Thuban (α Draconis) Worksheets

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Key Facts & Information

  • Thuban (α Draconis) is a double star system located in the constellation of Draco. It is a relatively inconspicuous star in the night sky of the Northern Hemisphere, located around 303 light years away from our sun. Alpha Draconis is historically significant as it was the north pole star from the 4th to the 2nd millennium BCE. Although Thuban is designated as the alpha star, it is not the brightest star in its constellation and is 3.7 times fainter than the brightest star Eltanin (Gamma Draconis).
  • Thuban is a single-lined spectroscopic binary in which the primary star is the only star that can be detected in the spectrum.
  • The two stars also form an eclipsing binary.
  • Alpha Draconis is suspected to be a Maia variable star.
  • Thuban has slowly drifted away from the true north over the last 4,800 years and is one of the faintest pole stars.
  • It is suggested that Thuban will become once again the North Star in 20346 CE.
  • Alpha Draconis will continue to exist for many more millions of years and might become a blue giant star.

Nomenclature

  • Alpha Draconis is the star’s Bayer designation while 11 Draconis its flamsteed designation.
  • The traditional name Thuban derives from Arabic which means “large snake”.
  • It is named after the Dragon’s tail due to its position in the constellation of Draco and is sometimes known as Adib.
  • The International Astronomical Union (IAU) organized a Working Group on Star Names which approved the name Thuban for the star and it is now listed in the IAU Catalog of Star Names.
  • Thuban also has other designated stellar names: GSC 04174-01262, 2MASS J14042335+6422331, 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.
  • In Chinese, Thuban is known as the First Star of the Right Wall of Purple Forbidden Enclosure, which is an asterism formed by Thuban (Alpha Draconis), Kappa Draconis, Giausar (Lambda Draconis), 24 Ursae Majoris, Alpha Camelopardalis, 43 Camelopardalis and BK Camelopardalis.
  • The said asterism is located around the north celestial pole in the sky and represents the imperial palace.
  • The star also represents the Right Pivot and is called “You Shu”.

Properties

  • Thuban is a double star system around 303 light years away from the sun.
  • The primary star is more massive and several times bigger than our sun.
  • Thuban has a mass of around 2.8 solar masses and a radius of around 3.4 solar radii.
  • Based on its radius, this star is estimated to be around six times bigger than the sun.
  • Alpha Draconis is a white giant star with spectral class A0III.
  • Thuban A has been used as an Morgan-Keenan spectral standard for its spectral class.
  • The star has an apparent magnitude of around 3.64 and an absolute  magnitude of -1.20.
  • It is around 479 more luminous than our sun and has an average surface temperature of around 10,100 K, making it 1.7 times hotter than our sun.
  • Thuban has a surface gravity of around 3.5 cgs and a rotation of 26.2.
  • The companion star has a mass of around 2.6 solar masses.
  • Alpha Draconis B is around 40 times more luminous than our sun, thus, 1.83 magnitudes fainter than the primary star.
  • It is suggested that the companion star is a main-sequence star with spectral type Au and is slightly cooler than the primary star.
  • Other information about the companion star is still unknown.

Pole Star

  • Thuban is one of the stars that switch turns as the North Star during the precession cycle of Earth. It was the closest star to the north pole visible to the naked eye.
  • Alpha Draconis was the Pole star from 3942 BCE, when it superseded Iota Draconis, until 1793 BCE, during the creation of some of the largest pyramids in Egypts, when it was superseded by Kappa Draconis.
  • It was closest to the pole in 2830 BCE, the closest among the other pole stars, less than 10 arcminutes away from the pole.
  • Thuban remained within one degree of celestial north for about 200 years afterwards.
  • It was just five degree off the pole 900 years after its closest encounter.
  • Alpha Draconis was one of the faintest pole stars.
  • It was considered as the pole star until around 1800 BCE, when the much brighter Kochab (Beta Ursae Minoris) dethroned it as it began to approach the pole as well.
  • The current north star is Polaris and Thuban was preceded by Edasich (Iota Draconis), and succeeded by Kochab, and Kappa Draconis which is 1 degree closer to the pole compared to Kochab.
  • Thuban has slowly drifted away from the true north over the last 4,800 years.
  • The following are the pole stars and near north stars in decreasing brightness: Vega (Alpha Lyrae), Deneb (Alpha Cygni), Polaris (Alpha Ursae Minoris), Kochab (Beta Ursae Minoris), Alderamin (Alpha Cephei), Fawaris (Delta Cygni), Errai (Gamma Cephei), Edasich (Iota Draconis), Iota Cephei, Alfirk (Beta Cephei), Thuban (Alpha Draconis), Iota Herculis, Kappa Draconis, and Tau Herculis.

