# Homework Help: Binary system calculations.

1. Mar 1, 2013

### Lengalicious

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?

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2. Mar 1, 2013

### TSny

Your work looks correct to me.

Under the assumption that ma >> mb, you have ra << rb. So, I think you can safely neglect ra relative to rb.

3. Mar 1, 2013

### Lengalicious

Lol that does make sense, silly me, thanks. Hopefully that's correct. :)