Binary Star Orbits

Each of the two stars orbits around the same point which is called the center of mass of the system. Kepler's Laws govern how each star orbits the center of mass.

In this example, the two stars travel on elliptical orbits, so that sometimes they are further apart than at other times. Can you figure out which star is the more massive? How can you tell?

Apastron is the point where the stars are furthest apart and are travelling slowest in their orbits.

Periastron is the point where the stars are closest together and are travelling fastest in their orbits.

When one star passes directly in front of the other, as viewed from Earth, we seen an eclipsing binary.

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