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Cosmic Challenge 61 Cygni: Piazzi's Flying Star

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Cosmic Challenge

61 Cygni: Piazzi's Flying Star

September 2017

Phil Harrington

This month's suggested aperture range

Giant Binoculars, 3- to 5-inch (75 to 125mm telescopes)








61 Cygni

Binary star

21 06.9

+38 44.8


5.2, 6.0


The star 61 Cygni is not bright, nor is it visually distinctive. To the eye alone, it looks just like any other 5th-magnitude point of light deep in the Milky Way flowing through the Swan.

But looks can be deceiving! This unremarkable looking star is indeed quite remarkable for its unusually high rate of proper motion. By watching and plotting it against the backdrop of stars over the course of relatively few years, its position shifts at an extraordinarily fast pace. At present, 61 Cyg has a proper motion of more than 5 arc-seconds per year.

Above: Summer star map from Star Watch by Phil Harrington.


Above: Finder chart for this month's Cosmic Challenge.

Chart adapted from Cosmic Challenge by Phil Harrington.
Click on the chart to open a printable PDF version in a new window.

Why so fast? For one, it's nearby. At just 11.4 light years away, 61 Cyg is the 4th closest naked eye star (albeit only from under dark skies) to our solar system. The three closer stars - Alpha Centauri, Sirius, and Epsilon Eridani - don't show such a high rate of motion, however. So again, why 61 Cyg? While those others may be closer, 61's velocity is higher. The system has a net space velocity of 108 km/s relative to the Sun. That causes 61 to really hit the gas and go!

The Italian astronomer Giuseppe Piazzi (1746-1826), who is also credited with discovering the first asteroid (excuse me, "dwarf planet"), Ceres, was also the first to notice 61 Cyg's rapid motion after completing a 10-year study in 1804. Piazzi called it the "Flying Star," a name that sticks to this day.

Curiously, Piazzi made no mention of the fact that 61 Cyg is a binary star, however, even though both stellar members must have been visible through his telescope. It wasn't until 1830 that German astronomer Friedrich von Struve (1793-1864) announced that 61 Cyg was a binary system.

Eight years after Struve, another German astronomer, Friedrich Bessel (1784-1846), measured 61 Cyg's parallax shift, becoming the first to use that trigonometric method to calculate a star's distance. His estimate of 10.4 light years is impressively close to the modern value of 11.4 light years.

We now know that 61 Cygni is a pair of orange (type K) main sequence stars, both of which are smaller, cooler, and older than our Sun. The primary sun, 61 Cygni A, shines at magnitude 5.2, while 61 Cygni B shines at magnitude 6.0. Each is separated from the other by about 30 arc-seconds. I can just make out that 61 is "oval" through my 8x40s, but resolve them in my 10x50s. Others, apparently with sharper eyes and/optics than mine, have reported a clean separation at 8x. Give it a go and post your experiences in this article's discussion forum.

If you can't quite resolve the pair just yet, take comfort in knowing that time is on your side. As the two stars continue in their 650-year orbit of each other, the gap between 61 Cyg A and B will continue to widen from our perspective. As the diagram below shows, the pair will be at their widest in about the year 2100, when the apparent separation will be 34".

Above: The apparent path of 61 Cyg B around 61 Cyg A.  The pair is heading toward widest separation in approximately 83 years.

The real challenge presented by 61 Cygni is not in splitting the binary, however. Rather, it's monitoring and detecting their collective proper motion over the course of several years. The chart below shows the pair's path from 1900 to 2100. Notice how 61 Cygni A and B passed to either side of an 11th-magnitude field star between 2010 and 2015. That star, GSC 3168:590, actually appeared to lie between the 61 components back in 2011. For a moment, 61 Cyg was a faux triple star system. Again, if you were watching the star during that period, share your experiences in our discussion forum.

Above: 61 Cygni showing proper motion at one year intervals. Photo credit: By IndividusObservantis (Own work) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons.

The 61 pair has continued on by now, leaving GSC 3168:590 behind. Use the finder chart above to follow the stars' progress by marking their precise locations every year or so. Doing so will let you see for yourself, just as Piazzi saw more than 200 years ago, that 61 Cygni truly is the flying star.

Have a favorite challenge object of your own?  I'd love to hear about it, as well as how you did with this month's test.  Contact me through my web site or post to this e-column's discussion forum.

Remember, half of the fun is the thrill of the chase.  Game on!

About the Author:

Phil Harrington writes the monthly Binocular Universe column in Astronomy magazine and is the author of 9 books on astronomy.  Visit his web site at www.philharrington.net to learn more.

Phil Harrington's Cosmic Challenge is copyright 2017 by Philip S. Harrington.  All rights reserved.  No reproduction, in whole or in part, beyond single copies for use by an individual, is permitted without written permission of the copyright holder.


  • skypilgrim, Lewis Cason, okiestarman56 and 2 others like this


A lovely , low-power orange pair in the telescope.
    • PhilH likes this

These really are a nice low power pair.  I have been meaning to start a "near neighbors" observing list so I took a look tonight with the 80mm ED at 17 and 120x in town.  The color was quite orange, even more than many supposedly very red stars appear to me.  But there was some wildfire smoke/haze that had the moon looking yellow...and the aperture was small.


I will take a look in darker skies with more aperture and do a sketch so that I can track the movement in subsequent years. 

    • Knasal likes this

GSC designation  
Note on GSC  : the GSC Identification is made of a plate number (5 digits) and star number on the plate (5 digits); note that, in the literature, a dash separates generally the two parts.


GSC 3168-590 from the VizieR Result Page

Thanx, Phil. I'll have to put this on my list. I did that, photographically, with Pluto about 25 years ago, when I had a good imaging scope. Yep, the dot moved, astounding that Clyde picked that outa the haystack! Things that change up there... in a lifetime... adds perspective to our little corner of the Cosmos.  Tom

    • Jon Isaacs likes this

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