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| Alcor & Sidus Ludoviciana |
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| It's impossible to write a comprehensive article on Mizar and leave out its two famous neighbors. The first one, Alcor (80 g UMa), easy to spot with the naked eye at distance of 11 arcminutes, was sometimes considered a fifth member of the Mizar system. This notion was made less likely by the Hipparcos distances to Mizar (78 light years) and Alcor (81 light years) which have the uncertainty of only one light year each. Nonetheless, the stars share the motion in space and were formed together as close members of the nearest star cluster, Collinder 285, better known as the Ursa Major moving cluster. It was recognized in 1869 by the English astronomer Richard A. Proctor who noticed that five stars of the Big Dipper (Beta, Gamma, Delta, Epsilon and Zeta UMa) hurried together towards the solar apex in Hercules rather than away from it like most other stars in the sky. The current view of Collinder 285 is that of a sparse doomed cluster, 300 or 400 million years old, with some 50 members scattered all over the sky. The five Proctor stars and a few fainter ones (like Alcor) form its nucleus. |
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| Myth and truth |
| Four richly storied stars in this 15' field were within reach of the simple telescopes of Galileo Galilei and his friend Benedetto Castelli. These are Mizar A (Zeta_1 UMa, 2.27 mag), the famous 20.5-day spectroscopic binary discovered in 1889 and imaged with the NPOI in Arizona a century later, Mizar B (Zeta_2 UMa, 3.95 mag), its visual companion 15 arcseconds away, also a spectroscopic pair of stars which orbit each other twice a year, Alcor (80 g UMa), a single but variable star (one of the Hipparcos discoveries, 4.04 – 4.07 mag), and finally Sidus Ludoviciana (HD 116798, 7.59 mag), a star once considered a new planet. (1) Mizar A (2) Mizar B (3) Alcor (4) Sidus Ludoviciana |
| A planet that wasn't Observed already by Benedetto Castelli in 1616, an 8th-magnitude star in the Mizar field became known as Sidus Ludoviciana a century later. [16] Johann Georg Liebknecht (1679 – 1749), professor of theology and mathematics at the German university of Giesen, noticed this star on December 2, 1722, while observing with a nonachromatic telescope six feet long. Believing he detected its slow motion among stars, Liebknecht became convinced it was a new planet and named it hastily Sidus Ludoviciana (Ludwig's star) in honor of his monarch, the Landgrave Ludwig of Hessen-Darmstadt. Liebknecht was however much less lucky than Galileo with his Medicean stars (Jupiter's four satellites) or William Herschel who named the genuine seventh planet, Uranus, after his sovereign King George III. While both of them enjoyed the favor of the mighty patrons as well as recognition by fellow astronomers, Liebknecht's pamphlet announcing the find was sharply criticized, to put it mildly, by his colleagues who dismissed it as nonsense. Apart from this curious tale that can enliven any public observation of Mizar through a telescope, Sidus Ludoviciana is a run-of-the-mill field star which has nothing to do with the Ursa Major cluster. A comparison of its visual magnitude with that of Alcor (which has the same spectral type A5 V) shows it's roughly five times farther. |