Everyone familiar with the night sky knows of Mizar and Alcor, two stars in the handle of the Plough in Ursa Major that appear very close to one another. What nobody realised until now was that Alcor, the fainter of the two, is itself a binary system, which has been discovered by astronomers using the Hale Telescope at Palomar Observatory.

The astronomers, calling themselves Project 1640 and hailing from the American Museum of Natural History, the University of Cambridge and NASA’s Jet Propulsion Laboratory, spotted the tiny companion to Alcor – a cool red dwarf star a quarter of the mass of the Sun – using the 5.1 metre Hale Telescope’s adaptive optics and coronagraph, which blots out the glare of the primary star in the Alcor system.

“Right away I spotted a faint point of light next to the star,” says Neil Zimmerman, a graduate student at Columbia University who is doing his PhD dissertation at the Museum of Natural History. “No one had reported this object before, and it was very close to Alcor, so we realised it was probably an unknown companion star.”

To prove this, they took advantage of a technique that had never been used in this way before: parallactic motion. As the Earth orbits the Sun, the position of nearby stars appears to shift slightly relative to more distant background stars; it’s like when you hold your finger up close before you, and you shut one eye, then switch eyes, and your finger appears to move relative to the background because of the angle between it and your eyes. Called parallax, this is usually used for measuring distances to stars (Alcor and its companion are 80 light years away), but Zimmerman and his colleagues realised that if this new star followed Alcor precisely in its parallactic motion, it would confirm that they are gravitationally bound. When they came back to check 103 days later, they found that they were moving together. This is a much faster method of discovering if two objects are orbiting one another, rather than waiting to watch them orbit (the red dwarf orbits Alcor A once every 90 years).

The Alcor and Mizar system is a fascinating one. Mizar itself can be split into two by a telescope, and each component of Mizar is also spectroscopic binary (meaning that the individual stars are too close to be separated, but their existence becomes apparent in shifts in the stars’ spectra), making four stars. Plus, there is some debate as to whether Alcor (and now its red dwarf companion) are orbiting the Mizar quartet. However, they are all moving through space in the same direction, having all been born together about 500 million years ago along with the other stars in the Plough.

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