My understanding is that the particulate matter in the various layers of earth’s atmosphere get in the path of the photons traveling toward us; the twinkling is the particles briefly diffracting the light as they pass that path; if the atmosphere is more dense with material (dust, ions, water vapor, smog, etc.) the apparent effect would be more twinkling. I have heard that in outer space, there is little-to no twinkling effect as there are far fewer particles in zero atmospheric conditions.

The stars always look brighter to me on nights with a new moon (no visible moon).

The concept in astronomy is called seeing—the better the seeing, the clearer (and less twinkly the stars). @sndfreQ is really close—it’s actually thermal diffraction (like heat coming off a road), because the star’s light is coming from such a singular point, the tiniest heat differentials in the atmosphere make things twinkle.

@steve6 you are also correct—the moon tends to light up the atmosphere (at least in lower altitudes & lower humidites), which reduces the contrast of the stars against the sky—I was up at 6000ft on a cool, very dry night—the moon was quite lovely against a nearly black sky.

the clearer the path between you and the star
the brighter it looks