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Thuban, α Dra
Draco constellation map.svg
Red circle.svg
Location of Thuban (circled)
Observation data
Epoch J2000       Equinox J2000
Constellation Draco
Right ascension  14h 04m 23.3498s
Declination 64° 22 33.062
Apparent magnitude  (V)3.6452
Spectral type A0III
U−B color index -0.08
B−V color index -0.04
Variable type suspected Maia [1]
Radial velocity (Rv)-13.0 km/s
Proper motion (μ)RA: -56.34  mas/yr
Dec.: 17.21  mas/yr
Parallax (π)10.76 ± 0.17  mas
Distance 303 ± 5  ly
(93 ± 1  pc)
Absolute magnitude  (MV)1.20 [2]
Period (P)51.4167 days [3]
Semi-major axis (a)0.46 AU [1]
Eccentricity (e)0.4355 ± 0.0042 [3]
Inclination (i)90 [1] °
Semi-amplitude (K1)
47.9340 ± 0.2990 [3] km/s
α Dra A
Mass 2.8 [1]   M
Radius 3.4 [4]   R
Luminosity 479 [5]   L
Surface gravity (log g)3.5 [6]   cgs
Temperature 10,100 [5]   K
Metallicity 0.20 [6]
Rotation 26.2 [6]
α Dra B
Mass 2.6 [3]   M
Luminosity40 [3]   L
Other designations
Alpha Draconis, GSC  04174-01262, 2MASS  J14042335+6422331, 11 Dra, HD  123299, AG+64° 666, PLX  3209, TYC  4174-1262-1, BD+65° 978, FK5  521, HIP  68756, PPM  18861, GC  19019, HR  5291, IRAS  14030+6436, SAO  16273
Database references

Thuban /ˈθjbæn/ , [7] designation Alpha Draconis (α Draconis, abbreviated Alpha Dra, α Dra), is a star (or star system) in the constellation of Draco. A relatively inconspicuous star in the night sky of the Northern Hemisphere, it is historically significant as having been the north pole star from the 4th to 2nd millennium BCE.


Even though Johann Bayer gave Thuban the designation Alpha, its apparent magnitude of 3.65 means it is 3.7 times fainter than the brightest star in the constellation, Gamma Draconis (Eltanin), whose apparent magnitude is 2.24.


α Draconis (Latinised to Alpha Draconis) is the star's Bayer designation.

The traditional name Thuban is derived from the Arabic word ثعبانthuʿbān, 'large snake' (e.g. a python or a legendary draconian serpent). It is sometimes known as the Dragon's Tail and as Adib /əˈdb/ . [7] In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN) [8] to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016 [9] included a table of the first two batches of names approved by the WGSN; which included Thuban for this star. It is now so entered in the IAU Catalog of Star Names. [10]

In Chinese, 紫微右垣 (Zǐ Wēi Yòu Yuán), meaning Right Wall of Purple Forbidden Enclosure , refers to an asterism consisting of Alpha Draconis, Kappa Draconis, Lambda Draconis, 24 Ursae Majoris, 43 Camelopardalis, Alpha Camelopardalis and BK Camelopardalis. [11] Consequently, the Chinese name for Alpha Draconis itself is 紫微右垣一 (Zǐ Wēi Yòu Yuán yī, English: the First Star of Right Wall of Purple Forbidden Enclosure.), [12] representing 右樞 (Yòushū), meaning Right Pivot. [13] 右樞 (Yòushū) is westernized into Yu Choo by R.H. Allen with the same meaning. [14]


Given good viewing conditions, Thuban is relatively easy to spot in the night sky, due to its location in relation to the Big Dipper (aka the Plough) asterism of Ursa Major. While it is well known that the two outer stars of the 'dipper' point to the modern-day pole star Polaris, it is less well known that the two inner stars, Phecda and Megrez, point to Thuban, just 15 degrees of arc from Megrez. Thuban is not bright enough to be viewed from badly light-polluted areas.

Pole star

Precession of the equinoxes for the Pole Star. Thuban is toward the right of the image, below the -2000 mark. Precession N.gif
Precession of the equinoxes for the Pole Star. Thuban is toward the right of the image, below the -2000 mark.

