Binary Stars

Find the distance between the stars in a binary system if the overall mass of the system is M= 4.6x10^30  kg and the period of revolution is 8.8 x 10^7  s. Give your answer in Gm (1Gm=10^9m).

So I'm all like: "Easy, use Keplers second law yeah?"

r^3 / t^2 = GM/ (4*Pi^2)  ; where M = (m1 + m2) = mass of the system

so I get r = 0.881 gigameters or 8.81x10^8 meters

But the answer is apparently 391.9 gigameters.

What did I do wrong?

Britany Spears

>>1
Assumed the stars are the same mass?

>>3
How would it differ if they weren't the same mass?

Third law, not second.  And it's been a while since I've had to use it, but I think you fucked up the equation (and I'm too lazy to do any math right now).

>>4
Oh, and don't listen to >>3, he's an idiot.