Decoherence over long distances - thought expt
Suppose 10,000 years from now, we perform the ultimate macroscopic superposition experiment: We're somehow able to prepare an entire star, complete with a large orbiting planet, as a pure quantum state. It is surrounded by a black shell, of radius 1 light-day, which is maintained at a temperature of 100mK. All other measures are put into place to ensure that the system remains a closed system.
Suppose then we go up to the shell, open a tiny porthole, and observe the (now open) system with a telescope. Would the decoherence of the star/planet (in our reference frame) be rapid, resulting in the observation of a classical situation immediately upon opening the porthole? If so, wouldn't this create the appearance of a violation of locality? Or, would it take the system a day or more to "know" that it is open to an outside environment, and if so, what do you suppose that would look like through the porthole?
I'd be interested in hearing how an answer might be arrived at mathematically (in terms of diagonalization etc.). Thank you.