The handle of the Big Dipper just got stronger! Astronomers have found an additional star located in the Dipper’s gripper that is invisible to the unaided eye. Alcor, one of the stars that makes the bend in the Big Dipper’s handle has a smaller red dwarf companion orbiting it. Now known as “Alcor B,” the star was found with an innovative technique called “common parallactic motion,” and was found by members of Project 1640, an international collaborative team that gives a nod to the insight of Galileo Gallilei.
“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 at the Museum of Natural History. “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 year.
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The collaborative team that found the star includes astronomers from 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.
Alcor is a relatively young star twice the mass of the Sun. Stars this massive are relatively rare, short-lived, and bright. Alcor and its cousins in the Big Dipper formed from the same cloud of matter about 500 million years ago, something unusual for a constellation since most of these patterns in the sky are composed of unrelated stars. Alcor shares a position in the Big Dipper constellation with another star, Mizar. In fact, both stars were used as a common test of eyesight—being 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.
In 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 found the star had the same motion as Alcor, proving it was a companion star.
Alcor and its smaller companion Alcor B are both about 80 light-years away and orbit each other every 90 years or more. The team was also able to determine Alcor B is a common type of M-dwarf star or red dwarf that is about 250 times the mass of Jupiter, or roughly a quarter of the mass of our Sun. The companion is much smaller and cooler than Alcor A.
“Red dwarfs are not commonly reported around the brighter higher mass type of star that Alcor is, but we have a hunch that they are actually fairly common,” says Oppenheimer. “This discovery shows that even the brightest and most familiar stars in the sky hold secrets we have yet to reveal.”
The team plans to use parallactic motion again in the future. “We hope to use the same technique to check that other objects we find like exoplanets are truly bound to their host stars,” says Zimmerman. “In fact, we anticipate other research groups hunting for exoplanets will also use this technique to speed up the discovery process.”