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Vega, the brightest star of the Summer Triangle, shines in the east on May evenings, with a bright and beautiful radiance that can't be missed. The star made astronomical history in 1836 when it became one of the first to have its distance measured, the magnitude of the feat as impressive as the magnitude of the star itself.
A good two centuries before the unveiling of Vega's distance, Galileo (1564-1642) actually suggested how to compute stellar distances - but lacked the technology to follow through on his own idea. In fact, the success of his "double star" method only came about after a long incubation, not to bear fruit until Wilhelm Struve (1793-1864), Friedrich Bessel (1784-1864) and Thomas Henderson's (1798-1844) individual yet nearly simultaneous triumphs in the nineteenth century.
Galileo's proposed "double star" solution is as simple as it is ingenious. An observer watches a double star through a telescope over the course of a year, noting any change of angular distance between the pair of stars. For a sample of how the procedure works, simply walk outside on a starry night. Shut your right eye and align your finger with any star, looking at it with your left eye. Then shut your left eye, opening your right. You'll notice that your finger appears to move in relationship to that star.
Your right eye represents the Earth's change of position after six months, and your finger shows how the nearer star is expected to shift in respect to the more distant one. If you can accurately measure this shift (parallax), then you can also calculate the nearer star's distance.
Graphic courtesy of Wikipedia
But to keep things in perspective, remember that the parallax of your finger (at an arm's length) equals about six degrees. In contrast, the parallax of even the nearest star is extremely minute, spanning less than one second of arc (1/3600 of a degree). A parallax of one second (parsec) = 206,265 astronomical units = 3.26 light-years.
William Herschel (1738-1822) tried to discern stellar parallax by Galileo's method, but without success. His work with double stars, however, led to important discoveries. For starters, Herschel found out that many double stars are NOT chance alignments of two physically unrelated stars - as most everyone had assumed at the time. Rather, many doubles are binaries: two stars orbiting a common center of mass. It became obvious that the brighter stars are not necessarily any closer than the dimmer ones - shattering a common preconception.
Wilhelm Struve, aware of Herschel's past difficulties, carefully selected his double star, studying Vega and its dim, far-off companion star with the state of the art Fraunhofer telescope. He found that Vega's parallax amounted to about 1/8th second of arc (the diameter of a penny at twenty miles), the angle revealing Vega to be some 25 light-years or 150 trillion miles away.
by Bruce McClure
April 2011 Feature * June 2011 Feature
Cambridge Illustrated History of Astronomy by Michael Hoskin
Parallax, The Race to Measure the Cosmos by Alan W. Hirshfeld