I just heard this simple reasoning in a documentary film:
Light rays from distant stars are perfectly parallel.
This is pretty interesting thought. In nature, it is hard to find something really perfect. For example, there are no isolated systems -- only quasi-isolated. But here it looks like indeed the light rays are perfectly parallel.
Maybe this can be confirmed by calculations? -- e.g. that angle between two light rays cannot be infinitely small (Planck constant will be probably involved?).