calculating semi major axis

[goldsax]

need help please in calculating semi major axis(SMA) of a binary star..
if for example
the orbital distance of star A is 50AU and star B is 100 AU from the centre of mass what would the SMA be?

is it as simple as the half way point ? ie 75AU?

thanks in advance

[Drakkith]

Maybe this will help: http://en.wikipedia.org/wiki/Semi-major_axis#Orbital_period

[goldsax]

thanks for the link, i have had good rethink re. this..
can a binary system that has a circular orbit have a SMA?
or is that called a radii when it is not elliptical?
so for the question concerned would the SMA simply be 100AU?
cheers

[goldsax]

if the speeds of the 2 stars are constant, would r ( distance between M and m fig. b ) remain the same .

if so would r be the SMA? and not the distance between the centre of mass and the orbit of m (fig. a)?

oh i am getting confused!!!!!!!

[Drakkith]

From the article I linked above :

For the special case of a circle, the semi-major axis is the radius.

If this is a simple two body problem with no other objects then the distance between M and m would stay the same.

[goldsax]

many thanks for your guidence!!!