Thuban (Alpha Draconis)
Thuban (Alpha Draconis) is a star in the constellation Draco, magnitude 3.7.
The pole star of the pyramid builders — a modest star in the Dragon that will guide the north again in AD 20,346.
When the pyramids of Giza were built, the sky turned around Thuban. Earth’s axis slowly wobbles, and from 3942 BC until 1793 BC this star in Draco was the naked-eye star nearest the north celestial pole. In 2830 BC it sat less than a sixth of a degree from the pole — closer than Polaris is today.
The wobble has since carried the pole away towards Polaris, leaving Thuban a modest star in the Dragon’s tail. But the cycle turns: in AD 20,346 Thuban will again be the pole star. Its name comes from the Arabic thu‘bān, a great serpent. Chinese astronomers called it Yòushū, the Right Pivot — fitting for a star the sky once turned upon.
Thuban is a white giant, about 120 times as luminous as the Sun, that has stopped fusing hydrogen in its core. A fainter partner circles it every 51 days, and NASA’s TESS satellite has caught the two eclipsing each other.
Thuban (Alpha Draconis) is 300 light-years away: the light we see from it set off around 1725, 40 years after Isaac Newton published his laws of motion and gravity.
Did you know?
- Bayer’s puzzling Alpha. Johann Bayer made Thuban the Alpha of Draco in 1603, yet today it is 3.7 times fainter than Eltanin, the Dragon’s brightest star.
- A standard white giant. Astronomers use Thuban as a reference: it is a standard star for the A0 III spectral type, against which other white giants are compared.
- Seen at last. Thuban’s partner sits just a few thousandths of an arcsecond from it. The Navy Precision Optical Interferometer finally detected it directly.
A modest white 4th-magnitude star in the Dragon’s tail, roughly halfway between Mizar and Kochab. Or follow Phecda through Megrez, the inner stars of the Plough’s bowl, about 15 degrees onward. It is hard to find from light-polluted skies.
When and where to see it
It is best placed in the evening sky around May, when it crosses the meridian at about 10 pm local time (around midnight in April). It can be seen from everywhere north of 26°S and never sets north of 26°N.
At its highest it stands 76° up from 50°N, 61° up from 35°N, 26° up from the equator, below the horizon from 35°S.
Facts
- Type
- Star
- Constellation
- Draco
- Designations
- Alpha Draconis, α Dra, 11 Dra, HR 5291, HD 123299, HIP 68756
- Magnitude
- 3.7
- Position
- RA 14h 4.4m, Dec +64° 23′ (J2000)
- Spectral type
- A0III
- Colour
- white (B−V -0.04)
- Distance
- 300 light-years
- Discovered
- Known since antiquity
Open AstroPilot to see when Thuban (Alpha Draconis) is highest tonight from your location, how easy it is with your telescope or binoculars, and an eyepiece view.