Mizar — A Fresh Look at an Old Friend

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 doubles up

Mizar and its naked-eye companion star Alcor are easily found at the bend of the Dipper's handle.

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.

Mizar and companion Alcor in a 4.5-inch (11.4-cm) telescope. Bob King

Mizar and companion Alcor in a 4.5-inch (11.4-cm) telescope at 45x.
Bob King

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.

Sought-for planet reveals Alcor companion

Alcor and its dim red dwarf companion Alcor B.
University of Rochester

Erik Mamajek, Associate Professor of Physics and Astronomy at the University of Rochester, discovered Alcor's companion while looking for extrasolar planets:

“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.

Fingerprints of a hidden companion

Discovery plate of the first known spectroscopic binary, Zeta Ursa Majoris, better known as Mizar. Top plate taken on Mar 29, 1887; bottom taken Apr 5, 1887. E.C. Pickering noted that the K line of ionized calcium was double on the earlier plate but single later. In time he concluded that Mizar A was a binary system.
Harvard College Observatory

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.

Doppler shift to the rescue!

In A, the two stars that make up Mizar A are moving perpendicular to our line of sight and show no Doppler shift. Spectral lines appear single. In B, the two stars are now moving in opposite directions from Earth. The difference in their Doppler shifts — one red, the other blue — causes their merged lines to separate into pairs, one for each star, revealing two stars where only one can be seen in a telescope.
Bob King

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.

The two components of Mizar A are both nearly identical-sized class A stars. This image was made in the mid-1990s with an optical interferometer capable of extremely high resolution. A_J. Benson et al_NPOI Group_USNO_NRL

The two components of Mizar A are both nearly identically-sized spectral class A stars. This image was made in the mid-1990s with an optical interferometer capable of extremely high resolution.
J. Benson et al., NPOI Group, USNO, NRL

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).

If we could see all six!

Model of the Mizar system (not to scale!) showing each of the three pairs that compose the sextuple. All are main sequence A-class stars similar to Vega or Altair except for Alcor's red dwarf companion.
Bob King

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?

Wondering how to find the Big Dipper? Use a Sky & Telescope star wheel to find your way!

Explore the Night with Bob King, Observing
Bob King

About Bob King

Amateur astronomer since childhood and long-time member of the American Association of Variable Star Observers (AAVSO), Bob King also teaches community education astronomy and writes the blog Astro Bob. The universe invites us on an adventure every single night. All we need do is look up. My book "Night Sky with the Naked Eye" was just published and is now available on Amazon and BN. It covers all the great things you can see at night with just your eyeballs. No equipment needed!

4 thoughts on “Mizar — A Fresh Look at an Old Friend

    1. Bob KingBob King Post author

      Hi Justin,
      I have been unable to find the period of Alcor’s companion, however Alcor revolves around the Mizar quadruple with a period of about 750,000 years.

  1. Jorge LuisJorge Luis

    Mizar and alcor an astronomical test to examine the vision !!! very good article bob, I have a telescope 4.5 ( 11.4 cm), tonight observe this double! which eye you recommend? I’m 20 mm and 32 mm 3x barlow ….. greetings from san diego, Venezuela
    my site : 114milimetros.blogspot.com

  2. Sam Storch

    For more than five decades now of observing, I’ve known the star between Alcor and Mizar as Sidum Ludovicianum, or “Ludwig’s Star.” See Bob King’s view, above.

    When you show the view to a “newbie,” you can always stimulate some interest by having them work to spot both Mizar A and B, then Alcor, calling them the “horse and rider,” which is what those names mean. Then, have them notice Sidum Ludovicianum. That’s another story to tell, if time permits.

    There is a lot you can do with this view.

    Call me just an old planetarium presenter guy at heart who still looks at the “real thing.”

All comments must follow the Sky & Telescope Terms of Use and will be moderated prior to posting. Please be civil in your comments. Sky & Telescope reserves the right to use the comments we receive, in whole or in part, and to use the commenter’s username, in any medium. See also the Terms of Use and Privacy Policy.