Why do stars twinkle but the Sun doesn't (I'm asking this because the Sun is also a star)
Stars twinkle because they are effectively a point of light. This point of light can be distorted and magnified by movement of patches of varying density in the atmosphere. These act as lenses causing the twinkle effect.
If you get to space, the stars don't twinkle. Stars appear as a point only because they are so very very distant.
Planets twinkle less because they appear as little discs of light. If one point is being made brighter by the atmosphere perhaps another is being made less bright. By being a disc, the twinkling is smoothed out by the average amount of light making it to your eye.
The sun is a very large disc because it is closer than the other stars. The sun is so large in the sky that the distortion of the light is not very noticeable. However when the sun is setting and its light has to travel through a lot more air to reach the ground, you will notice a shimmering around the edge. This is due to the same kind of atmospheric distortion.
I think you should explain briefly _why_ stars are "points" and the sun is not. It's simple, but still..
I've altered the OP's answer to clarify that they're points because they're far away, then added some further explanation.
IOW, it *does* twinkle, but we don't notice because it's size and brightness overwhelms the minor aberrations which are "twinkles"?
Around sunrise & sunset, sometimes you can actually see the image of the Sun being distorted by the atmosphere: https://gph.is/15yEkPv
Is the term "disc" in this context common? I've never heard that to mean "slightly larger than a point" (which is what I gather you meant by it)..
@maxathousand: Merriam–Webster s.v. _disk_ (1a *the seemingly flat figure of a celestial body: the solar disk*), Oxford s.v. _disc_ (2 *An object or part resembling a disc in shape or appearance* with numerous example sentences, e.g. *Venus will appear as a small disc moving across the sun between 10.45 a.m. and 4.51 p.m.*)
@chirlu Thank you, I had not heard it used in this sense before. My understanding (which could very well be wrong—I'm not even an *amateur* astronomer) is that planets and stars tend to be more spherical than "flat", or disc-like, so this struck me as an odd choice of word, but apparently it's not uncommon.
@maxathousand: They are of course spheres (mostly), but they appear flat when seen from sufficiently far away.