There are many stars in the sky which look like single points of light to the unaided eye. But these special stars are in fact doubles, or even sometimes triples or more, when viewed through binoculars and telescopes.
Optical Doubles
However, double stars are not always “real” – sometimes they are what is termed “optical” doubles.
This is when two stars just appear to be very close side-by-side when we see them from Earth, when in fact, they are at different distances from us and are not really close at all.
The double star appearance is nothing more than a line-of-sight effect.
Physical Doubles
However, most double stars we see are “physical” doubles. They really are a pair of stars that are close together and linked by gravity.
Each star orbits around a common centre of gravity, situated between the stars.
There are some well-known examples of double stars.
Mizar (zeta Ursae Majoris), a 2nd magnitude star in the Plough asterism, within the constellation of Ursa Major, has a 4th magnitude companion called Alcor. This may be easily seen with binoculars or a small telescope and can been seen with the unaided eye, if your eyesight is good.
Being able to see Mizar as a double, was used during the First World War as a test of eyesight for soldiers. I’m not sure whether it was a good or bad thing, to pass the test…
Another example and a beautiful sight in the sky, is Albireo (Beta Cygni) in the constellation of Cygnus. It is a 3rd magnitude orange giant together with a 5th magnitude blue-green companion. The colours make Alibireo a wonderful sight through a small telescope.
The separation between the components of double stars is measured in terms of angular separation, as viewed from Earth.
Many doubles have angular separations of a few seconds of arc (a second of arc is 1/3600th of a degree of arc). The closer the separation, the larger the telescope needed to “split” them, meaning that you can see the component stars.
Another good example is Epsilon Lyra in the constellation of Lyra. A small telescope or binoculars will easily show it as a pair of 5th magnitude white stars.
But look more closely with a powerful telescope, and each of the pair of stars can itself be seen as a double star. Epsilon Lyrae is sometimes called “The Double Double”.
Some double stars are so close together that the component stars cannot be seen through telescopes. Analysis of their light with a spectroscope however, reveals their double nature. These double star are called “spectroscopic binaries”.
