The Big Dipper has a secret, invisible to the unaided eye, according to a new paper published in The Astrophysical Journal, which says that one of the stars that makes the bend in the ladle's handle, Alcor, has a smaller red dwarf companion.
Newly discovered Alcor B orbits its larger sibling and was caught in the act with an innovative technique called "common parallactic motion" by members of Project 1640, an international collaborative team that includes astrophysicists at the American Museum of Natural History, the University of Cambridge's Institute of Astronomy, the California Institute of Technology, and
NASA's Jet Propulsion Laboratory.
able to distinguish "the rider from the horse"—among ancient people.
One of Galileo's colleagues observed that Mizar itself is actually a double, the first binary star system resolved by a telescope. Many years later, the two components Mizar A and B were themselves determined each to be tightly orbiting binaries, altogether forming a quadruple system.
"We used a brand new technique for determining that an object orbits a nearby star, a technique that's a nice nod to Galileo," says Ben R. Oppenheimer, Curator and Professor in the Department of Astrophysics at the Museum. "Galileo showed tremendous foresight. Four hundred years ago, he realized that if Copernicus was right—that the Earth orbits the Sun—they could show it by observing the "parallactic motion" of the nearest stars. Incredibly, Galileo tried to use Alcor to see it but didn't have the necessary precision."
If Galileo had been able to see change over time in Alcor's position, he would have had conclusive evidence that Copernicus was right. Parallactic motion is the way nearby stars appear to move in an annual, repeatable pattern relative to much more distant stars, simply because the observer on Earth is circling the Sun and sees these stars from different places over the
Now, Alcor, which is near the four stars of the Mizar system, also has a companion. This March, members of Project 1640 attached their coronagraph and adaptive optics to the 200-inch Hale Telescope at the Palomar Observatory in California and pointed to Alcor. "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. "No one had reported this object before, and it was very close to Alcor, so we realized it was probably an unknown companion star."
The team retuned a few months later and re-imaged the star, hoping to prove that the two stars are companions by mapping the tiny movement of both in relation to very distant background stars as the Earth moves around the Sun, in other words, by mapping its parallactic motion. If
the proposed companion were just a background star, it wouldn't move along with Alcor.