# Thuban

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This article is about the star. For AMD's Thuban processor core, see [Phenom II](/source/Phenom_II).

**Thuban** (/θjuːbæn/),[1] with [Bayer designation](/source/Bayer_designation) **Alpha Draconis** or **α Draconis**, is a [binary star](/source/Binary_star) system in the [northern](/source/Northern_celestial_hemisphere) [constellation](/source/Constellation) of [Draco](/source/Draco_(constellation)). A relatively inconspicuous star in the night sky of the [Northern Hemisphere](/source/Northern_Hemisphere), it is historically significant as having been the north [pole star](/source/Pole_star) from the 4th to 2nd millennium BC.

[Johann Bayer](/source/Johann_Bayer) gave Thuban the designation [Alpha](/source/Alpha) and placed it as the only member of his *secundae* magnitude class in Draco, although its current [apparent magnitude](/source/Apparent_magnitude) of 3.65 means it is 3.7 times fainter than the brightest star in the constellation, [Gamma Draconis](/source/Gamma_Draconis) (Eltanin), which Bayer placed in his *tertiae* magnitude class although its current apparent magnitude is 2.24.[2]

## Nomenclature

*α Draconis* ([Latinised](/source/Latinisation_of_names) to *Alpha Draconis*) is the star's [Bayer designation](/source/Bayer_designation).

The traditional name *Thuban* is derived from the [Arabic](/source/Arabic) word ثعبان **ṯuʿbān** (*large snake*—*e.g.*, a python or a legendary, draconian serpent). It is sometimes known as the *Dragon's Tail*, and as *Adib* /@diːb/.[1] In 2016, the [International Astronomical Union](/source/International_Astronomical_Union) organized a [Working Group on Star Names](/source/Working_Group_on_Star_Names) (WGSN)[3] to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016[4] 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.[5]

In [Chinese](/source/Chinese_astronomy), 紫微右垣 (*Zǐ Wēi Yòu Yuán*), meaning *[Right Wall of Purple Forbidden Enclosure](/source/Purple_Forbidden_enclosure)*, refers to an [asterism](/source/Asterism_(astronomy)) consisting of Alpha Draconis, [Kappa Draconis](/source/Kappa_Draconis), [Lambda Draconis](/source/Lambda_Draconis), [24 Ursae Majoris](/source/24_Ursae_Majoris), [43 Camelopardalis](/source/43_Camelopardalis), [Alpha Camelopardalis](/source/Alpha_Camelopardalis) and [BK Camelopardalis](/source/BK_Camelopardalis).[6] Consequently, the [Chinese name](/source/Chinese_star_name) for Alpha Draconis itself is 紫微右垣一 (*Zǐ Wēi Yòu Yuán yī*, English: *First Star of Right Wall of Purple Forbidden Enclosure*),[7] representing 右樞 (*Yòushū*), meaning *Right Pivot*.[8]

## Visibility

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

### Pole star

Due to the [precession](/source/Precession_(astronomy)) of [Earth](/source/Earth)'s rotational axis, Thuban was the naked-eye star closest to the [North Pole](/source/North_Pole) from 3942 BC, when it superseded [Tau Herculis](/source/Tau_Herculis) as the pole star, until 1793 BC, when it was superseded by [Kappa Draconis](/source/Kappa_Draconis). It was closest to the pole in 2830 BC, when it was less than 10 [arcminutes](/source/Arcminutes) away from the pole.[9] 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](/source/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](/source/Declination) of , [RA](/source/Right_ascension) . After moving nearly 47° off the pole by [10,000 AD](/source/List_of_future_astronomical_events#After_10,000_AD), Thuban will gradually move back toward the north celestial pole. In [20,346 AD](/source/List_of_future_astronomical_events#After_10,000_AD), it will again be the pole star, that year reaching a maximum declination of , at right ascension .[citation needed]