Due to the precession of Earth's rotational axis, Thuban was the naked-eye star closest to the north pole from 3942 BCE, when it superseded Iota Draconis as the Pole Star, until 1793 BCE, when it was superseded by Kappa Draconis. It was closest to the pole in 2830 BCE, when it was less than ten arc-minutes away from the pole. [15] It remained within one degree of celestial north for nearly 200 years afterwards, and even 900 years after its closest approach, was just five degrees off the pole. Thuban was considered the pole star until about 1800 BCE, when the much brighter Beta Ursae Minoris (Kochab) began to approach the pole as well.

Having gradually drifted away from the pole over the last 4,800 years, Thuban is now seen in the night sky at a declination of 64° 20' 45.6", RA 14h 04m 33.58s. After moving nearly 47 degrees off the pole by 10000 CE, Thuban will gradually move back toward the north celestial pole. In 20346 CE, it will again be the pole star, that year reaching a maximum declination of 88° 43 17.3, at right ascension  19h 08m 54.17s.[ citation needed ]

Preceded by Pole Star Succeeded by
Iota Draconis c. 3900–1800 BCE Kochab

Binary system

Thuban is a single-lined spectroscopic binary. Only the primary star can be detected in the spectrum. The radial velocity variations of the primary can be measured and the pair have a somewhat eccentric orbit of 51.4 days. Making some assumptions based on the faintness of the secondary, the stars are likely to be about 0.46 astronomical units apart and the secondary is a little less massive than the primary. [1] The secondary is likely to be a main sequence star slightly cooler than the primary, possibly an A2 spectral class. [3]

The secondary star was detected in high spatial resolution observations using the Navy Precision Optical Interferometer. The secondary star is 1.8 magnitudes (at 700 nm) fainter than the primary star and was detected at separations ranging from 6.2 to 2.6 milli-arcseconds. [16] Eclipses were detected using data obtained with the Transiting Exoplanet Survey Satellite (TESS). [17] The presence of eclipses places Thuban into the class of binaries known as eclipsing binaries.


Thuban has a spectral class of A0III, indicating its similarity to Vega in temperature and spectrum, but more luminous and more massive. It has been used as an MK spectral standard for the A0III type. [18]

Thuban is not a main sequence star; it has now ceased hydrogen fusion in its core. That makes it a white giant star, being 120 times more luminous than the Sun. It is 300 light years away and its brightness is only decreased by 0.003 magnitudes by intervening gas and dust. [2]

The United States Navy ship USS Thuban (AKA-19) was built in 1943 and decommissioned in 1967.

Thuban has featured in works of fiction.

Related Research Articles

Spica Star in the constellation Virgo

Spica, designated α Virginis, is the brightest object in the constellation Virgo and one of the 20 brightest stars in the night sky. Analysis of its parallax shows that it is located 250 ± 10 light years from the Sun. It is a spectroscopic binary star and rotating ellipsoidal variable; a system whose two stars are so close together they are egg-shaped rather than spherical, and can only be separated by their spectra. The primary is a blue giant and a variable star of the Beta Cephei type.

Acrux star in the constellation Crux

Acrux, designated α Crucis is a multiple star system 321 light-years from the Sun in the constellation of Crux and part of the asterism known as the Southern Cross. With a combined visual magnitude of +0.76, it is the brightest object in Crux and the 13th brightest star in the night sky. It is the southernmost first-magnitude star, 2.3 degrees more southerly than Alpha Centauri.

Achernar Star in the constellation Eridanus

Achernar is the primary component of the binary system designated Alpha Eridani, which is the brightest star in the constellation of Eridanus, and the ninth-brightest in the night sky. The two components are designated Alpha Eridani A and B. As determined by the Hipparcos astrometry satellite, it is approximately 139 light-years (43 pc) from the Sun.

Castor (star) star in the constellation Gemini

Castor is the second-brightest object in the zodiac constellation of Gemini and one of the brightest stars in the night sky. It has the Bayer designation α Geminorum, which is Latinised to Alpha Geminorum and abbreviated Alpha Gem or α Gem. It appears singular to the naked eye, but it is actually a sextuple star system organized into three binary pairs, made up of the stars Castor Aa, Castor Ab, Castor Ba, Castor Bb, Castor Ca, and Castor Cb. Although it is the 'α' (alpha) member of the constellation, it is fainter than 'β' (beta) Geminorum, Pollux.

Gamma Cephei Binary star in the constellation Cephei

Gamma Cephei is a binary star system approximately 45 light-years away in the constellation of Cepheus. The primary is a stellar class K1 orange giant or subgiant star; it has a red dwarf companion. An exoplanet has been confirmed to be orbiting the primary.

