# How is the Hanbury-Brown and Twiss effect used to measure the size of stars?

I understand what an Hanbury Brown and Twiss (HBT) interferometer does, but how can this be used to measure the apparent angular diameter of some object?

What is the mathematical explaination?

It can be shown[1][2] that the visibility $V$ at a baseline $d$ is equal to: $$V^{2}=\gamma^{2}= \frac{\langle I_1 * I_2 \rangle}{\langle I_1 \rangle \langle I_2 \rangle}$$
Where $I_1$ and $I_2$ are the measurements at two separated detectors, and the angle brackets indicate time averages.