Random Walk
Parallax in Motion
Four hundred years after Galileo first turned a homemade spyglass to the heavens, the somewhat larger Hale Telescope at Caltech’s Palomar Observatory has discovered a new star, Alcor B, in the Big Dipper—using a technique he foresaw. Says team leader Ben Oppenheimer (PhD ’99), of the American Museum of Natural History in Manhattan, “Galileo realized that if Copernicus was right—if the earth orbits the sun—he could show it by observing the parallactic motion of the nearest stars. He tried to use Alcor to see this, but didn’t have the necessary precision.” Parallactic motion is the way that nearby stars appear to move slightly, relative to the firmament, over the course of a year as we see them from different points in Earth’s orbit.
Alcor B is a red dwarf about 250 times the mass of Jupiter, or one-quarter that of our sun. The star around which it orbits, Alcor A, is a relatively young star twice the mass of the sun. The Alcors share their position—second star from the end in the Dipper’s handle—with another star, Mizar. (In fact, the ability to see both stars—being able to distinguish “the rider from the horse”—was a common test of eyesight among ancient peoples.) One of Galileo’s colleagues observed that Mizar itself is actually a double, making it the first binary star system resolved by a telescope. Many years later, Mizar A and B were each found to be tightly orbiting binaries, altogether forming a quadruple system.
Last March, members of Project 1640, a collaborative hunt for planets orbiting other stars, attached their coronagraph—a device for blocking out a star’s light, allowing faint objects nearby to be seen—to the Hale and aimed it at Alcor. “Right away I spotted a faint point of light next to the star,” says Neil Zimmerman, a grad student at Columbia who is doing his dissertation at the museum.
The team returned 103 days later, hoping to find that the two stars had moved as one. If the proposed companion was just a background star, it wouldn’t necessarily be keeping Alcor company any more. But the two had, indeed, moved in tandem, says Oppenheimer. “Our technique is much faster than the usual way of confirming that objects in the sky are physically related.” If the putative pair is too far away for parallax, you may have to watch them for years in order to measure any motion.
The Alcors are about 80 light-years away, and over one year they appear to move in an ellipse about 0.08 arc seconds long. This is about 1,000 times smaller than the eye can discern—but easily detectable with the coronagraph’s adaptive optics, which remove the distorting effects of atmospheric turbulence and thus allow very fine position measurements to be made.
“We hope to use this technique to check that any potential exoplanets we find are truly bound to their host stars,” says Zimmerman.
“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 hold secrets we have yet to reveal.”
The Project 1640 collaboration includes researchers from the American Museum of Natural History, the University of Cambridge, and the Space Telescope Science Institute, as well as Associate Professor of Astronomy Lynne Hillenbrand, Senior Faculty Associate in Astronomy Charles Beichman, postdocs Sasha Hinkley and Justin Crepp, adaptive optics engineer Antonin Bouchez (PhD ’04), and Member of the Professional Staff Richard Dekany (BS ’89) from Caltech, and Gautam Vasisht (PhD ’96), Rick Burruss, Michael Shao, Lewis Roberts, and Jennifer Roberts of JPL. Project 1640 is funded by the National Science Foundation.
—SK
