Alcor, 1 half of the first known binary star system, has its own surprise star companion.
In ancient times, people with exceptional vision discovered that one of the brightest stars in the Big Dipper was, in fact, two stars so close together that most people cannot distinguish them. The two stars, Alcor and Mizar, were the first binary stars—a pair of stars that orbit each other—ever known.
Modern telescopes have since found that Mizar is itself a pair of binaries, revealing what was once thought of as a single star to be four stars orbiting each other. Alcor has been sometimes considered a fifth member of the system, orbiting far away from the Mizar quadruplet.
Now, an astronomer at the University of Rochester and his colleagues have made the surprise discovery that Alcor is also actually two stars, and is apparently gravitationally bound to the Mizar system, making the whole group a sextuplet. This would make the Mizar-Alcor sextuplet the second-nearest such system known. The discovery is especially surprising because Alcor is one of the most studied stars in the sky.
Benedetto Castelli, Galileo's protege and collaborator, first observed with a telescope that Mizar was not a single star in 1617, and Galileo observed it a week after hearing about this from Castelli, and noted it in his notebooks, says Mamajek. Those two stars, called Mizar A and Mizar B, together with Alcor, in 1857 became the first binary stars ever photographed through a telescope. In 1890, Mizar A was discovered to itself be a binary, being the first binary to be discovered using spectroscopy. In 1908, spectroscopy revealed that Mizar B was also a pair of stars, making the group the first-known quintuple star system.
Mamajek is continuing his efforts to find planets around nearby stars, but his attention is not completely off Alcor and Mizar. "You see how the disk of Alcor B doesn't seem perfectly round?" says Mamajek, pointing toward an image of Alcor and its new companion. "Some of us have a feeling that Alcor might actually have another surprise in store for us."
http://www.rdmag.com/News/2009/12/General-Science-First-known-binary-star-discovered-to-be-triplet-and-more/
