The relationships between objects in space are governed by gravitational forces.  Matter has mass, and mass is associated with gravity.  Large bodies, such as stars, have a lot of gravitational attraction, and it is the sun's gravity that keeps the planets from just wandering away into space.  The gravity of the earth and the gravity of the moon affect each other's orbits, hold the two bodies together, and generate tides.

    Gravity is a very important force, and astronomers and physicists have developed formulas that describe what it does.  However, so far, we do not know what gravity really is or why it works.  Scientists of the future, here is a challenge for you!

    When stars form, sometimes they form as one star in the center of a protoplanetary disk, but sometimes two or more stars may form there.  Many of the stars that we see appear to be members in stellar groupings.  Two stars that are bound to each other gravitaionally are called binary stars, and larger groups of three, four, or even five gravitationally linked stars have been reported.

     For us as world builders, our questions are about the interactions of stars and planets in these groups.  Here is some information about how they work.

     Groups of stars. or twin planets, rotate around their common center of mass.  This is the point where all the gravitational forces  balance.   Let's think about this in terms of a mobile.

Here are some balls that we are going to use to make a mobile.  It is going to hang from one point by a piece of string.  We can use straws on which to hang the balls.

     You can see for yourself that this mobile is not working.  The top bar should be level horizontally, and it is leaning down toward the two big balls.

     If we can get all the bars level, the mobile will be in balance and can move freely in the breeze.  We will need to move the strings back and forth on the straws to find the points of balance.

     Although groups of stars are not tied together, if they are orbiting in the right paths and situated at the right distance from the common center of mass (the place where our top string would attach in this mobile) they will also be able to move around their common center of mass with no problems.

     If you have time and want to play with this idea, make a mobile and think about how all the different pieces balance around the common center of mass.  You will see that it takes a lot of fiddling around to get it to work perfectly.  When dealing with large, orbiting, rapidly moving stars, the job is no easier!

     Systems with more than two bodies becoming increasingly complex both mathematically and physically.  They also become less stable: another star passing by and disturbing the path of one star in the group can cause the whole group to break up.  The following pages in this section will deal with pairs of stars and in planets.  Binary stars and twin planets can have long term, stable relationships which may permit life to appear on the worlds.