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
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
bound to each other gravitaionally are called binary stars, and larger groups of three, four, or even five
gravitationally linked stars have
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
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
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
1998,2003, 2004. Elizabeth Anne Viau. All rights
reserved. This material may be used by individuals for instructional
purposes but not sold. Please inform the author if you use it