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I have received a question about how to calculate the mass of Cygnus x-1 (black star). Since we are able to find the the mass through this Wikipedia page I know that we can find the mass through Newton's 2nd law or it would be better to look through Kepler's 3rd law.

My teacher has told me to describe this formula, on this page.

I just don't understand where $\sin$ comes from and how one can describe this formula.

We know that it is a binary system and almost circular.

I apologize for my English. I'm not from America or any other English speaking countries :)

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Think about how velocities of stars are measured. Do you measure the whole velocity via this method, or do you measure a part of the velocity? – Jerry Schirmer Dec 12 '12 at 20:35

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