Preceded by Pole star Succeeded by Tau Herculis c. 3900–1800 BC Kochab

## Binary system

Thuban is a single-lined [spectroscopic binary](/source/Spectroscopic_binary). For a long time, only the primary star could be detected in the [spectrum](/source/Stellar_spectrum). The radial velocity variations of the stars can be measured and the pair have a somewhat eccentric orbit of 51.4 days.[10] The secondary is a [main-sequence star](/source/Main-sequence_star) slightly cooler than the primary, with an A1 spectral class.[10]

The secondary star was detected in high spatial resolution observations using the [Navy Precision Optical Interferometer](/source/Navy_Precision_Optical_Interferometer). The secondary star is 1.8 magnitudes (at 700 [nm](/source/Nanometre)) fainter than the primary star and was detected at separations ranging from 6.2 to 2.6 [milliarcseconds](/source/Milliarcseconds).[11] Eclipses were detected using data obtained with the [Transiting Exoplanet Survey Satellite](/source/Transiting_Exoplanet_Survey_Satellite) (TESS).[12] The presence of eclipses places Thuban into the class of binaries known as [eclipsing binaries](/source/Eclipsing_binaries).[10]

## Properties

Thuban has a [spectral class](/source/Spectral_class) of A0III, indicating its similarity to [Vega](/source/Vega) in temperature and [spectrum](/source/Electromagnetic_spectrum), but more [luminous](/source/Luminosity) and more massive. It has been used as an MK spectral standard for the A0III type.[13]

Thuban is not a main-sequence star; it has now ceased [hydrogen](/source/Hydrogen) [fusion](/source/Nuclear_fusion) in its core. That makes it a white [giant star](/source/Giant_star), being 120 times more [luminous than the Sun](/source/Solar_luminosity). It is 300 light-years away and its brightness is only decreased by 0.003 of a magnitude by intervening gas and dust.[14]

## References

1. Kunitzsch, Paul & Smart, Tim (2006). *A Dictionary of Modern star Names: A Short Guide to 254 Star Names and Their Derivations*. 2nd rev. ed. Cambridge, Massachusetts: Sky Pub. ISBN 978-1-931559-44-7.

1. Johann Bayer (1603). [*Uranometria: Omnivm Asterismorvm Continens Schemata, Nova Methodo Delineata, Aereis Laminis Expressa*](https://books.google.com/books?id=ivZeAAAAcAAJ)

1. ["IAU Working Group on Star Names (WGSN)"](https://www.iau.org/science/scientific_bodies/working_groups/280/). Retrieved 22 May 2016.

1. ["Bulletin of the IAU Working Group on Star Names, No. 1"](http://www.pas.rochester.edu/~emamajek/WGSN/WGSN_bulletin1.pdf). Retrieved 28 July 2016.

1. ["IAU Catalog of Star Names"](http://www.pas.rochester.edu/~emamajek/WGSN/IAU-CSN.txt). Retrieved 28 July 2016.

1. (in Chinese) *中國星座神話*, written by 陳久金. Published by 台灣書房出版有限公司, 2005, ISBN 978-986-7332-25-7.

1. (in Chinese) [香港太空館 - 研究資源 - 亮星中英對照表](http://www.lcsd.gov.hk/CE/Museum/Space/Research/StarName/c_research_chinengstars_t_z.htm) [Archived](https://web.archive.org/web/20100819122914/http://www.lcsd.gov.hk/CE/Museum/Space/Research/StarName/c_research_chinengstars_t_z.htm) 2010-08-19 at the Wayback Machine, Hong Kong Space Museum. Retrieved 2010-11-23.

1. (in Chinese) [English-Chinese Glossary of Chinese Star Regions, Asterisms and Star Name](http://www.lcsd.gov.hk/CE/Museum/Space/Research/StarName/e_research_chinengstarzone_b.htm#PrupleForbiddenEnclosure) [Archived](https://web.archive.org/web/20100810114313/http://www.lcsd.gov.hk/CE/Museum/Space/Research/StarName/e_research_chinengstarzone_b.htm) 2010-08-10 at the Wayback Machine, Hong Kong Space Museum. Retrieved 2010-11-23.

1. Moore, Patrick (1998). [*The Observer's Year*](https://archive.org/details/observersyear36600moor). [doi:10.1007/978-1-4471-3613-2](https://doi.org/10.1007/978-1-4471-3613-2). ISBN 978-3-540-76147-1.

