Seeing Doubles - Brief Article

Observe the stars one by one, two by two, four by four ...

At some time or other, you've probably found yourself staring up at the stars and marveling, "Look at all those points of light!" Maybe for emphasis you added an adjective: "Look at all those individual points of light!" If so, you were wrong.

Contrary to appearances, if not common sense, the majority of "stars" are not individual points. They're systems of multiple stars so far from Earth that to the naked eye, each system appears to be a single entity. The illusion is especially convincing because it reinforces an insidious prejudice, one so fundamental to the way we think about the universe that we might easily overlook it. In fact, for thousands of years, we did. Because we see a singleton, the Sun, in our daytime sky, we assume that the stars in our nighttime sky must be the same.

But the Sun is actually an exception. When British astronomer William Herschel undertook, in the late eighteenth century, the first comprehensive telescopic survey of stars, he started finding doubles everywhere. At first he figured that such pairs consisted of two stars vastly distant from each other but coincidentally lying along a single line of sight--a visual phenomenon we now call optical doubles. And some were. But over time, he observed that most were two stars interacting through their mutual gravitational attraction--physical doubles. Not only did this surprise Herschel, but it provided stunning support for the idea that Newton's theory of gravitation applied beyond our solar system--that it just might be universal, after all.

One especially prominent example--the second point of light from the end of the handle in the Big Dipper--provides a sort of history of our evolving understanding of double stars. This "star" is actually two stars, an optical double consisting of Mizar (the brighter star) and Alcor. (The pair are circled by the red "o" in the photograph at left.) From the earliest days of sky watching, the ability to perceive this optical double served as a test of eyesight among Arabs and Native Americans. But Mizar itself is not an individual star. Most standard sources credit Italian astronomer Giovanni Battista Riccioli with being, in 1650, the first to resolve Mizar (through a telescope) into Mizar A and Mizar B, though recent research indicates that the honor might belong to Benedetto Castelli and date back to 1617--less than a decade after the invention of the telescope. Either way, to Mizar goes the distinction of being the first telescopic binary.

Then in 1889, Harvard astronomer Edward C. Pickering examined the lines in the stars' spectra and found that one of the two, Mizar A, was itself a binary, thereby bestowing on it the distinction of being the first spectroscopic binary. Later analysis revealed the same to be true of Mizar B. A spectroscopic binary, by definition, should frustrate even the most powerful telescope, but in 1996 the Navy Prototype Optical Interferometer focused on the Mizar A binary--which, for the record, comprises Mizar Aa and Mizar Ab--and produced what was at the time the highest-resolution image in the history of optical astronomy.