You might be tempted to pass up this familiar star for more exotic quarry, but take another look at a multiple star with a most interesting history.
Mizar, the bright double star in the bend of the Big Dipper's handle, has racked up a few firsts over the years. First double star discovered (1617). First binary photographed in a telescope (1857). First spectroscopic binary (1890). First stellar pair seen by generations of novice star gazers.
From the northern U.S. it's visible all year long, but puts in its best appearance in spring skies, riding the Big Dipper like a rocket to the zenith.
Most people with reasonably good vision can easily spot Mizar's 4th-magnitude companion star, Alcor, 11.8′ to the east. Arabic peoples knew them as the Horse and Rider, a wonderful image which neatly matches their naked-eye appearance.
Through my 4.5-inch reflector at 45x, Mizar is easily one of the prettiest low-power doubles in the sky. Even low magnification cleaves it into two trembling white gems separated by 14″ with 2nd-magnitude Mizar A attended by 4th-magnitude Mizar B.
Both Mizar and Alcor lie about 80 light-years from Earth and share a common proper motion across the sky, yet their great distance from one another made it difficult to determine if they formed a true gravitationally-bound binary star. That issue appears to have been resolved in 2009, when observations made by two independent teams of astronomers not only revealed that Alcor possessed a dim red dwarf companion, but that it was indeed tethered to Mizar. Just barely. The two are separated by 0.5-1.5 light-years.
“We were trying a new method of planet hunting and instead of finding a planet orbiting Alcor, we found a star,” said Mamjek. But long before Mizar-Alcor was found to be a triple system, the spectroscope forced Mizar to reveal its hand.
American astronomer Edward Pickering, who served as director of the Harvard College Observatory from 1877 until his death in 1919, discovered that Mizar A was itself a very close binary star. Too close to split for most telescopes then and now, its duplicity was only revealed by the spectroscope.
Pickering examined spectra taken of the star, noting that the K line of ionized calcium was double on one date but single on a later date. In a spectroscopic binary, two stars orbit their common center of mass just like the rest of the double clan, but because they're too close to split, they appear as a single point of light.
When the two stars lie across our line of sight, their spectral lines overlap and we see a series of single dark lines. A few days later, the stars are moving toward and away from us along our line of sight. Light from the star moving toward us is shifted toward the blue end of the spectrum, while light from the star moving away is shifted toward the red. The difference in speeds separates the overlapping lines into pairs of lines, one set for each star.
That was 1890. In 1908, further spectroscopy showed Mizar B to also be a close pair of orbiting stars, making the system fully sextuple!
The two components of Mizar A, separated by just 7 or 8 thousands of an arcsecond, are both about 35 times as bright as the Sun and revolve around each other once every 20.5 days.
The Mizar B pair is comprised of two slightly cooler and fainter A class stars each about 1.6 times as massive as the Sun. While it might seem that the Mizar B and Mizar A pairs must be far apart to split them so easily in a small telescope, the gap between them is only as wide as 8 times Pluto's distance from the Sun (30 billion miles).
No new companions have been discovered since 2009, but Mamajek hasn't given up on planet hunting around Alcor. He points out that Alcor's disk isn't perfectly round. Does a planet or perhaps another star hide in its glare? Do I hear septuple?
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