[Robert J. Sawyer] Science Fiction Writer
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Alpha Centauri Information

The orbital mechanics of the Alpha Centauri system, as presented in my novel Illegal Alien, were worked out with the help of Jean-Louis Trudel, who was at the University of Toronto at the time I wrote the book, and Edward F. Guinan of the Department of Astronomy and Astrophysics at Villanova University, one of the world's foremost experts on Alpha Centauri. Both are thanked on the acknowledgments page for the novel.

A Few Facts About the Alpha Centauri System

  • Separation between Alpha Centauri A and B varies from 11 to 35 Astronomical Units; they take 80 years to orbit around each other.

  • Stable planetary orbits are only found within 1/5 of the distance of closest approach, meaning 2 AU is the limit for stable orbits around either A or B.

  • From a planet orbiting Centauri A at 1 AU, the apparent magnitude of Centauri B ranges from -18.1 to -20.6

  • From a planet orbiting Centauri B at 1 AU, the apparent magnitude of Centauri A ranges from -21.9 to -19.4

  • Proxima Centauri is currently 13,000 AUs from A and B; no orbital parameters of it are known.

Astronomical Data

                           Sol         A Cen A      A Cen B     Proxima
 Color                   Yellow       Yellow       Orange         Red
 Spectral type               G2           G2           K1          M5
 Temperature              5800K        5800K        5300K       2700K
 Mass                       1.0         1.09         0.90        0.10
 Radius                     1.0         1.2          0.8         0.2
 Brightness                 1.0         1.54         0.44        0.00006
 Apparent magnitude       -26.7         0.0          1.4        11.1
 Absolute magnitude         4.85        4.38         5.74       15.54
 Distance (light-years)     0           4.35         4.35        4.22
 Age (billions of years)    4.6         5            5          **

   ** Age of Proxima is unknown; M-class stars live for many billions of 
   years.  However, Proxima is a flare star, which could imply that it's one 
   billion years old; on the other hand, if it was formed with A and B, 
   obviously it is also 5 billion years old.

Some Sources Used for the Orbital Calculations in Illegal Alien

  • "Does Alpha Centauri Have Intelligent Life?" by Ken Croswell in the April 1991 issue of Astronomy Magazine.

  • "Proxima Centauri: Rotation, Chromospheric Activity, and Flares" by E. F. Guinan and N. D. Morgan, presented at the 1996 meeting of the American Astronomical Society.

  • "Rotation and Stellar Activity of the Stars of the Alpha Centauri Triple Star System" by J. E. Jay, E. F. Guinan, N. D. Morgan, S. Messina, and D. Jassour, presented at the 1997 meeting of the American Astronomical Society.

  • "The Stability of Planets in the Alpha Centauri System" by Paul Wiegert and Matt Holman in the April 1997 Astronomical Journal.

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