61 Cygni

61 Cygni

Also known as Bessel’s Star, Piazzi’s Flying Star, GJ 820 A/B, Struve 2758A/B,

ADS 14636 A/B, V 1803 Cyg A/B, GCTP 5077.00A/B

     Finally. A nearby star that just maybe/possibly/might be visible by naked eye alone.

     Well, with a combined apparent magnitude of 4.8, perhaps barely so, or at least from Howard County. It should be an easy catch from any dark sky site, other than for its location within the relatively crowded star fields of the summer Milky Way. And in addition to being of interest due to its proximity to the Sun, 61 Cygni is also a significant star both historically and in its own right.

     First of all, 61 Cygni is actually two stars (A and B), gravitationally bound in orbits quite similar to those of Struve 2398’s two components. It takes 653 years for them to circle about a common center of gravity at an average distance of 85 AU (approximately the diameter of Pluto’s Orbit). At their closest, they can be separated by as little as 51 AU, whilst at the other extreme they move as far apart as 119 AU. The two stars are both main sequence orange dwarfs of spectral class K, somewhat smaller than the Sun (0.70 and 0.63 solar masses, respectively), and correspondingly smaller and cooler. They rank among the coolest, dimmest main sequence stars capable of being seen without optical aid. Both stars appear to be the same age, and are older than the Sun (perhaps as much as ten billion years old, but more probably closer to six). Apparently not members of the Milky Way’s thin disk population (see the posting for Lalande 21185), they are “just passing through” from their probable origin in the galactic halo. Evidence for this is both their low metallicity (about one quarter of the Sun’s), and their velocity with respect to the solar system – a whopping 67 miles per second, moving roughly in the direction of Orion’s Belt. Remarkably similar to each other, it is safe to assume that A and B were formed as a binary pair.

Both have quite similar rotation periods (35 and 37) days, with their sunspot cycles being 7 and 11 years respectively. 61 Cygnus A is classed as a BY Draconis variable, meaning that its sunspot cycle is so extreme (compared to the Sun’s) that it can fluctuate by as much as 0.5 magnitude as it rotates, turning its massive starspots toward and away from out view. Meanwhile, the smaller B component shares a characteristic with the even dimmer red dwarfs of being a flare star, exhibiting at intervals a sudden and massive brightening across the spectrum. Such activity makes both stars poor candidates for a place to find stable enough conditions for life to exist on any hypothetical planet (for which there is no evidence).

     But 61 Cygni is also quite significant for the role it has played in astronomical history. It first came to prominence in the year 1804 due to painstaking observations by Italian Catholic priest, Giuseppe Piazzi (perhaps better known for his discovery three years previously of the asteroid Ceres). Piazzi noted that the star had detectible motion relative to the stars around it, and christened it the “Flying Star”. Such movement is known as “proper motion”. It took another eight years for the German mathematician Friedrich Wilhelm Bessel to bring Piazzi’s discovery to the attention of the worldwide astronomical community. It was Bessel who realized that 61 Cygni’s rapid movement across the sky indicated that it was likely among the closest stars to the sun. This gave him the idea of using the star to determine the distance to the stars in general.

     For until the 19th Century, no one had any real idea just how far away the stars were. This had been a question bedeviling astronomers since ancient times. The great Ptolemy, foiled in every attempt to determine the figure, finally came to the reasonable conclusion that for all practical purposes they should be considered as infinitely distant. In more modern times, it was generally accepted since Copernicus that measurements ought (at least in theory) to be able to be made using parallax. Parallax is the method by which the human brain is able to perceive what our eyes see in three dimensions. Hold up your thumb at arm’s length and look at it, first using only your left eye, then your right. You will notice an apparent jumping back and forth of your thumb relative to objects in the background. Of course the thumb isn’t really moving, but the angle of observation is changing as you move from one eye to the other. Bessel proposed doing the same thing with a nearby star, using opposite sides of the Earth’s orbit as the “eyes” (see illustration below).

     Bessel kept up exacting measurements of 61 Cygni’s differing aspects at different times of the year from 1812 to 1838, by which time he was able to determine that it appeared to wobble back and forth with respect to the more distant stars by 0.3136 arcseconds, suggesting a distance of 10.4 light years. He was off by one light year (61 Cygni being 11.4 ly distant). But despite the 8.8% error, 61 Cygni became the first star ever to have its distance from the Earth known to any degree of accuracy whatsoever.

     In 1911, American astronomer Benjamin Boss expanded upon 61 Cygni’s astronomical importance by discovering that the star was one of at least 26 that shared nearly identical velocities with respect to the Sun, and proposed that this “co-moving group” was physically related (perhaps the remnants of a former open star cluster). The various members of this group of fellow travelers are scattered all across our skies, in constellations as various as Taurus, Virgo, Cygnus, Mensa, and Columba.

     To observe 61 Cygni, you need nothing more than a clear sky and a pair of binoculars. Start by locating the constellation Cygnus. If that’s too difficult, then look for the huge “Summer Triangle” of Deneb, Vega, and Altair. If you can’t locate that, then it’s time to consult a good star atlas. In any case, once you’re looking in the direction of Cygnus, mark the northwest corner of the Northern Cross asterism (formed by Deneb, Albireo, Delta and Epsilon Cygni, with Sadr at the center). 61 Cygni would serve as the fourth corner of an imaginary quadrilateral at the upper left of the cross (although it would be by far the dimmest of the four corner stars). If your sky is moonless and sufficiently dark, you may even be able to spot it with your eye alone. If suburban light pollution is an issue, a good pair of binoculars should do the trick. In fact, due to their 30+ arcseconds separation, a pair of 7X50 binos ought to be enough to split the double star into its A and B components.

     But by all means don’t stop there. Pull out your telescope, and get a good look at this fascinating star system, and recall to mind how much astronomical history is displayed before your eyes in this object. Besides, after this “rest stop”, it’s on to one of the most difficult stars of all on our journey through the Solar Neighborhood – EZ Aquarii.