Thuban (Alpha Draconis): The Dragon’s Forgotten North Star

Thuban, Alpha Draconis (α Dra), is a spectroscopic binary star system located in the constellation Draco. Even though it has the Bayer designation Alpha, it is not Draco’s brightest star. The title belongs to Eltanin, while Thuban ranks a modest eighth. With an apparent magnitude of 3.68, it is a relatively inconspicuous star in the Dragon’s tail, easy to miss between the Big Dipper’s handle and the Little Dipper’s bowl. It lies at a distance of 284 light years from Earth.

Thuban once had a much more prominent role. From 3942 BCE to 1793 BCE, when civilization itself was just taking shape, it was the north pole star, the fixed point of light around which the ancient night sky seemed to turn. The Egyptians saw it as more than a navigational marker. As one of the “imperishable stars” that never set, Thuban was believed to steer the soul of pharaohs to their place in the heavens.

Thuban (Alpha Draconis), image credit: ESO/Digitized Sky Survey 2 (CC BY 4.0)

Inside the Thuban star system

The Alpha Draconis star system is one of the brightest detached eclipsing binary stars known, along with Delta Velorum and Gamma Persei.

Composed of two A-type stars much hotter and larger than the Sun, Thuban was previously classified as a single-lined spectroscopic binary system, which means that only the spectral lines of the primary component were visible. The presence of the unseen companion could be inferred from the primary star’s motion toward and away from Earth, signalled by its lines periodically shifting towards the blue and then towards the red. The motion indicated that the star was orbiting around a common centre of mass in a binary system.

Alpha Draconis was traditionally given the spectral type A0III and even served as the spectral standard for this class.

More recent work, notably by Pavlovski et al. in 2022 and follow-up studies, resolved the Thuban system as an eclipsing double-lined spectroscopic binary (SB2). The detection of the secondary component allowed astronomers to classify the two stars as A0IV and A1V, indicating a subgiant and a main sequence star.

Alpha Draconis A and Alpha Draconis B orbit each other with a period of 51.417350 days and an eccentricity of 0.43. The stars are separated by 5.52 milliarcseconds in the sky, corresponding to an average distance of 61 million kilometers. They cannot be resolved by optical telescopes.

Alpha Draconis A: The evolved primary component

The primary component, formally named Thuban and designated Alpha Draconis A, has the stellar classification A0IV, indicating a blue-white subgiant. It has a mass 3.186 times that of the Sun and has expanded to a size of 4.932 solar radii. With an effective temperature of around 10,225 K, the star is on the hotter end of the A0 class. It is 240 times more luminous than the Sun.

Thuban is still a young star, with an estimated age of 280 million years. Pavlovski et al. reported it to be an evolved post-TAMS (Terminal-Age Main Sequence) star with a tiny convective core.

Both stars in the Alpha Draconis system follow the evolutionary path of intermediate-mass stars. Neither is massive enough to go out as a supernova at the end of its life cycle.

Alpha Draconis B: The fast-spinning companion

The companion, Alpha Draconis B, is a blue-white main sequence star of the spectral type A1V. It has a mass of 2.431 solar masses and shines with 47.5 solar luminosities, with a surface temperature of 9,930 K. It has a radius of 2.326 solar radii and spins at 168 km/s. The star is believed to be 345 million years old.

Alpha Draconis B is 1.8 magnitudes fainter than the primary component and accounts for only 17.48% of the system’s total luminosity. It was resolved using interferometry with the Navy Precision Optical Interferometer (NPOI), photometry with the TESS space observatory, and high-resolution spectroscopy with the HERMES spectrograph on the 1.2-m Mercator telescope at the La Palma observatory. The star’s unambiguous detection was reported in 2022.

Thuban’s hidden eclipses: The eclipsing binary discovery

Thuban A and B periodically eclipse each other as they orbit around the common centre of mass, causing the system’s overall brightness to vary.

Alpha Draconis was revealed to be an eclipsing binary system in 2019, based on a survey with the Transiting Exoplanet Survey Satellite (TESS). Bedding et al. reported that the system was a detached eclipsing binary that exhibits primary and secondary eclipses with depths of 9% and 2%, indicating that the eclipses are partial, with an inclination of slightly less than 90°. The eclipses last only six hours.

North pole star at the dawn of history

Thuban is one of the stars that take their turn as the North Star during the Earth’s precession cycle. It last held the title from 3942 BCE to 1793 BCE, spanning the period from the New Stone Age to the end of Egypt’s Middle Kingdom and the early Minoan civilization on the island of Crete. Alpha Draconis was closest to the pole in 2830 BCE.

The star has slowly drifted away from true north over the last 4,800 years but will gradually move back and be the North Star again in 20346 CE.

