NASA/MIT/TESS The Thuban star at night.

Astronomers have long understood that a star called Thuban, which may have served as the north star for the ancient Egyptians, is actually a pair of stars.

NASA astronomers recently discovered the two stars eclipse one other.

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On a clear night, the star Thuban is easy to spot. It’s sandwiched between the bowl of the Little Dipper to the left and the handle of the Big Dipper to the right.

For the ancient Egyptians, it may have served as the north star.

About 4,700 years ago, when the Egyptians were building the earliest pyramids, Thuban was likely the axis around which all the other stars appeared to spin at night. Some researchers think the pyramids were built to point directly toward Thuban for that reason.

Today, the North Star is Polaris, a brighter star to Thuban’s right, because of the way the tilt of Earth’s axis (and where it points) has changed over time.

NASA astronomers still monitor the old north star – and many, many others – via the Transiting Exoplanet Survey Satellite (TESS), a space telescope whose main purpose is to hunt for exoplanets. The two-year TESS mission watches the night sky in 24-by-90-degree strips, lingering on each portion for 27 days at a time.

As it searched for signs of exoplanets, TESS recently discovered that Thuban undergoes eclipses.

That’s because Thuban is actually a pair of stars. The larger star of the pair is more than four times bigger than the sun and 70% hotter, with a surface temperature of about 17,500 degrees Fahrenheit. Its partner is five times fainter than the larger star in the pair and about half its size.

The two stars orbit one another at an average distance of about 38 million miles and complete a full circle every 51 days. Astronomers had never found any evidence that the stars eclipse one another in those orbital paths until now.

“The first question that comes to mind is: ‘How did we miss this?'” Angela Kochoska, a postdoctoral researcher at Villanova University, said in a statement to NASA.

She presented her findings at the annual meeting of the American Astronomical Society on Monday and attempted to answer that question: “The eclipses are brief, lasting only six hours, so ground-based observations can easily miss them,” Kochoska said. “Because the star is so bright, it would have quickly saturated detectors on NASA’s Kepler observatory, which would also mask the eclipses.”

Astronomers now understand that Thuban and its partner are among the brightest “eclipsing binaries” – pairs of stars that pass in front of one another, leaving the other in a shadow. In this case, the two stars never completely conceal one another, but portions of them become shrouded from view.

Observing Thuban during these eclipse events could help astronomers get a more accurate measurement of the masses and sizes of both stars.

Kochoska said she expected TESS to discover more eclipses in other parts of the sky as it continues to scan the cosmos.

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