Expert: Jayendra Upadhye - 4/30/2008

Question
Hello
how do astronomers measure diameter of a Binary star orbit ?
Thank you

Answer
Hi Omid,
Some ways are as under.
1 - Spectroscopy tells them all about the wobble in the spectrum of either star as they go to and fro about the center of gravity of the system.
The periodicity of the wobble (spectral lines move towards the red end and blue end as the stars approach the line of sight and recede).
gives us the period of the orbit.

2 - The Brightness and the composition tell us something about the mass of the star. (more massive stars are bluer at a certain reference age, determined by presence of various elements that appear over age)

3 - By using thses facts, they factor them in and get the orbital parameters. Also of the pair is close enough, telescopes like hubble can actually make out the current separation.

That coupled with linear velocities of recession / approach can be used to judge the whole orbit.

regards

Please do rate the answer if you find it meaningful.
Jayen