When you look up at a clear night sky, you can see thousands of stars. They appear to twinkle just like our nursery rhymes told us. The actual term for this is astronomical scintillation. That wouldn't work so well in the nursery rhyme, would it? Let us just stick with twinkling. The fact is that stars don't twinkle, it is because of our atmosphere that they appear to do so. Our atmosphere is made up of various layers, namely, the troposphere, stratosphere, mesosphere, thermosphere, exosphere. The temperature in each layer is different. Hence, the density is different in each layer. Refraction is the process by which light bends when moving from one medium to another. The different layers act as lenses and refract the light of stars. But these lenses aren't stationary. These lenses move around and change shape due to the dynamic nature of the atmospheric layers. So, the refracted starlight keeps changing its direction. The direction can either be towards our eyes or any path apart from that. When these two cases keep alternating it looks like the star is twinkling. One might think, The sun at the center of our solar system is also a star, However, it doesn't twinkle. This is because the other stars are extremely far away from us, due to which they appear very small, even when viewed through a telescope. It is like seeing a dot, all the light is coming from a single point. Hence, the light coming from the other stars is easily disturbed by our atmosphere. But, the sun is much closer to us. Thereby, it appears much bigger than the rest of the stars that twinkle. It looks more like a disk. Hence, the effect of the atmosphere on the light from the sun is negligible. This is why the sun doesn't twinkle.