Our view of stars as single points of light in the night sky is somewhat deceptive. Astronomers who scan the sky nightly with their telescopes have found that about one-third of all the stars they can see are double stars.
In general, the two stars appear to revolve around their common center of gravity, some very slowly, others rather rapidly. Some arrangements are even more complex. Many stars are multiple, some forming double-double stars or even more complex families.
In the evening sky right now is an excellent example of a double star and a multiple star system. Both are conveniently located in the same area of the sky, passing nearly overhead between 8 and 9 p.m. local daylight time.
The double, called Albireo, is one of the most beautiful in the entire night sky, considered by many to be the finest double star in the heavens for a small telescope. Located in the constellation of Cygnus, the Swan, Albireo marks the swans beak.
able --> A small telescope or even a pair of steadily held binoculars will readily split Albireo into two tiny points of light, the brighter one a rich yellowish-orange, the other a deep azure blue, both placed very close together. An absolutely stunning view will come with a telescope magnifying between 18 and 30 power. In Volume Two of his famous "Celestial Handbook," Robert Burnham, Jr. (1931-1993) wrote:
"No more than 30 power is required on a good 6-inch (telescope) to show this superb pair as two contrasting jewels suspended against a background of glittering star-dust."
Albireo is believed by astronomers to be a physical pair, although there has never been evidence of any orbital motion between these two colorful stars. The projected separation between the two is just over 400 billion miles. At least 55 solar systems could be lined-up edge-to-edge, across the space that separates the components of this famous double star.
Nearby to the west is the small constellation of Lyra, the Lyre, which was supposed to represent Apollos harp. In Lyra is the star Epsilon, quite close to the brilliant blue star Vega.
Epsilon Lyrae is better known as the "double-double" star.
Here is a difficult test of good eyesight: Exceptionally good vision on a clear, dark night with no light pollution will reveal Epsilon as two tiny stars (designated Epsilon 1 and Epsilon 2) that are very close together. The separation is 3.5 arc minutes, which is approximately one-ninth the apparent diameter of the full Moon. This feat is probably right at the limit of perfect vision.
Binoculars will make the two stars clearly visible. In a 3-inch telescope each of these two stars are themselves shown to be double stars. Sir William Herschel (1738-1822) was the first to notice them in 1779:
"A very curious double-double star. At first sight it appears double at some considerable distance, and by attending a little we see that each of the stars is a very delicate double star."
So here, in what initially might appear as a single speck of light in the sky, we have a system of four stars, revolving intricately about each other. The two stars that make up Epsilon 1 take at least several hundred years or more to orbit each other. An even longer interval of nearly a thousand years has been assigned to the two stars that make up Epsilon 2.
Meanwhile, the pairs appear to revolve about a common center of gravity with a period that probably is on the order of a million years or more.
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