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See Ancient “North Stars" Eclipse Each Other

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.

Thuban in TESS image
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'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.
Primary Eclipses

February 22, 2020 4:44February 21, 2020 22:44February 22, 2020 5:44
April 13, 2020 14:48April 13, 2020 9:48April 13, 2020 16:48
June 4, 2020 0:52June 3, 2020 19:52June 4, 2020 2:52
July 25, 2020 10:56July 25, 2020 5:56July 25, 2020 12:56
September 14, 2020 21:01September 14, 2020 16:01September 14, 2020 23:01
November 5, 2020 7:05November 5, 2020 1:05November 5, 2020 8:05
December 26, 2020 17:09December 26, 2020 11:09December 26, 2020 18:09
Secondary Eclipses

February 9, 2020 5h:59mFebruary 8, 2020 23:59February 9, 2020 6:59
March 31, 2020 16h:3mMarch 31, 2020 11:03March 31, 2020 18:03
May 22, 2020 2h:7mMay 21, 2020 21:07May 22, 2020 4:07
July 12, 2020 12h:11mJuly 12, 2020 7:11July 12, 2020 14:11
September 1, 2020 22h:15mSeptember 1, 2020 17:15September 2, 2020 0:15
October 23, 2020 8h:19mOctober 23, 2020 3:19October 23, 2020 10:19
December 13, 2020 18h:23mDecember 13, 2020 12:23December 13, 2020 19:23
If you catch an eclipse, let us know in the comments!

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