Sky Object of the Month – August 2014

Sky Object of the Month – August 2014

61 Cygni  – Double Star in Cygnus

by Glenn Chaple

            Our August “Sky Object of the Month” takes us on an 11.4 light year journey to the binary star 61 Cygni. Discovered in 1753 by the English astronomer James Bradley, 61 Cygni has historical significance as the first star (besides the sun) whose distance was accurately determined.

In 1792, the Italian astronomer Giuseppe Piazzi observed 61 Cygni and noted that its location was noticeably different from what Bradley had recorded. “Piazzi’s Flying Star” was zipping across the sky at a rate of 5 seconds of arc per year – fast enough to cross a distance equal to a moon diameter in just 360 years! Reasoning that 61 Cygni must be a relative neighbor to Earth, astronomers began an earnest effort to calculate its distance through an accurate parallax determination. The feat, finally accomplished by the German astronomer Friedrich Wilhelm Bessel in 1832, is one of the greatest in modern astronomy.

61 Cygni is an easy split, but not an easy find. The accompanying chart shows its location relative to a trapezium formed by xi (x), nu (n), sigma (s), and tau (t) Cygni. There’s no need for high magnification. The same low power (25 – 50X) normally used to locate sky objects will easily bridge the 31.6 magnitude (should read “arc-second) gap separating its magnitude 5.2 and 6.1 components. Both sport K-type spectra and appear golden yellow in the eyepiece. They complete an orbit in a little over 650 years

I could write more about this celebrated system, but an article on 61 Cygni, written by Alan MacRobert coincidentally appears on pages 50 and 51 of the current (August, 2014) issue of Sky and Telescope. MacRobert includes a simple observing project to document 61 Cygni’s proper motion against the background stars.

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