديّ ''al-jadiyy'', meaning "the billy goat". [[Arab]] astronomers however named the star ''Al Kaukab al Shamāliyy'', or "the star of the north". For [[Hinduism|Hindu mythology]], Polaris was known first as Grahadhāra, then as Dhruva (a devotee of [[Vishnu]]). <ref>http://penelope.uchicago.edu/Thayer/E/Gazetteer/Topics/astronomy/_Texts/secondary/ALLSTA/Ursa_Minor*.html</ref>
==The System==
Polaris is actually a ternary system of three known stars. '''Polaris A''', which is the north star visible from [[Earth]], is a yellow-white giant or [[supergiant]] of spectral class F7 II-Ib. The star is around five times the mass of our [[Sun]] with a radius 30 times as great.<ref name="mass">http://arxiv.org/abs/astro-ph/0609759</ref> Because of the enormous size, Polaris is actually some 2200-2500 times as visually luminous as our own Sun, with an average surface temperature of 6600 K. The star's estimated [[metallicity]] (abundance of elements heavier then [[hydrogen]] and [[helium]]), is 112 percent that of our Sun, based on the abundance of [[iron]] measured.<ref>http://adsabs.harvard.edu/abs/2001A%26A...373..159C</ref>
Polaris A is also a [[Cepheid variable]] star, which pulsates in luminosity with precise regularity. Such stars are used as standard candles for determining distance, especially on an extragalactic scale. As such, Polaris as studied extensively by astronomers. The variation in Polaris is only by 0.03 magnitudes, which occur over a period of 3.97 days.
'''Polaris B''' was first discovered by [[William Herschel]] in 1780. As an eighth magnitude star, it can be spotted using a small [[telescope]]. The star is a yellow-white dwarf of spectral type F3 V. Polaris B orbits its primary with at an average distance of 2400 AU, taking some 42,000 years to complete one orbit. The mass of the star is estimated to be 1.5 times that of our Sun.<ref>http://arxiv.org/abs/astro-ph/0002406</ref>
In 1929 it was discovered that the Polaris system had a third star, now named '''Polaris Ab''', which is very close to the primary star Polaris A. Polaris Ab is a yellow-white dwarf of spectral class F7 V. The mass is estimated to be around 1.4 times that of the Sun.<ref name="mass">http://arxiv.org/abs/astro-ph/0609759</ref> The star orbits the primary at an average distance of 18.5 AU, or about the distance of [[Uranus]] from the Sun. However the orbit is highly eccentric, running between 27 and 6.7 AU.<ref>http://hubblesite.org/newscenter/archive/releases/2006/02/fastfacts/</ref>
==Polaris as a Pole Star==
At present, Polaris is the closest visible star to the north celestial pole, only 0.7 degrees away. As such, the star is almost in a direct line with the Earth's [[axis]] of rotation. From an observer on Earth, the star appears virtually motionless as the night goes on, with all the other celestial objects in the northern sky seemingly orbiting it. At present, due to the [[precession of the equinoxes]], the celestial pole is actually moving closer to Polaris, and will reach it's closest in the year 2105 before pulling away.<ref>http://stars.astro.illinois.edu/sow/polaris.html</ref>
Because of the 26,000 year axial precession of the Earth, Polaris was not always the pole star, nor will it be in the future. Past pole stars include [[Beta Ursae Minoris]] from 1500 BC until 500 AD, Alpha Draconis ([[Thuban]]) in around 3000 BC, and [[Vega]] around 12,000 BC. In the future, [[Deneb]] will become the north star around 8000 AD and Vega in around 14,000 AD. Eventually in 27,800 AD Polaris will be the north star once again, but it will not be as close to the celestial pole this next time around, due to the star's proper motion.<ref>Norton, Arthur P. (1973). Norton's Star Atlas. Edinburgh: Sky Publishing. p. 10. ISBN 0-85248-900-5.</ref>
[[Category:Astronomy]]
==References==
{{reflist}}