World Builders™                                                                        Session One  --  Astronomy                                        Binary Star Systems The stars orbit around their common center of mass.

Some of the stars in the universe are members of binary star systems.   My sources give different percentages, but there are enough of them so that you could plan for a binary star system for your worlds.

Binary stars travel with a partner, another star that is linked to them by gravity.

The stars do not orbit around each other.  They orbit around their common center of mass.

The center of mass is where the fulcrum would be if the stars were balanced on a seesaw.

This is how it works with people.

The weight is the same and the distance is the same on each side.

The board is horizontal because . .

the weights and the distances are the same on both sides of the  Center of Balance.

Now let's do it with stars.

Here is a pair of binary stars:

Each star has about the same mass as our sun.

The center of mass is where the fulcrum would be if this were a seesaw.

Notice that we are talking about mass here.

Mass is about how much matter there is.

Check the page on Weight, Mass and Density to see how weight and mass are related.

The important idea here is that if the mass is the same and the distance is the same on both sides of the center of mass, this system is stable and will work.

So . . .

What happens if the masses are not the same?

Let's look at the seesaw again.

Here is a mom with a little child.

The seesaw will not work if the fulcrum is in the middle.

See where the Center of Balance is.

weight * distance = weight * distance

Again, this works for stars and also for twin planets.

This system has a large star and a tiny star, but it is balanced.

Here is another stable relationship.

mass * distance = mass * distance

4 * 2 = 1 * 8

What happens if a distance changes?

mass * distance = mass * distance

4 * 1 = 1 * 4

So long as the numbers are equal on both sides, the stars may be near each other or distant from one another.

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