The Astrogators’ Guide to

Gliese 581

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    Not visible to the naked eye, this star does not have an idiosyncratic name (such as Altair or Vega). It is of interest because of its planetary system, described at the end of this article.

Location in Space

Radial Distance:

    Parallax = 0.16091±0.00262 arc-seconds, which leads to;

        1. 20.3±0.3 lightyears (6.2±0.1 parsecs).

        2. 1,278,843±20,627 AU.

Equatorial Coordinates:

    Right Ascension; 15 hr, 19 min, 26.825 sec - 8.2234t sec.

    Declination; -7 deg, 43 min, 20.209 sec - 9.452t 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) by 230 degrees and then south (down) by 7 -3/4 degrees.

Ecliptic Coordinates:

    Ecliptic Latitude; +10.256˚ - 41.27t arc-sec.

    Ecliptic Longitude; 310.48˚ - 116.626t 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 310 degrees, and then shift your gaze northward by 10 -1/4 degrees along a line perpendicular to the Ecliptic into the constellation of Libra.

Galactic Coordinates:

    Galactic Latitude; +40.02˚ - 53.037t arc-sec.

    Galactic Longitude; 354.093˚ - 111.77t arc-sec.

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

    Look toward the radio source Sagittarius-A by casting your gaze across the Orion-Sagittarius Gap toward the Sagittarius-Carina Arm of the galaxy and at a point about six degrees south of the Ecliptic on the west (right) side of Sagittarius and two degrees south of X Sagitarii (the point of the arrow in the Archer’s bow). Then move your gaze in an easterly direction along the plane of the Milky Way by 354 degrees. Then tilt your gaze by 40 degrees in a northerly direction, more or less upward and to your right, at right angles to the plane of the Milky Way.

Annual Proper Motion

    in Right Ascension = -1.23351 arc-sec/yr = 7.6774 AU/yr = 36.3944 km/sec.

    in Declination = -0.09452 arc-sec/yr = 0.5883 AU/yr = 2.7888 km/sec.

        Total Proper Motion = 1.2371 arc-sec/yr = 7.6995 AU/yr = 36.50107 km/sec in a direction 265.62˚ counterclockwise from due celestial north, 250.51˚ counterclockwise from due Ecliptic north, and 244.61˚ counterclockwise from due Galactic north.

    in Radial Distance = -9.5±0.5 km/sec = 2.004 AU/yr.

        Total motion = 7.956 AU/yr = 37.7153 km/sec.

    From the present; in 40,500 years Gliese 581 will reach its perihelion 19.65 lightyears (1,242,239 AU) from Sol in the western part of the constellation of Libra, near the border with southern Virgo, after crossing 14.53 degrees of sky.

Orientation in Space

We generally only get this information for binary stars.

The Star Itself

        Diameter; 403,651 km (0.29 Sol).

        Harvard Class; M3V (3,480±48 Kelvins).

        Age; 7 - 11 billion years

        Mass; 0.31 Sol.

        Brightness; 0.013 Sol.

        Habitable zone: 0.178±0.009 AU, 57±5 days.

        Surface composition: not found. BY Draconis variable.

The Planetary System

    Gliese 581 has three confirmed planets and a debris disc analogous to our solar system’s Kuiper Belt and Oort Cloud. The following list tells what we know about them:

    1. Gliese 581e; The smallest of the confirmed planets, 581e ponders a minimum mass of 1.7±0.2 times Earth’s mass, but may ponder twice as much if the inclination of its orbit matches that of the debris disc (30 – 70 ). The planet revolves about its sun once every 3.1490±0.0002 days on an orbit that has a semi-major axis spanning 0.02815±0.00006 AU and an eccentricity of 0.00 – 0.06, that of an almost perfect circle.   

    2. Gliese 581b; The biggest of the confirmed planets, 581b ponders a minimum mass of 15.8±0.3 Earth masses, but may ponder up to twice as much if the inclination of its orbit matches that of the debris disc. The planet revolves about its sun once every 5.3686±0.0001 days on an orbit that has a semi-major axis spanning 0.04061±0.00003 AU and an eccentricity of 0.00 – 0.03.

    3. Gliese 581c; This planet ponders a minimum mass of 5.5±0.3 Earth masses, but may ponder up to twice as much of the inclination of its orbit matches that of the debris disc. The planet revolves about its sun once every 12.914±0.002 days on an orbit that has a semi-major axis spanning 0.0721±0.0003 AU and an eccentricity of 0.00 – 0.06.

    4. The debris disc; Tilted at an inclination between 30 and 70 , the debris disc contains orbits whose semi-major axes span the range between 25±12 AU and 60 AU. Objects following those orbits revolve about their sun in periods that range from 83,381 days (228.29 years) give or take 59,430 days (162.7 years) to 310,016 days (878.78 years).

    Measurements imply that the disc contains upwards of ten times as many comets as does the debris disc surrounding Sol. That material will support the construction of free-floating cities around Gliese 581, the ideal habitations for a space-faring civilization.

 

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