# Pole Star

Triple
Star Pole Star (α UMi)
Finding the North Star
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Position of the North Star
Observation
data epoch : J2000.0
Constellation Little Bear
Vis. Brightness
(total)
1.97 likes
Astrometry
Radial velocity −17 km / s
parallax  (7.56 ± 0.48) mas
distance  (431 ± 27) ly
((132 ± 8) pc )
Absolute visual
brightness (total)
M vis
−3.6 mag
Proper movement
Rec. Share: 44.22 mas / a
Dec. portion: −11.74 mas / a
Individual data
Names Aa , Ab , B
Observation data:
Right ascension Aa 2 h 31 m 49 s
From
B. 2 h 30 m 42 s
declination Aa + 89 ° 15 ′ 51 ″
From
B. + 89 ° 15 ′ 38 ″
Apparent
brightness
Aa 2.0 m
From approx. 9.2 m
B. 8.6 m
Typing:
Spectral class Aa F7 Ib-IIv
From F6 V
B. F3 V
BV color index Aa 0.60
From
B. 0.42
UB color index Aa 0.38
From
B. 0.01
Astrometry:
Absolute
visual
brightness
M vis
Aa like
From like
B. approx. +3 mag
Physical Properties:
Dimensions Aa 4.5 +2.2 -1.4 M
From 1.26 +0.14 -0.07 M
B. approx. 1.4 M
From R
B. approx. 1.4 R
Luminosity Aa approx. 2500 L
From approx. 3 L
B. approx. 3.9 L
Effective temperature Aa K
From K
B. approx. 6900 K
Designations and catalog entries
Bayer-Bez. α UMi
Flamsteed-Bez. 1 UMi
Bright Star Cat. HR 424
HD catalog HD 8890
Tycho catalog TYC 4628-237-1
Hipparcos catalog HIP 11767
Other
names
Polaris, FK5 907
Bonn diam. Aa BD + 88 ° 8
From BD + 88 ° 8
B. BD + 88 ° 7
SAO catalog Aa SAO 308
From SAO 308
B. SAO 305
The Pole Star (α UMi A) is a Cepheid with a very small amplitude.

The Pole Star is the brightest star in the constellation Kleiner Bär (also popularly known in German as the Kleiner Wagen ). Since its apparent magnitude of 2 mag is relatively high and it is close to the north pole of the sky , it is a suitable means of determining true north .

## Surname

A large number of names have been passed down for this star, which reflects its great importance in the most diverse cultures: Stella Polaris or just Polaris and North Star ; the ancient Greeks called it Phoenice ("the Phoenician"), other names are fishing star , Cynosaura , Cynosura (Greek Κυνόσουρα "tail of the dog"), Lodestar , Mismar , Navigatoria , Tramontana , Çulpan and Poljarnaja . Another name for the polar star is also the north polar star.

The systematic name in astronomy is α Ursae Minoris (Alpha Ursae Minoris) or α UMi for short.

The IAU Working Group on Star Names (WGSN) defined the proper name Polaris as the standardized proper name for this star on June 30, 2016 .

## Triple star system

Polaris is a visual double star . The companion with the apparent magnitude of 9 mag at a distance of 18.4 ″ was discovered in 1780 by Wilhelm Herschel . The main star itself is again double, which was only able to be resolved optically in 2006 with the help of the Hubble space telescope. The two components have an angular distance of 0.17 arc seconds.

For some time the main star was considered to be a pulsation-variable star of the Cepheid class with a brightness fluctuation between 1.92 mag to 2.07 mag and a period of 3.9 days. However, since around 1980 these fluctuations have slowly subsided and are now only around 0.01 to 0.02 mag. The North Star is moving away from the earth at a speed of 17 km / s. Until its variability was discovered , it also served as a reference star (zero point) for the scale of apparent brightness.

The components of close binary star are systematically as Polaris Aa and Polaris Ab denotes the companion as Polaris B . The main star of the system, Polaris Aa, is a supergiant and about 2000 times brighter than the sun. It is around 430 light years away from us. Polaris Ab is a dwarf star that orbits Polaris Aa in a retrograde orbit at a distance of about 20 AU (3 billion kilometers, about the distance of the planet Uranus from the Sun) within about 30 years . Two other potential companions 43 "(C) and 83" (D) apart are very likely not part of the star system.

