JOHN SANFORD / SCIENCE PHOTO LIBRARY JOHN SANFORD / SCIENCE PHOTO LIBRARY
Optical photograph showing the naked-eye double stars Mizar and Alcor (left). Mizar, the brighter of the two stars, is also known as Zeta Ursae Majoris. It lies at the bend in the handle of the Plough or Big Dipper in Ursa Major. The two stars orbit each other only once every ten million years. When viewed through a small telescope, Mizar itself is seen to be a double star, the two components (Zeta 1 & Zeta 2 Ursae Majoris) orbit each other with a period of at least 20,000 years. Spectroscopic observations of Zeta 1, Zeta 2 and Alcor indicate that all three are in fact themselves double star systems. Zeta 1 and Zeta 2 have periods of 175 days and 20 days respectively.
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