Faint Star Orbiting the Big Dipper's Alcor Discovered
Next time you spy the Big Dipper, keep in mind that there is another star, invisible to the unaided eye, contributing to this constellation.
According to a new paper published in The Astrophysical Journal, 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.
"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 year.
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 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.
"Alcor, a star in the middle of the Big Dipper's handle, has a newly found red dwarf companion (circled in green). Project 1640 astronomers discovered the faint star by blocking out almost all of Alcor's light with a coronagraph. The halo of speckles around the coronagraph's occulting mask is caused by the wave-like properties of light from Alcor's residual glare. The actual diameters of both stars take up just a tiny fraction of a pixel. (Credit: Project 1640/AMNH and Digital Universe Atlas)"
Source: American Museum of Natural History
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