Path of the northern celestial pole, image credit: Wikimedia Commons/Miraceti, original authors: GregBenson and Wereon (CC BY-SA 3.0)

Most accurate North Star

Of all the pole stars, Thuban comes closest to the north celestial pole – within 0.2° – but it is also fainter than most other pole stars.

In comparison, Polaris, the current North Star, comes within 0.5° of the north celestial pole and has an apparent magnitude of 1.98. Polaris is very useful in navigation because it is both relatively bright and located very close to the pole. Like Thuban, it can be easily identified using the stars of the Big Dipper.

The same cannot be said for the current south pole star, Sigma Octantis (Polaris Australis), which is barely visible at magnitude 5.47 and useless in navigation.

The brightest stars that serve as indicators of true north – Vega (mag. 0.026) and Deneb (mag. 1.25) – only come within 5° and 7° of the pole and are not as accurate markers of true north as Thuban and Polaris. However, as the fifth and 19th brightest stars in the sky, they are much more prominent and easier to identify.

Animation of the cycle of precession of Earth’s axis, depicting the orientation of the axis in relation to the North Ecliptic Pole. Image: Wikimedia Commons/Tfr000 (CC BY-SA 3.0)

Other north pole stars

As the North Star, Thuban was preceded by Edasich (Iota Draconis, mag. 3.29) and succeeded by the brighter Kochab (Beta Ursae Minoris, mag. 2.08), one of the stars of the Little Dipper, and the fainter Kappa Draconis (mag. 3.82). Kochab and Kappa Draconis share timing as near-north stars, coming within 6° and 7° of the north celestial pole.

Other than Polaris, Thuban, Edasich, Vega, Deneb, Kochab and Kappa Draconis, the stars that serve as North Stars over the course of Earth’s 25,770-year precession cycle are Errai (Gamma Cephei), Iota Cephei and Alfirk (Beta Cephei), Alderamin (Alpha Cephei), Fawaris (Delta Cygni), Tianbang (Iota Herculis), and Asuusiha (Tau Herculis).

North Stars (marked red), image (cropped from original): Roberto Mura (CC BY-SA 3.0)

Egyptians and the ancient cult of the pole star

Thuban was the north pole star during the Old Kingdom of Egypt, when the Egyptians built some of their largest pyramids, including the Great Pyramid of Giza. Constructed around 2560 BC as the tomb for the pharaoh Khufu, the Great Pyramid took roughly a quarter of a century to build and remained the world’s tallest human-made structure for over 3,700 years.

The sides of the pyramid are very closely aligned to true cardinal directions, and the northern shaft of the King’s Chamber was calculated to point at Thuban, while the southern shaft pointed at Orion’s Belt. The pyramid’s precise north-south orientation is believed to have been achieved using circumpolar stars in the region of the north celestial pole, including Thuban, the nearest visible star to the pole at the time.

In ancient Egypt, circumpolar stars were called “imperishable stars,” and they were considered particularly important for the pharaoh’s afterlife. The pyramid of Pharaoh Unas (c. 2375–c. 2345 BC) contains the following text:

“The (glorified) humanity bewail you after the Imperishable Stars have carried you. Enter, then, into the place where your father is, where Geb is! He gives you that which was on the brow of Horus, so that you become powerful and full of glory through it, so that you become the one at the head of the Westerners through it. (…) Look at me! You have seen the forms of the children of their fathers, who know their spell, who are now Imperishable Stars.”

For a civilization that associated eternal life with a star that never set, Thuban was more than just a marker of true north. It symbolized a promise of what awaited the pharaoh among the stars.

The name Thuban (pronunciation: /ˈθjuːbæn/) comes from the Arabic word thuʿbān, denoting a large snake. Thuban has also been known as Adib, derived from the Arabic Al Dhi’bah, meaning “the hyenas,” and as the Dragon’s Tail, the latter indicating its position in the constellation Draco.

The origin of the name Thuban is linked with the old name of Eltanin (Gamma Draconis), ra’s al-tinnin (“the serpent’s head”), used during the Arabic Almagest period. In the Middle Ages, the name was transliterated and corrupted and ultimately became the name of Beta Draconis (Rastaban).

In early modern times, the form Rastaben was mistakenly attributed to the Arabic word thu’ban.

The International Astronomical Union’s (IAU) Working Group on Star Names (WGSN) approved the name on June 30, 2016. The name formally applies only to the component Alpha Draconis A.

Alpha Draconis in Chinese lore

The Chinese know Alpha Draconis as 紫微右垣一 (Zǐ Wēi Yòu Yuán yī), the First Star of Right Wall of Purple Forbidden Enclosure. The Chinese asterism called the Right Wall of Purple Forbidden Enclosure is formed by Thuban with Kappa Draconis, Giausar (Lambda Draconis), 24 Ursae Majoris, Alpha Camelopardalis, 43 Camelopardalis, and BK Camelopardalis.

