Polaris
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 Polaris as seen by the Hubble Space Telescope. |
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| Observation data Epoch J2000 Equinox J2000 |
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|---|---|
| Constellation | Ursa Minor |
| Right ascension | 02h 31m 48.7s |
| Declination | +89° 15′ 51″ |
| Apparent magnitude (V) | 1.97 |
| Characteristics | |
| Spectral type | F7 Ib-II SB |
| U-B color index | 0.38 |
| B-V color index | 0.60 |
| Variable type | Cepheid variable |
| Astrometry | |
| Radial velocity (Rv) | -17 km/s |
| Proper motion (μ) | RA: 44.22 mas/yr Dec.: -11.74 mas/yr |
| Parallax (π) | 7.56 ± 0.48 mas |
| Distance | 430 ± 30 ly (132 ± 8 pc) |
| Absolute magnitude (MV) | -3.63±0.14[1] |
| Details | |
| Mass | 7.54±0.6[1][2] M☉ |
| Radius | 30 R☉ |
| Luminosity | 2200 L☉ |
| Temperature | 7200 K |
| Metallicity | 112% solar[3] |
| Rotation | ~17 km/s |
| Age | ? years |
| Other designations | |
Polaris (α UMi / α Ursae Minoris / Alpha Ursae Minoris, commonly North(ern) Star or Pole Star, and sometimes Lodestar) is the brightest star in the constellation Ursa Minor. It is very close to the north celestial pole (42′ away as of 2006[update][citation needed]), making it the current northern pole star.
Polaris is about 430 light-years from Earth. It is a multiple star. α UMi A is a six solar mass[4] F7 bright giant (II) or supergiant (Ib). The two smaller companions are: α UMi B, a 1.5 solar mass[4] F3V main sequence star orbiting at a distance of 2400 AU, and α UMi Ab, a very close dwarf with an 18.5 AU radius orbit. There are also two distant components UMi C and UMi D.[5] Recent observations show that Polaris may be part of a loose open cluster of type A and F stars.[clarification needed]
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), which had been theorized in earlier observations (Moore, J.H and Kholodovsky, E. A.). In January 2006, NASA released images from the Hubble telescope, directly showing all three members of the Polaris ternary system. The nearer dwarf star is in an orbit of only 18.5 AU (2.8 billion km;[6] about the distance from our Sun to Uranus) from Polaris A, explaining why its light is swamped by its close and much brighter companion.[7]
Polaris is a classic 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). 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."
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[edit] Pole Star
Because in the current era[8] 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.[citation needed] In more recent history it was referenced in Nathaniel Bowditch's 1802 book, The American Practical Navigator, where it is listed as one of the navigational stars.[9] 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 over 25,800 years.[8] Other stars along this circle would have served as the pole star in the past and will again in the future, including Beta Ursae Minoris from 1500 BC to 500 AD, Thuban around 2500 BC and Vega 12,000 years ago.[8]
[edit] Other names for Polaris
Polaris has numerous traditional names: Alruccabah, Cynosura, Phoenice, Tramontana, Angel Stern, Navigatoria, Star of Arcady, Yilduz, Mismar, Polyarnaya. Cynosūra is from the Greek κυνόσουρα "the dog’s tail". Yilduz is from the Turkish word for "star" (see Yildun).
[edit] Cultural significance
- To the Bedouin people of the Negev and Sinai, Polaris is known as الجديّ al-jadiyy, "the billy goat". It and Suhail (= Canopus, α Car) are the two principal stars used for nomadic wandering at night. Because it was circumpolar and hence always visible from the Northern Hemisphere, it became associated with a steadfast nature, as opposed to Suhail, which disappears below the horizon and hence 'flees'.[10]
- In Hindu mythology Polaris is termed as Dhruva, meaning fixed.
- It was reported by Homer M. Brown, the Patriarch of the Granite, Utah Stake of the Church of Jesus Christ of Latter-day Saints, that when his grandfather asked Joseph Smith where the Ten Lost Tribes were, Smith pointed to Polaris.[11][unreliable source?] Because of this, after the advent of the flying saucer craze in the 1950s, some Mormons[who?] came to believe as part of Mormon folklore that the Ten Lost Tribes will return from the star Polaris in flying saucers.[12][dubious ] The tenth Latter Day Saint Article of Faith states that Mormons believe in the “literal gathering of Israel and the restoration of the Ten Tribes” (see House of Joseph (LDS Church)).
[edit] See also
[edit] Notes
- ^ a b Wieland page 9: Table 5 gives mass of component A as 6.0 ±0.5 and P as 1.54 ±0.25 solar masses
- ^ Polaris: Mass and Multiplicity, http://arxiv.org/abs/astro-ph/0609759, retrieved on 2009-04-13 (URL for full PDF) (Evans et al support Wieland's prediction: "preliminary mass of 5.0 ± 1.5 M⊙ for the Cepheid and 1.38 ± 0.61 M⊙ for the close companion.)
- ^ Cayrel de Strobel, G.; Soubiran, C.; Ralite, N. (2001). " Catalogue of [Fe/H] determinations for FGK stars: 2001 edition ". A&A 373: 159–163. http://adsabs.harvard.edu/abs/2001A%26A...373..159C.
- ^ a b Wieland page 3: masses of A and P ... (6.0+1.54M⊙)
- ^ Wieland page 2
- ^ There's More to the North Star Than Meets the Eye
- ^ Evans, N. R.; Schaefer, G.; Bond, H.; Bono, G.; Karovska, M.; Nelan, E.; Sasselov, D. (January 9, 2006). "Direct detection of the close companion of Polaris with the Hubble Space Telescope". American Astronomical Society 207th Meeting.
- ^ a b c Norton, Arthur P. (1973). Norton's Star Atlas. Edinburgh: Sky Publishing. p. 10. ISBN 0-85248-900-5. "4500 years ago it was Thuban (α Draconis); 8000 years hence it will be Deneb"
- ^ Nathaniel Bowditch: The American Practical Navigator, 2002 Bicentennial Ed., Chapter 15 Navigational Astronomy, page 248, Figure 1530a. Navigational stars and the planets
- ^ Bailey, Clinton (1974). "(abstract) Bedouin Star-Lore in Sinai and the Negev". Bulletin of the School of Oriental and African Studies, University of London 37 (3): 580–96. http://links.jstor.org/sici?sici=0041-977X%281974%2937%3A3%3C580%3ABSISAT%3E2.0.CO%3B2-Q (abstract). Retrieved on 2008-01-14.
- ^ Smith, Robert W. (1943) The Last Days: Scriptural and Secular Prophecies Pertaining to the Last Days. Salt Lake City: Pyramid Press, p. 218
- ^ A 1987 Sunstone Magazine[specify] cartoon shows someone emerging from a flying saucer that has just landed proclaiming “We’re from the Ten Lost Tribes and we want to see Disney World!”
[edit] References
- Wieland, R.; H. Jahreiss, C. Dettbarn, H. Lenhardt, H. Schwan, Polaris: astrometric orbit, position, and proper motion, http://arxiv.org/abs/astro-ph/0002406, retrieved on 2009-04-13 (Link to download full paper as PDF)
[edit] External links
| Preceded by Kochab & Pherkad |
Pole Star 500–3000 |
Succeeded by Alrai |
