The Big Dipper Adds a Star

It's no secret that at community star parties I bring small, squat telescopes specifically to attract little kids in the crowd. And one of my favorite targets is the paired stars Alcor and Mizar in the Big Dipper's handle.

I tell the kids this is a "star with a secret." With just the slightest optical aid, they can make out both stars, along with an unrelated field star known as Sidus Ludoviciana that lurks nearby, creating a satisfying little triangle.

Then I ask them to train their attention on the brightest of the three (bumping up the magnification if needed), and they quickly realize that Mizar is actually a double star. I cap off my little spiel by noting that each of those two stars is itself double. So one star by eye in the Big Dipper's handle is really six stars.

Now I can make that seven!

Astronomers recently discovered that Alcor is also a binary. Neil Zimmerman and Ben Oppenheimer, affiliated with Columbia University and New York's American Museum of Natural History, led the 19-member observing team that made the find. They've announced their discovery online and in a forthcoming issue of the Astrophysical Journal.

Curiously, the observers weren't looking for double stars when they slewed the giant 5-meter eye at Palomar Observatory over to Alcor. They've been hunting for extrasolar planets at near-infrared wavelengths as part of an effort called Project 1640. Their detector uses adaptive optics to steady the atmospheric seeing and a coronagraph mask to block the light from individual stars, in the hope of glimpsing much fainter planets that would otherwise be hopelessly lost in the star's glare.

Alcor's companion first turned up last March 16th. "“Right away I spotted a faint point of light next to the star,” says Zimmerman in a press release. “No one had reported this object before, and it was very close to Alcor, so we realized it was probably an unknown companion star.”

However, another explanation is that it's an unrelated star coincidentally within 1 arcsecond of our line of sight to Alcor. Ordinarily it might have taken decades to determine whether the two stars were indeed twirling in slow motion around each other. But Alcor is quite close to us, interstellarly speaking, just 80 light-years away, and that allowed the team to use a novel technique to get a definitive answer much more quickly.

They reimaged the pair on June 27th, when Earth had swung to a different side of its orbit, and compared the two images. If the newfound star were much farther away, its position would have shifted relative to Alcor due to parallax. (It wouldn't have been much, the observers note, only about 0.034 arcsecond.) But it didn't, confirming that the two stars were truly together in space.

Alcor B turns out to be an M dwarf with only about a quarter the Sun's mass. Alcor, by comparison, is a much hotter and brighter A star with twice the Sun's mass.

It's wonderfully fitting that this discovery comes during the International Year of Astronomy, celebrating Galileo's telescopic discoveries made 400 years ago. In confirming the Copernican theory that Earth revolved around the Sun, Galileo insightfully realized that such changes in stars' positions should be possible. In fact, the great Italian astronomer attempted to discover a shift in relative positions among Alcor, Mizar A and B, and Sidus Ludoviciana (also designated HD 116798), but his telescopes just weren't up to the task.