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**1. Homework Statement**

1) Astronomers discover a binary star system that has period of 90 days. The binary star system consists of two equal mass stars each with a mass twice the sun's. They rotate about the center of mass at the midpoint between them. How far apart are the two stars?

2) Three stars each with a mass of our sun, form an equilateral triangle with sides 1* 10^12 m long. The triangle has to rotate because otherwise the stars would collide. What is the period of rotation?

**2. Homework Equations**

**3. The Attempt at a Solution**

For #1,

There doesn't seem to me to be any clear pivot or mass around which the stars orbit, and that's what baffles me. I thought about pretending they were connected by a massless string and calculating the moment of inertia, and then using rotational kinetic energy, but I'm not sure how to start

#2 I think I'll have a better grasp on this after figuring out #1