## Polar star data

The components of Alpha Ursae Minoris

The North Star is currently only about 0.7 ° from the northern celestial pole and is therefore visible all year round in the northern hemisphere ( circumpolar ), but never in the southern hemisphere . Due to its proximity to the pole, it has long been used as a free-eyed orientation and navigation aid. With its help, you can roughly determine the direction to the geographic North Pole. For observers without a telescope it seems to be in the same place all year round; its height in the sky corresponds roughly to the (northern) degree of latitude on which the observer is located. As a star with an apparent brightness of the second magnitude, however, it does not immediately catch the eye and is by no means, as is often mistakenly assumed, the brightest star in the night sky visible from Earth . The nearest brighter stars are about 30 ° away in the constellations Cassiopeia and Great Bear ; the part of the Great Bear that makes up the Big Dipper is often used to help locate the North Star (see below).

In the southern starry sky there is no star of comparable brightness near the pole; Polaris Australis (σ  Octantis ), a much weaker star with an apparent magnitude of 5 mag, can be seen as the counterpart of the Pole Star.

## Find in the sky

Orientation of the Big Dipper to the horizon around midnight in summer in Germany

The constellation Great Bear or Big Dipper is very clear and can be seen in the sky all year round in northern latitudes. If you lengthen the imaginary connecting line between the two bright rear stars of the Big Dipper (over its "rear axle") by about five times, you get almost directly to the North Star, which is about 1.5 moon in diameter next to the imaginary line.

${\ displaystyle {\ overline {\ alpha _ {UMa} \; \ alpha _ {UMi}}} \ quad = \ quad 5 \ cdot {\ overline {\ alpha _ {UMa} \; \ beta _ {UMa}} }}$

Another variant is to draw a line between the first and third drawbar star (depending on the season and observer position) of the Big Dipper and the center star of Cassiopeia . The North Star is roughly in the middle of this connecting line.

With both methods there is the possibility of mistakenly mistaking the Pole Star for one of the two “rear axle stars” of the Little Dipper. On the other hand, it is helpful that a comparably bright star can neither be seen below nor above the Pole Star.

The location of the Big Dipper and Cassiopeia (as well as that of all other constellations) depends, as mentioned, on the season, the time and the latitude of the observer (see also sidereal time ).

The coordinates of the North Star:

## Polarstern in past and future

Apparent movement of the stars around the celestial pole in the immediate vicinity of the North Star

The earth's axis in space is not stable, but performs a slow circular motion . With a period of approx. 25,700 years, the Platonic year , it moves around the pole of the ecliptic , which lies in the constellation of the Dragon . This process is called precession ; it leads to a shift in the celestial poles. Arab astronomers therefore originally considered the star Kochab to be the polar star, and Turkish astronomers Yildun . Around 2800 BC Thuban served as a pole star.

The North Star is currently still moving slightly towards the celestial pole due to this shift in coordinates ; The closest distance to it will be in the year 2102 with 0 ° 28 '31 " 0.4753 °, after which it will slowly move away from it. In about 12,000 years, Vega , the main star in the constellation Lyra , will again become the Pole Star It was already in the Stone Age about 14,000 years ago. ${\ displaystyle \ approx}$

List of the respective polar stars:

Path of the celestial north pole (= north pole of the earth's axis of rotation) around the ecliptic pole. + 2000 = year 2000 of our calendar: The drawbar
star of the Little Dipper fits this time as the Pole Star
Polar stars of the north
approx year Pole Star
12000-11500 BC Chr. Wega (Alpha Lyrae)
10500 BC Chr. 90 Herculis
7500 BC Chr. Tau Herculis
5000 BC Chr. HR 5635 (Bearkeeper constellation)
2900-2795 BC Chr. Thuban (Alpha Draconis)
1300 BC Chr. Thuban (Alpha Draconis), Kochab (Beta UMi), Kappa Draconis
850-1000 HR 4892 , HR 4893 (Giraffe constellation), Kochab (Beta UMi)
2000 Stella Polaris (Alpha UMi)
4000 Errai (Gamma Cephei)
6800 Alkurhah (Zeta Cephei)
7500 Alderamin (Alpha Cephei), Nü Cephei
10800 26 Cygni
11400 Delta Cygni
Path of the south celestial pole (= south pole of the earth's axis of rotation) around the ecliptic pole
Polar stars of the south
approx year Pole Star
11800 BC Chr. Nü Puppis
10950 BC Chr. HR 2203
10500 BC Chr. Eta Columbae
7930 BC Chr. Delta Caeli
3650 BC Chr. Lambda Horologii
3000-2900 BC Chr. Alpha Hydri
2000 BC Chr. Eta Hydri
100-1 BC Chr. Beta Hydri
2000 Sigma Octantis
4000 Delta Chamaeleontis , Gamma Chamaeleontis
5000 I Carinae
5700 Omega Carinae
6750 Ypsilon Carinae
8100 Tureis (Iota Carinae)
9240 Delta Velorum
10800 V Puppis
11200 J Puppis

## Symbolic use

A schematic representation of the position in the sky can also be found on the flag of Alaska .