Alpha Draconis: astronomers discover that it’s a binary eclipsing system

The binary system of Alpha Draconis (also known as Thuban), a system visible even to the naked eye and well studied 270 light years away from us, is actually an eclipsing binary system. Eclipsing binary systems are those in which the two stars, from our point of view, eclipse each other, i.e. they pass one in front of the other. This surprised the same researchers who discovered this important feature using data from NASA’s TESS telescope.

The system is made by a primary star 4.3 times the size of the Sun, and a companion star probably half the size of the primary star.

“The first question that comes to mind is ‘How did we miss this?'”, says Angela Kochoska, a researcher at Villanova University in Pennsylvania who presented her findings at the annual meeting of the American Astronomical Society in Honolulu.

Probably this feature has never been detected until now because eclipses, being quite short (they last about six hours) can be easily lost from Earth and, being the star so bright, the thing appeared “hidden” also other space detectors. This is one of the brightest known eclipsing binary systems in which the two stars are widely separated and interact only gravitationally (they don’t exchange materials).

These systems are very important for astronomers because it is possible to measure their masses and dimensions with unprecedented accuracy.

Researchers have found that stars orbit every 51.4 days at an average distance that is slightly greater than the average distance between mercury and the Sun. From our point of view, however, neither is ever completely covered by its companion and so it is a partial eclipse.