Systems are a star(s) with orbiting planets. These are the various star systems available from the Galaxy Map and organized by cluster. Each system contains at least one location you can travel to, usually a planet, but in some cases another ship or small moon.
If a system has a planet for which the semi-major axis (i.e., in game terms, orbital radius) and orbital period are given, the star's standard gravitational parameter (Newton'sgravitational constant multiplied by solar mass) may be derived from Kepler's Third Law. That is,
Mass Effect gives planetary orbital radius, a, in terms of AU, and orbital period, T, in terms of Earth years. From these, we can compute the mass of the star:
(This according to Astronomy Today, 3rd Edition, pub. Prentice Hall Inc., 1998; pp 48-9.)
In order to obtain the highest possible precision for the stellar mass, the attributes of the system planet with the highest precision orbital radius and period should be used in the computation.