The Astrogators’ Guide to

61 Cygni

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    Also called Bessel’s Star, after Friedrich Wilhelm Bessel (1784 Jul 22 – 1846 Mar 17), who became the first astronomer to measure a stellar parallax when he determined the distance to this star. Its age and its speed relative to Sol imply that it is properly a member of the galactic halo and, thus, that any planets it may have are little more than gas balls containing a smaller proportion of heavy elements than do Jupiter and Saturn.

Location in Space

Radial Distance:

    Parallax = 0.28718±0.00151 arc-seconds, which leads to;

        1. 11.36 lightyears ±0.06 lightyear (3.48±0.02 parsecs)

        2. 718,242.91 AU ±3776.64 AU

Equatorial Coordinates:

    Right Ascension; 21 hr, 06 min, 54.6041 sec + 27.55t sec.

    Declination; +38 deg, 44 min, 44.649 sec + 320t arc-sec.

        [elapsed time measured in centuries, Jan 2000 is t = 0]

    Imagine standing on the north side of a plane in which Earth’s equator lies with the north celestial pole directly overhead and look toward the First Point of Aries (now in Pisces just southeast of the Circlet). Then look eastward (to the left) along the celestial equator by 316.73 degrees and then north (up) by 38.74 degrees. 61 Cygni lies about 1/3 of the way from Tau Cygni to Nu Cygni on the sky.

Ecliptic Coordinates:

    Ecliptic Latitude; +51 deg, 53 min, 24 sec + 88.75t arc-sec.

    Ecliptic Longitude; 203 deg, 03 min + 514.9t arc-sec.

        [elapsed time measured in centuries, Jan 2000 is t = 0]

Look toward the First Point of Aries (the point on the sky that the sun occupies on the first day of spring), shift your gaze eastward along the Ecliptic (the line that the sun traces through the Zodiac in the course of a year) by 203.05 degrees, and then shift your gaze northward (+) by 51.89 degrees along a line perpendicular to the Ecliptic.

Galactic Coordinates:

    Galactic Latitude; -5.8274 deg +453t arc-sec.

    Galactic Longitude; 82.3154 deg + 260t arc-sec.

        [elapsed time measured in centuries, Jan 2000 is t = 0]

    Look toward the radio source Sagittarius-A, move your gaze in an easterly direction along the plane of the Milky Way by 82.32 degrees, and then tilt your gaze by 5.83 degrees in a southerly direction at right angles to the plane of the Milky Way.

Annual Proper Motion

    in Right Ascension = 4.13 arc-sec/yr = 14.382 AU/yr = 68.1905 kilometers per second.

    in Declination = 3.2 arc-sec/yr = 11.151 AU/yr =52.835 kilometers per second.

        Total Proper Motion = 5.225 arc-sec/yr = 18.21 AU/yr = 86.27 kilometers per second in a direction 52.25 degrees counterclockwise from due celestial north, 80.22 degrees counterclockwise from due Ecliptic north, and 29.85 degrees counterclockwise from due Galactic north.

    in Parallax = -63.9 kilometers per second (13.48 AU/yr).

        Total motion = 22.66 AU/yr = 107.42 kilometers per second.

    From the present; 35,316 years ago 61 Cygni became an eclipsing binary (for about one century) as its orbital plane passed over the Sol-61 Cygni line and in 18,859 years 61 Cygni will reach its perihelion 9.127 lightyears (577,125 AU) from Sol in the northwestern part of the constellation of Cepheus after crossing 36.5 degrees of sky.

Orientation in Space

    Orbit size: 84.518 AU = the semi-major axis of the combined orbits of the two stars in mutual revolution about their common barycenter. The ellipse of the orbit has eccentricity, e=0.49 ± 0.03. The minimum (periastron) and maximum (apastron) separations between the two stars = 43.1 - 125.93 AU.

