Aaron- It's always a challenge to answer a basic question on long-established facts, since we get out of the habit of thinking about how we know these basics. Here's a tiny fragment of what could be an overwhelmingly long response.
In the cases where the light is indeed reflected sunlight (planets and the moon), the brightness varies substantially depending on the positions of the Sun and the reflector. Venus is much brighter when the Sun is mostly shining on the side we see than when it's mostly shining on the opposite side. (In fact, the planets are so close that with even a crude telescope, you can see that part of the planet is lit up by the Sun and the rest isn't, just like you see by eye for the Moon.) The stars show no such brightness variations.
The precise set of spectral colors in distant stars shows a phenomenon called redshift, which comes in a simple way (Doppler shift) from the motion of those stars away from us. The frequencies of spectral lines coming from individual atoms is reduced by an amount that depends on how fast the stars are moving away. (There are also some blue-shifted stars, moving toward us.) Again, you could mimic that with a set of reflectors moving away from us, but the rates of motion required are so large that each reflector would very soon be at an enormous distance. As the reflector recedes from us, any sunlight reflected would rapidly become vanishingly small. (Reflected light drops off inversely as the fourth power of the distance.) Yet the brightness stays about constant for most stars. Since actual stars, rather than reflectors, are enormously bright sources, they are already
so far away from us that the increasing distance makes no noticeable change in the intensities (which fall off inversely as the square of the distances) over our observation time.
Different stars have different types of light, with different colors due to the different surface temperatures. You might argue that this is because the Sun's light reflects off different color surfaces, but that would take some very special set of surfaces to mimic the set of color spectra expected from stars of different types. Also, many stars show weird bursts of energy, as expected for collapsing stars. Again, it would take a very special set of reflectors to mimic that.
I could go on and on, about binary pairs, etc. but that's enough for now, especially since I suspect that you were playing devil's advocate rather than really having doubts about stars.
(published on 05/16/2013)