June 2, 2020

AstroEvent: The Passage of 61 Cygni.

To the naked eye observer, the positions of the stars will appear pretty much the same on the day you die as the day you were born; stellar motion doesn’t add up to much over the span of a human lifetime. This week, however, we’ll look at an interesting configuration that just might show some changes through the telescope over the coming years; 61 Cygni. No less an authority than Burnham’s Celestial Handbook lists 61 Cygni as “historically one of the most interesting objects in the heavens.” A good double star for small telescopes, 61 Cygni lies within a few degrees of the bright star Deneb and is currently placed high in the west for northern hemisphere observers immediately after sunset.  Sometimes known as Bessel’s Star or Piazzi’s Flying Star, 61 Cygni attracted the attention of astronomers around 1800 after Giuseppe Piazzi noted a large proper motion for the pair of 5.22” per year towards a direction of position angle 52°. This is extremely fast, currently the seventh fastest known. The pair itself is just above the naked eye visibility threshold at about +5.2 magnitude, and are currently separated by 30+ arc seconds in its 653 year orbit. In fact, the pair of orange-hued stars will reach maximum apparent separation around 2100 A.D., and thus will continue to separate throughout our lifetimes. An interesting fact about the pair came to our attention via a letter published in the November 2010 Sky & Telescope magazine submitted by Richard Stanton of Three Rivers, California: Component A of the pair is currently “flying by” a distant 11th magnitude background star, and tracking its motion over the next few years could provide an interesting challenge. The constellation Cygnus is well placed in the summer months, but you can start acquainting yourself with the pair tonight. The coordinates of the pair are;

Right Ascension: 21h 06m 54s

Declination: +38° 44’ 45”

The background star should currently be approaching a position angle of 26° and a separation of 5” the summer of 2011 and will be at its closest apparent approach on the following year at a separation of less than 3”. Sketching or tracking the pair would be an interesting exercise in observing proper motion… an even more intriguing feat would be to construct a stop motion animation of the motion of the pair. Do give 61 Cygni a look over the next few years, and marvel at the slow change of movement in the heavens!

The astroword for this week is: Proper Motion. This is the apparent shift of stars against the background as seen from our particular vantage point in space. As we wheel about the core of our galaxy, nearby stars appear to slowly shift in position due to their differing relative motion. The measured proper motion is cumulative between the observed stars’ true radial motion and that of our own solar system; generally, the higher the proper motion, the nearer the star is to us. Think of observing a flock of birds passing by; the birds closer will appear to move faster. It was this fact that brought 61 Cygni to the attention of astronomers in the early 19th century; it wound up on a short list of target stars due to its large proper motion, as it was suspected to be nearby in the galactic hood. It achieved historical notoriety in 1838, when Friedrich Wilhelm Bessel successfully measured its stellar parallax at a tiny 0.29” (this was over the span of six months!) giving 61 Cygni a distance of 10.3 light years, close to the accepted value of 11.4. 61 Cygni was the first star to have its stellar parallax measured, and is now known to be the 14th nearest star system from our own Sun. As you look at the tiny pair this holiday season, remember its place in astronomical history and the role it played in discovering that the universe was indeed a vast place!