This ancient north star likely helped guide the building of Egyptian pyramids

Although it’s not the brightest star in the sky, Polaris serves as our guiding North Star. It’s a steady star in the Ursa Minor constellation to guide by, remaining constant while the rest of the north sky moves.

This is because Polaris is closest to Earth’s north pole, so it appears that the other stars in the sky rotate around it.

But Polaris hasn’t always been our North Star. And during the construction of the earliest Egyptian pyramids 4,700 years ago, the pole star was Thuban, or Alpha Draconis, found in the Draco constellation. The star is 270 light-years from Earth.

There is evidence now that Thuban was used to guide the construction of the pyramids, given how their placement, and even air shafts, align with the stars.

The North Star changed because Earth’s spin axis itself wobbles slowly over the course of 26,000 years, which alters the position of our pole and where it points in the sky.

Now, using NASA’s planet-hunting TESS mission, which stares at large sections of sky and starlight for 27 days at a time, astronomers realized something new about Thuban.

The former North Star is part of a binary system, meaning it has a much fainter companion star it orbits. TESS helped astronomers discover that the stars regularly eclipse each other.

The findings were presented this week at the 235th meeting of the American Astronomical Society in Honolulu.

“The first question that comes to mind is ‘How did we miss this?’ ” said Angela Kochoska, a postdoctoral researcher at Villanova University in Pennsylvania. “The eclipses are brief, lasting only six hours, so ground-based observations can easily miss them. And because the star is so bright, it would have quickly saturated detectors on NASA’s Kepler observatory, which would also mask the eclipses.”

Previously, the only hint of this behavior came from a report that small brightness changes could be seen over the course of an hour in Thuban. At the time, they thought maybe the star was pulsating. TESS measurements of the binary star system helped astronomers discover the mutual eclipses and rule out pulsating.

The stars are 38 million miles apart, about the distance between Mercury and the sun, and complete an orbit around each other every 51.4 days.

Thuban, the brightest of the pair, is 4.3 times the size of our sun and is 70% hotter than it at 17,500 degrees Fahrenheit. The fainter star is half the size of Thuban and only about 40% hotter than the sun.