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Alpha Ursae Minoris
Polaris as seen by the Hubble Space Telescope. NASA photo. |
Observation data
Epoch J2000 |
| 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 |
| B-V color index |
0.60 |
| U-B color index |
0.38 |
| 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 |
431 ly (132 pc) |
| Absolute magnitude (MV) |
-3.64 |
| Details |
| Mass |
6 M☉ |
| Radius |
30 R☉ |
| Luminosity |
2200 L☉ |
| Temperature |
7000 K |
| Metallicity |
? |
| Rotation |
~17 km/s |
| Age |
? years |
| Other designations |
Polaris, Cynosura, Alruccabah, Phoenice, Lodestar, Pole Star, Tramontana, Angel Stern, Navigatoria, Star of Arcady, Yilduz, Mismar, Polyarnaya, 1 Ursae Minoris, HR 424, BD +88°8, HD 8890, SAO 308, FK5 907, GC 2243, ADS 1477, CCDM 02319+8915, HIP 11767. |
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 (2006 only 42'), making it the current northern pole star.
Name and star physics
"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".
Polaris is a really giant but very distant star (431 lightyears) and dominant part of a trinary system: a large yellow Cepheid variable (α UMi A) is orbited distantly (about 2700 AU or 380 billion km) by a bright yellow dwarf (α UMi B) and by a close other dwarf. Polaris B can be seen with even a modest telescope and was first noticed by William Herschel in 1780. In 1929 a small doppler shift in the spectrum of Polaris A was discovered, caused by a very close 2nd 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.
Triple star with strange variations
The Polaris triple star 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 F3 V main sequence star, orbiting in 2700 AU distance, and C a very close dwarf on a 20 AU orbit.
The main star is a Population II star (typical for the "old" galactic halo) and a cepheid variable, the pulsations of which cause it to cycle steadily. Around [[1900], the star varied ± 8% relating to the average luminosity (0.15 magnitudes in total) with a 3.97 day period. As of 2005, the variations are about 2% from peak to trough. Further the star is about 15% brighter (on average) than it was in 1900, 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".
Polaris and the North Pole
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.
Although Shakespeare wrote "I am as constant as the northern star", Polaris will not always be the pole star. This is due to precession of the equinox over thousands of years. Other stars have been the pole star in the past and will be again in the future, including Thuban and Vega.
Polaris is due to become an even more accurate pole star in the near future — the distance between Polaris and the pole will reach a minimum (of just under 1/2 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 a position high up 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 46th 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.
Polaris in fiction
In astrology Polaris is one of the Behenian fixed stars and has the symbol 
.
Polaris' role as the North Star (a signifance attached to it not least by somewhat outdated thinking) has led to it figuring in fiction as well. Among other mentions, Polaris, in DC Comics, is the star around which Thanagar — home planet of the Hawks — orbits.
External links
