This animation illustrates a preliminary model of the Thuban system, now known to be an eclipsing binary thanks to data from NASA’s Transiting Exoplanet Survey Satellite (TESS). The stars orbit every 51.4 days at an average distance slightly greater than Mercury’s distance from the Sun. We view the system about three degrees above the stars’ orbital plane, so they undergo mutual eclipses, but neither is ever completely covered up by its partner. The primary star is 4.3 times bigger than the Sun and has a surface temperature around 17,500 degrees Fahrenheit (9,700 C), making it 70% hotter than our Sun. Its companion, which is five times fainter, is most likely half the primary’s size and 40% hotter than the Sun. Thuban, also called Alpha Draconis, is located about 270 light-years away in the northern constellation Draco.
Credit: NASA's Goddard Space Flight Center/Chris Smith (USRA)
Astronomers using data from NASA’s Transiting Exoplanet Survey Satellite (TESS) have shown that Alpha Draconis, a well-studied star visible to the naked eye, and its fainter companion star regularly eclipse each other. While astronomers previously knew this was a binary system, the mutual eclipses came as a complete surprise.
The eclipses are brief, lasting only six hours, so ground-based observations can easily miss them.
The system ranks among the brightest-known eclipsing binaries where the two stars are widely separated, or detached, and only interact gravitationally. Such systems are important because astronomers can measure the masses and sizes of both stars with unrivaled accuracy.
Alpha Draconis, also known as Thuban, lies about 270 light-years away in the northern constellation Draco. Despite its “alpha” designation, it shines as Draco’s fourth-brightest star. Thuban’s fame arises from a historical role it played some 4,700 years ago, back when the earliest pyramids were being built in Egypt.
At that time, it appeared as the North Star, the one closest to the northern pole of Earth’s spin axis, the point around which all of the other stars appear to turn in their nightly motion. Today, this role is played by Polaris, a brighter star in the constellation Ursa Minor. The change happened because Earth’s spin axis performs a cyclic 26,000-year wobble, called precession, that slowly alters the sky position of the rotational pole.
Discovering eclipses in a well-known, bright, historically important star highlights how TESS impacts the broader astronomical community. In this case, the high precision, uninterrupted TESS data can be used to help constrain fundamental stellar parameters at a level we’ve never before achieved.
GIF. The long orbital period of the Thuban system (circled) required multiple sectors of TESS data to reveal the eclipses. This discovery, together with future observations and modeling, will help astronomers better understand the stars and their orbits.