Question concerning Michelson stellar interferometer

[arrektor]

Hello there, I have a question concerning Michelson stellar interferometer.
It is about stars ejecting mass in a thin luminous shell of gas expanding to a size greater than that of star itself.
So it says in the exercise:
a michelson interferometer is used as a Fourier transform spectrometer to examine radiation from an area of sky neart he star so as to include contributions from the front and back of the shell but not from the star itself. the visibility curve obtained for a spectral line of wavelength 656 nm is given( sketch of sinc function).
How can I estimate the velocity of expansion of the shell and the apparent width of the component lines?

Thank you

 

[Andy Resnick]

This is a good question. As a hint, Michaelson interferometers measure *temporal* coherence, not *spatial* coherence. So, what is the visibility curve actually measuring?

 

[arrektor]

I suppose that it has to do with the temporal coherence of the source so this would mean that what we see in the graph is the change in visibility at different times during its expansion?