61 Cygni

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Cygnus, the swan, soars gracefully across the sky tonight. Its brightest stars form a shape like a lower-case letter "T," with the brilliant blue-white star Deneb at its top.

One of the swan's other stars is much fainter than Deneb, but it has an important place in astronomy history: It was the first star other than the Sun whose distance astronomers measured. It's known as 61 Cygni.

Astronomers had tried to measure the distances to stars by measuring their parallax. It occurs because Earth goes around the Sun. So astronomers view the stars from a slightly different perspective in July than in January. This causes a slight shift in a star's apparent position compared to the other stars. The closer the star, the larger the shift should be.

At first, astronomers aimed their efforts to measure parallax at bright stars, like Deneb. They thought these stars looked bright because they were nearby.

But the star that opened the way to parallax was faint 61 Cygni, which is barely visible to the unaided eye.

In 1804, Sicilian astronomer Giuseppe Piazzi discovered that the star is fairly close to Earth. That meant that 61 Cygni's parallax ought to be large, making it fairly easy to measure. In 1838, a Prussian astronomer, Friedrich Wilhelm Bessel, did just that. He made 61 Cygni -- which is 11 light-years from Earth -- the first star outside the solar system with a known distance.

Script by Ken Croswell, Copyright 2008

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