B is for Binary Stars

I was thinking about Star Wars and the twin suns rising above Tatooine. These would be called binary stars.

What defines a binary star?

A binary star consist of two star. The bigger one being called the primary and the smaller is called the secondary or companion. The two stars then orbit around their common center of mass. If both star were the same mass then they would orbit around a point half way between them. With one having more mass the common center of mass would be closer to that star. A good example in this animated image from Wikipedia. You can see that both the stars are moving and orbit a single point.

Source Wikipedia User: Silver Spoon

How many are there?

They estimate that about 1/3 of the star systems in the Milky Way have 2 or more stars in them.

Are there planets around them?

Kepler 16b is orbiting two stars. The image above the purple dot is Kepler 16b. This type of orbit is called “P-type” or “circumbinary”. The fun bit is that this planet has been given the nickname Tatooine. Unfortunately it has the mass of Saturn so its not exactly habitable. The other type is where the planet orbits only one of the stars. This is called a “S-type”. Gamma Cephei has a planet in an “S-type” orbit around its primary star.


Wikipedia Binary Stars

Wikipedia Kepler 16

Wikipedia  Gamma Cephei

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