The table below gives the primary and secondary eclipses that occur in 2020. But if you're up for the observing challenge, Kochoska has calculated the times of the upcoming eclipses visible throughout 2020. The changes in Thuban's magnitude as it eclipses (and is eclipsed by) its smaller companion are small, as noted above. This giant star is several hundred times brighter than the Sun, though dim in our view for being 300 light-years away. While Thuban is known to be a binary, if you observe the system, you're really just observing the primary. So back when the Egyptians were just starting to build their pyramids, it was Thuban, not Polaris, that aligned with Earth's rotation axis.Īs such, all the stars of the northern sky would have appeared to revolve around Thuban, giving it a place of celestial importance for ancient Egyptians, who may have aligned the Great Pyramids based on its position.
HUBBLE SPACE TELESCOPE IMAGES NORTH STAR FULL
Thuban is famous not for its brightness, but for the role it played in the Egyptian sky some 4,700 years ago: North Star.Įarth precesses, wobbling like a top as it spins, though its wobble takes 26,000 years to come full circle. These long observations with very precise measurements enabled TESS to find the subtle variations in Thuban's brightness. Near the poles, these sectors overlap in what's termed the continuous viewing zone. The satellite monitors large swaths of sky for 27 days at a time, splitting each celestial hemispheres into 13 sectors each. TESS, though, is designed to look at bright, nearby stars. Thuban was actually too bright for Kepler to look at without saturating its detector. So perhaps it's no surprise that the small, brief dips in brightness were missed until now.Įven the space-based Kepler mission didn't catch the star's variations. What's more, the eclipses are only six hours long. The variations are small in part because the stars don't fully eclipse each other. However, the primary eclipse causes a change in brightness of only about 0.1 magnitude the secondary eclipse is marked by an even smaller dip, 0.02 magnitude. So even though Thuban isn't exactly the brightest star in the sky - at magnitude 3.7 it's the eighth-brightest star in Draco - the discovery at first took astronomers by surprise. Thuban's eclipses occur twice every 51.4 days.
When two stars in a binary system eclipse each other, their overall brightness as seen from Earth fades and increases again in a cyclic way. “The first question that comes to mind is ‘how did we miss this?’” says Angela Kochoska (Villanova University). Turns out, the two stars eclipse each other as seen from Earth. At the recent American Astronomical Society meeting in Honolulu, astronomers announced that TESS had uncovered something rather surprising about the well-known double star in Draco named Thuban (also called Alpha Draconis). But as it's examining stars for the subtle dips that mark a planet's passage in front of its star, it can catch lots of other stellar goings-on, too. NASA's TESS mission has a primary goal of looking for exoplanets. Now data from NASA's TESS show its two stars undergo mutual eclipses. The star Thuban, also known as Alpha Draconis, has long been known to be a binary system.
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