1. The problem statement, all variables and given/known data Hi, I'd really appreciate being pointed in the right direction with this past exam question, as I just have no idea how to approach it and can't find similar examples in my notes anywhere. A nearby star has a spectrum similar to the sun, suggesting it has the same mass. The Doppler shift of lines in the stellar spectrum vary sinusoidally with a period of 10^6s and velocity amplitude of 90m/s The question then asks for things like the mass of the 'invisible companion' to the star, and then lead onto finding the distance from one star to the other as they eclipse, etc. What I'm really asking, is what information can I get using the period, velocity amplitude, and mass? Any help appreciated! 2. Relevant equations Can the Virial Theorem be used? (1/2)I''=2U+Ω where I= Σmr^2, U is kinetic energy and Ω is potential energy. v=(2pia)/P I could find the velocity of the galaxy if I knew the semi major axis, then maybe combine this with the doppler shift information given in the question? 3. The attempt at a solution Some ideas above..