    We refer the following to the plane of the sky, which we define as the plane comprising all straight lines that cross our line of sight through the barycenter of the system under study, crossing it at a right angle, and that pass through that barycenter. One of those lines coincides with the system’s line of nodes, the line where the system’s orbital plane crosses the plane of the sky. As a matter of definition, astronomers refer to the node where the system’s secondary crosses the plane of the sky moving at least partly away from Earth as the ascending node.

    Inclination; the angle between the plane of the stars’ orbits and the plane of the sky. Imagine laying the orbit flat on our line of sight to the target star system. In that orientation the stars would appear to us to move from side to side on a straight line. We define that inclination as 90 and we measure the actual inclination of the system by an angle between 0 and 180 . If the system has an inclination less than 90 , the stars appear to revolve counterclockwise about our line of sight (which we imagine always passing through the system’s barycenter) and we call the orbit prograde. If the system has an inclination greater than 90 , the stars appear to revolve clockwise about our line of sight and we call the orbit retrograde.

i=51 ± 2 degrees.

    Position angle of the secondary’s ascending node; the angle between the Equatorial north vector and the line of nodes, measured counterclockwise from the line pointing Equatorial north toward the system’s ascending node.

Ω=178 ± 2 degrees.

    Longitude of Periastron (or Argument of Periastron); the angle between the line of nodes and the orbit’s major axis (line of apsides), measured from 0 to 360 in the prograde direction (the direction of the secondary’s motion) in the plane of the true orbit, from the secondary’s ascending node to the secondary’s periastron

ω=149 ± 6 degrees.

    On a piece of stiff paper draw an ellipse of eccentricity equal to that of the orbit and draw an arrow indicating the direction of the star’s motion on the orbit that the ellipse represents. Imagine standing on the Equatorial plane with the Equatorial north pole directly overhead. Look toward the target star and so hold the paper that the line of apsides coincides with your line of sight and the north vector of the orbit (defined by the right-hand rule: when your right thumb, extended in a thumbs-up gesture, points north, the fingers of that hand curl in the same way that the body moves on its orbit) points Ecliptic north. Designate the farther focus of the ellipse as the barycenter, the system’s center of mass. In that orientation your ellipse has a position angle of +270 degrees, an inclination of 90 degrees, and a longitude of periastron of 90 degrees. Rotate the paper counterclockwise about the line of apsides by 88 degrees (clockwise if you get a negative number). Turn the paper 121 degrees in the retrograde direction about the orbit’s north vector. And then tilt the paper 39 degrees about the line of nodes, turning the paper by the left-hand rule, the appropriate rule stating that when the thumb of the left hand points from the orbit’s ascending node to its descending node, the fingers curl in the direction you must turn the paper. With the paper in that position you have a picture of the orbit of the system’s secondary star. The ellipse represents the orbit of the system’s primary star if you change the position of the barycenter from one focus of the ellipse to the other.

    Orbital Period: 678±34 years.

        Time of Periastron passage: calculated out to AD 2300 from Twentieth Century. The presentation format is:

            1. AD 1709 ± 16 years.

            2. AD 2387 ± 50 years.

The Stars Themselves

    Star A:

        Diameter; 1,002,168 km (0.72 Sol).

        Harvard Class; K5V (4,640 Kelvins).

        Age; 10 billion years

        Mass; 0.70 Sol.

        Brightness; 0.215 Sol.

        Habitable zone: 0.45 AU, 138±14 days.

        Surface composition: Not found. The star is a BY Draconis variable.

    Star B:

        Diameter; 932,573 km (0.67 Sol).

        Harvard Class; K7V (4,440 Kelvins).

        Age; 10 billion years

        Mass; 0.63 Sol.

        Brightness; 0.15 Sol.

        Habitable zone: 0.355 AU, 103±10 days.

        Surface composition: Not found. The star is a flare star.

Planetary system properties:

    stable planetary orbits lie within 1/5 of closest approach of components, 8.6 AU in this case.

    In "Mission of Gravity" and subsequent stories the science-fiction writer Hal Clement made 61 Cygni the home star of the fabulous planet Mesklin, a planet 16 times as massive as Jupiter and rotating on its axis once every 17.75 minutes.


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