User Guide

LUMINOSITY
Once we determine the actual distance of a star, several other bits of data fall into
place. We can determine how bright the star actually is — its absolute magnitude
— by comparing its apparent magnitude from Earth and its distance. It turns out
that neither Alpha Centauri A nor Alpha Centauri B is among the stars with the
greatest absolute magnitude; their apparent brightness relies a great deal on how
close they are to us. For the sake of any colonization efforts, that turns out to be
a good thing — both stars have habitable zones (which isn’t to say that either star
has a planet orbiting it, much less one actually within the habitable zone).
MASS
The fact that Alpha Centauri is a binary system gives us a great deal more
information about its stars. We know how far away it is, so we can calculate
the average distance between Alpha Centauri A and Alpha Centauri B. If we
know this average distance and their period of revolution (how long it takes
them to complete one revolution around each other), we can determine the
total mass of the system. If we can measure how much each moves relative to
more distant stars, we can calculate which one is more massive, and by how
much. And if we know all of this, we can calculate whether their orbit around
each other is closer to a circle or to a long, narrow oval.
SOLAR SYSTEM COMPARISONS
So how similar are Alpha Centauri A and Alpha Centauri B to the Sun?
Sun Alpha Centauri A Alpha Centauri B Alpha Centauri C
Mass 1.00 1.08 0.91 0.12
Luminosity 1.00 1.45 0.45 0.00006
Spectral Class G2 G2 K5 M5
Surface Temp 1.00 1.00 0.73 0.50
Diameter 1.00 1.09 0.70 0.25
A NEW SUN
APPENDIX 5
214