Zero Genie

The ancient pole star known as Thuban turns out to be an eclipsing binary. If you’re up for a challenge, you can see the eclipses for yourself.

The star Thuban, also known as Alpha Draconis, has long been known to be a binary system. Now data from NASA’s TESS show its two stars undergo mutual eclipses.
NASA / MIT / TESS

NASA’s TESS mission has a primary goal of looking for exoplanets. But as it’s examining stars for the subtle dips that mark a planet’s passage in front of its star, it can catch lots of other stellar goings-on, too. At the recent American Astronomical Society meeting in Honolulu, astronomers announced that TESS had uncovered something rather surprising about the well-known double star in Draco named Thuban (also called Alpha Draconis). Turns out, the two stars eclipse each other as seen from Earth.

“The first question that comes to mind is ‘how did we miss this?’” says Angela Kochoska (Villanova University).

When two stars in a binary system eclipse each other, their overall brightness as seen from Earth fades and increases again in a cyclic way. Thuban’s eclipses occur twice every 51.4 days. So even though Thuban isn’t exactly the brightest star in the sky — at magnitude 3.7 it’s the eighth-brightest star in Draco — the discovery at first took astronomers by surprise.

However, the primary eclipse causes a change in brightness of only about 0.1 magnitude; the secondary eclipse is marked by an even smaller dip, 0.02 magnitude. The variations are small in part because the stars don’t fully eclipse each other. What’s more, the eclipses are only six hours long. So perhaps it’s no surprise that the small, brief dips in brightness were missed until now.

Even the space-based Kepler mission didn’t catch the star’s variations. Thuban was actually too bright for Kepler to look at without saturating its detector.

TESS, though, is designed to look at bright, nearby stars. The satellite monitors large swaths of sky for 27 days at a time, splitting each celestial hemispheres into 13 sectors each. Near the poles, these sectors overlap in what’s termed the continuous viewing zone. These long observations with very precise measurements enabled TESS to find the subtle variations in Thuban’s brightness.

Thuban, the Ancient Pole Star

Thuban is famous not for its brightness, but for the role it played in the Egyptian sky some 4,700 years ago: North Star.

Earth precesses, wobbling like a top as it spins, though its wobble takes 26,000 years to come full circle. So back when the Egyptians were just starting to build their pyramids, it was Thuban, not Polaris, that aligned with Earth’s rotation axis.

As such, all the stars of the northern sky would have appeared to revolve around Thuban, giving it a place of celestial importance for ancient Egyptians, who may have aligned the Great Pyramids based on its position.

See for Yourself

Stellarium, with additions.

While Thuban is known to be a binary, if you observe the system, you’re really just observing the primary. This giant star is several hundred times brighter than the Sun, though dim in our view for being 300 light-years away.

The changes in Thuban’s magnitude as it eclipses (and is eclipsed by) its smaller companion are small, as noted above. But if you’re up for the observing challenge, Kochoska has calculated the times of the upcoming eclipses visible throughout 2020.

The table below gives the primary and secondary eclipses that occur in 2020. The bolded lines are observable from the North American Central Time Zone (CT) and/or the Central European Time Zone (CET); the lines in italics mark eclipses where an observer would miss the beginning but could still catch the end.

If you catch an eclipse, let us know in the comments!