Brightest Stars

Mizar and Alcor in the bend of the Big Dipper

Chart: White dots and blue lines tracing the shape of the Big Dipper.
On March evenings, the Big Dipper is in the northeast. The famous star pair Mizar and Alcor is second to the end of the Dipper’s handle. Look closely, and you’ll see the two points of light, Alcor.

Mizar and Alcor

Mizar and its fainter companion star Alcor are one of the most famous double stars in the sky. You’ll spot Mizar first, as the middle star of the Big Dipper‘s handle. Look closely, and you’ll see fainter Alcor right next to Mizar.

Historically, Mizar and Alcor are a test of eyesight. But even people with less-than-perfect eyesight can see the two stars, especially if they’re looking in a dark, clear sky. This pair of stars in the Big Dipper’s handle has the nickname of horse and rider. If you can’t see fainter Alcor with the unaided eye, use binoculars to see Mizar’s nearby companion.

Mizar alone is a quadruple star

Mizar is perhaps the Big Dipper’s most famous star, glorified in the annals of astronomy many times over. Apart from Alcor, Mizar by itself is a double star. In fact, it was the first double star known. An Italian astronomer brought it to the attention of Galileo in 1617. A third Italian astronomer, Giovanni Battista Riccioli, wrote about Mizar as a double star.

Few, if any, astronomers back then even dreamed that double stars were anything other than chance alignments of physically unrelated stars. Yet, in 1889, a spectroscope revealed that the brighter component of Mizar’s two stars consisted of two stars itself. This made Mizar the first binary star ever discovered by spectroscopic means.

Later, Mizar’s dimmer telescopic component also showed itself to be a spectroscopic binary, meaning that Mizar consists of two sets of binaries, making it a quadruple star.

Starfield with two bright bluish stars at center, Mizar and Alcor.
Mizar and Alcor. Mizar is the brighter of the two. Image via Fred Espenak/

And Alcor is double

As for Alcor, scientists long believed that Mizar and Alcor were not gravitationally bound and did not form a true binary star system. Not until 2009 did our knowledge expand. Two groups of astronomers independently reported that Alcor is itself a binary, consisting of Alcor A and Alcor B. Astronomers now believe that the Alcor binary system is gravitationally bound to the Mizar quadruple system. That makes this “double” star six stars in all, but we can only see two with the unaided eye.

Mizar and Alcor have proven to not only be a test of human eyesight, but a test of the limits of our technological vision as well.

Starfield with two bright stars with a golden glow, brighter of the two on the right.
Located in the handle of the Big Dipper, Mizar (brighter at right center) and Alcor (fainter and centered) are one of the most famous visual double stars in the sky. Image via ESO/ Online Digitized Sky Survey.

Bottom line: Famous star pair Mizar and Alcor is easy to find in the handle of the Big Dipper. Mizar is really four stars, and Alcor is two stars. So what we see as two stars are really six in one!

March 27, 2022
Brightest Stars

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Bruce McClure

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