High overhead on early fall evenings, in a busy region of the Milky Way near Deneb, is a rather ordinary double star. With a small telescope at 60x you will see 61 Cygni as a bright warm white primary with a slightly dimmer secondary, also warm white, lying about 30” to the south-southeast.
61 Cygni is more interesting for another of its characteristics. Like Barnard’s Star, 61 Cygni has an unusually high proper motion, causing it to move visibly against the more distant stellar background over the course of relatively few years. At present, 61 Cygni has a proper motion of more than 5” per year.
Since 1838, 61 Cygni also holds a grand place in astronomical history. That year, it became the first star to have its parallax measured. Using the Fraunhofer heliometer at Königsberg Observatory, German astronomer Friedrich Wilhelm Bessel determined that 61 Cygni has a parallax of 0.31” (very close to the modern value of 0.29”), implying that the star was 10.3 light years from Earth (today’s accepted value is 11.41 light years). The Royal Astronomical Society awarded him its Gold Medal for this achievement, which marked the first step towards accurately measuring stellar distances.
The very high proper motion of 61 Cygni results from three factors. First, the star actually moves very fast, clipping along at 67 miles (108 kilometers) per second relative to the Sun. Second, 61 Cygni’s motion is almost perpendicular to our line of sight. Third, the star is very close to us (it is in fact the fourth nearest star that is visible to the naked eye for mid-northern latitudes, after Sirius, Epsilon Eridani, and Procyon A).
For backyard observers whishing to monitor 61 Cygni’s proper motion over the course of several years, the second chart below shows the pair’s path from 1900 to 2100. Notice how the two components of 61 Cygni, A (magnitude +5.20) and B (magnitude +6.05), are passing either side of the 11th-magnitude star GSC 3168:590.
Finder map – field width 35°, stars to magnitude +6.5.
Map – 61 Cygni’s proper motion from 1900 to 2100.
