Epoch J2000 Equinox J2000
|Right ascension||14h 04m 23.3500s|
|Declination||64° 22′ 33.062″|
|Apparent magnitude (V)||3.67|
|Spectral type||A0IV + A1V|
|U−B color index||−0.08|
|B−V color index||−0.049±0.005|
|Variable type||suspected Maia|
|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|
|Period (P)||51.417350 d|
|Semi-major axis (a)||5.52 mas|
|Longitude of the node (Ω)||252.6°|
|Periastron epoch (T)||JD 2453498.8|
| Argument of periastron (ω)|
| Semi-amplitude (K1)|
|α Draconis A|
Alpha Draconis A
|Surface gravity (log g)||3.555 cgs|
|Rotational velocity (v sin i)||25.4 km/s|
|α Draconis B|
Alpha Draconis B
|Surface gravity (log g)||4.090 cgs|
|Rotational velocity (v sin i)||168 km/s|
Thuban ( // ), with Bayer designation Alpha Draconis or α Draconis, is a binary star system in the northern 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 BC.
Johann Bayer gave Thuban the designation Alpha and placed it as the only member of his secundae magnitude class in Draco, although its current apparent magnitude of 3.65 means it is 3.7 times fainter than the brightest star in the constellation, Gamma Draconis (Eltanin), which Bayer placed in his tertiae magnitude class although its current 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 // . In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN) to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016 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.
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. 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), representing 右樞 (Yòushū), meaning Right Pivot. 右樞 (Yòushū) is westernized into Yu Choo by R.H. Allen with the same meaning.
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.
Due to the precession of Earth's rotational axis, Thuban was the naked-eye star closest to the north pole from 3942 BC, when it superseded Tau Herculis as the pole star, until 1793 BC, when it was superseded by Kappa Draconis. It was closest to the pole in 2830 BC, when it was less than 10 arcminutes away from the pole. It remained within one degree of celestial north for nearly 200 years afterwards, and even 900 years after its closest approach, was just 5° off the pole. Thuban was considered the pole star until about 1800 BC, 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 4800 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° off the pole by 10,000 AD, Thuban will gradually move back toward the north celestial pole. In 20,346 AD, 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|
|Tau Herculis||c. 3900–1800 BC||Kochab|
Thuban is a single-lined spectroscopic binary. For a long time, only the primary star could be detected in the spectrum. The radial velocity variations of the stars can be measured and the pair have a somewhat eccentric orbit of 51.4 days.The secondary is a main-sequence star slightly cooler than the primary, with an A1 spectral class.
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 milliarcseconds.Eclipses were detected using data obtained with the Transiting Exoplanet Survey Satellite (TESS). 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.
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 of a magnitude by intervening gas and dust.
Spica, designated α Virginis, is the brightest object in the constellation of 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, designated α Crucis, is a multiple star system 321 light-years from the Sun in the constellation of Crux. It is the most southerly star 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 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 from the Sun.
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 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 is a double star and, despite its 'alpha' designation, it is 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.
Mu Draconis is a multiple star system near the head of the constellation of Draco. With a combined magnitude of 4.92, it is visible to the naked eye. Based on parallax estimates by the Hipparcos spacecraft, it is located approximately 89 light-years from the Sun.
The Cor Caroli system is a binary star designated Alpha Canum Venaticorum or α Canum Venaticorum. The International Astronomical Union uses the name "Cor Caroli" specifically for the brighter star of the binary. Alpha Canum Venaticorum is the brightest point of light in the northern constellation of Canes Venatici.
Alpha Sagittarii, also named Rukbat, is a star in the constellation of Sagittarius.
Epsilon Carinae, officially named Avior, is a binary star in the southern constellation of Carina. At apparent magnitude +1.86 it is one of the brightest stars in the night sky, but is not visible from the northern hemisphere. The False Cross is an asterism formed of Delta Velorum, Kappa Velorum, Iota Carinae and ε Carinae. It is so called because it is sometimes mistaken for the Southern Cross, causing errors in astronavigation.
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 about 151 light-years from the Solar System.
Alpha Delphini is a multiple star system in the constellation of Delphinus.
Delta Velorum is a triple star system in the southern constellation of Vela, near the border with Carina, and is part of the False Cross. Based on parallax measurements, it is approximately 80.6 light-years from the Sun. It is one of the stars that at times lies near the south celestial pole due to precession.
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.
Xi Draconis is a double or binary star in the northern circumpolar constellation of Draco. It has an apparent visual magnitude of 3.75. Based upon parallax measurements, it is located at a distance of 112.5 light-years from the Sun. At this distance, the apparent magnitude is diminished by 0.03 from extinction caused by intervening gas and dust.
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, 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.
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 460 light-years away, and is 1,400 times brighter than the Sun.
Alpha Camelopardalis, Latinized from α Camelopardalis, is a star in the northern constellation of 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 Beta Camelopardalis and CS Camelopardalis, respectively. It is the farthest constellational star, with a distance of approximately 6,000 light-years from Earth based on parallax measurements.
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.