The Purple Forbidden Enclosure is located in the area of the sky around the north celestial pole and it represents the imperial palace. Thuban represents the Right Pivot (右樞,Yòu Shū).

Bayer designation Alpha: Was Thuban once brighter?

Even though it is considerably fainter than Eltanin (Gamma Draconis), Draco’s brightest star, Thuban was given the designation Alpha by the German astronomer Johann Bayer in his star atlas Uranometria (1603).

When assigning the Greek letter designations, the celestial cartographer was guided by the order of magnitude of stars in individual constellations. He listed Thuban as the only second magnitude star in Draco and Eltanin as a third magnitude star. This suggests that the star may have been much brighter than it is today.

Since astronomers have not identified a mechanism that would account for such a change, and Thuban shows no evidence of being a variable star capable of dimming this dramatically over just a few centuries, Bayer’s classification remains an open puzzle.

Thuban in the constellation Draco, “Draco and Ursa Minor”, plate 1 in Urania’s Mirror, a set of celestial cards accompanied by A familiar treatise on astronomy… by Jehoshaphat Aspin. London. (PD)

How to find Thuban

Thuban is easy to identify but challenging to see from light-polluted areas. The star lies about halfway between Mizar, the middle star of the Big Dipper’s handle, and Kochab and Pherkad, the stars that form the outer side of the Little Dipper’s bowl.

An imaginary line extended from Phecda through Megrez, the inner stars of the Big Dipper’s bowl, passes close to Alpha Draconis.

At declination +64°, Thuban is best seen from the northern hemisphere. It never rises for observers south of the latitude 25° S. For most northern observers, the star is circumpolar, i.e. it never sets and can be seen throughout the year. The best time to see it is during the northern hemisphere summer, when Draco appears the highest in the sky.

The location of Thuban (Alpha Draconis), image: Stellarium (annotated for this article)

Draco: The constellation that hosts Alpha Draconis

Thuban lies in the constellation Draco. Draco is the eighth largest constellation in the sky and the fourth largest northern constellation, occupying an area of 1,083 square degrees. However, most of its stars are not particularly conspicuous.

Winding around Ursa Minor and the north celestial pole, Draco is one of the northernmost constellations in the sky and largely invisible from the southern hemisphere. The entire constellation is visible north of the latitude 7° S.

Draco was one of the 48 Greek constellations catalogued by Claudius Ptolemy of Alexandria in his Almagest in the 2nd century CE. In Greek mythology, it is associated with the dragon Ladon from the tale of Heracles’ Twelve Labours. In Greek lore, Ladon was responsible for guarding the golden apples of the Hesperides and was defeated by Heracles in the service of King Eurystheus of Tiryns.

Eltanin (Gamma Draconis), the brightest star of Draco, is an orange giant that shines at magnitude 2.23 from a distance of 154 light-years. It forms the head of Draco with the yellow bright giant Rastaban (Beta Draconis), orange giant Grumium (Xi Draconis), and A-type binary star Kuma (Nu Draconis).

Other bright stars in Draco include the yellow giants Athebyne (Eta Draconis) and Altais (Delta Draconis), the blue giant Aldhibah (Zeta Draconis), the orange giant Edasich (Iota Draconis), and the binary system Chi Draconis.

Draco constellation map by IAU and Sky&Telescope magazine (Roger Sinnott & Rick Fienberg) (CC BY 3.0)

The constellation contains many notable deep sky objects. The best-known ones include the Cat’s Eye Nebula (NGC 6543), the Spindle Galaxy (NGC 5866, Messier 102), the disrupted spiral galaxy Arp 188 (the Tadpole Galaxy), the Splinter Galaxy (NGC 5907), the Dolphin Galaxy (NGC 6670), the Draco Group (Draco Triplet), and the Draco Dwarf, one of the faintest satellites of the Milky Way.

The best time of the year to observe the stars and deep sky objects in Draco is during the month of July, when the constellation appears higher above the horizon in the early evening.

The 10 brightest stars in Draco are Eltanin (Gamma Dra, mag. 2.23), Athebyne (Eta Dra, mag. 2.73), Rastaban (Beta Dra, mag. 2.79), Altais (Delta Dra, mag. 3.07), Aldhibah (Zeta Dra, mag. 3.17), Edasich (Iota Dra, mag. 3.29), Chi Draconis (mag. 3.57), Thuban (Alpha Dra, mag. 3.6452), Grumium (Xi Dra, mag. 3.75), and Giausar (Lambda Dra, mag. 3.85).

Thuban – Alpha Draconis

Alpha Draconis A

Alpha Draconis B