The star that ancient Egyptians once used as their North Star is part of a binary pair of stars that periodically eclipse one another, recent observations show.
Known as Alpha Draconis, or Thuban, the star is located 270 light-years from Earth in the constellation of Draco, the dragon (opens in new tab). Thuban is estimated to be five times brighter and nearly double the size of its stellar companion.
Using NASA's Transiting Exoplanet Survey Satellite (opens in new tab) (TESS), astronomers found that Thuban and its fainter companion star eclipse, or pass in front of each other from Earth's perspective, every 51.4 days at an average distance of about 38 million miles apart, NASA officials said in a statement.
"The first question that comes to mind is 'how did we miss this?'" Angela Kochoska, a postdoctoral researcher at Villanova University in Pennsylvania, said in the statement (opens in new tab). "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 (opens in new tab), which would also mask the eclipses."
Thuban is bright enough to be seen by the naked eye. Around 2600 B.C., when the ancient Egyptians were building the earliest pyramids, Thuban appeared as the North Star. At the time, it was the closest star located near Earth's geographic north pole (opens in new tab), where the planet's axis of rotation intersects with its surface. However, Earth's axis shifts or "wobbles" over time — a cyclic movement known as precession — which is why Polaris (opens in new tab) is now the North Star.
Using TESS, the researchers were able to track changes in the stellar pair's brightness for 27 days at a time, revealing that Thuban and its companion are among the brightest "eclipsing binaries (opens in new tab)" — pairs of stars that pass in front of one another. During the eclipses, neither star completely conceals the other, according to the statement.
Observing the Thuban system during these eclipses could help astronomers get a more accurate measurement of the mass and size of each star. Members of the team plan to use ground-based follow-up observations to further study the pair, and they expect that TESS (opens in new tab) will reveal more eclipsing binaries in other parts of the sky.
"Discovering eclipses in a well-known, bright, historically important star highlights how TESS impacts the broader astronomical community," Padi Boyd, the TESS project scientist at NASA's Goddard Space Flight Center in Greenbelt, Maryland, said in the statement. "In this case, the high precision, uninterrupted TESS data (opens in new tab) can be used to help constrain fundamental stellar parameters at a level we've never before achieved."
Kochoska presented the team's findings at the 235th meeting of the American Astronomical Society (opens in new tab) (AAS) in Honolulu on Jan. 6. The study was published Oct. 31, 2019 (opens in new tab), in the Research Notes of the AAS.
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