1. The problem statement, all variables and given/known data See attachment. 2. Relevant equations 3. The attempt at a solution Ok, so for Qd) my attempt was: d = vt = 75x10^3 * (2*2.8*86400*365) = 1.32x10^13 m, (circumference of orbit) because the star periodically shifts from red to blue shifted frequencies every 2.8 years so I guess the full orbit to be 2*2.8 years. Then 2∏r = 1.32x10^13, -> r = 2.1x10^12m? Now for Qe) All I could figure is that the relevant eqn is (2∏/T)2 = G*(ma + mb)/(ra + rb)3 But with the information I have, 1stly from the assumpt that ma >> mb I can say that: (2∏/T)2 = G*(ma)/(ra + rb)3 right? Considering X-major is ma. Now I can rearrange for the mass but I still have missing information, I just have radius of the orbit for x-minor, but not x-major. I even have the orbital period. Just not ra. Can someone give me a helping hand here, how do I find ra? Also have I done everything thus far correctly?