The star used by ancient Egyptians as their North Star has just been discovered to be one part of a pair of binary stars that periodically eclipse one another, according to NASA scientists.

The team’s astronomical findings were published Oct. 31, 2019 in the Research Notes of the American Astronomical Society (AAS). Using NASA's Transiting Exoplanet Survey Satellite (TESS), astronomers found that from Earth's perspective, Thuban, and its fainter companion star, pass in front of each other (eclipse) every 51.4 days at an average distance of about 38 million miles apart.

Also known as Alpha Draconis, the star is located 270 light-years from Earth in the constellation of Draco, the dragon , and Thuban is estimated to be five times brighter and nearly double the size of its stellar companion, and while it is not a particularly bright star to us it held a special place in the hearts of ancient stargazers.

Stella pyramid alignments. ( Aliaksei / Adobe Stock)

Ancient Dragon Star of The Egyptians

Caused by the gravitational pull of the Sun and the Moon on the Earth the 26,000-year-long precession cycle determines which star is seen at the north celestial pole at any given time. Several historical sources claim Thuban lay almost exactly at the north celestial pole in the year 2787 BC and this can be confirmed with modern astronomical models. Thuban, therefore, as a part of the constellation ‘Draco’, the Dragon, lay at the fixed celestial north pole when the Egyptians were building the pyramids.

In the 1960s, the so-called ‘air shafts’ in the Great Pyramid of Giza, that were once thought to serve for ventilation as the pyramids were being built, were found to align to the star Thuban at the north celestial pole, around which the heavens appeared to turn. And now, scientists find as the dragon star flies around the universe at a certain stage in its cycle, viewed from Earth, it is eclipsed by another star.

Thuban or Alpha Draconis (shown here in an image of the northern sky) was the North Star at the time of the building of the pyramids. ( NASA)

How Did We Miss This?

In a NASA statement, scientist Dr. Angela Kochoska, a postdoctoral researcher at Villanova University in Pennsylvania, says the first question that comes to mind is “how did we miss this?” And proposing an answer she said that the eclipses only last six hours so they are easily missed, and furthermore, the star is so bright the detectors on NASA's Kepler observatory would have been saturated, ‘masking the eclipses.’

Using TESS, the stellar pair's brightness was tracked for 27 days at a time revealing that Thuban and its companion are among the brightest ‘eclipsing binaries’, which are pairs of stars that pass in front of one another, but neither star completely concealing the other, the astronomers said in a statement. And it is by observing these eclipses that astronomers hope to get a more accurate measurement of the mass and size of each star.

Precision Precession Targeting Takes Time

Discovering eclipses in a well-known, bright, historically important star highlights how TESS impacts the broader astronomical community, said Padi Boyd, the TESS project scientist at NASA's Goddard Space Flight Center in Greenbelt, Maryland. In this case, the high precision TESS data can be used to help constrain fundamental stellar parameters at a level we've never before achieved, said the NASA scientist, whose forefathers in the ancient world regarded eclipses as powerful omens that foretold death and destruction.

The modern pole star, Polaris, formerly Phoenice, will get as close as it will get to the celestial north pole on March 24, 2100, but Thuban’s precision, as seen from Earth, will only be beaten by the star Alderamin, which will take its turn at the apparently fixed north pole around 7500 AD.

Top image: Pole starts used to align the pyramids at Giza. Source: Aliaksei / Adobe Stock

By Ashley Cowie