Alpha Librae star in the constellation Libra

Alpha Librae, is a double star and despite its 'alpha' designation the second-brightest star system in the constellation of Libra. The two components are designated α¹ Librae and α² Librae. The system bore the traditional name of Zubenelgenubi, though the International Astronomical Union now regards that name as only applying to α² Librae.

Alpha Comae Berenices star in the constellation Coma Berenices

Alpha Comae Berenices is a binary star in the constellation of Coma Berenices, 17.8 parsecs (58 ly) away. It consists of two main sequence stars, each a little hotter and more luminous than the Sun.

Alpha Pavonis star in the southern constellation Pavo

Alpha Pavonis, formally named Peacock, is a binary star in the southern constellation of Pavo, near the border with the constellation Telescopium.

Alpha Piscium variable star

Alpha Piscium (α Piscium) is a binary star system in the equatorial constellation of Pisces. Based upon parallax measurements made by the Hipparcos spacecraft, it is located approximately 311 light-years from the Sun.

Kappa Velorum Binary star in the constellation Vela

Kappa Velorum is a binary star system in the southern constellation of Vela. The two components are designated Kappa Velorum A and B.

Eta Boötis Star in the constellation Boötes

Eta Boötis is a binary star in the constellation of Boötes. Based on parallax measurements obtained during the Hipparcos mission, it is approximately 37 light-years distant from the Sun. Since 1943, the spectrum of this star has served as one of the stable anchor points by which other stars are classified. It forms a double star with the star BD+19 2726.

Alpha Corvi, also named Alchiba, is an F-type main-sequence star and the fifth-brightest star in the constellation of Corvus. Based on parallax measurements made during the Hipparcos mission, it is approximately 49 light-years from the Sun.

Beta Cephei star in the constellation of Cepheus

Beta Cephei is a triple star system of the third magnitude in the constellation of Cepheus. Based on parallax measurements obtained during the Hipparcos mission, it is approximately 690 light-years distant from the Sun. It is the prototype of the Beta Cephei variable stars.

Lambda Draconis star

Lambda Draconis, also named Giausar, is a solitary, orange-red star in the northern circumpolar constellation of Draco. It is visible to the naked eye with an apparent visual magnitude of +3.85. Based upon an annual parallax shift of 9.79 mas as seen from the Earth, the star is located around 333 light years from the Sun.

Alpha Equulei star in the constellation Equuleus

Alpha Equulei, officially named Kitalpha, is a star in the constellation of Equuleus. It is a high proper motion star only 190 light years away.

Kappa Draconis Star in the constellation Draco

Kappa Draconis, Latinized from κ Draconis, is a blue giant star located in the northern circumpolar constellation of Draco. At an apparent magnitude of 3.88, it is barely visible to the naked eye when artificial lighting from cities is present. Nevertheless, it is a powerful star, approximately five time as massive as the Sun. It is about 490 light-years away, and is 1,400 times brighter than the Sun.

Alpha Camelopardalis star

Alpha Camelopardalis is a star in the constellation Camelopardalis, with an apparent visual magnitude of 4.3. It is the third-brightest star in this not-very-prominent circumpolar constellation, the first and second-brightest stars being β Camelopardalis and CS Camelopardalis, respectively. It is the farthest constellational star, with a distance of 6,000 light-years from Earth.

Delta Cygni third-magnitude star in the constellation Cygnus

Delta Cygni is a binary star of a combined third-magnitude in the constellation of Cygnus. It is also part of the Northern Cross asterism whose brightest star is Deneb. Based upon parallax measurements obtained during the Hipparcos mission, Delta Cygni is located roughly 165 light-years distant from the Sun.

BK Camelopardalis star

BK Camelopardalis is a solitary variable star in the northern circumpolar constellation of Camelopardalis, near the constellation border with Cassiopeia. It is visible to the naked eye as a faint, blue-white hued star with an apparent visual magnitude that fluctuates around 4.74. The star is located approximately 540 light years away from the Sun based on parallax. It is a proposed member of the Cassiopeia–Taurus group of co-moving stars.

Phi Draconis variable star

Phi Draconis is a fourth-magnitude variable star in the constellation Draco. It has the Flamsteed designation 43 Draconis. It is also a triple star system where the brightest component is a chemically peculiar Ap star.


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Coordinates: Celestia.png 14h 04m 23.3498s, +64° 22′ 33.062″