Seeing Double in the Summer Sky

By Dennis Mammana

August 7, 2014 4 min read

Week of August 10-16, 2014

At first glance, every star looks pretty much the same. I know this because stargazers tell me all the time. But if you take a few minutes to look more closely with binoculars or a small telescope, you'll discover that this just isn't true.

For example, you will find that there are stars that appear single to the unaided eye, but are actually composed of two or more stars close together. Could this be coincidence or might there be some physical relationship between the two?

Often such stars aren't related at all, but appear simply along the same line of sight. These are called "double" stars. However — and perhaps most surprising to stargazers — is that roughly half of all stars in our galaxy are part of systems in which two stars orbit a common center of gravity. These are called binary stars.

Perhaps the easiest to spot lies within the seven stars that form the bowl and handle of the Big Dipper. Look at the middle star of the Dipper's bent handle to see if you can spot the double star here. These two are known by their proper names Mizar and its fainter companion Alcor — also known as "the horse and rider."

While Alcor and Mizar form a "double" star, Mizar itself is actually part of a physical binary system. Aim a small telescope in its direction and you should be able to see its faint companion nearby.

You can find an even more beautiful example of a binary star system near the head of the summer constellation Cygnus, the swan. It's known to astronomers as Beta Cygni, but most of us just know it by its proper name: Albireo.

To the eye, Albireo appears as a single star, no matter how good your vision. But with a small telescope you can resolve its light into that of two separate stars: one blue and one yellow.

And if you'd really like to challenge your vision, check out a star named Epsilon Lyrae. To find it, you'll need to look straight up after dark this week. The brightest star in that direction is Vega, the main star in the constellation Lyra, the harp. To its southeast lie four faint stars that form the shape of a tiny parallelogram, and just to its northeast lies another faint star. This is Epsilon Lyrae.

If you have binoculars, aim them in its direction to confirm that there really are two stars there. Even with optical aid, the two stars of Epsilon Lyrae appear very near to each other, and that makes them perfect to test the resolving power of our unaided eyes. In fact, these stars appear separated by an amount very close to the threshold of the human eye.

The best time to try this test is on a night when the stars do not appear to twinkle very much. Look in the direction of Epsilon Lyrae — you may actually have to glance slightly off to the side of it to get your best view — and try to see if you can spot both stars very close together. It's tough, but not impossible if you've got good vision.

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