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 A pole star or polar star is a star, preferably bright, nearly aligned with the axis of a rotating astronomical body. Currently, Earth's pole stars are Polaris (Alpha Ursae Minoris), a bright magnitude-2 star aligned approximately with its ...

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 Apparent magnitude () is a measure of the brightness of a star or other astronomical object observed from Earth. An object's apparent magnitude depends on its intrinsic luminosity, its distance from Earth, and any extinction of the object's ...

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.

Nomenclature

''α Draconis'' ( Latinised to ''Alpha Draconis'') is the star's Bayer designation. The traditional name ''Thuban'' is derived from the

Arabic Arabic (, ' ; , ' or ) is a Semitic language spoken primarily across the Arab world.Semitic languages: an international handbook / edited by Stefan Weninger; in collaboration with Geoffrey Khan, Michael P. Streck, Janet C. E.Watson; Walter ...

word ' ('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 The International Astronomical Union (IAU; french: link=yes, Union astronomique internationale, UAI) is a nongovernmental organisation with the objective of advancing astronomy in all aspects, including promoting astronomical research, outreac ...

organized a

Working Group on Star Names The International Astronomical Union (IAU) established a Working Group on Star Names (WGSN) in May 2016 to catalog and standardize proper names for stars for the international astronomical community. It operates under Division C – Education ...

(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, (), meaning '' Right Wall of Purple Forbidden Enclosure'', refers to an asterism consisting of Alpha Draconis,

Kappa Draconis 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 ...

Lambda Draconis Lambda Draconis (λ Draconis, abbreviated Lam Dra, λ Dra), 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 ...

, 24 Ursae Majoris,

43 Camelopardalis 43 Camelopardalis is a single star in the northern circumpolar constellation of Camelopardalis, located roughly 1,060 light years away from the Sun based on parallax. It is visible to the naked eye as a faint, blue-white hued star wi ...

Alpha Camelopardalis 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 constell ...

and

BK Camelopardalis BK Camelopardalis is a 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 ma ...

. Consequently, the

Chinese name Chinese names or Chinese personal names are names used by individuals from Greater China and other parts of the Chinese-speaking world throughout East and Southeast Asia (ESEA). In addition, many names used in Japan, Korea and Vietnam are ofte ...

for Alpha Draconis itself is (, en, the First Star of Right Wall of Purple Forbidden Enclosure), representing (), meaning ''Right Pivot''. 右樞 (Yòushū) is westernized into ''Yu Choo'' by R.H. Allen with the same meaning.

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 The Big Dipper ( US, Canada) or the Plough ( UK, Ireland) is a large asterism consisting of seven bright stars of the constellation Ursa Major; six of them are of second magnitude and one, Megrez (δ), of third magnitude. Four define a "bowl" ...

(aka the Plough) asterism of

Ursa Major Ursa Major (; also known as the Great Bear) is a constellation in the northern sky, whose associated mythology likely dates back into prehistory. Its Latin name means "greater (or larger) bear," referring to and contrasting it with nearby Ursa ...

. While it is well known that the two outer stars of the 'dipper' point to the modern-day

Polaris, it is less well known that the two inner stars,

Phecda Gamma Ursae Majoris (γ Ursae Majoris, abbreviated Gamma UMa, γ UMa), formally named Phecda , is a star in the constellation of Ursa Major. Since 1943, the spectrum of this star has served as one of the stable anchor poi ...

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

Due to the

precession Precession is a change in the orientation of the rotational axis of a rotating body. In an appropriate reference frame it can be defined as a change in the first Euler angle, whereas the third Euler angle defines the rotation itself. In oth ...

Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...

's rotational axis, Thuban was the naked-eye star closest to the

north pole The North Pole, also known as the Geographic North Pole or Terrestrial North Pole, is the point in the Northern Hemisphere where the Earth's rotation, Earth's axis of rotation meets its surface. It is called the True North Pole to distingu ...

from 3942 BC, when it superseded Tau Herculis as the pole star, until 1793 BC, when it was superseded by

. 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 Beta Ursae Minoris (β Ursae Minoris, abbreviated β UMi, Beta UMi), formally named Kochab , is the brightest star in the bowl of the Little Dipper asterism (which is part of the constellation of Ursa Minor), and only slightly fainter tha ...

(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 , RA . 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 , at right ascension .

Binary system

Thuban is a single-lined spectroscopic binary. For a long time, only the primary star could be detected in the

spectrum A spectrum (plural ''spectra'' or ''spectrums'') is a condition that is not limited to a specific set of values but can vary, without gaps, across a continuum. The word was first used scientifically in optics to describe the rainbow of colors ...

. 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 In astronomy, the main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Hert ...

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 Navy Precision Optical Interferometer (NPOI) is an American astronomical interferometer, with the world's largest baselines, operated by the Naval Observatory Flagstaff Station (NOFS) in collaboration with the Naval Research Laboratory (NRL) ...

. 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 A minute of arc, arcminute (arcmin), arc minute, or minute arc, denoted by the symbol , is a unit of angular measurement equal to of one degree. Since one degree is of a turn (or complete rotation), one minute of arc is of a turn. The n ...

. 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 A binary star is a system of two stars that are gravitationally bound to and in orbit around each other. Binary stars in the night sky that are seen as a single object to the naked eye are often resolved using a telescope as separate stars, in wh ...

Properties

Thuban has a

spectral class In astronomy, stellar classification is the classification of stars based on their spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a prism or diffraction grating into a spectrum exhibiting the ...

of A0III, indicating its similarity to Vega in temperature and

, 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 Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic ...

fusion Fusion, or synthesis, is the process of combining two or more distinct entities into a new whole. Fusion may also refer to: Science and technology Physics *Nuclear fusion, multiple atomic nuclei combining to form one or more different atomic nucl ...

in its core. That makes it a white

giant star A giant star is a star with substantially larger radius and luminosity than a main-sequence (or ''dwarf'') star of the same surface temperature.Giant star, entry in ''Astronomy Encyclopedia'', ed. Patrick Moore, New York: Oxford University Press ...

, 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.

Conspiracy theory

According to British conspiracy theorist

David Icke David Vaughan Icke (; born 29 April 1952) is an English conspiracy theorist and a former footballer and sports broadcaster. He has written over 20 books, self-published since the mid-1990s, and spoken in more than 25 countries. In 1990, Icke ...

, Alpha Draconis is the origin of blood-drinking, shape-shifting reptilians who lurk in underground bases and plot against humanity (with the aid of powerful figures including royalty).

References

External links

* Jim Kaler's Stars, University of Illinois
THUBAN (Alpha Draconis)
* NASA's

*

{{DEFAULTSORT:Thuban A-type giants Suspected variables Spectroscopic binaries Draco (constellation) Draconis, Alpha Durchmusterung objects Draconis, 11 123299 068756 5291 Northern pole stars Stars named from the Arabic language