1. Pavlovski, K.; Hummel, C. A.; Tkachenko, A.; Dervişoğlu, A.; Kayhan, C.; Zavala, R. T.; Hutter, D. J.; Tycner, C.; Şahin, T.; Audenaert, J.; Baeyens, R.; Bodensteiner, J.; Bowman, D. M.; Gebruers, S.; Jannsen, N. E.; Mombarg, J. S. G. (2022). "Dynamical parallax, physical parameters, and evolutionary status of the components of the bright eclipsing binary α Draconis". *Astronomy & Astrophysics*. **658**: A92. [arXiv:2111.03887](https://arxiv.org/abs/2111.03887). [Bibcode:2022A&A...658A..92P](https://ui.adsabs.harvard.edu/abs/2022A%26A...658A..92P). [doi:10.1051/0004-6361/202142292](https://doi.org/10.1051/0004-6361/202142292). [S2CID 243847585](https://api.semanticscholar.org/CorpusID:243847585)

1. Hutter, D. J.; Zavala, R. T.; Tycner, C.; Benson, J. A.; Hummel, C. A.; Sanborn, J.; Franz, O. G.; Johnston, K. J. (2016-11-01). "Surveying the Bright Stars by Optical Interferometry. I. A Search for Multiplicity among Stars of Spectral Types F-K". *The Astrophysical Journal Supplement Series*. **227** (1): 4. [arXiv:1609.05254](https://arxiv.org/abs/1609.05254). [Bibcode:2016ApJS..227....4H](https://ui.adsabs.harvard.edu/abs/2016ApJS..227....4H). [doi:10.3847/0067-0049/227/1/4](https://doi.org/10.3847/0067-0049/227/1/4). [ISSN 0067-0049](https://www.worldcat.org/issn/0067-0049). [S2CID 118803592](https://api.semanticscholar.org/CorpusID:118803592)

1. Bedding, Timothy R.; Hey, Daniel R.; Murphy, Simon J. (2019-10-31). "A Dance with Dragons: TESS Reveals α Draconis is a Detached Eclipsing Binary". *Research Notes of the AAS*. **3** (10): 163. [arXiv:1910.12449](https://arxiv.org/abs/1910.12449). [Bibcode:2019RNAAS...3..163B](https://ui.adsabs.harvard.edu/abs/2019RNAAS...3..163B). [doi:10.3847/2515-5172/ab5112](https://doi.org/10.3847/2515-5172/ab5112). [ISSN 2515-5172](https://www.worldcat.org/issn/2515-5172). [S2CID 213224065](https://api.semanticscholar.org/CorpusID:213224065)

1. Morgan, W. W.; Abt, Helmut A.; Tapscott, J. W. (1978). "Revised MK Spectral Atlas for stars earlier than the sun". *Williams Bay: Yerkes Observatory*. [Bibcode:1978rmsa.book.....M](https://ui.adsabs.harvard.edu/abs/1978rmsa.book.....M)

1. Murphy, Simon J. & Paunzen, Ernst (2017). "Gaia's view of the λ Boo star puzzle". *Monthly Notices of the Royal Astronomical Society*. **466** (1): 546. [arXiv:1612.01528](https://arxiv.org/abs/1612.01528). [Bibcode:2017MNRAS.466..546M](https://ui.adsabs.harvard.edu/abs/2017MNRAS.466..546M). [doi:10.1093/mnras/stw3141](https://doi.org/10.1093/mnras/stw3141). [S2CID 119328659](https://api.semanticscholar.org/CorpusID:119328659)

## External links

- Jim Kaler's Stars, University of Illinois: [THUBAN (Alpha Draconis)](http://stars.astro.illinois.edu/sow/thuban.html)
- NASA's: [History of Precession](http://www-istp.gsfc.nasa.gov/stargaze/Sprecess.htm)
- : [Precession of the Equinoxes](http://www.crystalinks.com/precession.html)

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Adapted from the Wikipedia article [Thuban](https://en.wikipedia.org/wiki/Thuban) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Thuban?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.