Mellita quiquiesperforata

The "stars" that you found inside the sand dollar -- were there five of them, and were they shaped like "doves?" These are the five parts of the sand dollar mouth! The scientific name is Mellita quiquiesperforata. The 5 holes are called lunules and may serve a couple of different purposes. They help prevent the living animals from being washed out of the sand (something to do with hydrodynamics of the water currents). And they help the sand move upwards as the animal buries itself.

Their 5 mouth parts form what's referred to as "Aristotle's lantern".
That's the 5 doves you hear rattling around in dead sand dollars.

When alive, they are covered with tiny brownish green spines that help the animal move and bury. Lots of things each sand dollars, including flounders and sea stars.
Sand Dollar Doves