Alpha Draconis, also known as Thuban, is located about 300 light years away in the northern Draco constellation.
Despite its “alpha” designation, it shines as Draco’s fourth-brightest star. Thuban’s fame arises from a historical role it played some 4,700 years ago, back when the earliest pyramids were being built in Egypt.
Could the new astronomical observations shed light on what motivated the builders of the Egyptian pyramids?
Astronomers using data from NASA’s Transiting Exoplanet Survey Satellite (TESS) were surprised when they discovered that the bright star Alpha Draconis and its fainter, previously known companion actually undergo mutual eclipses.
Egyptology students already know that the ancient Egyptians were skilled astronomers who regularly observed the sky and tracked various celestial objects.
Previous research has revealed that the ancient Egyptian papyrus calendar 86637, Cairo, is the oldest preserved historical document from naked eye observations of a variable star, the Eclipsing binary algol – a manifestation of Horus, a god and a king.
It was confirmed that the ancient Egyptians knew about the variability of the ‘Demon Star’ Algol 3,000 years before Western astronomers.
Astronomers suggest that it is now possible that the first eclipse of the ancient North Star played a key role in building the Egyptian pyramids.
About 4,700 years ago, the star Thuban may have served as the Northern Star. It seemed to be the closest to the north pole of the earth’s axis of rotation, the point around which all other stars appear to rotate in their nocturnal motion.
Today this role is played by Polaris, the brightest star in the constellation Ursa Minor. The change happened because the earth’s axis of rotation performs a 26,000-year cyclic oscillation, called precession, which slowly changes the position of the sky at the rotational pole.
The North Star or Thuban is actually a pair of stars and the largest star between the pairs is four times larger and warmer than the Sun. The surface temperature of the larger star is about 9,700 degrees Celsius. Its mate, which is five times weaker, is probably half the size of the primary and is 40% warmer than the sun. Previous studies have suggested that Thuban exhibited slight changes in brightness that lasted about an hour, suggesting the possibility that brightest star in the system was pulsing.
Angela Kochoska, postdoctoral researcher at Villanova University in Pennsylvania, said:
The eclipses are brief, lasting only six hours; therefore terrestrial observations can easily lose them. And because the star is so bright, it would quickly saturate detectors at NASA’s Kepler Observatory, which would also conceal eclipses.
Many scientists think ancient Egyptians relied on the bright stars Big Dipper and Little Dipper to align their pyramids in a north-south direction with an accuracy of up to 0.05 degrees. However, it’s still debated how the pyramid builders took accurate measures for the alignment. Since the tilt of Earth’s axis has changed over time, so have the positions of the stars.
Using sophisticated astronomical software, scientists were able to turn back the astronomical clock and study the sky at the moment the pyramids were built. These studies showed that the two stars revolved around the opposite pole in the Old Kingdom sky, and an imaginary line joined these stars across the north pole.
This means that when the two stars placed vertically above each other both mark the true north position for the pyramid builders. In this case, if the calculation is correct, construction of the Great Pyramid of Giza began between 2485 and 2475 BC.
Kochoska, who presented the findings at the 235th meeting of the American Astronomical Society in Honolulu on Jan. 6, said she is planning a follow-up study to look into additional eclipses that TESS can see.
Padi Boyd, a TESS project scientist at NASA’s Goddard Space Flight Center, said in a statement:
The discovery of eclipses in a known, bright and historically important star highlights how TESS affects the wider astronomical community. In this case, uninterrupted, high-precision TESS data can be used to help constrain fundamental stellar parameters to a level never before reached.