Stellar System

  • Thuban star system is single-lined spectroscopic binary system meaning that only the spectral lines of the primary component are visible.
  • The two stars have an orbital period of 51.5 days and an orbital eccentricity of 0.43.
  • The two components are separated from each other by about 0.46 AU.
  • This system is an eclipsing binary system that lasts for only 6 hours.
  • The eclipses are only partial, with an inclination of less than 90 degrees and depths of 9% and 2%.

Formation

  • Thuban was formed from an interstellar cloud of gas and dust.
  • The swirling gas and dust were pulled together by the gravity giving birth to the star.
  • Based on its metallicity, which is about -0.20, the interstellar medium where Thuban was formed was metal-poor.
  • It has ceased hydrogen fusion inside its core and is no longer a main-sequence star.
  • The exact age of Thuban is still unknown.

Draco

  • Thuban is located in the constellation of Draco.
  • The constellation of Draco is the 8th largest constellation in the night sky and the 4th largest northern constellation, with an area of 1,083 square degrees.
  • This constellation never sets and is visible throughout the year for observers in northern latitudes.
  • Thuban is not easily spotted from light-polluted areas.
  • It lies about halfway between Mizar (middle star of Big Dipper’s handle) and Kochab and Pherkad (stars that form the outer side of the Little Dipper’s bowl).
  • Drawing an imaginary line extended from Phecda through Megrez (inner stars of the Big Dipper’s bowl) will point towards Thuban.
  • The 10 brightest stars of the Draco constellation are:
    • Eltanin (Gamma Dra, 2.23 mag)
    • Ethebyne (Eta Dra, 2.74 mag)
    • Restaban (Beta Dra, 2.79 mag)
    • Altais (Delta Dra, 3.07 mag)
    • Aldhibah (Zeta Dra, 3.17 mag)
    • Edasich (Iota Dra, 3.29 mag)
    • Chi Draconis (3.57 mag)
    • Thuban (Alpha Dra, 3.65 mag)
    • Grumium (Xi Dra, 3.75 mag)
    • Epsilon Draconis (3.83 mag)
  • Alpha Draconis is actually just 15 degrees of arc from Megrez.
  • The constellation of Draco is home to a lot of interesting deep sky objects such as the Cat’s Eye Nebula, the exoplanets Kepler-10b, Kepler-10c and TrES-2b, the lenticular galaxy NGC 5866, the disrupted spiral galaxy Tadpole, the massive galaxy cluster Abell 2218, and the Draco Dwarf Galaxy, one of the faintest satellites of our own Milky Way.
  • The best time to observe the celestial objects in the constellation of Draco is during the month of July.
  • February Eta Draconis is a meteor shower that was discovered on 4th February 2011.
  • Observers noted six meteors sharing a common radiant in a short period and its parent is a previously unknown long-period comet.

Popular Culture

  • The USS Thuban (AKA-19) is an andromeda-class attack cargo ship of the United States Navy built in 1943 and decommissioned in 1967, named after the star.
  • Thuban has also featured in many works of fiction, including the following:
    • “Futurama” is an animated science fiction comedy series in 2010 created by Matt Groening.
    • In its episode “That Darn Katz” it is revealed that the domestic house cat is originally from the planet Thuban IX.
    • The “Mass Effect 2” is a 2010 role playing game developed by BioWare and published by the Electronic Arts.
    • Players in this game can explore the Thuban planetary system.
    • “Way Station” is a novel written by Clifford Simak in 1964.
    • The protagonist in the novel (Enoch Wallace) entertains a visitor from the liquid planet Thuban IV.